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CHEMICAL RESEARCHAND DEVELOPMENTFOR MOLTENSALT BREEDERREACTORS W. R. Grimes ABSTRACT kq .; c

Results of the 15-year program of chemical research and development for molten salt reactors are summarized in this document. These results indicate that 7LiF-BeFz-LJFb mixtures are feasible fuels for Such mixtures show satisfactory phase bethermal breeder reactors. havior, they are compatible with Hastelloy N and moderator graphite, and they appear to resist radiation and tolerate fission product accumulation. Mixtures of 7LiF-BeF2-ThF4 similarly appear suitable as blankets for such machines. Several possible secondary coolant mixtures are available; NaF-NaBF3 systems seem, at present, to be the most likely possibility. Gaps in the technology are presented along with the accomplishments, and an attempt is made to define the information (and the research and development program) needed before a Molten Salt Thermal Breeder can be operated with confidence.

NOTICE

4 .”: ’y a

This document contains information of a preliminary nature and was prepared primarily for internal use at the Oak Ridge National Loboratory. It is subject to revision or correction and therefore does not represent a final report. The information is not to be abstracted, reprinted or otherwise given public dissemination without the approval of the ORNL potent branch, Legal and Information Control Department.

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A. Makes any worronty or rrpresentation, expressed or implied, with respect to the occuracy, completeness, or usefulness af th. information contained in this report, or that the use of ony information, apparatus. method, or process disclosed i n this report may not infringe privately owned rights; or B. A s s u m s any liabilities with r e s any information, apparatus, method,

or for damages resulting from the use of D.

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CONTENTS

. . . . . . . . . .. . . . . . . . . Selection of MSBR S a l t Mixtures . . . . . . General Requirements for t h e Fluids . .

Abstract

3 3

Choice of Fuel and Blanket Composition

....... Fuel and Blanket Compositions . . . . . Choice of Coolant . . . . . . . . . . . Physical P r o p e r t i e s of MSBR Liquids . . Oxide-Fluoride E q u i l i b r i a

29 30

34 37 37

.. S t a b i l i t y of UF3 and UF4 . . . . . . . Oxidation (Corrosion) of Hastelloy N .

Chemical Compatibility of MSRE Materials

Compatibility of Graphite with Fluorides

45 46

Radiation E f f e c t s .. .. . Behavior of Fission Products i n Molten S a l t s Physical Chemistry of F i s s i o n Products

. .

N e t Oxidizing P o t e n t i a l of F i s s i o n Process

......... . General . . . . . . . . . . . . . . . . . . Corrosion i n MSRE . . . . . . . . . . . . . . Behavior of Fission Products . . . . . . . . Molten-Salt Production Technology . . . . . , . . Production Process . . . . . . .,. . . . . . MSRE Salt-Production Econmics . . . . . . . Chemical Behavior i n MSRE

Separations Processes i n MSBR Fuels and Blankets

Possible Separation of R a r e Earths from Fuel

......... Proposed Program of Chemical Development . . . . References ...... . ..........

MSBR In-Line Analysis Program

L _ -E G A L N O T I C E f h l m r e p - r a s prepared as UL account of Government mpon.orad m r k . Nelther the Onlted the ~mmimsion.nor MYperwn u?% on behall of the Commlsslon: ~ t e . y & s .ogwuranty o r representation, sxpresmd o r implid, with respect to the acwracy completeness. o r usetulness d the information contained in this repon, or that the use ’ of L y informailon. appu’atlu. method, o r process d l r l o s d in this rep- rmy w t Lntrue privately 04 rlKhtS; O r B &,sues any lhblllues with respect to the use of. o r for damages msdfrom the o;w ~nformatlon,apparatus. metbod. o r process a r ~ o ~ in e d this report. 00 behalf of the Commlsslon” bcldes ULY em&-.b0se, **psr-

p~oyeeo r contractor of the ~ommlssion.or employse of such contractor. to the extent that mch employee o r contractor of the Commission, o r employse of much contractor prepares. dlsmmfnrtes, o r provideo access to. m y Information W s m t to his emplogmsllt or contract rithme Commission. o r his employmmt wlih much cpntrndor.

57 57 65 68 68 69 71 82

82 86 89 89 102 120

133

CHEMICAL RESEARCH AND DEVELOPMENT FOR MOLTEN-

SALT BREEDER REACTORS Use of molten f l u o r i d e s as f u e l s , blankets, and coolants o f f e r s a promising and v e r s a t i l e route t o thorium breeder r e a c t o r s .

Mixtures con-

t a i n i n g f i s s i l e and/or f e r t i l e materials have been s t u d i e d i n considerable d e t a i l , and shown t o possess liquidus temperatures, phase s t a b i l i t y , and physical p r o p e r t i e s which are s u i t a b l e f o r t h e purpose.

These fluo-

r i d e mixtures appear t o be compatible with s t r u c t u r a l metals and with

g r a p h i t e s u i t a b l e f o r use i n a Molten S a l t Breeder Reactor; such compatib i l i t y seems assured under i r r a d i a t i o n a t MSBR conditions.

Cheap, low-

m e l t i n g f l u o r i d e coolants f o r MSBR have not y e t been demonstrated but

promising leads a r e a v a i l a b l e ; t h e r e l a t i v e s i m p l i c i t y of t h e coolant problem lends assurance t h a t a reasonable s o l u t i o n can be found.

A reference design f o r a 1000 MW(e) Molten S a l t Breeder Reactor has

r e c e n t l y been published.â&#x20AC;&#x2122;

The s t a t e of knowledge of molten s a l t s as

materials f o r use i n t h a t r e a c t o r and i n a t t r a c t i v e a l t e r n a t i v e s or improvements i s described i n some d e t a i l i n t h e following pages.

An at-

tempt i s made t o define those areas where a d d i t i o n a l knowledge i s necess a r y o r very d e s i r a b l e and t o estimate t h e e f f o r t required t o obtain t h i s knowledge for a molten s a l t breeder r e a c t o r and a breeder r e a c t o r experi-

ment. SELECTION OF MSBR SALT MIXTURES~+

General Requirements f o r t h e Fluids A molten s a l t r e a c t o r makes t h e following s t r i n g e n t minimum demands

upon i t s f l u i d f u e l .

me f u e l must consist of elements of low (and prefer-

ably very low) capture cross s e c t i o n f o r neutrons t y p i c a l of t h e energy spectrum of t h e chosen design.

The f u e l m u s t dissolve more than t h e

c r i t i c a l concentration of f i s s i o n a b l e m a t e r i a l at temperatures s a f e l y below t h e temperature a t which t h e f u e l leaves t h e heat exchanger.

The mix-

t u r e must be thermally stable and i t s vapor pressure must be low over t h e operating temperature range.

The f u e l mixture must possess heat t r a n s f e r

and hydrodynamic p r o p e r t i e s adequate f o r i t s s e r v i c e as a heat-exchange fluid.

It must be r e l a t i v e l y non-aggressive toward some otherwise suit-

able material--presumably

a metal--of

able moderator material.

The f u e l must be s t a b l e toward r e a c t o r radia-

construction and toward some s u i t -

t i o n , m u s t be a b l e t o survive f i s s i o n of t h e uranium--or material--and

other fissionable

must t o l e r a t e f i s s i o n product accumulation without s e r i o u s

d e t e r i o r a t i o n of i t s u s e f u l p r o p e r t i e s . I f such r e a c t o r s a r e t o produce economical power we must add t o t h i s

l i s t t h e need f o r r e a c t o r temperatures s u f f i c i e n t l y high t o achieve genuinely high q u a l i t y steam, and w e must provide a s u i t a h l e l i n k ( a secoadary coolant) between t h e fuel c i r c u i t and t h e steam system.

must a l s o be assured of a genuinely low f u e l cycle c o s t ; t h i s presupposes

a cheap f u e l and an e f f e c t i v e turn-around of t h e unburned f i s s i o n a b l e m a t e r i a l o r (more reasonably) an e f f e c t i v e and economical decontamination and reprocessing scheme f o r t h e f u e l . If t h e r e a c t o r i s t o be a breeder w e must impose even more s t r i n g e n t

l i m i t s on permissible p a r a s i t i c neutron captures by t h e r e a c t o r m a t e r i a l s and provide s u f f i c i e n t f e r t i l e m a t e r i a l e i t h e r i n a breeder blanket o r i n t h e f u e l ( o r i n both).

If a blanket i s used it must be separated from

bi L

t h e f u e l by some material of very low neutron cross s e c t i o n . The demands imposed upon t h e coolant and blanket f l u i d s d i f f e r i n

obvious ways from t h o s e imposed upon t h e f u e l system. w i l l be considerably less i n t h e blanket--and

ant--than

i n the fuel.

Radiation i n t e n s i t y

markedly less i n t h e cool-

Efficiency of t h e blanket mixture as a heat t r a n s -

f e r agent may be r e l a t i v e l y unimportant, but a high concentration of f e r t i l e material i s e s s e n t i a l and an e f f e c t i v e recovery of bred m a t e r i a l i s l i k e l y t o be v i t a l . Choice of Fuel and Blanket Composition General Considerations The compounds which are permissible major c o n s t i t u e n t s of f u e l s or blankets f o r thermal breeders are those t h a t can be prepared from beryl-

l i u m , bismuth, boron-11, carbon, deuterium, f l u o r i n e , lithium-7, n i t r o gen-15, oxygen, and t h e f i s s i o n a b l e and f e r t i l e isotopes.

A s minor

c o n s t i t u e n t s one can probably t o l e r a t e compounds containing t h e elements l i s t e d i n Table 1. O f t h e known compounds containing u s e f u l concentrations of hydrogen

( o r deuterium) only t h e hydroxides of t h e a l k a l i metals, t h e s a l i n e hydrides of l i t h i u m and calcium, and c e r t a i n i n t e r s t i t i a l hydrides ( z i r -

conium hydride, f o r example) show adequate thermal s t a b i l i t y i n t h e 1000째F t o 1300째F temperature range.

[Acid f l u o r i d e s (NaHF2, f o r example] might

be permissible i n low concentrations a t lower temperatures.

The hy-

d r i d e s are very s t r o n g reducing agents and are most u n l i k e l y t o be u s e f u l components of any uraniferous l i q u i d f u e l system.

A l k a l i hydroxides dis-

solve extremely small q u a n t i t i e s of uranium compounds at u s e f u l r e a c t o r temperatures and are very corrosive t o v i r t u a l l y all u s e f u l metals a t

c"'

6 such temperatures.

One concludes , t h e r e f o r e , t h a t hydrogen-rich com-

pounds, which might provide self-moderation t o molten f u e l s , a r e not use-

ful i n p r a c t i c a l f u e l o r blanket mixtures. The non-metals carbon, nitrogen , s i l i c o n , sulfur, phosphorus, and oxygen each form only high melting and generally unsuitable binary compounds w i t h t h e metals of Table 1. From t h e s e non-metals, however, a wide v a r i e t y of oxygenated anions a r e a v a i l a b l e .

Nitrates , n i t r i t e s ,

sulfates, and s u l f i t e s can be dismissed as lacking adequate thermal stab i l i t y ; s i l i c a t e s can be dismissed because of undesirably high viscosi-

ties.

Phosphates , b o r a t e s , and carbonates a r e not s o easy t o eliminate

without study, and phosphates have, i n f a c t , received some a t t e n t i o n . The s e v e r a l problems of thermal s t a b i l i t y , corrosion, s o l u b i l i t y of urani-

um and thorium compounds, and, e s p e c i a l l y , r a d i a t i o n s t a b i l i t y would seem t o make t h e use of any such compounds very doubtful. When t h e oxygenated anions a r e eliminated only f l u o r i d e s and chlorides remain.

Chlorides o f f e r mixtures t h a t a r e , i n general, lower

melting than f l u o r i d e s ; i n a d d i t i o n UCl3 i s probably more s t a b l e than UF3 w i t h respect t o t h e analogous t e t r a v a l e n t compounds.

For thermal r e a c t o r s ,

f l u o r i d e s appear much more s u i t a b l e f o r reasons which include (1)usefuli

ness of t h e element without isotope s e p a r a t i o n , ( 2 ) b e t t e r neutron economy, (3) higher chemical s t a b i l i t y , ( 4 ) lower vapor p r e s s u r e , and

(5) higher heat capacity per u n i t weight o r volume.

Fluoride mixtures

a r e , accordingly, p r e f e r r e d as fuel and blanket mixtures f o r thermal reactors.

The f l u o r i d e i o n is capable of some moderation of neutrons;

t h i s moderation i s i n s u f f i c i e n t f o r thermal r e a c t o r s w i t h cores of reason-

able s i z e .

An a d d i t i o n a l moderator m a t e r i a l i s , accordingly, required.

W I

4 -

7 Table 1. Elements o r Isotopes Which May be Tolerable i n High Temperature Reactor Fuels

Material

Absorption Cross Section (Barns )

Nitrogen-15

0.000024

Oxygen

0.0002

Deuterium

0.00057

Carbon

0.0033

Fluorine

0.009

Beryllium

0.010

Bismuth

0.032

Lithium-7

0.033

Boron-11

0.05

Magnesium

0.063

Silicon

0.13

Lead

0.17

Zirconium

0.18

Phosphorus

0.21

Aluminum

0.23

Hydrogen Calcium

0.33 0.43

Sulfur

0.49

Sodium

0.53

Chlorine-37

0.56

Tin

0.6

Cerium

0.7

Rubidium

0.7

8 Choice of Active Material Uranium Fluoride.

- Uranium hexafluoride

i s a highly v o l a t i l e com-

pound c l e a r l y unsuited as a component of high-temperature l i q u i d s .

U02F2,

though r e l a t i v e l y n o n v o l a t i l e , is a strong oxidant which should prove very d i f f i c u l t t o contain,

Fluorides of pentavalent uranium ( U F

5' U2F9'

e t c . ) a r e not thermally s t a b l e and should prove p r o h i b i t i v e l y corrosive i f they could be s t a b i l i z e d i n solution. Uranium t e t r a f l u o r i d e (UF4) i s a r e l a t i v e l y s t a b l e , non-volatile, non-hygroscopic m a t e r i a l , which i s r e a d i l y prepared i n high p u r i t y .

melts at 1035OC, but t h i s freezing point i s markedly depressed by s e v e r a l useful diluent fluorides.

Uranium t r i f l u o r i d e (UF ) i s s t a b l e , under 3

i n e r t atmospheres, t o temperatures above 1000째C, but it disproportionates

at higher temperatures by t h e r e a c t i o n 4UF3

3uF4 +

uo.

Uranium t r i f l u o r i d e is appreciably l e s s s t a b l e i n molten f l u o r i d e s o h -

t ions. 435

It i s t o l e r a b l e i n r e a c t o r f u e l s only i n s o f a r as t h e e q u i l i b r i -

um a c t i v i t y of uranium metal i s s u f f i c i e n t l y low t o avoid r e a c t i o n with t h e moderator g r a p h i t e or alloying with t h e container metal. concentrations of UF

Appreciable

( s e e below) a r e t o l e r a b l e i n LiF-BeF2 mixtures such

as those used i n MSRE and proposed f o r MSBR.

I n g e n e r a l , however, uranium

t e t r a f l u o r i d e must be t h e major uraniferous compound i n t h e f u e l . Thorium Fluoride.

A l l t h e normal compounds of thorium a r e quadri-

v a l e n t ; ThF4 (melting at 1115OC) must be used i n any thorium-bearing fluoride m e l t .

Fortunately, t h e marked freezing point depression by useA

f u l d i l u e n t s noted above f o r uranium t e t r a f l u o r i d e a p p l i e s also t o thorium

9 tetrafluoride. Choice of Fluoride Diluents The f u e l systems f o r thermal r e a c t o r s of t h e MSRE and MSBR types

r e q u i r e low concentrations (0.2 t o 1 mole

of uranium, and t h e proper-

t i e s ( e s p e c i a l l y t h e melting temperature) of such f u e l s w i l l be e s s e n t i a l l y those of t h e d i l u e n t mixture.

Blanket mixtures (and perhaps f u e l

systems f o r one-region breeders) w i l l r e q u i r e considerable concentrations of high-melting ThF4.

The f u e l s must, i f they are t o be compatible w i t h

l a r g e steam t u r b i n e s , be completely molten at 975OF (525OC). Simple consideration of t h e nuclear p r o p e r t i e s leads one t o p r e f e r as d i l u e n t s t h e f l u o r i d e s of B e , B i , 7 L i , M g , Pb, Z r , C a , Na, and Sn ( i n

t h a t order).

Equally simple considerations (Table 2 ) of t h e s t a b i l i t y of

d i l u e n t f l u o r i d e s toward reduction by common s t r u c t u r a l metals, 6 ,7 howe v e r , serve t o eliminate BiF3, PbF2, and probably SnF2 from consideration. i

No s i n g l e f l u o r i d e can serve as a u s e f u l d i l u e n t f o r t h e a c t i v e

fluorides.

BeF i s t h e only s t a b l e compound l i s t e d whose melting p o i n t 2

i s c l o s e t o t h e required l e v e l ; t h i s cmpound i s t o o viscous f o r use i n t h e pure state.

The very stable f l u o r i

of t h e a l k a l i n e e a r t h s and of y t t r i u m an

cerium do not seem t o be u s e f u l major c o n s t i t u e n t s of low melting f l u i d s .

Mixtures containing about 10 mole % of a l k a l i n e e a r t h f l u o r i d e with BeF2 m e l t below 5OO0C, but t h e v i s c o s i t y of such melts i s c e r t a i n l y t o o high f o r s e r i o u s consideration. Some of t h e p o s s i b l e combinations of a l k a l i f l u o r i d e s have s u i t a b l e f r e e z i n g points.8

Equimolar mixtures of LiF and KF m e l t a t 490째C, and

mixtures with 40 mole

% LiF and 60 mole % RbF melt at 470OC.

The t e r n a r y

Table 2.

Relative S t a b i l i t y a of Fluorides

For Use i n High Temperature Reactors

Compound

Free Energy of Formation at 1000째K (kcal/F atm)

Me It i n g Point ("C)

Absorption Cross Sectionb f o r Thermal Neutrons (barns )

S t r u c t u r a l Metal Fluorides CrF2 FeF2 NiF2

3.1 2-5

-74 -66 5 -58

1330

4.6

-125

1330

-125 -124 -123

848

0.43 0.033' 1.17

1100 930

D i luent Fluorides ca2 LiF Be2 SrF2 CeF3 YF3 NF2 RbF NaF KF BeF2 ZrF4

m3 SnF2 PbF2 BiF3

-118

1280 1400 1430

-113

1144

-113

1270

-112 -112 -109 -104

792 995 856 548

-94

903 1404 213 850 727

-90 -62 -62 -50

1.16 0.7 1.27 0.063 0.70 0.53

1.97 0.010 0.180 0.23 0.6 0.17 0.032

Active Fluorides -101

-95 3 -100.4

1111 1035

1495

8Reference state i s t h e pure crystalline s o l i d ; t h e s e values are, accordi n g l y , only very approximately those f o r s o l u t i o n s i n molten mixtures. b0f M e t a l l i c ion. %ass s e c t i o n f o r 7L i .

. . -.

i i

systems LiF-NaF-KF and LiF-NaF-RbF have lower melting regions than do these binaries.

A l l t h e s e systems w i l l d i s s o l v e UF4 at concentrations up

t o s e v e r a l mole % at temperatures below 525OC.

"hey might w e l l prove use-

f u l as r e a c t o r f u e l s i f no mixtures with more a t t r a c t i v e p r o p e r t i e s were

available.

Mixtures with useful melting p o i n t s over r e l a t i v e l y wide ranges of

8 composition are a v a i l a b l e i f ZrF4 i s a major component of t h e system.

j I

Phase r e l a t i o n s h i p s NaF-ZrF4 system show low melting p o i n t s over t h e

i n t e r v a l 40 t o 55 mole % ZrF4.

A mixture of UF,4 with NaF and ZrF4 served

as fule f o r t h e A i r c r a f t Reactor Experiment.

The lowest melting binary mixtures of t h e usable d i l u e n t f l u o r i d e s

are t h o s e containing BeF2 with NaF or LiF. LiF-NaF-BeF2

, . i

(The t e r n a r y system

has been examined i n some d e t a i l , but it seems t o have no

important advantage over e i t h e r binary.) ,

Since Be o f f e r s t h e b e s t cross

s e c t i o n of t h e d i l u e n t s (and 7L i ranks very h i g h ) , f u e l s based on t h e LiF-BeF2 di-luent system were chosen f o r MSRE and are proposed f o r MSBR.

The binary system LiF-BeF2 has melting p o i n t s below 5OO0C over t h e concentration range from 33 t o 80 mole % BkF2. LiF-BeF

The p r e s e n t l y accepted

system diagram, presented i n Fig. 1, i s characterized by a s i n g l e

e u t e c t i c (52 mole % BeF2, melting at 360'~) between BeF2 and 2LiF*BeF2. t

The compound 2LiF-BeF2 melts incongruently t o LiF and l i q u i d a t 458OC. LiF*BeF2 i s formed by t h e r e a c t i o n of s o l i d BeF2 and s o l i d 2LiF=BeF2beOW 280Oc.

i f

. 4

900 848

800

700

r t

ORNL-DWG 66-7632

Li F+ LIQUID

-Y

60C

555

E t

4 OC LiF+2LiF-BeF2

30C

-2 N J

20(

F - B e F u?

+ LiF-BeF2

2LiF*BeF2+BeF2 (HIGH QUARTZ TYPE) I I I 280 LIF.BeF2 I + BeF2 (HIGH QUARTZ TYPE)

LiF.BeF2+ BeF2 (LOW QUARTZ TYPE),

220

I 90

BeF2

BeF2 (mole %)

Fig. 1. The System LiF-BeF2

i [

LiF-BeF2 Systems with Active Fluorides

e u t e c t i c containing very l i t t l e UF4.'

The phase diagram of t h e BeF2-UF4 system (Fig. 2 ) shows a s i n g l e

That of t h e LiF-UF4 system (Fig. 3)

shows t h r e e compounds, none of which m e l t s congruently androne of which shows a low temperature l i m i t of s t a b i l i t y . lo The e u t e c t i c mixture of 4LiF-UF4 and LiF*UF4 occurs a t 27 mole % UF4 and m e l t s a t 4gOOC. nary system LiF-BeF

as Fig. 4.' '

The ter-

UF4, of primary importance i n r e a c t o r f u e l s , i s shown

The system shows two e u t e c t i c s .

These are at 1 mole % UF4

and 52 mole % BeF2 and at 8 mole % UF4 and 26 mole % BeF2; they m e l t at

350 and 4 3 5 O C , respectively.

Moreover, t h e system shows a very w i d e range

of compositions melting below 525OC. The system BeF ThF4 i s very similar t o t h e analogous UF4 system.

The LiF-ThF4 system (Fig. 5) contains four compounds.12 .The compound

3LiF-ThF4 m e l t s congruently a t 58OOC and forms e u t e c t i c s a t 57OoC and 22 'r

mole % ThF4 and 5 6 0 ' ~ and 29 mole % ThF4 with LiF and with LiF*ThF4, respectively.

The compounds LiF'ThF4, LiF-2ThF4, and LiF' 4ThF4 m e l t in-

congruently a t 595OC and 890Oc. Fig.

6 ) shows only a s i n g l e e u t e c t i c w i t h

and 1.5 mole % ThF4 m e l t i

The t e r n a r y system ZiF-BeF2-ThF4 (see

6Oc.l'

t h e composition 47.0 mole

% LiF

I n s p i t e of s m a l l ' d i f f e r e n c e s due

t o t h e phase f i e l d s of LiF*2ThF4, 3LiF*ThF4, and 4LiF*UF4, t h e systems represented by Figures 4 and

'6 are very similar. \

ORN L-LR-DWG

28598A

e -

i - .

ORNL-DWG 47457A

4 400

IO00

900 c

0 Y

I-

. 600

500

400 LiF

UF4 (mole XI

60 ’

e System LiF-UFq

ORNL-DWG 66-7634 4035

Fig.

The System LiF-BeF2-UF4

w, t

- 1

ThF4 and UF4 form a continuous series of s o l i d s o l u t i o n s with n e i t h e r I

maximum nor minimum.

The LiF-ThF4-UF4 system ( s e e Fig. 7) shows no ter-

nary compounds and a s i n g l e e u t e c t i c 1 3 (which contains 1.5 mole % ThF4 with 26.5 mole $ UF4 and freeees a t 488Oc).

Most of t h e area on t h e dia-

I I

gram i s occupied by primary phase f i e l d s of t h e s o l i d s o l u t i o n s UFb-ThF4, LiF*4UF4-LiF*4ThF4, and LiF*UF4-LiF*ThF4. Liquidus temperatures decrease,

‘

g e n e r a l l y , t o t h e LiF-UF4 edge of t h e diagram.

It i s c l e a r from examination of t h e diagrams shown t h a t f u e l systems - m e l t i n g below 500°C are a v a i l a b l e over a wide range of compositions i n t h e

LiF-BeF2-UF4

system.

Since (see Fig. 6) up t o 28 mole % of ThF4 can be

melted a t temperatures below llOO°F, blanket systems with very l a r g e ThF4 concentrations can be obtained. ,-.

Moreover, t h e very g r e a t s i m i l a r i t y i n

behavior of ThF4 and UF permits f r a c t i o n a l replacement of ThF4 by UF4 with

l i t t l e e f f e c t on freezing temperature over t h e composition range of f

i n t e r e s t as f u e l .

Fuels f o r s i n g l e region r e a c t o r s should, accordingly,

be a v a i l a b l e i n t h e LiF-BeF2-ThF4-Lk4

quaternary system.

Phase behavior i n t h e t e r n a r y systems LiF-BeF -UF4 and LiF-BeF2-ThF4 2 has, as a consequence of s

d e t a i l and t h e phase d i a g

s c i t e d above, been examined i n considerable

If,’as i s likely, fuels

ll defined.

and blankets f o r two-region breeders can be chosen from t h e s e t e r n a r i e s then t h e only necessary add

n a l study of phase behavior i s a more de-

t a i l e d examination of l i q u i

and e s p e c i a l l y of s o l i d u s r e l a t i o n s h i p s and

c r y s t a l l i z a t i o n path behavior i n t h e regions near t h o s e chosen as f u e l and as blanket compositions. /

. $

ORNL-LR-DWG-26535B

Fig.

The System LiF-ThF4

, ORNL-LR-DWG 37420AR4

ThG 4414

TEMPERATURE IN OC COMPOSITION IN mole To

. -

526 BeF2

555

u /

Fig.

7. The

System

LiF-ThF -UF 4 4

Oxi de-Fluori de E q u i l i b r i a The phase behavior of pure f l u o r i d e systems i s such t h a t adequate f u e l s and blankets seem assured, but t h e behavior of such systems i s markedly a l t e r e d by appreciable concentrations of oxide.

Since a l l commercial

f l u o r i d e preparations contain some oxide (and water which r e a c t s w i t h t h e f l u o r i d e s at high temperature t o produce oxide) methods must be devised t o remove t h i s contaminant t o safe l e v e l s before use of t h e f l u o r i d e mixt u r e i n t h e reactor.

Avoiding contamination by oxide of t h e molten mix-

t u r e s during r e a c t o r operation and maintenance w a s possible i n p r i n c i p l e but , before t h e excellent operating experience with MSRE, w a s not at a l l certain i n practice.

Accordingly, c a r e f u l s t u d i e s of oxide-fluoride equi-

l i b r i a i n f l u o r i d e melts have been made t o e s t a b l i s h (1)t h e e f f e c t of contaminant oxide on MSRE f u e l , and ( 2 ) t h e ease of removal of oxide t o t o l e r a b l e l e v e l s p r i o r t o r e a c t o r usage of t h e m e l t s . 8

Measurements of r e a c t i o n e q u i l i b r i a between water vapor i n hydrogen c a r r i e r gas with LiF-BeF2 m e l t s over a wide composition.interva1 have been examined i n d e t a i l by Mathews and Baes. 14â&#x20AC;&#x2122;15Equilibrium quotients f o r t h e reaction

(where 2, g , and d refer t o l i q u i d , gaseous, and dissolved states and s i n d i c a t e s t h a t Be0 w a s present as a s a t u r a t i n g s o l i d phase) w e r e measured from 500 t o 7OO0C over t h e composition range %eF2 = 0.3 t o 0.8. results are summarized by

wherein a , b y and c a l l were l i n e a r functions of l/TÂ°K,

The

a = 3.900

- 4.418(10 3/ T I ,

b = 7.819

- 5.44O(lO5/T), +

c = -12.66

5.262(10 3/TI.

I n t h e same i n v e s t i g a t i o n , measurements were made upon melts not s a t u r a t e d with BeO.

I n addition t o t h e r e a c t i o n H2Â°(g)

2F-(d)

* 02- ( d )

(3)

2HF(â&#x201A;Ź5)

f o r t h e formation of oxide i o n , it became evident, both from t h e s e measure-

ments and from t h o s e upon Be0 s a t u r a t e d m e l t s , t h a t hydroxide i o n a l s o w a s formed

Because of l i m i t a t i o n s inherent i n t h e t r a n s p i r a t i o n method used, t h e eqirilibrium q u o t i e n t s f o r these two r e a c t i o n s w e r e less accurately determined than w a s t h e previous one f o r Be0 s a t u r a t e d melts ( c a . f.lo%, r e s p e c t i v e l y , compared t o

+ 5%).

They were s u f f i c i e n t t o show, however,

t h a t both oxide and hydroxide i n c r e a s e i n s t a b i l i t y with increasing temperature.

The s t a b i l i t y of hydroxide with respect t o oxide, however,

decreases w i t h increasing temperature.

Hydroxide can, accordingly, be

r e a d i l y decomposed i n these f l u o r i d e m e l t s by sparging with an i n e r t gas (e.g., hydrogen). OH- + F- Z HF

(g)

+ 0

, log

K = 5.23

- 6.56(103/T)

Similar measurements have a l s o been made by Baes and Hitch

> @ 3x

which t h e 2LiF-BeF2 contained added ZrF4. With

(5) 16

ZrO2

xzrF4

i s t h e s t a b l e s a t u r a t i n g oxide a o l i d , and hence t h e following equilibrium may be w r i t t e n 4

LJ 4(d)

Zro2 ( s )

4HF( g )

It w a s a l s o found t h a t t h e e q u i l i b r i a ( 3 and 4) f o r t h e formation of oxide

and hydroxide ions were s h i f t e d t o t h e r i g h t with increasing x

ZrF4

; i.e.,

i n t h e d i r e c t i o n of g r e a t e r s t a b i l i t y of t h e s e ions. These r e s u l t s are consistent with previous observations t h a t LiF-BeF

m e l t s are r e a d i l y f r e e d of oxide contamination by' treatment with gaseous mixtures of H

and HF.

The measured equilibrium q u o t i e n t s i n 2LiF*BeF2

were uged t o c a l c u l a t e t h e e f f i c i e n c y of HF u t i l i z a t i o n i n such a treatment as a function of temperature and HF p a r t i a l pressure with the assumpt i o n t h a t equilibrium i s maintained between t h e gas stream, t h e molten This c a l c u l a t i o n (Fig. 8) shows t h a t t h e

s a l t , and any Be0 s o l i d present.

e f f i c i e n c y i n t h e removal of oxide t o a f i n a l value of 16 ppm

(x02- = 3.3 x

i s q u i t e high over a wide range of conditions.

By combination of r e a c t i o n s (1)and

(61, it

is possible t o calculate

t h a t both Be0 and Zr02 w i l l coexist at equilibrium with 2LiF-BeF contain2 ing oxide i o n

ZrO

(7)

(SI

-4 mole

approximately 3 x 10

when ZrF

4 i s present a t

fraction.

+ 2Be0

With l a r g e r amounts of added ZrF4, Zr02 becomes t h e less s o l u b l e -

( s t a b l e ) oxide. When a molten mixtu with appreciable q u a n t i t i p r e c i p i t a t i o n of UO

resul

an-d i f it i s maintained i forms t r a n s p a r e n t ruby c r y s t

kid t

t a i n i n g only LiF, BeF2, and UF4 i s t r e a t e d r e a c t i v e oxide (such as H 0, CO= FeO) 2 3' The UO

2 so produced i s s t o i c h i o m e t r i c ,

a c t with t h e m e l t f o r s u f f i c i e n t t i m e it of U02.00.

Such p r e c i p i t a t i o n has been

ji I

1 .

'5 /,

ORN L-DWG

100 h

6 5 - 4187

80 % I

W I-

‘

.> , 0z 0

60 50 P

40 30 10-2

lo-’

100

INITIAL OXIDE CONTENT ( mole/kg ) Fig.

8. Efficiency of Removal of Oxide from 2LiFoBeF2 by Treatment with HF

assumed t o present a danger f o r t h e MSRE s i n c e slow p r e c i p i t a t i o n of U02 followed by a sudden entrance of t h e m a t e r i a l i n t o t h e core could permit uncontrolled increases i n r e a c t i v i t y .

Precautions were taken with t h e

MSRE t o assure cleanliness of t h e system, t h e f u e l mixture, and t h e cover

gas, but it w a s a n t i c i p a t e d t h a t some inadvertent contamination of t h e Accordingly, it w a s decided t o include ZrF4 i n t h e

system might occur.

MSRE f u e l composition s i n c e measurements of t h e metathesis r e a c t i o n Z r O 2(s)

UF4(d)

* ZrF4(d)

"2(s)

have shown t h a t t h e mole r a t i o of ZrF4 t o UFh a t equilibrium with both somewhat with temperature and melt composition

U02 and Zr02Âś while varyi

remains very f a r below t h a t chosen f o r t h e f u e l s a l t . considerable amount of Z r *

--an

As a consequence a

amount e a s i l y detected by chemical analysis

of t h e f u e l salt--would be p r e c i p i t a t e d by oxide contamination before an appreciable quantity of UO

ould p r e c i p i t a t e . 4 Âś 5

I n connection with t h

s t u d i e s , it w a s ascertained t h a t , contrary

,17only

t o published U02-Zr02 phase diagrams

very d i l u t e s o l i d s o l u t i o n s

a r e formed i n t h e temperature range 500-7OO0C.

Because of t h e obvious

i importance of t h i s t o

he MSRE, experiments have been c a r r i e d out i n which

I i i! I

both UO2* Zr02 mixture were e q u i l i b r a t e d with LiF-

s o l i d solutions p r

2 melts.

r e d by fusion

The r e s u l t i n g

e diagram18 f o r I

t h e UO Z r O system o 22 i n Fig. 9.

e r a t u r e i n t e r v a l of real concern i s shown

: i

The oxide concentrat

2LiF*BeF2 s a t u r a t e d with Be0 was estimated

t s f o r reactions (1)and ( 3 ) t o be:

by combining t h e equi log x 2- = -0.04 0

2.96 x IO3/,

1 i i

(9)

i -

-26

ORNL-DWG 66-963

MOLE PERCENT U02 400

99.5

99.0

98.5

400 90 80 70 60 50 40 30 20 10 0

MONOCLINIC

i.5

4.0

0.5

S.S.

MONOCLINIC S.S. a

MOLE PERCENT ZrO2 LEGEND: LIQUID-LAMBERTSON AND MUELLER (23) COHEN AND SCHANER (ABOVE 1600%) (BELOW 46OO0C)}(t61 c MUMPTON AND ROY ( 2 2 ) (BELOW t60O0C) ORNL PROPOSED

---

0 "WET" CHEMICAL ANALYSIS X

ACTIVATION ANALYSIS FOR U

Fig. 9.

Phase Behavior in System U02-Zr02 i

,The. s o l u b i l i t y

%eF2

increased w i t h temperature, but nd s t r o n g dependence on

w a s found.

I n t h e s e measurements t h e mole f r a c t i o n of oxide a t Be0

s a t u r a t i o n probably w a s less than 0.002. From t h e s i m i l a r measurements i n ZrF4- containing m e l t s ' 6

the solubili-

t y product of Zr02 could be e s t i m a t e d . ' With increasing xZrF4 , t h e concent r a t i o n of oxide a t Zr02 s a t u r a t i o n a t first f a l l s as would be expected from t h e equilibrium ZrO

4+ 2(s)

2(d)

(d)'

However, it then l e v e l s o f f and subsequently rises w i t h f u r t h e r increases I

in x

ZrF4

(Fig. 10).

This could be caused, at least i n p a r t , by t h e forma-

t i o n of a c m p l e x i o n , Z r O

o r it could be caused e n t i r e l y by t h e influence o$ t h e changing m e l t composition on t h e a c t i v i The p l o t i n Fig. 10 i n d i c

o e f f i c i e n t s of t h e species Z r 4+ and 02-

approximately t h e "oxide tolerance" of MSRE

f u e l s a l t - f l u s h s a l t mixtures ; i. e. mixtures

can

, the

amount of dissolved oxide t h e s e

contain w i t h

oxide t o l e r a n c e increases

f u e l composition ( x

ZrF4

with temperature, e s p e c i a l l y near t h e i n d i c a t i n g t h a t any excess oxide present

'might be removed by c d l l e c t i n g ZrO

on a r e l a t i v e i y cool s u r f a c e i n t h e

MSRE system. These s t u d i e s have defined r e l a t i v e l y w e l l t h e situa,tion i n LiF-BeF2

d i n LiF-BeF

ZrFh m e l t

e i n i t i a l purification

They have been of real value i n assessing

ess (see below) and i n assuring t h e inad-

v e r t e n t p r e c i p i t a t i o n of U02 should prove no problem i n MSRE.

I1 - F

ORNL-DWG 65-2542R

lo-’

z 0

I-

a 10-2 Q

c z w V

0 -

0 x

10-3

0.004

0.01

0.02

0.05

0.1

0.2

0.5

Z r b CONCENTRATION (mole/kg 1 Fig. 10.

S o l u b i l i t y of Z r 0 2 i n LiF-BeF2-ZrF4 Melts as Function of ZrF4 Concentration

29 Fuel and Blanket Compositions The f u e l chosen f o r MSRE w a s a mixture of 7LiF, BeF2, ZrF4 and UF4 c o n s i s t i n g of 65-29.1-5-0.9

mole %, r e s p e c t i v e l y , of t h e s e materials.

The

ZrF4 w t : added, as i n d i c a t e d above, t o eliminate t h e p o s s i b i l i t y of prec i p i t a t i o n of U02 through inadvertent contamination of t h e system with r e a c t i v e oxide.

[The general precautions regarding c l e a n l i n e s s i n MSRE

and t h e apparent success of t h o f u e l preparation and handling procedures f o r t h a t operation have gone f a r t o remove apprehension from t h i s source. No samples removed from MSRE have contained more than 100 ppm of oxide, and no p r e c i p i t a t e d oxides have been observed on examination by o p t i c a l

microscopy.

Since chemical reprocessing techniques (probably d i s t i l l a -

t i o n ) w i l l c e r t a i n l y be applied t o t h e MSBR f u e l system and s i n c e such a reprocessing scheme can be expected t o remove oxides, it seems very l i k e l y t h a t ZrF4 need not be a c o n s t i t u e n t of MSBR f u e l . On t h e basis of information presented above t h e reference f u e l s e l e c t -

ed f o r use i n t h e MSBR i s a t e r n a r y mixture of 7LiF-BeF2-233UF4 (68.331.5-0.2

mole

$1 ( s e e Fig. 4) which e x h i b i t s

proximately 45OOC.

a l i q u i d u s temperature of ap-

Equilibrium c r y s t a l l i z a t i o n of t h i s fuel mixture

proceeds according t o t h e following sequence:

t u r e i n t e r v a l 450 t o 438'C,

On cooling i n t h e tempera-

2LiF-BeF2 i s deposited from t h e f u e l .

438Oc, t h e s a l t mixture s o l i d i f i e s and produces a mixture of t h e two c r y s t a l l i n e phases, 2LiF*BeF2 and LiF*UF4, comprised of approximately 89 wt

% 2LiF*BeF2 and approximately 11 w t % LiF-UF4. The blanket s a l t s e l e c t e d f o r t h e MSBR i s t h e 7LiF-BeF2-ThF4 t e r n a r y

mixture (71-2-27 mole %) (see Fig. 6 ) , which e x h i b i t s a l i q u i d u s temperat u r e of approximately 5 6 0 ~ ~Equilibrium . c r y s t a l l i z a t i o n of t h i s blanket

30 mixture i s as uncomplicated as t h a t of t h e f u e l .

Only t h e two s o l i d

phases, LiF-ThF4 and a s o l i d s o l u t i o n of 3LiF*ThF4which,incorporates Be

i n both i n t e r s t i t i a l and s u b s t i t u t i o n a l sites, a r e formed during s o l i d i f i c a t i o n , and t h e s e s o l i d s a r e coprecipitated throughout t h e c r y s t a l l i z a t i o n of t h e salt.

11 Choice of Coolant

The secondary coolant i s required t o remove heat from t h e f u e l i n t h e primary heat exchanger and t o t r a n s p o r t t h i s heat t o t h e power generating system.

I n t h e MSBR t h e coolant must t r a n s p o r t heat t o s u p e r c r i t i c a l

steam at minimum temperatures only modestly above 700'F;

i n MSRE t h e h e a t

w a s r e j e c t e d t o an a i r cooled r a d i a t o r a t markedly higher temperatures.

The coolant m u s t be possessed of adequate heat t r a n s f e r p r o p e r t i e s and must be compatible with Hastelloy N s t r u c t u r e s .

It should not r e a c t

e n e r g e t i c a l l y with f u e l o r with steam, it should c o n s i s t of m a t e r i a l s whose leakage i n t o t h e f u e l would not n e c e s s i t a t e expensive separations procedures, and it should be r e l a t i v e l y inexpensive.

To assure easy com-

p a t i b i l i t y with t h e steam generation c i r c u i t t h e melting temperature of t h e coolant should be below (and preferably considerably below) 700'F. Other demands ( e s p e c i a l l y i n t h e neutron economy and i n r a d i a t i o n s t a b i l i t y a r e a s ) a r e c l e a r l y less s t r i n g e n t than those upon f u e l and blanket

mixtures. The coolant mixture chosen f o r MSRE and apparently shown t o be s a t i s f a c t o r y i n t h a t a p p l i c a t i o n i s BeF2 with 66 mole % of 7LiF.

Use of t h i s

mixture would r e q u i r e some changes i n design of equipment f o r t h e MSBR s i n c e i t s l i q u i d u s temperature i s 851'~; material.

moreover, it i s an expensive

The e u t e c t i c m i x t u r e of LiF with BeF2

(48 mole % LiF) melts a t

31 near 700'F

(see Fig. 1) but it i s both viscous and expensive.

The alkali

metals, e x c e l l e n t coolants with real promise i n o t h e r systems, are und e s i r a b l e here s i n c e they r e a c t vigorously with both f u e l and steam.

Less

noble metal coolants such as Pbo o r B i o might be t o l e r a t e d , but they may not prove compatible with Hastelloy N. Several binary chloride systems are known'' below ( i n some cases much below) 700'F.

t o have e u t e c t i c s melting

These binary systems do n o t , how-

e v e r , appear e s p e c i a l l y a t t r a c t i v e s i n c e they contain high concentrations of chlorides [TlCl, ZnC12, BiC12, CdC12, o r SnCl

1 , which

duced and , accordingly , corrosive o r chlorides [ A l C l BeC12] which are very v o l a t i l e .

are e a s i l y reH f C14 , or

ZrC14,

The only binary systems of s t a b l e , non-

v o l a t i l e chlorides are those containing L i C 1 ; L i C 1 - C s C 1 ( 330'C

% C s C 1 ) , LiC1-KC1 (355'C a t 42 mole % RbC1).

KC1)

LiC1-RbC1

(312'C

a t 45 mole

a t 45 mole %

Such systems would be r e l a t i v e l y expensive i f made from 7 L i C l , and

they could l e a d t o s e r i o u s contamination of t h e f u e l i f normal L i C l were used. Very f e w f l u o r i d e s o r mixtures of f l u o r i d e s a r e known t o melt a t t e m p e r a t u r e s below 37OoC.

Stannous f l u o r i d e (SnF ) m e l t s at 212OC.

This

material i s probably not s t a b l e during long term s e r v i c e i n Hastelloy N ; moreover, i t s phase diagrams with s t a b l e f l u o r i d e s (such as NaF o r KF) probably show high melting p o i n t s at r e l a t i v e l y low a l k a l i f l u o r i d e concentrations. Coolant mixtures of most i n t e r e s t a t present are those based on fluoborates of t h e a l k a l i metals.

The binary system NaF-NaBF4 is describ-

ed19'20 as having a e u t e c t i c ( a t 60 mole

% NaBF4) melting at 580'F.

Pre-

liminary unpublished s t u d i e s a t t h i s Laboratory suggest s t r o n g l y t h a t t h i s

published diagram i s i n e r r o r , and t h a t t h e NaF-NaBF4 e u t e c t i c melts at near 716OF.

There i s some evidence t o suggest t h a t b o r i c oxide sub-

s t a n t i a l l y lowers t h e freezing point of NaF-NaBF4 mixtures and we believe t h a t t h e Russian workers may have used q u i t e impure m a t e r i a l s .

It is l i k e -

l y , however, t h a t t h e m a t e r i a l (perhaps even with a moderate amount of B 0 ) may be useful.

2 3

It should prove s u f f i c i e n t l y s t a b l e t o r a d i a t i o n

f o r s e r v i c e as coolant, and t h e equilibrium pressure due t o

should prove s a t i s f a c t o r i l y low.

Estimates of t h e h e a t t r a n s f e r and f l u i d

p r o p e r t i e s of t h i s m a t e r i a l appear a t t r a c t i v e .

The e x t r a o r d i n a r i l y high

cross s e c t i o n of boron should permit small leaks i n t h e heat exchanger t o be recognized immediately, and removal of t r a c e s of BF

continued treatment with HF should be possible.

from t h e f u e l by

Compatiblity of t h e

NaF-NaBF4 mixture w i t h Hastelloy N w i l l probably be s a t i s f a c t o r y ( s e e subsequent s e c t i o n s ) , but such compatibility remains t o be demonstrated. I f t h e NaF-NaBF4 e u t e c t i c system proves unsuitable by v i r t u e of i t s

freezing p o i n t , preliminary d a t a ( s e e Fig. 11) suggests t h a t freezing p o i n t s below 70O0F can be obtained i n t h e t e r n a r y system NaF-KF-BF

Should experience prove t h e NaP-NaBF4 mixture ( o r i t s close r e l a t i v e s ) unsuitable, coolant compositions which w i l l meet t h e low liquidus tempera-

ture s p e c i f i c a t i o n m a y be chosen i n t h e NaF-BeF2 8,19 NaF-LiF-BeF2, 8 J 9 or KF-ZrF4-AlF3

systems.

These materials a r e almost c e r t a i n l y compatible

with Hastelloy N , and they possess adequate s p e c i f i c h e a t s and l o w vapor pressures ( s e e s e c t i o n below).

They ( e s p e c i a l l y those including LiF) a r e

moderately expensive, and t h e i r v i s c o s i t i e s a t low temperature a r e c e r t a i n i

ORNL-OWG 66-7633

TEMPERATURE I N O C COMPOSITION IN MOLE To DENOTES SOLID SOLUTION

. .

NoF

995

. Fig. ll. Th

E'710

tern NaF-KF-BF

856

(Preliminary)

34 Physical Properties of MSBR Liquids Estimates of some of t h e physical p r o p e r t i e s of t h e proposed MSBR blanket and f u e l s a l t s are l i s t e d i n Table 3.

Estimated values f o r f o u r

possible secondary coolants are given i n Table 4.

Table 3.

Composition and P r d p e r t i e s of Fuel and Blanket S a l t s

Composition (mole I )

Fuel LiF BeFi

Blanket

65.9 33.9

LiF

mF4

0.2

BeF2

uF4

L i q u i d u s Temperature : OC

Physical P r o p e r t i e s :

Density, l b / f t 3 ’

457 855

. .

A t 600Oc

125

280

Heat Capacity, Btu lb’l(oF)-l

0.55

Viscosity, centipoise

8.6

Vapor Pressure, mm Thermal Conductivity, w a t t s / ( Oc-cm)

560 1040

Negligible

0.011

0.22 21

Negligible

0 077

.w 1

35 l y higher than are d e s i r a b l e . o r even A1F

It i s p o s s i b l e t h a t s u b s t i t u t i o n of ZrF4

f o r some of t h e BeF2 w i l l provide l i q u i d s of lower v i s c o s i t y

a t no real expense i n l i q u i d u s temperature.

Table 4.

Composition and P r o p e r t i e s of Four Possible Secondary Coolants

Composition (mole I )

7.7 LiF 5 4 NaF NaBFk 96 NaE3F4 83.65 NaF 53 8.65 BeF2 42 KBF4 Na;F

LiF

NaF

BeF2 36

57 BeF2 43 NaF

Liquidus Ternperature : OC

380

370

318

328

340

716

700

604

622

634

130

138

136

139

Physical Propert i e s at 8 5 0 ~ ~

4540c)& Density, l b / f t 3

Heat Capacity Btu. lbW1( OF)-'

0.4

0.45

0.47

0.44

Viscosity, centipoise Vapor Pressure at 1125OF (607째C)b, mm 3 1 O C Thermal Conductivi t y (watts/OC-cm)

0.008

253' 0.0075

Negligible Negligible

Negligible

0.01

&Mean temperature of coolant going t o t h e primary heat exchanger. bHighest normal operating t & p e r a t u r e of coolant. 'Represents

decomposition pressure due t o MBF4

-F

+ MF.

0.01

36 The d e n s i t i e s were c a l c u l a t e d from t h e molar volumes of t h e pure components by assuming t h e volumes t o be additive.

The heat c a p a c i t i e s were

estimated by assuming t h a t each gram atom i n t h e mixture c o n t r i b u t e s 8 The value of 8 i s t h e approximate aver-

c a l o r i e s p e r degree centigrade.

age from a s e t of s i m i l a r f l u o r i d e m e l t s .

The v i s c o s i t y of t h e f u e l and coolants C , D, and E were estimated from other measured LiF-BeF2 and NaF-BeF2 mixtures; 23924'25 t h e v i s c o s i t y of t h e blanket salt w a s estimated from measurements contained UF4 i n s t e a d of ThF4.

of mixtures which

The v i s c o s i t y of coolant A could not be

r e l i a b l y estimated because of t h e absence of measurements on t h i s compoHowever, t h e v i s c o s i t y of t h e major components, NaBF4, i s about

sition.

14 cp at The vapor pressures of t h e f u e l , b l a n k e t , and coolants C , D, and E

a r e considered n e g l i g i b l e ; e x t r a p o l a t i o n of measurements on similar mixt u r e s y i e l d e d pressures less than 0.1 millimeter. of BF

The p a r t i a l pressure

above t h e fluoroborate coolant mixture was c a l c u l a t e d from measure-

ments on pure NaBF4

by assuming t h a t N a F , NaBF4, and KBF4 form an i d e a l

( i n t h e sense of Raoult's Law) s o l u t i o n . The values given a r e unlikely t o be i n e r r o r t o an extent s u f f i c i e n t t o remove t h e f l u i d from consideration.

It i s c l e a r from t h e ' f a c t t h a t

e s t i m a t e s , r a t h e r than experimentally determined values, are used i n t h e s e t a b l e s t h a t a program must be devoted t o measurement of physical propert i e s f o r t h e p e r t i n e n t materials.

CHEMICAL COMPATIBILITY OF MSRF: MATERIALS

Successful operation of t h e MSRE requires compatibility of t h e molten f u e l mixture with unclad graphite and Hastelloy N during years of r a p i d c i r c u l a t i o n of t h e f u e l through an appreciable temperature gradient.

Such

compatibility must, moreover, be assured while t h e f i s s i o n process produces

i t s i n t e n s e r a d i a t i o n f i e l d and t h e buildup of f i s s i o n product species. To evaluate t h e s e implied problems has required a l a r g e research and deI

velopment program i n which many t e s t s have been conducted over a period of s e v e r a l years.

Details and s p e c i f i c findings of t h e l a r g e program of corrosion testing are presented as a separate paper i n t h i s s e r i e s . p a t i b i l i t y of t h e MSBR materials

is'

I n b r i e f , com-

assured by choosing as m e l t constitu-

e n t s only f l u o r i d e s t h a t are thermodynamically s t a b l e toward t h e moderator g r a p h i t e and toward t h e s t r u c t u r a l metal, Hastelloy N , a n i c k e l a l l o y cont a i n i n g about 16%Mo, 7% C r , and 5% Fe.

The f u e l and blanket components

(LiF, BeF2, UF4, and ThF4) a r e much more s t a b l e than t h e s t r u c t u r a l m e t a l f l u o r i d e s (NiF2, FeF2, and CrF ) ; accordingly, t h e f u e l and blanket have 2 a minimal tendency t o corrode t h e m e t a l .

Such s e l e c t i o n , combined with

proper p u r i f i c a t i o n procedures, provides l i q u i d s whose c o r r o s i v i t y i s within t o l e r a b l e limits.

The chemical p r o p e r t i e s of t h e materials and t h e

nature of t h e i r several i n t e r a c t i o n s , both with and without r a d i a t i o n and f i s s i o n , are described b r i e f l y i n t h e following. S t a b i l i t y of UF3 and UF4 Pure, c r y s t a l l i n e uranium t r i f l u o r i d e i s s t a b l e , under an i n e r t at-

mosphere , t o temperatures i n excess of 1000째C , but it disproportionates at

s u f f i c i e n t l y high temperatures by

4UF3

* 3UF4 + uo.

Long, who s t u d i e d t h e r e a c t i o n UF4

+ $2 f UF3 + HF

obtained dat a28 which when combined with o t h e r accepted values i n d i c a t e t h a t t h e f r e e energies ( i n kcal/mole) f o r t h e pure c r y s t a l l i n e m a t e r i a l s can be represented by 9

= -351 + 52.8 x

AF'

TOK

UF3 and f %F3

f - AFUF4 = +97.0 - 15.6 x lom3 TOK.

However, uranium t r i f l u o r i d e i s appreciably less s t a b l e i n molten f l u o r i d e s o l u t i o n s than i n t h e c r y s t a l l i n e state.

Long's d a t a f o r t h e r e a c t i o n i n

2LiF*BeF2 s o l u t i o n y i e l d t h e following

f o r a c t i v i t y coef-

f i c i e n t s of t h e m a t e r i a l s in t h i s s o l u t i o n log

= -1.62 + 3.77 x

TOK

TOK.

uF3 and log YUF

= -0.99

1.31

Uranium t r i f l u o r i d e i s permissible i n r e a c t o r f u e l s only i n s o f a r as t h e equilibrium a c t i v i t y of Uo which r e s u l t s is s u f f i c i e n t l y low t o avoid r e a c t i o n with t h e moderator g r a p h i t e o r appreciable a l l o y formation with t h e Hastelloy N.

Use of t h e a c t i v i t y c o e f f i c i e n t s shown above t o p r e d i c t

at 1000째K (727OC) t h e a c t i v i t y of uranium i n equilibrium with m e l t s cont a i n i n g various U+3/U+4

r a t i o s leads t o t h e d a t a of Table 6.

It i s obvi-

ous t h a t l a r g e q u a n t i t i e s of UF4 must be reduced i f appreciable uranium

a c t i v i t i e s a r e t o be obtained. UF4 were reduced t o UF

would form, f o r example, i f 68% of t h e

Table 6. Calculated Values of t h e Fraction of t h e Total Uranium i n Solution Present i n t h e Trivalent State (UF3/Total U ) i n Equilibrium at 1000째K w i t h Various Phases

( T o t a l uranium i n s o l u t i o n = 1 mole $)

Phase

UF3/Total U( % )

Uo Activity

' 99

U Metal

1.0

3 x

lo-'

uc2

10-7

N i alloy

2 x

N i alloy

20 1

I n f u e l processing, hydrogen reduction of t h e f u e l mixtures (as described i n t h e s e c t i o n on Production Technology below) should l e a d t o reduction of no more than about 2% of the UF4.

as 2UF4

C r % CrF2

+ 2UF3

would increase t h e UF

n e g l i g i b l e e x t e n t above this value.

Corrosion reactions such 3

concentration t o a

Thus, under r e a c t o r conditions, it

seems c l e a r t h a t t h e reduction of t h e UF4 normally encountered would i n t r o duce no problems; only through d r a s t J c and v i r t u a l l y unimaginable reduct i o n could s e r i o u s consequences arise.

40 Oxidation (Corrosion) of Hastelloy N3-5 Blood3'

has made a c a r e f u l study of t h e r e a c t i o n MF2(d)

H2(d)

+ 2HF

'(c)

(g)

where M represents C r , Fe, o r N i , c , g , and d i n d i c a t e t h a t t h e Species i s c r y s t a l l i n e s o l i d , gaseous, o r dissolved i n molten LiF-BeF2 mixture.

His

d a t a (Table 7) when combined15 with accepted values f o r HF, y i e l d f r e e Table 7.

Experimentally Determined Equilibrium Constants f o r t h e Reaction

Mixture Containing 62 mole % LiF

i n LiF-BeF, Temperature

5 for

CrF2

% for

1000째K

4.4

1-9

800Oc

1.3

0.80

7OO0C

7.5

10-5

0.53

600Oc

1.2

10-5

0.13

where

% for

FeF2

NiF2

105

1.5 x i o4

2 'HF

%= 'MF2

'H2 ~~

energies of formation (along with those of Long f o r UF4 and UF3) i n Table 8.

Table 8. Free Energiesa f o r Solutes i n Molten 2LiF*BeF2 ( 773-l0OO0K)

Solute

A 2 (kcal/mole)

(lOOO째K) (kcal/F-)

U4+

+ 4F-

444.6

58.1 x loq3 T째K

96.6

U3+

+ 3F-

336.7

40.5 x

TOK

98.7

36.3 x

T째K

55.3

Ni2+

2F-

146.9

Fe2+

+ 2F-

154.7

Cr2+

+ 2F-

- 21.8 x TOK 171.8 - 21.4 x loT3 TOK

61.5 75.2

a The reference s t a t e i s t h a t hypothetical s o l u t i o n with t h e s o l u t e at u n i t mole f r a c t i o n and with t h e

a c t i v i t y c o e f f i c i e n t it would have at i n f i n i t e dilution. These d a t a r e v e a l c l e a r l y t h a t chromium i s much more r e a d i l y oxidized than i r o n o r nickel.

Accordingly, any oxidative a t t a c k upon Hastelloy N

should be expected t o show s e l e c t i v e a t t a c k on t h e chromium.

Such oxida-

t i o n and s e l e c t i v e a t t a c k follows from reactions such as t h e following:

1. Impurities i n t h e m e l t

+ NiF2

CrF + N i , o r 2

+ 2HF

-t

CrF2

+ H2

Oxide films on t h e metal NiO

+ BeF2

NiF2

Be0

followed by r e a c t i o n of NiF2 with C r

Reduction of UF4 t o UF

C r + 2UF4 $ 2UF3

+ CrF2

42 Reactions implied under (1)and (2) above w i l l proceed e s s e n t i a l l y t o completion at a l l temperatures within t h e MSBR c i r c u i t .

Accordingly,

such reactions can l e a d ( i f t h e system i s poorly cleaned) t o a noticeable r a p i d i n i t i a l corrosion rate.

however, t h e s e r e a c t i o n s do not give a sus-

t a i n e d corrosive a t t a c k . The r e a c t i o n of UF4 with C r , on t h e other hand, has an equilibrium constant with a s m a l l temperature dependence; hence, when t h e salt i s forced t o c i r c u l a t e through a temperature g r a d i e n t , a possible mechanism e x i s t s f o r mass t r a n s f e r and continued a t t a c k . I f n i c k e l , i r o n , and molybdenum are assumed t o be completely i n e r t

d i l u e n t s f o r chromium (as i s approximately t r u e ) , and i f t h e c i r c u l a t i o n rate in t h e MSBR i s very rapid, t h e corrosion process can be simply de-

scribed.

A t high f l a w rates, uniform concentrations of UF

and CrF2 a r e

maintained throughout t h e f l u i d c i r c u i t ; t h e s e concentrations satisfy ( a t some intermediate temperature) t h e equilibrium constant f o r t h e r e a c t i o n . Under t h e s e steady-state conditions, t h e r e exists some temperature i n t e r mediate between t h e maximum and minimum temperatures of t h e c i r c u i t , at which t h e i n i t i a l s w f a c e composition of t h e s t r u c t u r a l metal i s at equilibrium w i t h t h e fused s a l t .

Since t h e equilibrium constant f o r t h e chemi-

c a l r e a c t i o n increases w i t h increasing temperature, t h e chranium concent r a t i o n i n t h e a l l o y surface tends t o decrease a t temperatures higher than T and tends t o i n c r e a s e at temperatures lower than T.

LiF-KF-UF4,

[ I n some melts (NaF-

f o r example) AG f o r t h e m a s s t r a n s f e r r e a c t i o n i s q u i t e l a r g e ,

and t h e equilibrium constant changes s u f f i c i e n t l y as a function of temp e r a t u r e t o cause formation of d e n d r i t i c chromium c r y s t a l s i n t h e cold zone.

For MSBR f u e l and o t h e r LiF-BeF2-UF4 mixtures, t h e temperature

43 dependence of t h e mass-transfer r e a c t i o n i s small, and the equilibrium i s s a t i s f i e d a t r e a c t o r temperature conditions without t h e formation of

c r y s t a l l i n e chromium. Thus, i n t h e MSBR, t h e rate of chromium removal from t h e s a l t stream

by deposition at cold-fluid regions i s c o n t r o l l e d by t h e rate at which >

chromium diffuses i n t o t h e cold-fluid w a l l ; t h e chromium concentration g r a d i e n t tends t o be small, and t h e r e s u l t i n g corrosion i s w e l l within tolerable l i m i t s .

I n t h e h o t - f l u i d region, t h e a l l o y s u r f a c e becomes

depleted i n chromium, and chromium from t h e i n t e r i o r of t h e w a l l diffuses toward t h e surface.

This rate of d i f f u s i o n i s dependent on t h e chromium

concentration g r a d i e n t .

Since d i f f u s i o n occurs by a vacancy process and

i n t h i s p a r t i c u l a r s i t u a t i o n i s e s s e n t i a l l y monodirectional, an excess of vacancies can accumulate i n t h e depleted region.

These vacancies p r e c i p i -

t a t e i n areas of d i s r e g i s t r y , p r i n c i p a l l y a t g r a i n boundaries and impuriI

t i e s , t o form voids.

The voids i n t u r n agglomerate and grow i n s i z e w i t h

increasing t i m e and temperature.

The r e s u l t i n g subsurface voids are not

interconnected with each o t h e r o r with t h e surface. The mechanisms described above lead t o such observations as ( a ) t h e

complete independence of corrosion rate from flow rate f o r a given system *

and ( b ) t h e i n c r e a s e i n â&#x20AC;&#x2DC;corrosion with i n c r e a s e i n temperature drop as w e l l as with i n c r e a s e i n mean temperature within a system. The results of numerous long-term tests have shown t h a t Hastelloy N has e x c e l l e n t corrosion r e s i s t a n c e t o molten f l u o r i d e mixtures at tempera-

tures w e l l above t h o s e a n t i c i p a t e d i n MSBR.

The a t t a c k from mixtures

similar t o t h e MSBR f u e l a t temperatures as high as 1300Â°F i s b a r e l y observable i n t e s t s of as long as 12,000 h r .

A f i g u r e of 0.5 m i l / y r might

44 be expected.31

Even l e s s corrosion occurs i n t h e blanket where t h e UF4

concentration is very low.

Further, t h e mechanical p r o p e r t i e s of

Hastelloy N are v i r t u a l l y unaffected by long-time exposure t o t h e molten f l u o r i d e f u e l and blanket mixtures.

Corrosion of t h e container metal by

t h e r e a c t o r fâ&#x20AC;&#x2122;uel and blanket does not seem t o be an important problem i n t h e MSBR. This encouraging s t a t u s f o r metal-salt compatibility c e r t a i n l y applies

t o t h e coolant mixture i f a reasonable NaF-BeF2 o r NaF-LiF-BeF2 mixture i s chosen.

It i s l i k e l y t h a t t h e NaF-NaEiF4 coolant mixture w i l l a l s o prove

compatible with INOR-8,but no d e t a i l e d experimental proof of t h i s i s available.

The f r e e energy change f o r t h e chemical r e a c t i o n

is about +30 k c a l at 8 0 0 ~32 ~ . The r e a c t i o n i s , t h e r e f o r e , q u i t e u n l i k e l y

t o occur, and similar r e a c t i o n s with Fe, Mo, and N i a r e much less so.

a d d i t i o n , t h e above r e a c t i o n becomes even l e s s l i k e l y (perhaps by 10 k c a l

o r so) when one considers t h e e n e r g e t i c s of formation of t h e compound NaBF4 and d i l u t i o n of t h e NaBF4 by NaF.

However, t h e following r e a c t i o n

i s almost c e r t a i n l y t h e one t o be expected.

Thermochemical d a t a f o r t h e

borides of C r , N i , Mo, and Fe do not seem t o have been e s t a b l i s h e d .

Very

s t a b l e borides such as TiB2 and ZrB2 show f r e e energies of formation of

-67 and -68 kcal/mole ( o r about -34 kcal/boron atam) a t 800Â°K.33

The

borides of Mg (MgB and MgB4) show f r e e energies of formation of l e s s than 2 -10 k c a l p e r boron

Unless t h e borides of t h e Hastelloy N constitu-

e n t s are very s t a b l e , it would appear t h a t t h e a l l o y w i l l prove r e s i s t a n t

t o t h i s coolant.

However, such compatibility must be demonstrated by ex-

periments. Compatibility of Graphite with Fluorides Graphite does not react chemically with molten f l u o r i d e mixtures of t h e type t o be used i n t h e MSBR.

6 suggest

Available thermodynamic data

t h a t t h e most l i k e l y reaction: 4VF4 + C Z CF4 + 4W3

-a

should come t o equilibrium a t CF4 pressures below 10

atm.

4 concen-

t r a t i o n s over graphite-salt systems maintained f o r long periods at elevated temperatures have been shown34 t o be below t h e l i m i t of detection ( > 1 ppm) of t h i s compound by mass spectrometry.

Moreover, graphite has

been used as a container material f o r many NaF'-ZrF4-UF4, LiF-BeF2-UF4, I

and

other salt mixtures with no evidence of chemical i n s t a b i l i t y . The MSBR w i l l contain perhaps 20 t o n s of graphite.

Several p o t e n t i a l

problems i n addition t o t h a t of chemical s t a b i l i t y have been considered. These include (1)hazardous increase i n uranium content of core through permeations of t h e graphite by fuel, ( 2 ) reaction of f u e l material with oxygenated gaseous species desorbed from t h e g r a p h i t e , and ( 3 ) carburizat i o n of t h e Hastellay N s t r u c t u r e by carbon dissolved, suspended, or other-

w i s e carried i n the circulating salt.

These p o s s i b i l i t i e s have been

s t u d i e d experimentally and found t o be inconsequential or t o have p r a c t i c a b l e s o l u t i o n s . 4,5 Graphite i s not wetted by MSR f u e l mixtures (or by o t h e r similar mix-

tures) at elevated temperatures.

hd II

The extent t o which g r a p h i t

i s permeat-

ed by t h e f u e l i s , accordingly, defined by well-known r e l a t i o n s h i p s among

46 applied pressure, surface tension of t h e nonwetting l i q u i d (about 130 dynes/cm), and t h e pore s i z e spectrum of t h e g r a p h i t e specimen.

However,

s i n c e t h e void volume of t h e g r a p h i t e m a y be about 16% of t h e core f u e l volume, d e t a i l e d t e s t i n g of permeation behavior has been necessary.

Typi-

c a l t e s t s 3 5 with MSRE g r a p h i t e have exposed evacuated specimens t o MSRE f u e l mixtures a t 1300'F;

applied pressures were set at 150 l b , a value of

t h r e e times t h e r e a c t o r design pressure. change w i t h time a f t e r a f e w hours.

The observed permeation d i d not

I n t h e s e t e s t s 0.18% of t h e g r a p h i t e

bulk volume was permeated by t h e s a l t ; such permeation i s w e l l within t h a t considered t o l e r a b l e during MSRE operation.

Specimens permeated t o t h i s

extent have been given 100 cycles between 390 and 1300째F without detectable change in p r o p e r t i e s o r appearance. Radiation E f f e c t s 1 ,3-5 3 34 3 36 A considerable body of information about t h e s t a b i l i t y and compati-

b i l i t y of MSBR m a t e r i a l s under i r r a d i a t i o n from f i s s i o n i n g f u e l has been obtained.

These s t u d i e s were motivated by t h e concern t h a t neutrons, b e t a

and gamma r a y s , and f i s s i o n fragments might cause r a d i a t i o n damage t o f u e l , metal, and g r a p h i t e s t r u c t u r a l components.

Fission fragments, which should

produce l o c a l i z e d regions of dense i o n i z a t i o n and r a d i o l y s i s i n t h e molten salt , might a f f e c t f u e l s t a b i l i t y and corrosion behavior.

Early In-Pile Tests on NaF-ZrFL-UFL Fuels The e a r l i e s t s t u d i e s of r a d i a t i o n effects on molten f l u o r i d e systems

were done i n t h e molten-salt ANP program. mixtures and Inconel containers.

These tests used NaF-ZrF4-UF4

Such i r r a d i a t i o n s , w i t h melts and metal

chemically s i ~ i l a r t othose proposed f o r t h e MSBR, were performed over a i

47 wider range of power density and temperature than were used i n more recent i r r a d i a t i o n work i n support of t h e MSRE.

More than 100 s t a t i c capsule

1 4 neutrons

t e s t s were c a r r i e d out i n thermal neutron fluxes from 10l1 t o 10

-2 -1 cm s e c , with f i s s i o n power d e n s i t i e s from 80 t o 8000 w/c, at temperatures from 1500 t o 1 6 0 0 and ~ ~ f~o r i r r a d i a t i o n t i m e s from 300 t o 800 h r . Chemical, p h y s i c a l , and metallographic t e s t s i n d i c a t e d no maJor changes i n t h e f u e l o r t h e Inconel which could be a t t r i b u t e d t o t h e i r r a d i a t i o n

conditions.

Corrosion of Inconel w a s comparable t o t h a t found i n unir-

r a d i a t e d controls.

Three types of Inconel forced-circulation i n - p i l e

4 m e l t s at f i s s i o n power d e n s i t i e s of 400 t o 800 w/cc, maximum temperatures of 1500 t o 16oo0c, and f o r 235 t o loops were operated with NaF-ZrF -UF

475 h r at f u l l power; corrosive a t t a c k on t h e Inconel w a s no g r e a t e r than i n corresponding out-of-pile tests ( w a l l penetrations less than 3 m i l s ) . Early Tests on LiF-BeF2-Wh Fuels "he first i r r a d i a t i o n t e s t on an LiF-BeF4based f u e l w a s a graphite-

f u e l compatibility t e s t i n t h e MTR.

Two Inconel capsules containing

g r a p h i t e l i n e r s f i l l e d with LiF-BeF2-UF4

(62-37-1 mole

$1 were

irradiated

at 1250째F f o r 1610 and 1492 h r , and at average power d e n s i t i e s of 954 and 920 w/cc, respectively.

The exposure r e s u l t e d i n no apparent damage t o

t h e g r a p h i t e , and n e g l i g i b l e corrosion t o t h e Inconel which w a s exposed t o t h e s a l t through s m a l l holes i n t h e g r a p h i t e l i n e r .

I n t h e next t e s t , two small Hastelloy N capsules were f i l l e d with t h e same LiF-BeF2-W4 mixture and i r r a d i a t e d f o r 5275 h r i n a f l u x of 1 t o

1 4 neutrons cm-2 s e c-1 a t an i n i t i a l power d e n s i t y of 1170 w/cc and

2 x 10

a temperature of 1250'F t

an estimated 75% burnup.

The f a i l u r e of one

48 capsule at t h i s time forced termination of t h e experiment.

The r e s u l t s of

l a t e r t e s t s suggest t h a t f u e l r a d i o l y s i s at ambient r e a c t o r temperature during shutdowns may have contributed importantly t o t h e capsule f a i l u r e .

Two forced-circulation Hastelloy N loops containing t h e above LiFBeF2-UF4 mixture were a l s o i n s t a l l e d and operated i n t h e MTR.

These were

designed t o operate at 1300'F maximum temperature, 190 w/cc power d e n s i t y , and a l i n e a r f l o w v e l o c i t y of 2.5 f t / s e c .

Pump f a i l u r e terminated t h e

f i r s t t e s t af'ter 860 h r and t h e second a f t e r 1000 h r .

Metallographic ex-

amination of t h e metal from t h e first loop revealed a moderately eroded region (approximately 2 mils deep) i n one of t h e sharp bends i n t h e high-

flux region.

Metal specimens from t h e second loop showed a n e g l i g i b l e

degree of corrosive a t t a c k .

Since later in-pile t e s t s confirmed t h e good

corrosion r e s i s t a n c e of Hastelloy N, it i s suspected t h a t t h e f i r s t loop

w a s f a b r i c a t e d from substandard a l l o y . Testing; of MSRE Fuels The ORNLMTR-47-

s e r i e s of capsule i r r a d i a t i o n experiments w a s design-

ed t o t e s t t h e s t a b i l i t y and ccmpatibility of a c t u a l MSRE materials (graphi t e , Hastellay N, and f u e l s a l t ) under conditions approximately t h o s e of t h e MSRE, with emphasis on t h e i n t e r f a c i a l behavior of molten salt and graphite.

The capsules were r e l a t i v e l y l a r g e t o provide adequate speci-

mens of g r a p h i t e , f u e l , and Hastelloy N f o r thorough p o s t i r r a d i a t i o n examination. I n t h e 47-3 t e s t , four Hastelloy N capsules ( s e e Fig. 1 2 ) containing g r a p h i t e boats holding a pool of f u e l s a l t (BeF2-UF4-LIF-ThF4-ZrF4, 23.21.4-69.0-1.2-5.2

'mole

were i r r a d i a t e d f o r 1594 h r at maximum tempera-

t u r e s of 8OOOC andmaximum power d e n s i t i e s of 200 w/cc t o a burnup of about I

ORNL-LR-DWG

MATERIALS SPECIMENS

MATERIALS SPECIMENS,

‘

SALT FUEL

,GRAPHITE

BLADE

\THERMOCOUPLE

GRAPHITE BOAT

‘4

‘/2

INCH

Fig. 12. MSRE Graphite-Fuel Capsule Test

WELL

56754R

10%. Each capsule a l s o contained specimens of Hastelloy N, molybdenum,

and a p y r o l y t i c g r a p h i t e attached t o a g r a p h i t e blade dipping i n t o t h e shallow pool of f u e l .

Two of t h e g r a p h i t e boats were i n i t i a l l y impregnated

with t h e f u e l t o provide a more extreme t e s t - of graphite-fuel CompatibilXty When t h e caps'ules were dismantled, t h e frozen f u e l

at high temperature.

exhibited nonwetting contact angles with t h e g r a p h i t e b o a t s , t h e graphite blades, and t h e p y r o l y t i c g r a p h i t e specimens.

The g r a p h i t e s t r u c t u r e ap-

peared undamaged-v i s u a l l y and metallographically.

However , t h e r e were

d e f i n i t e observations t h a t f u e l r a d i o l y s i s had taken place (generation of F2 and CF4).

Because of t h e s e observations, subsequent tests s t u d i e d t h e

r a d i o l y t i c i n s t a b i l i t y of t h e f u e l i n d e t a i l :

- .

it w a s found t h a t only when

t h e irradiated f u e l w a s allowed t o f r e e z e and cool below 100째C d i d radiol y t i c decomposition t a k e place. ,

The 47-4 i r r a d i a t i o n assembly comprised of four l a r g e Hastelloy N cap-

s u l e s ( s e e Fig. 13), each containing a O.5-in.-diam rounded by a 0.2-in.

g r a p h i t e specimen sur-

annulus of fuel, and two s m a l l Hastelloy N capsules

containing a O.s-in.-OD

g r a p h i t e cup nearly f i l l e d with f u e l .

capsules contained about 25 g of f u e l ( BeF2-UF4-LiF-ThF4-ZrF4, 71.0-1.0-4.7mole

The l a r g e 22.6-0.7-

%). One of t h e smaller capsules contained 10 g of t h e

same f u e l ; t h e other 10 g of a similar f u e l of higher UF4 concentration

(1.4 mole %). "he capsules w e r e irradiated f o r 1553 h r a t temperatures up t o 8OOOC (gOO째C f o r t h e s m a l l 1:4 mole % UF4 capsule) , a t average power d e n s i t i e s from 40 t o 260 w/cc, and t o burnups from 5 t o 10%. There w a s aJ

gain evidence t h a t f u e l r a d i o l y s i s occurred at low temperatures during r e a c t o r shutdown; however, metallographic examination of t h e Hastelloy N capsule w a l l s showed no 'discernible corrosion, and t h e g r a p h i t e appeared

, -

bd 3

t ORNL-LR-DWG

Cr - Ai THERMOCOUPLE

677t4R

NICKEL THERMOCOUPLE WELL

NICKEL POSITIONING LUGS (2)

INCH

Fig., 13.

L N I C K E L POSITIONING LUG

Large Fuel Capsule from OEK-MTR

'47-4

52 undamaged except f o r t h e vapor-exposed region of t h e small high power density capsule. To i n v e s t i g a t e t h e f u e l r a d i o l y s i s f u r t h e r , two capsules i n t h e 47-5 assembly, of design similar t o t h e l a r g e 47-4 capsules, were equipped with gas l i n e s which permitted measurement of pressure within t h e capsule and withdrawal of cover gas samples while t h e i r r a d i a t i o n was proceeding.

Two

l a r g e s e a l e d capsules with widely d i f f e r e n t areas of g r a p h i t e and metal exposed t o t h e f u e l , and two s m a l l capsules containing fuel-impregnated g r a p h i t e rods suspended i n helium completed the-assembly.

Four of t h e

. capsules contained s a l t having t h e composition LiF-BeF 2-ZrF4-UF4 with mole r a t i o s of 67.36-27.73-4.26-0.66. (LiF-BeF

S a l t w i t h lower uranium concentration

ZrF4-UF4, 67.19-27.96-4.51-0.34)

swept capsules and i n t h e low-flux,

w a s used i n one of t h e gas-

impregnated-rod capsule.

The

47-5 t

capsules were i r r a d i a t e d f o r 4 4 2 <months a t average fluxes between I

2 x l O l 3 and 3 x' 1013 neutrons

-1 see

t o burnups between 7 and

158.

Gas samples were taken from t h e purged capsules under a v a r i e t y of operati n g conditions, with f u e l temperatures varying from 190 t o 1500'F

and power I

d e n s i t i e s from 3 t o 80 w/cc.

During r e a c t o r shutdowns, when t h e assembly ,

cooled t o about 35OC, pressure r i s e s were observed i n t h e capsules equipped with gas l i n e s , and gas samples i n d i c a t e d t h e presence of f l u o r i n e . . W i t h t h e r e a c t o r operating and t h e fuel molten, t h e i s o l a t e d capsules

showed no f l u o r i n e .

I n a f e w of t h e 60 gas samples, b a r e l y d e t e c t a b l e

4 (approximately 5 ppm) were found; t h e s e were probably due t o

t r a c e s of CF

incomplete flushing of t h e system s i n c e t h e l a s t r e a c t o r shutdown.

I n any

case, t h e observed minute rates of CF4 generation represented n e g l i g i b l e reduction of UF4 t o UF3 and, accordingly, an inconsequential p r a c t i c a l

I n l a t e r h o t - c e l l s t u d i e s of frozen i r r a d i a t e d f u e l , it was es-

problem.

t a b l i s h e d that t h e gas evolved w a s pure f l u o r i n e and t h a t t h e G value at

35OC w a s 0.02 molecules of f l u o r i n e per 100 ev of f i s s i o n product decay energy absorbed. v

The r a t e of r a d i o l y s i s was g r e a t e s t i n t h e temperature

range of 35 t o 5OoC; it dropped t o l o w values a t -7OOC and t o zero a t temperatures above 80Oc. The 47-6 t e s t was designed t o a l l a y any l i n g e r i n g doubts t h a t f u e l r a d i o l y s i s and i t s consequences could be eliminated by maintaining t h e f u e l molten even during r e a c t o r shutdown.

Four c y l i n d r i c a l , Hastelloy N

capsules were used (1i n . OD x 2.615 i n . long).

Heaters were provided f o r

all capsules, and-these turned on automatically when t h e fuel temperature

approached t h e liquidus temperature

maintaining t h e f u e l s a l t i p a molten

condition even when t h e r e a c t o r w a s ,shut down.

The capsules contained

c y l i n d r i c a l g r a p h i t e cores which were 0.5 i n . i n diameter and 1.35 i n .

long; t h e cores were surrounded by 0.2 i n . of f u e l s a l t and pierced by a c e n t r e Hastelloy N thermocouple well.

Two of t h e capsules ( s e e Fig. 1 4 )

were equipped with gas l i n e s and d i f f e r e d from each o t h e r only i n t h a t h a l f the graphite area in one-was replaced by a Hastelloy N extension of t h e

thermowell.

These caps

were charged with an LiF-BeF

s i m i l a r t o t h a t i n t h e 47-5 t e s t but containing 0.9 mole

s e a l e d capsules containe

ZrF4-UF4 f u e l UF4.

The two

1-size g r a p h i t e cores and similar f u e l s with

0.5 mole % and 4.0 mole The 47-6 assembly w a s i r r a d i a t e d i n t h e MTR f o r 1500 h r t o burnups

from 1%' ( i n salt contEining 0.5% UF4) t o 5% ( s a l t w i t h 4.0% UF4).

Gas

samples were taken a t steady operation with t h e purged capsules a t tempera-

aill

tures from 850 t o 1300째F and at power d e n s i t i e s from 20 w/cc t o 75 w/cc.

54 , , ORNL-DWG 6 4 - 4 O t O

THERMOCOUPLE 1

GAS CAP PURGE SUPPLY TUBE

Fig. 14. Fuel Capsule from ORNL-MTR-47-6

: -. i

55 The gas analyses detected no CF4 o r o t h e r fluorine-containing gases i n any of t h e 36 gas samples.

CF4 d e l i b e r a t e l y added t o t h e capsules during ir-

r a d i a t i o n was r a d i o l y t i c a l l y decomposed a t a rate which decreased w i t h t e m p e r a t u r e and seldom exceeded 4$/hr. 4

P a r t i c u l a r care w a s given t o t h e p o s t i r r a d i a t i o n examination of t h e g r a p h i t e specimens.

No uranium deposits were found by chemical a n a l y s i s ,

by delayed neutron counting of neutron a c t i v a t e d specimens, o r by x-radiography of t h i n s e c t i o n s .

It i s t h e r e f o r e c l e a r t h a t uranium deposition on

g r a p h i t e i s associated only w i t h f u e l r a d i o l y s i s at low temperatures, and t h a t t h e r e a c t i o n does not take p l a c e between g r a p h i t e and molten f i s s i o n -

ing fuel.

I n a d d i t i o n , v i s u a l , metallographic, and x-ray d i f f r a c t i o n ex-

aminations of t h e 47-6 g r a p h i t e specimens f a i l e d t o r e v e a l any d i f f e r e n c e s between t h e irradiated g r a p h i t e and u n i r r a d i a t e d c o n t r o l s .

Also , t h e

Hastelloy N capsule w a l l specimens from run 47-6 appeared unaffected by t h e exposure based on v i s u a l and lowipower magnification examination.

Metallographic examination of unetched specimens revealed no change i n w a l l thickness (less than 1 m i l change).

Conclusions fram In-Pile Testing of Molten S a l t s I n summary, t h e 47- series of i r r a d i a t i o n s t u d i e s has been g e n e r a l l y reassuring as t o t h e r a d i a t i o n s t a b i l i t y and compatibility of Hastelloy N, g r a p h i t e , and f u e l s based on l i t h i u m and beryllium f l u o r i d e s . r o s i o n t h a t i s known t o occur, i . e .

, that

seem t o be influenced by power density.

The cor-

due t o mass t r a n s f e r , does not

It has been shown t h a t t h e prin-

c i p a l d i s t u r b i n g e f f e c t s are consequences of low-temperature f u e l radiolys i s which i s easily suppressed by maintaining t h e irradiated f u e l at a

temperature above (conservatively) 20OoC.

On t h e basis of t h e 47- s e r i e s

t e s t s , t h e limits of t h i s reassurance i n regard t o r a d i a t i o n e f f e c t s on

MSBR m a t e r i a l s extend t o temperatures of about 1400째F and power d e n s i t i e s

of about 100 w/cc. The two previous loop tests on s i m i l a r LiF-BeF2 f u e l s , described above, extend t h e l i m i t s of assurance t o power d e n s i t i e s of 200 w/cc a t 1300'F w i t h regard t o corrosion of Hastelloy N i n t h e absence of graphite. There has been no i n d i c a t i o n of t h e 47- s e r i e s experiments t h a t g r a p h i t e introduces problems i n addition t o t h e expected one of Hastelloy N carburization (when t h e two a r e i n c l o s e c o n t a c t ) . The previous capsule tests with LiF-BeF2-UF4 f u e l s , c a r r i e d out w i t h no provisions t o circumvent the low-temperature f u e l r a d i o l y s i s e f f e c t , suggested t h a t salt power d e n s i t i e s of a t least 1 kw/cc may be permissible.

Also, the numerous t e s t s w i t h NaF-ZrF4-UF4 f u e l s i n Inconel a t temperatures

~ ~ power d e n s i t i e s up t o many kilowatts p e r cubic centimeter up t o 1 6 0 0 and exhibited t o l e r a b l e c m p a t i b i l i t y c h a r a c t e r i s t i c s .

With respect t o radia-

t i o n e f f e c t s , t h e r e i s no obvious chemical reason t o suppose t h a t a grossl y d i f f e r e n t salt behavior would be observed using MSBR m a t e r i a l s .

57 BEHAVIOR OF FISSION PRODUCTS I N MOLTEN

F i s s i o n products w i l l be produced i n a 2225-Mw(th) MSBR at a r a t e of about 2.3 kg/day.

I n t h e reference MSBR design, t h e f u e l salt volume i s

about 700 f t 3 and t h e f i s s i l e inventory about TOO kg; with t h e s e values t h e f i s s i o n product concentration a f t e r 50 days accumulation would be about

15% of t h e f i s s i l e concentration.

Thus, it i s c l e a r t h a t f i s s i o n product

concentrations can be s i g n i f i c a n t even with high processing rates , and t h a t f i s s i o n product behavior needs t o be considered i n specifying r e a c t o r operating conditions. Physical Chemistry of Fission Products F i s s i o n and i t s immediate aftermath m u s t be a v i o l e n t process; t h e very e n e r g e t i c major fragments are probably d e f i c i e n t i n e l e c t r o n s a t t h e i r j

o r i g i n , and, as they l o s e energy by c o l l i s i o n s , they undoubtedly produce a d d i t i o n a l i o n i z a t i o n within t h e medium.

It seems c e r t a i n , however, t h a t

e l e c t r i c a l charge i s conserved i n t h i s process; e l e c t r o n s and protons are n e i t h e r c r e a t e d nor destroyed by t h e f i s s i o n event.

It follows, t h e r e f o r e ,

t h a t when f i s s i o n of UF4 occurs i n an i n e r t environment [ a s i n a (hypot h e t i c a l ) completely i n e r t c o n t a i n e r ] t h e r e a c t i o n UF

* 2FP + 2+n + 4F-

must, i n a s t a t i s t i c a l sense, s a t i s f y t h e conditions t h a t (1)t h e s a l t be e l e c t r i c a l l y n e u t r a l , and ( 2 ) redox equilibrium be e s t a b l i s h e d among t h e numerous i o n i c species.

I n an i n e r t container such cation-anion equiva-

lence (and redox equilibrium) might be s a t i s f i e d with uranium valence

states above 4

and with p o s i t i v e i o n formation by Nb, Mo, T e , o r Ru.

The

MSBR container metal (Hastelloy N ) i s not completely i n e r t and t h e f u e l

58 contains a s m a l l concentration of UF f o r t h i s system.

so additional p o s s i b i l i t i e s e x i s t

Should t h e f i s s i o n product cations prove inadequate f o r

t h e f l u o r i d e ions plus t h e f i s s i o n product anions (notably I-), o r should they prove adequate only by assuming element valence states t o o high t o be thermodynamically compatible with Hastelloy N, t h e container metal would be constrained t o supply t h e c a t i o n deficiency. Thermochemical data from which t h e s t a b i l i t y of f i s s i o n product f l u o r i d e s i n complex d i l u t e s o l u t i o n s can be predicted a r e lacking i n many cases.

Such information which appears d e f i n i t e i s b r i e f l y described i n

t h e following sections. Rare Gases The f i s s i o n products krypton and xenon a r e v o l a t i l i z e d from high-temp e r a t u r e m e l t s as elements. 34'37

The s o l u b i l i t i e s of t h e s e gases i n mol-

t e n f l u o r i d e m i x t u r e s 38939s40 obey Henry's l a w , i n c r e a s e with increasing temperature, decrease with increasing atomic weight of t h e g a s , and vary somewhat with composition of t h e solvent.

Henry's l a w constants and h e a t s

of s o l u t i o n f o r t h e rare gases i n LiF-BeF2 miztures a r e shown i n Table 9. The p o s i t i v e heat of s o l u t i o n ensures t h a t blanketing o r sparging of t h e f u e l with helium o r argon i n a low-temperature region of t h e r e a c t o r cannot l e a d t o d i f f i c u l t y due t o decreased s o l u b i l i t y and bubble formation i n higher temperature regions of t h e system.

[There i s no evidence of

t r o u b l e from such source i n MSRE where t h e He i s applied i n t h e pump bowl

at t h e highest temperature i n t h e c i r c u i t . ] The very low s o l u b i l i t i e s of t h e s e gases suggest t h a t they should be r e a d i l y removed from r e a c t o r systems.

Only a s m a l l f r a c t i o n of t h e calcu-

l a t e d xenon poisoning w a s observed during operation of t h e A i r c r a f t Reactor

L d 7

59 Experiment41 where t h e only mechanism f o r xenon removal w a s t h e helium purge of t h e pump bowl. Table 9. S o l u b i l i t i e s and Heats of Solution f o r Noble Gases i n Molten LiF-BeF2 Mixtures a t 600% LiF-BeF2 (64-36 mole % )

K x 10

Gas

11.55 + 0.4

5.2

Neon

L.63 + 0.2

5.9

Argon

0.98 + 0.02

8.6

Xenon

0.233 5 0.01

12.1

Helium

aK = moles g a s / ( cm L

Heat of Solution (kcal/mole )

s o l v e n t ) ( atmosphere).

A somewhat more ambitious scheme f o r i n s u r i n g a low poison f r a c t i o n

f o r xenon (and krypton) i s o t o p e s i s t o remove t h e halogen precursors i o d i n e and bromine on a time cycle s h o r t compared t o t h e i r h a l f t i m e s f o r decay i n t o t h e noble gases.

Since 135Xe i s by f a r t h e worst poison of t h i s c l a s s ,

removal of i t s i o d i n e precursor would be most important; i t s decay h a l f -

t i m e i s such t h a t i t s residence time i n t h e r e a c t o r should be kept a t 1 hour o r less.

I n p r i n c i p l e I- (and Br-)

can be removed by t h e r e a c t i o n

where d and g i n d i c a t e t h a t t h e s p e c i e s i s d i s s o l v e d i n t h e melt o r e x i s t s i n t h e gaseous state.

Molten f l u o r i d e s s i m i l a r t o MSBR f u e l and spiked

with I- have been shown t o y i e l d t h e contained iodine r e a d i l y on contact with gaseous HI?. 42

These small-scale (and preliminary) s t u d i e s suggest

60 t h a t t h e removal s t e p i s chemically f e a s i b l e . Elements i n Periodic Groups I-A,

II-A,

II-B,

and IB-B

Rubidium, cesium, strontium, barium, zirconium, yttrium, and t h e lanthanides form very s t a b l e f l u o r i d e s .

These f i s s i o n products should, ac-

cordingly, e x i s t i n t h e molten f u e l i n t h e i r ordinary valence s t a t e s .

v a r i e t y of s t u d i e s of many types shows t h a t l a r g e amounts of ZrF4, t h e a l k a l i f l u o r i d e s , and t h e a l k a l i n e e a r t h f l u o r i d e s can be dissolved i n MSBR f u e l mixtures a t operating temperatures.

Since t h e t r i f l u o r i d e s are less

s o l u b l e , t h e s o l u b i l i t y behavior of t h e f l u o r i d e s of yttrium and t h e r a r e e a r t h s , 43'44 and of p l ~ t o n i u m ' ~ has been examined i n some d e t a i l .

The

s a t u r a t i n g phase from s o l u t i o n s i n LiF-BeF2 and r e l a t e d m i x t u r e s i s t h e simple t r i f l u o r i d e ; when more than one r a r e e a r t h is p r e s e n t , t h e s a t u r a t ing phase i s a ( n e a r l y i d e a l ) s o l i d s o l u t i o n of t h e t r i f l u o r i d e s . s o l i d s o l u t i o n s a r e known t o accommodate UF they would include PuF

as w e l l .

Such

and it i s very l i k e l y t h a t

The s o l u b i l i t i e s of t h e s e s o l i d s o l u t i o n s

depend strongly on composition of t h e m e l t ; t h e s o l u b i l i t i e s may be near t h e minimum value f o r MSBR f u e l compositions.

Even then, however, t h e

s o l u b i l i t y ( n e a r 0.5 mole % at MSBR operating temperatures) i s such t h a t many months would be required f o r t h e r e a c t o r t o s a t u r a t e i t s f u e l with t h e s e f i s s i o n products.

I n any case, reprocessing t o remove t h e r a r e

e a r t h s , and p a r t i c u l a r l y neodymium, i s required i n t h e i n t e r e s t of neutron economy. The above statements regarding rubidium and cesium do not apply t o t h a t f r a c t i o n of t h e s e elements o r i g i n a t i n g i n t h e g r a p h i t e as daughters of t h e r a r e gases which have permeated t h e moderator.

These a l k a l i metals

form cmpounds with g r a p h i t e at high temperature but t h e absolute amounts

61 are so small t h a t d i f f i c u l t i e s from t h i s source a r e unlikely.

Damage t o

t h e g r a p h i t e by t h i s mechanism w i l l , as a matter of course, be looked f o r i n a l l f u t u r e r a d i a t i o n and f i s s i o n s t u d i e s . Other F i s s i o n Products These products include molybdenum , ruthenium , technetium , niobium , and

tellurium produced i n r e l a t i v e l y high y i e l d s with rhodium, palladium, s i l v e r , cadmium, t i n , and antimony i n y i e l d s ranging from s m a l l t o t r i v i a l . The a v a i l a b l e thermochemical d a t a6 y 7 y 3 2 y 3 3 y 4 6

suggests t h a t t h e f l u o r i d e s

of t h e s e elements would be ( i f they were present i n t h e pure s t a t e ) reduced t o t h e metal by chromium at i t s a c t i v i t y i n Hastelloy N o r by UF

reasonable concentrations i n t h e f u e l salt. The high-yield noble metals (Mo, Nb, Ru, Tc, and T e ) have polyvalent

f l u o r i d e s which are generally q u i t e v o l a t i l e and moderately unstable. formation free energies f o r NbF

5’

The

MoF6, and uF6 may be c a l c u l a t e d with

r e l a t i v e l y good accuracy because of recent measurements a t Argonne of t h e h e a t s of formation of t h e s e compounds by f l u o r i n e bomb calorimetry. 47-49 The e n t r o p i e s and heat capacity d a t a also are a v a i l a b l e . 50 people a t Argonne have measured RuF

5’

While t h e

51 no entropy or heat capacity data

seem t o be a v a i l a b l e :

AH;98

Reference

-213.41 + 0.35

From t h e s e v d u e s and t h e a v a i l a b l e heat capacity data t h e following expressions f o r AG

were derived.

I n t h e case of RuF

6 5’ Glassner’s e a r l i e r

estimate w a s corrected t o be c o n s i s t e n t with t h e above AH AGf(N%F5,g) = -416.70 AGf(RuF5,g) = -200

AGf (MoF6,g) = -370.99

AGf(UF6,g)

measurement:

54.40(T/1000),

25(T/1000),

69.7(T/1000),

= -509.94 + 65.15(T/1000). --f

The following values of AG

have been reported previously f o r UF

and UF4

i n 2LiF-BeF2: A$(UF3,d)

= -336.73 + 40.54(T/1000),

A$(UF4,d)

= -444.61 + 58.13(T/lOOO).

From these free-energy values the following equilibrium constants have

been c a l c u l a t e d f o r t h e formation of t h e v o l a t i l e f l u o r i d e s by r e a c t i o n with w4(d)

i n t h e MSRE from t h e equation

lo@; K = a + b ( 1 0 3 1 ~ ) : Reaction

7.33

-26.82

13.76

-74.17

7.83

-60.38

6.15

-32.88

In Fig. 15, c a l c u l a t e d equilibrium p a r t i a l pressures of t h e gases a r e p l o t t e d vs t h e UF /UF4 r a t i o i n t h e melt. 3

m e l t is increased, NbF then RuF

As t h e oxidizing power of t h e

i s expected t o appear f i r s t , followed by MoF6, and

Uranium hexafluoride has a lower dependence on oxidizing power

because i t s reduction product i s UFh r a t h e r than t h e metal.

It w a s as-

sumed i n t h e case of NbF5, MoF6, and RUF5 t h a t t h e reduction product w a s t h e metal.

The W6,ShOdd not be formed i n s i g n i f i c a n t amounts u n t i l t h e

melt i s oxidizing enough t o produce RuF

5â&#x20AC;&#x2DC;

I f any s t a b l e intermediate

f l u o r i d e s of Nb, Mo, and Ru a r e formed i n t h e melt, t h e r e s u l t would be correspondingly lowered equilibrium gas pressures and lowered power dependences on t h e W4/UF3 r a t i o . T e l l u r i u m hexafluori

has not been included i n t h i s l i s t i n g , b u t t h i s

compound seems c e r t a i n t o be l e s s s t a b l e than any shown here.

No data

which would permit.inclusion of t h e f l u o r i d e s of technetium seem t o be i

available.

If t h e UF /IFh r a t i o i

would be expected t o v o l a t i

MSRE f a l l s s i g n i f i c a n t l y below

NbF5

ze if t h e niobium m e t a l i n equilibrium with

t h e fused s a l t were at a near u n i t a c t i v i t y .

Appreciable pressures of

MoF6, RuF5, W63 (and almost c e r t a i n l y of TcF5 o r TeF6) would require much more oxidizing conditions i n t h e m e l t . The a c t u a l s t a t e of t h e s e f i s s i o n products i s of moderate importance t o t h e effectiveness of molten s a l t r e a c t o r s as breeders, ,um, niobium, technetium, an

If t h e molybden-

henium e x i s t as m e t a l s ( o r perhaps as

i n t e r m e t a l l i c compounds)

l a t e t h e Hastellay N portions of t h e r e a c t o r

they w i l l be of l i t t l e co

nce as poisons, although they may prove a

s e r i o u s nuisance o r worse t o heat exchanger maintenance. s o l u b l e f l u o r i d e s then t h removable i n t h e process carbides o r by adhering i n Molybdenum can form Mo 2

If they e x i s t as

se l i t t l e t r o u b l e and a r e , i n p r i n c i p l e , cle.

They can causeâ&#x20AC;&#x2122;most t r o u b l e by forming

other way t o t h e g r a p h i t e moderator. i n t h e MSRE and MSBR temperature range;

t h e AG values f o r t h e s e compounds become negative a t about 45OoC and t h e

ORNL-DWG 67-773

IOQ

.j0-20 f

loo

IO2

Fig. 15.

404

406

408

40"

Equilibrium Pressures of Volatile Fluorides as Function of UFb/UF3 Ratio in MSRE Fuel.

65 compounds become more s t a b l e at increasing temperatures. 52

Niobium carbide

( e s s e n t i a l l y N'bC) has a l a r g e ( 3 3 k c a l ) negative heat of formation a t 298OK and i s c e r t a i n l y s t a b l e under r e a c t o r conditions.

Nothing appears t o be

known concerning carbides of technetium, but it seems c e r t a i n t h a t no

carbide formation i s expected from t h e platinum metals , s i l v e r , t e l l u r i u m , cadmium, antimony o r t i n . Net Oxidizing P o t e n t i a l of Fission Process 53 The f u e l exposure t e s t s have used 235U as f i s s i l e f u e l , w i t h thermal

-1 neutrons cm-2 sec

f l u x exposures of about 3 x

Table 10 shows the

r e l a t i v e y i e l d s of the s e v e r a l most important f i s s i o n products30 r e s u l t i n g from f i s s i o n of 235U i n a steady thermal f l u x of 3 x 1013 neutrons cm -1 sec f o r t h r e e s e l e c t e d time i n t e r v a l s .

-2

The l i s t e d f i s s i o n products cam\

p r i s e at l e a s t 97% of a l l those produced ( t o t a l y i e l d i s 2.0) at l i s t e d times, with no f i s s i o n product removal. Table 10.

Fission Yields from Thermal Fission of 235u

oth

11.6 days

Element

Br I Kr + X e Rb

Sr Ba Rare Earths Zr

= 3 x 1013 neutrons cm-2 s e c - l

0.00030 0.0359 0 297 0.0387 0.0971 0.144 0.105 0 528 0.318

0.00021 0.0145 0.301 0.0390 0.131 0.121 0.0684 0.560 0.318

1.564

Subto t a1 0.0040 0 201 0.0410 0.140

Nb Mo Tc Ru Total

T i m e Since S t a r t u p 116 days

0.00021 0.0125 0.301 0.0393 0.132 0.0980 0.0626 0.559 0.317

1.522

1.553 0.0139 0.201 0.0586 0.126

1 950

3.2 years

0.0028 0.242 0.0592 0.114

1.953

1.940

66 If t h e chemically a c t i v e f i s s i o n products shown i n Table 10 occur as

I-'

, Br-' , Rb+. , Cs+ , Sr2+,

Ba*+, Y",

L3+(rare e a r t h s ) , Zr4+, and Nb5+

and i f krypton, xenon, molybdenum, technetium, and ruthenium occur as elements, and i f no f i s s i o n product species a r e removed from t h e r e a c t o r , then t h e t o t a l f i s s i o n product y i e l d m u l t i p l i e d by t h e valence be 3.475 and 3.560 at 11.6 and 116 days, respectively.

(cXizi)

W i l l

If a l l krypton and

xenon nuclides of h a l f - l i f e g r e a t e r than 5 minutes are removed from t h e system before they decay, t h e comparable CX.Z. values become 3.21 and 3.26. 1 1

If all krypton and xenon nuclides with h a l f - l i v e s g r e a t e r than l m i n u t e are

removed before decay, t h e CXiZi days, respectively.

values a r e 3.06 and 3.09 a t 11.6 and 116

The above CXiZi

values (which seem inadequate t o

satisf'y t h e f l u o r i d e ions released by t h e f i s s i o n e d uranium) suggest t h a t

t h e f i s s i o n process i s per s e oxidizing t o UF

and u l t i m a t e l y t o Hastellay

Results of many i n - p i l e tests of compatibility of t h e materials, how-

ever, suggest t h a t f i s s i o n does not l e a d t o corrosion of t h i s container

metal. If, on t h e o t h e r hand, all t h e molybdenum formed MoF6 and t h e techne-

tium formed TcF then t h e f i s s i o n process would r e q u i r e more than 4 fluo-

r i d e ions p e r f i s s i o n event and t h e f i s s i o n process would p e r s e be reducing t o UF4.

Even f o r t h e r a t h e r u n r e a l i s t i c case where all xenon and

krypton species with h a l f - l i v e s i n excess of l m i n u t e were removed t h e CX.Z.

.1 1

values would be near 4.5.

of UF4 t o UF

This would r e q u i r e reduction of one mole

f o r each 2 moles of uranium f i s s i o n e d .

Both extremes ( t h a t i s a s t r o n g l y oxidizing o r a s t r o n g l y reducing a c t i o n of t h e f i s s i o n process) seem unlikely.

It seems l i k e l y t h a t a

f r a c t i o n of t h e molybdenum, niobium, and technetium exist as f l u o r i d e s

67 ( o f valence lower than t h i s maximum) and t h a t , accordingly, the net e f f e c t of f i s s i o n i s n e i t h e r markedly oxidizing nor markedly reducing t o t h e

Hastelloy N-UF4

system.

Should subsequent long-term tests at high burnup prove t h e f i s s i o n process t o be oxidizing t h e cure would seem t o be r e l a t i v e l y simple; i f t h e burned uranium were made up by addition of UF (or UF3 + UF4) t h e problem 3 would be solved.

S i m i l a r l y , i f t h e f i s s i o n processes were (unexpectedly)

reducing toward UF4 t h e makeup of burned uranium could be as a mixture of UF

(or UF6) w i t h W4.

68 I

CHEMICAL BEKAVIOR IN MSRE

General The Molten-Salt Reactor Experiment operated during s i x separate periods i n 1966; v i r t u a l l y all of t h e operating time accumulated after midMay w a s a t t h e maximum p o s s i b l e power of about 7.5 Mw.

The r e a c t o r accumu-

Additional operation i n

l a t e d approximately 11,200 Mwhr during t h e year.

1967 ( e s s e n t i a l l y all a t maximum power) l e d t o accumulation of an addition-

a l 21,000 Mwhr as of t h e scheduled shutdown on M a y 10, 1967. During periods of r e a c t o r operation, samples of t h e r e a c t o r salts were removed r o u t i n e l y and were analyzed f o r major c o n s t i t u e n t s , corrosion products and (less f r e q u e n t l y ) oxide contamination.

Standard samples of

f'uel are drawn three times p e r week; t h e LiF-BeF2 coolant s a l t i s sampled every two weeks. Current chemical analyses suggest no p e r c e p t i b l e composition changes f o r t h e salts s i n c e they were first introduced i n t o t h e r e a c t o r some 20 months ago. While analyses f o r ZrF4 and f o r UF4 agree q u i t e w e l l w i t h t h e m a t e r i a l

balance on q u a n t i t i e s charged t o t h e r e a c t o r tanks, t h e values f o r 'LiF

and

BeF2 have never done so; analyses f o r LiF have shown lower and f o r BeF2 have sham highervalues than t h e book value s i n c e s t a r t u p .

Table 11 shows

a comparison of current a n a l y s i s with t h e o r i g i n a l inventory value.

While

t h e discrepancy i n LiF and BeF2 concentration remains a puzzle, t h e r e i s

nothing i n t h e a n a l y s i s ( o r i n t h e behavior of t h e r e a c t o r ) t o suggest that

any changes have occurred. Routine determinations of oxide (by study of salt-H20-HF e q u i l i b r i a ) continue t o show low values (about 50 ppm) f o r 0

There i s no reason t o

--.

ti' P

b e l i e v e t h a t contamination of t h e f u e l has been s i g n i f i c a n t i n operations t o t h e present. Table 11. Current and Original Composition of MSRE Fuel Mixture Const it uent

Original Value (mole $1

Current Analysis (mole % )

7LiF

63.40

-+ 0.49

64.35

BeF2

30.63

2 0.55

29.83

5.14 5 0.12

5.02

ZrF4

0.821

uF4 _

_ ~-

-+ 0.008

0.803

MSRE maintenance operations have n e c e s s i t a t e d flushing t h e i n t e r i o r of t h e drained r e a c t o r c i r c u i t on four occasions.

a t i o n consisted o r i g i n a l l y of an 7LiF-BeF

The s a l t used f o r t h i s oper-

(66.0-34.0

mole %) mixture.

Analysis of t h i s s a l t before and a f t e r each use shows t h a t 215 ppm of uranium i s added t o t h e f l u s h s a l t i n each flushing operation, correspondi n g t o t h e removal of 22.7 kg of f u e l - s a l t residue (about 0.5% of t h e charge) from the r e a c t o r c i r c u i t . Corrosion i n MSFE The chromium concentration i n MSRE f u e l i s 64 ppm at present; t h e en-

t i r e operation seems t o have increased t h e chromium concentration only 26 ppm.

This increase corresponds t o removal of about 130 g of chromium from

t h e metal of t h e fuel c i r c u i t .

I f t h i s were removed uniformly it would

represent removal of chromium t o a depth of about 0 . 1 m i l .

Analyses f o r

i r o n and n i c k e l i n t h e system are r e l a t i v e l y high (120 and 50 ppm respect i v e l y ) and do not seem t o represent dissolved Fe

and Ni

While there

70 i s considerable s c a t t e r i n t h e s e analyses, t h e r e seems t o be no i n d i c a t i o n

of corrosion of t h e Hastellay N by t h e s a l t . The absence of corrosion--though

a w i d e v a r i e t y of out-of-pile f o r t h e following reasons.

i n general accord w i t h r e s u l t s from

corrosion tests--seems The UF

w a s markedly less than intended.

somewhat s u r p r i s i n g

concentration of t h e f u e l added t o MSRE

Careful reexamination of t h e production

records and study of t h e r e a c t i o n

b 2 2 +UF4$UF3+HF on simples of surplus fuel concentrate show t h a t t h e fuel salt had only a-

.3+

bout 0.16% of i t s uranium as U

N e a r l y 10 f o l d more than t h i s was intend-

ed. If--as

seems v i r t u a l l y certain--the

chromium content of t h e salt w a s

due t o 1 pr

+ UF4

an a d d i t i o n a l 1100 grams of added t h e i

&rF 2 2

2’

UF3

should have resulted.

v3” should have t o t a l e d about

1500 grams and as much as 0.65% of

t h e uranium i n t h e system could have been t r i v a l e n t . t o determine t h e

d’

With t h a t o r i g i n a l l y

A n attempt, however,

(by t h e H2-HF r e a c t i o n above) a f t e r 11,000 Mwhr of

MSRE operation i n d i c a t e d t h a t less than 0.1% of t h e uranium was t r i v a l e n t . F i s s i o n of t h e 550 grams of uranium (corresponding 11,000 Mwhr) could which ’ c e r t a i n l y not have oxidized more t h a n 40% of t h e 1350 grams of I? had apparently been oxidized.

The remaining 800 grams (approximately)

could have been oxidized by inadvertent contamination ( a s by 60 grams of H20 desorbed fram t h e moderator s t a c k ) .

However, t h e r a t e of corrosion

even by t h i s r e l a t i v e l y oxidizing f u e l melt remained imperceptibly slow.

71 Addition of beryllium metal (as 3" rods of 3/8" diameter i n a perfor a t e d basket of n i c k e l ) through t h e sampling system i n t h e pump bowl served as a convenient means of reducing U4+ t o U3'

during r e a c t o r operation.

t h i s form beryllium appears t o r e a c t a t about 1.25 grams per hour s o t h a t

some 600 grams of U3* are produced by an 8 hour treatment.

Some 30 grams

of Be have been added i n t h i s way t o c r e a t e an a d d i t i o n a l 1.6 kg of U3'. During t h e subsequent 20,000 Mwhr of operation (which burned 1 kg of urani-

um) t h i s 1.6 kg of U34 seems t o have been oxidized.

Again, it seems l i k e l y

t h a t t h e f i s s i o n process w a s responsible f o r oxidizing a s u b s t a n t i a l fract i o n , but not a l l of, t h i s m a t e r i a l . Additional beryllium w i l l be added t o MSRE f u e l as soon as power operof t h e a t i o n is resumed; it i s t e n t a t i v e l y planned t o reduce at l e a s t 1% U4+ t o U3+ at t h a t time.

The l a c k of corrosion i n MSRE by melts which appear t o be more oxidizi n g than t h o s e intended can be r a t i o n a l i z e d by t h e assumption (1)that t h e Hastellay N has been depleted i n C r (and Fe) at t h e surface s o t h a t only

Mo and N i are exposed t o a t t a c k , with C r (and Fe) r e a c t i n g only a t t h e slow rate at which it i s furnished t o t h e s u r f a c e by d i f f u s i o n , o r ( 2 ) t h a t t h e noble-metal f i s s i o n products (see s e c t i o n s following) are forming an adh e r e n t and p r o t e c t i v e p l a t e on t h e r e a c t o r metal. Behavior of F i s s i o n Products 54,55 H e l i u m i s introduced i n t o t h e pump bowl of MSRE a t a rate of about 4

l i t e r s per minute; t h i s he

serves t o s t r i p K r and X e from t h e f u e l i n

t h e pump bowl and t o sweep t h e s e gases t o t h e c h a r c o a l - f i l l e d t r a p s f a r downstream i n t h e e x i t gas system.

Since a r e l a t i v e l y small f r a c t i o n ( l e s s

than 10%) of t h e f u e l mixture is bypassed through t h e pump bowl, t h e efj

72 f i c i e n c y of removal of t h e f i s s i o n product gases should not be very high. However, t h e X e poisoning of MSRE at 7 Mw i s only about 0.3% i n AK/K, a value considerably l e s s than w a s a n t i c i p a t e d .

"his low poison l e v e l i s

probably due t o s t r i p p i n g of X e within t h e f u e l system i n t o helium bubbles which a r e known t o c i r c u l a t e ( a t perhaps 0.2% by volume) i n t h e f u e l salt. Samples of MSRE f u e l , drawn i n 10 t o 50 cc metal samples a t t h e sampli n g s t a t i o n in t h e pump bowl, have been r o u t i n e l y analyzed, by radiochemic a l techniques, f o r 1 4 f i s s i o n product isotopes and, i n some cases, f o r 239Np and 23gPu produced i n t h e f u e l .

I n general, t h e f i s s i o n product

species which are known t o possess s t a b l e f l u o r i d e s a r e present i n t h e c i r c u l a t i n g f u e l at approximately t h e expected concentration l e v e l s . monitors ('lSr

The b e s t

and 143Ce) with convenient h a l f - l i v e s , s t a b l e non-volatile

f l u o r i d e s , and no precursors of consequence t y p i c a l l y and c o n s i s t e n t l y show concentration l e v e l s sane 15% lower than those c a l c u l a t e d from power l e v e l s based upon heat balances f o r t h e r e a c t o r . Those elements whose f l u o r i d e s a r e knawn t o be r e l a t i v e l y unstable (molybdenum, niobium, ruthenium, t e l l u r i u m , and s i l v e r ) a r e found in t h e salt at considerably l e s s than t h e expected concentration. of amounts expected are based upon concentrations of "Sr, "Mo,

If c a l c u l a t i o n s

about 60% of t h e

30% of t h e lo3Ru, and about 30% of t h e 132Te appears i n t h e m e l t .

i s not y e t p o s s i b l e t o s t a t e with c e r t a i n t y whether t h e s e m a t e r i a l s a r e

present i n t h e s a l t as c o l l o i d a l metal ( o r alloy) p a r t i c l e s o r as s o l u b l e chemical s p e c i e s , though present evidence suggests t h a t t h e former i s t h e more l i k e l y . Iodine has been found (presumably as I-> a t nearly t h e expected concent r a t i o n i n t h e samples of fuel.

Examination of g r a p h i t e and metal samples

73 and, e s p e c i a l l y , of specimens from t h e vapor phase as described below do show s e v e r a l s u r p r i s e s . An assembly of MSRE graphite and Hastelloy N specimens w a s exposed on

t h e c e n t r a l s t r i n g e r within t h e MSRE core during i t s i n i t i a l operation. This assembly w a s removed during t h e J u l y 17 shutdown a f t e r 7800 Mwhr of r e a c t o r operation, and many specimens have been c a r e f u l l y examined. No evidence of a l t e r a t i o n of t h e g r a p h i t e w a s found under examination

by v i s u a l , x-radiographic

, and metallographic

examination.

Autoradiographs

showed t h a t penetration of radioactive m a t e r i a l s i n t o t h e g r a p h i t e w a s not uniform and disclosed a t h i n (perhaps 1- t o 2-mil) l a y e r of highly radioa c t i v e m a t e r i a l s on or near t h e exposed g r a p h i t e surfaces.

Examination of

t h e metal specimens showed no evidence of corrosion or other danger. Rectangular bars of graphite from t h e t o p ( o u t l e t ) , middle, and bottom ( i n l e t ) region of t h i s c e n t r a l s t r i n g e r were milled i n t h e hot c e l l t o remove s i x successive l a y e r s from each surface.

The removed l a y e r s were then

analyzed f o r s e v e r a l f i s s i o n product isotopes. The r e s u l t s of analysis of t h e outer l a y e r from t h e g r a p h i t e specimens a r e shown i n Table 12.

It i s c l e a r t h a t , w i t h the assumption of uniform

deposition on or i n a l l t h e moderator g r a p h i t e , appreciable f r a c t i o n s of

Mo, T e , and Ru and a l a r g e f r a c t i o n of t h e Nb a r e associated w i t h t h e graphite.

No analyses f o r Tc have been obtained,

The behavior of 14'Ba,

89Sr, 14'Ce,

144Ce,

all of which have

xenon or krypton precursors, can be account'ed f o r i n terms of l a w s of d i f fusion and h a l f - l i v e s of t h e precursors.

Figure

16 shows t h e change i n

concentration of t h e f i s s i o n product isotope with depth i n t h e graphite.

I t

Those isotopes (such as l4'â&#x201A;Ź3a) which penetrated t h e g r a p h i t e as noble gases

.....

-~.~

... ~, . . .-... ~. . .-

.......

i._.

......

........

. .

...................

.................

F i s s i o n Product Deposition on Surfacea of MSRE Graphite

Table 12.

Graphite Location Middle

Top

Isotope dPm/cm2

Per cent of Tot alb

dpm/cm2

Bottom

Percent of Tot a l b

dprn/cm2

Percent

of Totalb A

(x 109)

109)

( x loy)

9 9 ~ ~

39.7

13.4

51.4

17.2

34.2

11.5

132Te

32.2

13.8

32.6

13.6

27.8

12.0

lo 3Ru

8.3

11.4

7.5

10.3

4.8

6.3

95,

4.6

22.8

59.2

24.0

62.4

1311

0.21

0.16

0.42

0.33

0.25

952,

0.38

0.33

0.31

0.27

0.17

0.15

144~e

0.016

0.052

0.083

0.27

0.044

0.14

89~r

3.52

3.24

3.58

3.30

2.99

2.74

1403a

3.56

1.38

4.76

1.85

2.93

1.14

141Ce

0.32

0.19

1.03

0.63

0.58

0.36

137cs a

6.6

x 10-

Average of values i n

0.07

2.3

0.25

2.0

0.212

7- t o 10-mil c u t s from each of t h r e e exposed.graphite faces.

bPercent o f t o t a l i n r e a c t o r deposited on g r a p h i t e if each cm2 of t h e 2 x . 1 0 moderator had t h e same concentration as the specimen.

75 ORNL-DWG 67-774

40'~ I

1 ,

d2 W

t f

a az

LL 0

' 3

w 40 n 10 i

408

40 50 SURFACE OF GRAPHITE (mils)

r o f i l e of Fission Products aphite After 8000 MwJxr

76 show s t r a i g h t l i n e s on t h e logarithmic p l o t ; they seem t o have remained at t h e point where t h e noble gas decayed.

As eq:octed, t h e gradient f o r l4OE3a

with a 16-sec 140Xe precxrsor is much s t e e p e r than t h a t f o r 89Sr, which has a 3.2-min 8gKr precursar. Eration dependence.

A l l t h e oihers shown show a much s t e e p e r concen-

Generally t h e concentration drops a f a c t o r of 100 from e

t h e t o p 6 t o 10 mils t o t h e second layer. <

It i s possible t h a t carbide form-ation i s responsible f o r t h e depos i t i o n of Nb and possibly f o r t h a t of Mo, but it seems q u i t e unlikely f o r Ru and Te; t h e iodine probably got in as i t s t e l l u r i u m precursor.

Since

these materials have been shown t o appear i n t h e exit gas as v o l a t i l e species, it seems l i k e l y t h a t they entered t h e graphite by t h e same mechanism.

The p o s s i b i l i t y t h a t t h e strongly oxidizing f l u o r i d e s such as MoF6

were present r a i s e d t h e question as t o whether UF6 w a s accumulating i n t h e graphite.

An average of 0.23 yg/cm

w a s found i n t h e surface of t h e graph-

i t e ; much l e s s was present i n i n t e r i o r samples.

This amount of uranium,

equivalent t o less than 1 g i n t h e core, w a s considered t o be negligible. Table 13 shows t h e extent t o which various f i s s i o n product isotopes

are deposited on t h e Hastellay N specimens i n t h e core.

A large fraction

of t h e molybdenum and t e l l u r i u m and a s u b s t a n t i a l f r a c t i o n of t h e ruthenium

seem t o be so deposited.

It seems possible t h a t t h e 13'1 w a s c a r r i e d i n t o

t h e specimen as i t s tellurium precursor.

The values f o r 95Zr seem sur-

p r i s i n g l y high, s i n c e those f o r t h e 141Ce and 144Ce

with noble-gas pre-

cursors probably r e f l e c t t h e amount expected by d i r e c t r e m i l a t t h e mmenf of f i s s i o n . If t h e Nb and Tc are assumed+t o behave l i k e t h e Mo, Te, and Ru, it may i

3e noted t h a t t h e MSRE could have been uniformly p l a t e d durine i t s opera-

W f

i I ~

Table 13.

Deposition

of Fission

Products

on Hastelloy

Hastelloy

dpm/cm2

Percent of Total&

(x 109) "MO

212,

132Te

508

Location

Middle

Top

Isotope

N in MSRE Core

dpm/cm2

Bottom Percent of Totala

(x 109)

42.8 131

dpm/cm2

Percent of Total&

(x 109)

276

55.6

204

41.2

341

427

110

23.2

19.1

1.8

5.2

2.4

1.8

1.0

2.6

1.3

0.02

0.22

0.07

0.15

0.06

0.02

0.09

0.18

0.35

0.07

lo3Ru

35.5

29.3

25.5

131,

8.2

3.8

4.0

g5Zr

1.8

1.0

141Ce

0.05

144Ce

0.01

8Percent of total present in reactor which would deposit on the 1.2 x 106 cm2 of Hastelloy if deposition on all surfaces was the same as on the specimen.

t i o n w i t h s e v e r a l hundred angstroms of r e l a t i v e l y noble metals. The only gas-liquid i n t e r f a c e i n t h e MSRF: (except f o r t h e contact be-

tween l i q u i d and t h e g a s - f i l l e d pores of t h e moderator g r a p h i t e ) e x i s t s i n t h e pump bowl.

There a salt flow of about 60 gpm (5% of t h e t o t a l system

flow) contacts a helium cover gas which flows through t h e bowl at 4 l i t e r s / min.

Provisions f o r d i r e c t sampling of t h i s e x i t gas are planned but have

not y e t been i n s t a l l e d i n t h e MSRE. Samples of t h e l i q u i d f u e l a r e obtained by lowering a sampler, on a s t a i n l e s s steel cable, through t h i s cover gas and i n t o t h e l i q u i d .

It has

been p o s s i b l e , accordingly, t o d e t e c t chemically a c t i v e f i s s i o n product species i n t h i s cover gas by radiochemical analysis of t h e s t a i n l e s s s t e e l cable and i t s accessories which contact only t h e gas phase and by a n a l y s i s of s p e c i a l g e t t e r m a t e r i a l s which are attdched t o t h e cable.

Coils of

s i l v e r w i r e and specimens of Hastelloy N have generally been used as get-

ters f o r t h i s purpose.

No q u a n t i t a t i v e measure of t h e isotopes present i n

t h e gas phase i s p o s s i b l e , s i n c e no good estimate can be made of t h e gas

volume sampled.

The q u a n t i t y of m a t e r i a l deposited on t h e wire specimen

does not c o r r e l a t e well w i t h contact time ( i n t h e range 1 t o 10 min) o r with t h e g e t t e r m a t e r i a l s studied.

The quantity of m a t e r i a l deposited, however, i s r e l a t i v e l y l a r g e .

Table 14 i n d i c a t e s r e l a t i v e amounts found i n t y p i c a l t e s t s .

There i s no

doubt t h a t Mo, Ru, Te, (and from subsequent t e s t s , Nb) a r e appearing i n t h e helium gas of t h e pump bowl.

The q u a n t i t i e s a r e , moreover, surprising-

l y l a r g e ; i f t h e materials are presumed t o be vapors t h e p a r t i a l pressures

would be above 10â&#x20AC;&#x2122;â&#x20AC;&#x2122; f e r e n t behavior.

atmosphere.

The iodine isotopes show p e r c e p t i b l y d i f -

Iodine-135, whose tellurium precursor has a s h o r t half-

14.

Table

Q u a l i t a t i v e Indication of Fission Product i n MSRE E x i t Gas Amount"

Isotope

On N i

Fro% Liquid

On Hastelloy

132Te

lo5Ru

135,

1331

1.5

0.9

0.5

0.8

9 9 ~ ~

1311

%e u n i t of q u a n t i t y is t h a t amount of t h e isotope i n 1 g of salt. I

bOn s t a i n l e s s s t e e l cable.

80 l i f e , does not appear, while 1311and 1331,both of which have t e l l u r i u m

precursors of appreciable h a l f - l i f e , a r e found.

These findings--along

with

t h e f a c t t h a t t h e s e iodine isotopes a r e present i n t h e salt a t near t h e i r expected concentration--suggest

t h a t any iodine i n t h e vapor phase comes

as a r e s u l t of v o l a t i l i z a t i o n of t h e t e l l u r i u m precursors. Early attempts t o f i n d uranium on t h e wires (as from evolution of were unsuccessful.

w6)

More recent attempts--perhaps with t h e oxidation po-

t e n t i a l of t h e salt a t a higher level--have

shown s i g n i f i c a n t uranium depo-

s i t i o n corresponding t o s e v e r a l p a r t s per m i l l i o n i n t h e gas phase.

It i s

p o s s i b l e , but it seems u n l i k e l y , t h a t "salt spray" could account f o r t h i s observed uranium.

S a l t spray c e r t a i n l y does not account f o r t h e observed

noble metal species c a r r i e d i n t h e gas. "he behavior of t h e s e f i s s i o n product species i n t h e gas phase seems t o c o r r e l a t e poorly--if

a t all--with t h e UF /Wh r a t i o i n t h e f u e l melt.

Concentrations of Mo, Nb, Ru, and Te i n t h e gas phase seem t o increase ( o r decrease) together but were unaffected--within t h e data--by

t h e considerable s c a t t e r of

t h e d e l i b e r a t e addition of beryllium t o t h e MSRE melt.

The

concentrations of t h e s e elements i n t h e f u e l decrease ( a f t e r correction f o r radioactive decay) during r e a c t o r shutdowns ; such behavior would be expected i f they p l a t e out upon m e t a l l i c o r o t h e r surfaces.

Concentrations i n

t h e gas phase decrease somewhat more than those i n t h e salt but t h e differences seem much smaller than should be a t t r i b u t a b l e t o ( f o r example)

some r a d i a t i o n chemistry oxidation process t o produce MoF6, e t c . It seems most unlikely t h a t t h e s e d a t a can be reconciled as e q u i l i b r i -

um behavior of t h e v o l a t i l e f l u o r i d e s .

It i s p o s s i b l e t h a t t h e MSRE metal

i s p l a t e d with a noble-metal a l l o y whose thickness i s s e v e r a l hundred

81 4

angstroms, and it i s conceivable t h a t t h e UF4/UF3 r a t i o i s near 10 compound NbF

The

could show an appreciable pressure under t h e s e circumstances.

The o t h e r p o s s i b i l i t i e s such as MoF6, TeF6, and R u F would r e q u i r e much

higher W4/UF3 r a t i o s , and it seems most u n l i k e l y t h a t any s i n g l e redox pot e n t i a l can y i e l d t h e r e l a t i v e abundance observed f o r t h e s e isotopes. The recent findings of s i l v e r and palladium isotopes i n r e l a t i v e l y high concentration i n t h e gas phase seem ( s i n c e t h e s e elements c e r t a i n l y l a c k v o l a t i l e f l u o r i d e s ) t o case a d d i t i o n a l doubt on species such as MoF6, RuF

e t c . as t h e gas-borne species.

One p o s s i b l e explanation of t h e a v a i l a b l e d a t a i s t h e following:

The

noble metal species (Mo, Nb, Ru, T e , Ag, Pd, and probably Tc) occur--as thermodynamics predicts--in

t h e elemental state.

very rapidly become) i n d i v i d u a l metal atoms.

They o r i g i n a t e as ( o r

They aggregate at some f i n i t e

rate, probably alloying with one another i n t h e process and become insoluble

very minute c o l l o i d a l p a r t i c l e s which then grow at a slower rate.

These c o l l o i d a l p a r t i c l e s are not wetted by t h e f u e l , tend t o c o l l e c t a t gas-liquid i n t e r f a c e s , and can r e a d i l y be swept i n t o t h e gas stream of t h e helium purge of *he pump b o w l .

They tend t o p l a t e upon t h e m e t a l surfaces

of t h e system, t o form carbides (Nb and Mo only) with t h e g r a p h i t e , and

(as e x t r a o r d i n a r i l y f i n e "smoke") t o p e n e t r a t e t h e o u t e r layers of t h e moderator.

While t h e r e are d i f f i c u l t i e s with t h i s i n t e r p r e t a t i o n it seems

mgre p l a u s i b l e than o t h e r s suggested t o date. It is c l e a r t h a t f u r t h e r study and a d d i t i o n a l d a t a from MSRE and from s o p h i s t i c a t e d i n - p i l e loop tests w i l l 'be required before t h e d e t a i l s of f i s s i o n product behavior can be understood.

82 MOLTEN-SALT PRODUCTION TECHNOLOGY The f u e l and blanket salts of a molten-salt breeder r e a c t o r can be prepared by techniques similar t o those developed f o r t h e production of f l u o r i d e mixtures f o r t h e MSRE.

Commercially a v a i l a b l e f l u o r i d e salts,

which were used as s t a r t i n g m a t e r i a l s f o r t h e f l u o r i d e production process, required f u r t h e r p u r i f i c a t i o n only t o remove a l i m i t e d number of impurity species.

Chemical r e a c t i o n s used t o e f f e c t salt p u r i f i c a t i o n and methods

by which process conditions were c o n t r o l l e d are both adaptable t o t h e larger-scale production c a p a b i l i t i e s t h a t w i l l be required t o supply larges c a l e MSBR's. Production Process Fluoride mixtures required f o r t h e MSRE were prepared by a batch process i n a f a c i l i t y i n i t i a l l y designed t o support t h e various chemical and engineering tests of t h e program.

i s shown i n Fig. 17.

A layout of t h e production process

S t a r t i n g materials were weighed i n t o appropriate

batch s i z e s and simultaneously t r a n s f e r r e d by v i b r a t o r y conveyor t o a meltdown furnace assembly.

I n addition t o providing a molten charge t o each

of two adjacent processing u n i t s , t h e meltdown f a c i l i t y was u t i l i z e d f o r preliminary p u r i f i c a t i o n of t h e f l u o r i d e mixtures.

Beryllium-metal turn-

ings were added t o reduce structural-metal i m p u r i t i e s t o t h e i r i n s o l u b l e m e t a l l i c states.

The molten m i x t u r e w a s a l s o sparged with helium and hy-

drogen a t r e l a t i v e l y high f l a w rates t o remove i n s o l u b l e carbon by entrainment. Primary salt p u r i f i c a t i o n w a s achieved i n each of two batch processing u n i t s .

The melts were i n i t i a l l y sparged with a gaseous mixture of an-

hydrous HF i n hydrogen (1:lO v o l r a t i o ) .

Oxides, e i t h e r i n i t i a l l y present

ORNL -DWG 64-6998

PROTECTIVE CLOTHING STORAGE -

o r formed by r e a c t i o n of t h e f l u o r i d e salts with t h e i r adsorbed water on heating, were removed as water by t h e reaction 02-

+ H20.

WF c' 2F-

As shown by Fig. 18, t h e e f f i c i e n c y of t h i s r e a c t i o n i s q u i t e high.

Oxide

removal rates were determined by condensing water vapor from t h e gas eff l u e n t . i n a cold t r a p . Sulfides were a l s o removed (as H2S) by reaction with HF.

However,

any remaining sulfates m u s t be reduced by hydrogen o r added beryllium metal before s u l f u r removal by HF treatment 5 s e f f e c t i v e .

Although t h i s

impurity i s d i f f i c u l t t o remove, commercial vendors of f l u o r i d e salts used i n t h e MSiE were successful, through process development e f f o r t s , i n subs t a n t i a l l y reducing t h i s impurity from t h e i r products.

Consequently, sul-

fur removal from f l u o r i d e salts should not be an important consideration f o r f u t u r e production of fused f l u o r i d e mixtures f o r MSBR's. Nonequilibrium concentrations of structural-metal f l u o r i d e impurities ,-

t h a t a r e more e a s i l y r e d u c e d t h a n UF4 (e.g.,

NiF2 o r FeF2) would r e s u l t i n

t h e depletion of chromium a c t i v i t y i n t h e Hastelloy N a l l o y used as t h e Since t h e s e impurities are present i n

s t r u c t u r a l material i n t h e E R E .

f l u o r i d e r a w materials and may a l s o be introduced by corrosion of t h e process equipment, t h e i r concentrations i n t h e p u r i f i e d f l u o r i d e mixtures were an important process consideration. r i d e mixtures were sparged with. H

Following HF treatment, t h e fluo-

alone at' 7OO0C t o e f f e c t t h e conversion

of impurities t o insoluble metals by t h e reaction \

+ H2

Mo + 2HF.

Hydrogen w a s a l s o introduced during HF treatment t o reduce corrosion of t h e nickel salt-containment vessel.

Measurement of t h e HF concentration

ORNL-DWG 64-6996

5 HF INPUT CONCENTRATION

. o 0

Fig. 18.

15 20 OCESS TIME ('hr)

Fluoride Prbduction f o r MSRE; U t i l i z a t i o n P u r i f i c a t i o n of 7LiF-BeF2

86 -..

i n t h e gas e f f l u e n t during hydrogen sparging provided a convenient process control.

As shown by Fig. 19 t h e concentration of HF i n fhe g a s . e f f l u e n t

w a s i n d i c a t i v e of t h e i r o n concentration remaining i n t h e s a l t mixtures. A t 'the conclukon of t h e H2 treatment, r e s i d u a l q u a n t i t i e s of HF were

removed by sparging t h e melt with dry helium,

The p d i f i e d f l u o r i d e mix-

ture w a s then t r a n s f e r r e d t o i t s storage container.

A sintered'nickel

f i l t e r , i n s e r t e d i n t h e t r a n s f e r l i n e , removed entrained s o l i d s from t h e

melt.

Thus primary c o n t r o l of t h e production process was exercised by analysis of process gas streams.

F i l t e r e d samples of t h e s a l t mixtures were ob-

t a i n e d p e r i o d i c a l l y during t h e process f o r chemical analyses.

"his second-

ary c o n t r o l measure provided t h e b a s i s f o r acceptance of t h e s a l t batch f o r

A . L ~t h e

-'U

required f o r c r i t i c a l operation of t h e r e a c t o r could be

prepared as a concentrate mixture, 7LiF-UF4 (73-27 mole %), with UF4 t h a t

w a s highly enriched i n 235U.

This f a c i l i t a t e d compliance with nuclear

s a f e t y requirements and permitted an orderly approach t o c r i t i c a l i t y during f u e l i n g operations through incremental additions of 235UF4 t o t h e f u e l system of t h e reactor.

Since t h e density of 235U i n t h e concentrate mix-

ture i s r e l a t i v e l y high (2.5 g / c c ) , t h e f u e l i n g method employed f o r t h e MSRE should s u f f i c e f o r a l l p r a c t i c a l r e a c t o r systems. ~

MSRE Salt-Production Economics

The operation of t h e production f a c i l i t y f o r t h e preparation of MSRE m a t e r i a l s w a s conducted on a seven-day, t h r e e - s h i f t schedule at a budgeted c o s t of about $20,000 per month.

The r a w m a t e r i a l s c o s t f o r t h e 15,300 l b

of 7LiF-BeF2(66-34 mole %) used as t h e coolant and f l u s h s a l t was $?i.29

-- .

u p e r pound and t h a t of t h e f u e l salt (11,260 l b ) excluding 235U c o s t s w a s $10.13 per pound.

As c a l c u l a t e d from operating and r a w m a t e r i a l s c o s t s

(but not p l a n t amortization o r 235.U c o s t s ) t h e coolant and fl&h salt cost

$19.71 p e r pound and t h e f u e l s a l t . c o s t w a s $17.33 per pound.

Operating

c o s t s , as w e l l as r a w m a t e r i a l s c o s t s , should be s u b s t a n t i a l l y reduced f o r larger-scale production operations.

SEPARATIONS PROCESSES I N MSBR FUELS AND BLANKETS U s e of molten s a l t r e a c t o r s as thermal .breeders w i l l obviously r e q u i r e

e f f e c t i v e schemes f o r decontamination of t h e f u e l and f o r recovery of bred uranium from t h e blanket.

No provision, however, was made f o r on-stream

removal of f i s s i o n products from t h e MSRF,, and f u e l reprocessing has received less a t t e n t i o n t o d a t e than have more immediate materials problems of t h i s and s i m i l a r machines.

No d e t a i l e d reprocessing scheme h a s , accord-

i n g l y , been demonstrated. Recovery of uranium from molten f l u o r i d e s by v o l a t i l i z a t i o n as urani-

um hexafluoride and t h e subsequent p u r i f i c a t i o n of t h i s uF6 by r e c t i f i c a t i o n o r by svrption-desorption on NaF beds i s w e l l demonstrated.

Recovery

of bred uranium from blankets o r removal of uranium (where necessary t o f a c i l i t a t e o t h e r processing operations) from t h e f u e l , t h e r e f o r e , i s clearl y feasible.

Such v o l a t i l i t y processing i s described i n some d e t a i l else-

where56 i n t h i s series. More recent studies57 have shown t h a t t h e LiF, BeF2 (and ZrF4, i f p r e s e n t ) can be recovered q u a n t i t a t i v e l y , along with much of t h e uranium, by vacuum d i s t i l l a t i o n a t temperatures near 1000째C; very encouraging de-

contamination f a c t o r s from rare e a r t h f l u o r i d e s (which are l e f t behind i n t h e s t i l l bottoms) have been demonstrated.

As i s described elsewhere i n

t h i s series56 t h i s d i s t i l l a t i o n procedure combined with recovery of UF6 by f l u o r i n a t i o n shows real promise as a f u e l processing technique. Several other techniques have shown promise, a t least i n preliminary testing.

A b r i e f summary of t h e s e i s presented

i n t h e following.

Possible Separation of Rare Earths from Fuel

b, t

The rare earth f i s s i o n products, which are t h e most important nuclear

poisons i n a r e a c t o r from which xenon i s e f f e c t i v e l y removed, f o r n very

stable t r i f l u o r i d e s w i t h a p o r t i o n of t h a t F-

ium as UFL.

released as f i s s i o n of uran-

There i s no doubt, t h e r e f o r e , t h a t t h e s e f i s s i o n products a r e

dissolved i n t h e molten f u e l and a r e a v a i l a b l e f o r reprocessing. By Solid-Liquid E q u i l i b r i a

The limited s o l u b i l i t y of' t h e s e t r i f l u o r i d e s (though s u f f i c i e n t t o prevent t h e i r p r e c i p i t a t i o n under normal MSBR conditions ) suggested years When a LiF-BeF2-UF4 melt ( i n t h e MSBR

ago a p o s s i b l e recovery scheme.

concentration range) t h a t i s s a t u r a t e d with a s i n g l e r a r e e a r t h f l u o r i d e

(LaF3, f o r example) i s cooled slowly t h e p r e c i p i t a t e i s t h e pure simple trifluoride.

When t h e m e l t contains more than one r a r e e a r t h f l u o r i d e the

p r e c i p i t a t e i s a ( n e a r l y i d e a l ) s o l i d s o l u t i o n of t h e t r i f l u o r i d e s .

Ac-

cordingly, addition of an excess of CaF o r LaF t o t h e melt followed by 3 3 heating t o e f f e c t d i s s o l u t i o n of t h e added t r i f l u o r i d e and cooling t o eff e c t c r y s t a l l i z a t i o n e f f e c t i v e l y removes t h e f i s s i o n product r a r e e a r t h s

from solution.

It i s l i k e l y t h a t e f f e c t i v e removal of t h e rare e a r t h s

and y t t r i u m (along w i t h UF

and PIS ) can be obtained by passage of t h e

fuel through a heated bed of s o l i d CeF3 o r LaF 3'

The p r i c e , which i s al-

most c e r t a i n l y t o o high, i s t h a t t h e r e s u l t i n g f u e l s o l u t i o n i s s a t u r a t e d with t h e scavenger f l u o r i d e (LaF

o r CeF

whose cross s e c t i o n i s far from

n e g l i g i b l e ) at t h e temperature of contact. Since t h e rare earth f l u o r i d e s seem t o form with uranium t r i f l u o r i d e s o l i d s o l u t i o n s similar t o those described above it i s p o s s i b l e t o cons i d e r UF

as t h e scavenger material.

f u e l UF4 t o UF

It should be p o s s i b l e t o reduce t h e

and then by passage of t h e s o l u t i o n through a bed of UF

t o remove t h e contaminant r a r e earths; i n p r i n c i p l e , by c a r e f u l c o n t r o l of

i-i c

t h e column temperature (and, thereby, t h e s o l u b i l i t y of UF ) one could ob3 t a i n from t h e column a f u e l of t h e correct uranium concentration which could be returned t o t h e r e a c t o r a f t e r oxidation (by HF o r HF-H2 mixture) of UF3 t o UF4. b i l i t y of UF

While t h e process deserves f u r t h e r study, t h e g r e a t i n s t a -

i n s o l u t i o n s of high UF3/UFb r a t i o s and t h e g r e a t ease with

which t h e m e t a l l i c uranium a l l o y s with s t r u c t u r a l metals w i l l probably make t h e process . u n a t t r a c t i v e i n p r a c t i c e . Removal of r a r e e a r t h i o n s , and other i o n i c f i s s i o n product s p e c i e s , by use of c a t i o n exchangers a l s o seems an appealing p o s s i b i l i t y .

The ion

exchanger would, of course, need (1)t o be q u i t e insoluble, ( 2 ) t o be extremely unreactive ( i n a gross sense) with t h e melt, and (3 ) t o t a k e up r a r e e a r t h cations i n exchange ions of low neutron cross s e c t i o n .

For

r a r e e a r t h separations it would probably s u f f i c e i f t h e m a t e r i a l exchanged normal Ce3+ o r La3+ f o r t h e f i s s i o n product r a r e e a r t h s ; other separation schemes (such a s d i s t i l l a t i o n ) would be required t o remove t h e Ce3+ o r La3+ but they could operate on a much longer time cycle.

[The bed of CeF3

described above functions i n an ion exchanger; it f a i l s t o be t r u l y benef i c i a l because it i s t o o soluble i n t h e melt.] Unfortunately, t h e r e a r e not many m a t e r i a l s known t o be t r u l y s t a b l e t o t h e f u e l mixture.

Zirconium oxide i s s t a b l e ( i n i t s low temperature

form) t o melts whosejZr

8-k 4-k /U r a t i o i s i n excess of about 3.

It i s con-

ceivable t h a t s u f f i c i e n t l y d i l u t e s o l i d s o l u t i o n s of Ce 0 i n Zr02 would 2 3 be s t a b l e and would exchange Ce3'

f o r o t h e r r a r e e a r t h species.

Inter-

m e t a l l i c canpounds of r a r e e a r t h s with moderately noble metals ( o r r a r e e a r t h s i n very d i l u t e alloys with such metals) seem unlikely t o be of use

because they a r e unlikely t o be s t a b l e toward oxidation by UF4.

Compounds

92 with oxygenated anions (such as s i l i c a t e s and molybdates) a r e decomposed

by t h e f l u o r i d e melt; they, and simple oxides (Zr02 excepted) p r e c i p i t a t e U02 from the f u e l mixture.

It is p o s s i b l e t h a t r e f r a c t o r y compounds (such

as carbides o r n i t r i d e s ) of t h e r a r e e a r t h s e i t h e r alone o r i n s o l i d di-

l u t e s o l u t i o n with analogous uranium compounds, may prove useful.

A con-

s i d e r a b l e amount of exploratory research w i l l be required (and many of t h e obvious p o s s i b i l i t i e s have already been r e j e c t e d ) before such a technique can be given consideration. By Reduction

The rare e a r t h f l u o r i d e s are very s t a b l e toward reduction t o t h e metal.

For example, at 1000째K t h e r e a c t i o n

where c and 8 i n d i c a t e c r y s t a l l i n e s o l i d and l i q u i d , r e s p e c t i v e l y , shows

+ 32.4 k c a l f o r t h e f r e e energy of reaction.

With t h e LaF

t i o n and BeF2 i n concentrated s o l u t i o n i n LiF-BeF change i s , of course, even more unfavorable.

i n d i l u t e solu-

mixture t h e f r e e energy

However, t h e r a r e earth

metals form extremely s t a b l e solutions58 i n molten metals such as bismuth. Beryllium i s v i r t u a l l y i n s o l u b l e i n bismuth and forms no i n t e r m e t a l l i c canpounds w i t h t h i s metal.

Accordingly, t h e r e a c t i o n

where d i n d i c a t e s t h a t t h e species is dissolved i n 2LiF-BeF2, c i n d i c a t e s c r y s t a l l i n e s o l i d , and B i i n d i c a t e s a d i l u t e alloy i n bismuth, can be made t o proceed e s s e n t i a l l y t o completion.

Accordingly, LaF

and e x t r a c t e d i n t o molten B i from LiF-BeF2 mixtures.

can be reduced ,-

93 Since’ L i o a l s o forms s t a b l e s o l u t i o n s i n molten bismuth, 58,59 t h e process of reducing t h e r a r e e a r t h s with beryllium cause some reduction of LiF and e x t r a c t i o n of lithium by t h e bismuth. -’

I n p r a c t i c e , it i s more con-

venient t o use 7Li i n bismuth ( a t or j u s t below t h e concentration which y i e l d s c r y s t a l l i n e Be

a t equilibrium) as t h e reductant.

Figure 20 shows

t h e behavior of s e v e r a l r a r e e a r t h s when e x t r a c t e d from very d i l u t e s o h t i o n s i n 2LiF*BeF2 w i t

aring

i n simple equiiment. 6o

It is s t i l l

t o o e a r l y t o be s u r e t h a t

separations a v a i l a b l e a r e s u f f i c i e n t l y com-

p l e t e , e s p e c i a l l y f o r he

rare e a r t h s , f o r t h e method t o be competitive

with t h e d i s t i l l a t i o n p r

I n addition, it -is uncertain whether re-

covery of t h e 7Li w i l l be necessary and, i f >so, how’much recovery would be accomplished.

However, t h e process seems at t h i s preliminary s t a g e t o be

worthy -of f u r t h e r study.

It i s c l e a r t h a t reduc p r i p c i p l e , be accomplished

on processes of t h i s type can, at l e a s t i n ectrochemically with t h e molten bismuth as

t h e cathode and with some i n e r t anode a t which f l u o r i n e gas can be generated.

The concentrations of r a

tained i n the molten b i s

tal equilibrium as .desc

method o r d i r e c t chemica

e a r t h metals, l i t h i u m , and beryllium obbe i d e n t i c a l t o those obtained by chemi-

Whether one p

, e r s t h e electrolyt’ic

i b r a t i o n w i l l , accordingly, depend upon t h e

economics of t h e competing processes. i Recovery of Protactinium from Blanket

While removal of b r e feasible, -the p r i o r r e m 0 t o 233U

outside t h e neutro

t h e breeding economy.

rom t h e blanket by f l ~ o r i n a t i o nappears ~~ %a from t h e blanket t o

ermit i t s decay

I d would.be a most valuable contribution t o

Such a s e p a r a t i v e process must be simple, s i n c e it

ORNL- DWG 66- 2425

400

MOLE FRACTION OF RARE EARTH IN METAL MOLE FRACTION OF RARE EARTH IN SALT

Fig. 20.

E f f e c t of Lithium Concentration i n Metal Phase on t h e D i s t r i b u t i o n of Rare Earths Between LiF-EeF, (66-34 mole %) and Bismuth a t 60OOe. \

4000 \

95 must be capable of handling t h e e n t i r e blanket i n a time s h o r t compared with t h e 31.5 day half-time f o r decay of 233Pa. By Oxide-Fluoride E q u i l i b r i a Removal of Pa by d e l i b e r a t e a d d i t i o n t o LiF-BeF2-ThF4 blanket mix-

tures of BeO, Tho2, o r U02 has been demonstrated. 61,62

P r e c i p i t a t i o n of

an oxide of protactinium, adsorption of protactinium on t h e added oxide,

or (more l i k e l y ) formation of a s o l i d s o l u t i o n of protactinium oxide with the b e s t oxide, has been shown t o be e s s e n t i a l l y complete.

The process

has been demonstrated t o be r e v e r s i b l e ; treatment of t h e oxide-fluoride mixture with anhydrous HF dissolves t h e added ( o r p r e c i p i t a t e d ) oxide and

returns t h e protactinium t o s o l u t i o n from which it can r e a d i l y be reprecipitated.

It seems l i k e l y t h a t protactinium might be removed from t h e

blanket by passage of a s i d e stream through a tower packed w i t h Tho2 ( o r possibly B e O ) ; t h e protactinium, i n some u n i d e n t i f i e d form, would remain on t h e bed and would t h e r e decay t o 233U o u t s i d e t h e neutron f i e l d .

i t s passage through t h e packed bed of oxide t h e blanket m e l t becomes satur a t e d with oxide ion.

This oxide ion concentration would probably have t o

be diminished appreciably by treatment w i t h HF and then H

before t h e melt

could be returned t o t h e blanket stream. By Reduction

The p o s s i b i l i t y of recovery of protactinium from r e a l i s t i c LiF-BeF2-

ThF4 blanket mixtures by reduction has been examined experimentally with s u r p r i s i n g and encouraging results.63

N o real information e x i s t s as t o

t h e free energy formation of t h e f l u o r i d e s of protactinium.

Accordingly,

experiments were performed i n which t r a c e s of 233Pa were added t o LiF-BeF2ThF4 m e l t s , t h e m e l t s were c a r e f u l l y t r e a t e d w i t h HF and H2 t o i n s u r e

conversion of protactinium t o f l u o r i d e and i t s d i s s o l u t i o n i n t h e melt, and t h e s o l u t i o n subsequently treated with a strong reducing agent. experiments used "b2 i n l e a d o r ThBi t e s t s have used metallic thorium.

Some

i n bismuth as t h e reductant; o t h e r

I n each case; t h e protactinium remained

i n molten f l u o r i d e s o l u t i o n (as judged by radiochemical a n a l y s i s of f i l t e r ed samples) u n t i l t h e reducing agent was added and w a s removed, upon ad-

d i t i o n of reductant, t o very low concentration l e v e l s . t h e data f o r a t y p i c a l case.

Figure 2 1 shows

The removal has been shown t o be nearly

q u a n t i t a t i v e at both t r a c e s ( l e s s than p a r t per b i l l i o n ) l e v e l s and at r e a l i s t i c concentrations ( 50 ppm) of 23$a t r a c e d with 23%a.

The process

has also been shown t o be r e v e r s i b l e ; sparging of t h e system w i t h HF o r

HF-H2 mixtures r e t u r n s t h e protactinium q u a n t i t a t i v e l y t o t h e molten fluoride solution.

Recovery of t h e p r e c i p i t a t e d protactinium has proved t o be more difficult.

Attempts t o obtain t h e deposited protactinium i n molten B i o r Pb

have been generally unsuccessful i n equipment of i r o n , copper, niobium, o r s t e e l ; t h e deposited protactinium w a s only f l e e t i n g l y ( i f e v e r ) dissolved i n t h e molten metal.

When thorium w a s used as t h e reductant no ap-

p r e c i a b l e concentration of protactinium was found i n t h e excess thorium. Careful examination of sectioned apparatus shows some protactinium on t h e

v e s s e l w a l l s , and some appears t o remain suspended ( i n e a s i l y f i l t e r a b l e form) i n t h e salt.

The mechanism of removal of protactinium from t h e salt

mixture remains far from c e r t a i n .

It appears l i k e l y that t h e thorium ( o r

s l i g h t l y weaker reducing agent) reduces protactinium t o form a moderately stable i n t e r m e t a l l i c compound (perhaps with C r o r Fe) which i s f i l t e r a b l e ,

i s not dissolved by t h e molten lead o r bismuch, and i s r e a d i l y decomposed

ORNL-I YG 66-970

100

80 CI

WEIGHT OF SALT: 6009 WEIGHT OF LEAD: 9OOg

I 60

n '

WITHOUT ADDED THORIUM

WITH 0.1wt 70

C LL

0 0 Fig. 21.

Effect o

rim Metal on t h e

. by anhydrous HF. Attempts t o recover t h e protactinium by reduction with m e t a l l i c thor-

ium i n s t e e l equipment i n t h e presence of added i r o n surface ( s t e e l wool) have shown some promise.

64 I n a t y p i c a l experiment, some 320 grams of

LiF-ThF4 (27 mole % ThF4) containing 81 ppm (26 mg) of protactinium w a s reduced with thorium-in t h e presence of 4 grams of s f e e l wool.

The

L i F - W 4 , previously p u r i f i e d , w a s placed i n a welded n i c k e l r e a c t i o n

v e s s e l , i r r a d i a t e d ThF4 containing a known amount of 233Pa and 231Pa was added t o t h e mixture, and it w a s t r e a t e d first with a mixture of HF and H2 and then w i t h H

alone.

2 Four grams of s t e e l wool (grade 00, 0.068 m /g

surface area) w a s placed i n a low-carbon-steel vessel.

l i n e r i n s i d e another n i c k e l

The contents of t h i s v e s s e l were t h e n ' t r e a t e d w i t h p u r i f i e d hydro-

gen a t 800Oc f o r s e v e r a l hours t o remove as much as p o s s i b l e of t h e oxide /

s u r f a c e contamination of t h e s t e e l wool and l i n e r .

The two v e s s e l s were

then connected together a t room temperature and heated t o about 65ooc, and t h e salt w a s t r a n s f e r r e d t o t h e s t e e l - l i n e d vessel.

After two separate

exposures of t h e salt t o a s o l i d thorium s u r f a c e , as i n d i c a t e d i n Table 15, I

t h e s a l t w a s t r a n s f e r r e d back t o i t s o r i g i n a l container and allowed t o cool i n helium.

The s t e e l - l i n e d v e s s e l was cut up, and samples were sub-

mitted- f o r analysis. /

The d a t a i n Table 1 5 show t h a t 99% of t h e protactinium w a s precipi-

t a t e d i n a form t h a t would not pass through a s i n t e r e d copper f i l t e r a f t e r

a f a i r l y s h o r t exposure t o s o l i d thorium, but nearly 7% was i n t h e un-

f i l t e r e d salt t h a t w a s t r a n s f e r r e d back t o t h e n i c k e l v e s s e l a f t e r exposure t o thorium.

About 69 g of salt w a s associated with t h e s t e e l wool r -

i n t h e s t e e l l i n e r i n t h e form of a hard b a l l .

P a r t i a l separation of t h e

salt from s t e e l wool w a s e f f e c t e d by use of a magnet after crushing t h e b a l l , and t h e iron-rich f r a c t i o n had t h e higher protactinium concentration.

The s m a l l amount o f protactinium found on t h e v e s s e l w a l l i s e s p e c i a l l y

notable.

Coprecipitation of m e t a l l i c protactinium and i r o n (and possibly

n i c k e l ) would h e l p t o account f o r t h e manner i n which protactinium s e t t l e d out on, and adhered t o , t h e s t e e l wool surface. On t h e basis of p r e s e n t l y a v a i l a b l e information, thorium reduction of protactinium from molten breeder blanket mixtures i n t h e presence of s t e e l wool i s believed t o be a promising recovery method warranting f u r t h e r inves t i g a t ion. Recent experiments have shown, i n a d d i t i o n , t h a t t h e handling of protactinium i s s i m p l i f i e d somewhat i f g r a p h i t e serves as the container. When irradiated thorium metal (containing 233Pa) i s dissolved i n molten bismuth i n metal containers t h e protactinium disappears from t h e l i q u i d

metal s o l u t i o n r a p i d l y .

Similar experiments using g r a p h i t e v e s s e l s show

very slow n e g l i g i b l e decreases i n protactinium concentration ( a f t e r corr e c t i o n f o r r a d i o a c t i v e decay) with time. According&,

recent studies of reduction of protactinium f r o m m o l t e n

f l u o r i d e s o l u t i o n have been conducted i n v e s s e l s of g r a p h i t e .

An i n t e r -

e s t i n g assembly which has been s t u d i e d i n a preliminary way uses a cylind r i c a l g r a p h i t e c r u c i b l e (as a l i n e r i n s i d e a s t a i n l e s s s t e e l o r n i c k e l v e s s e l ) contain ng a pool of molten bismuth and a c e n t r a l c y l i n d r i c a l chimney of g r a p h i t e with i t s lower end immersed i n t h e bismuth pool.

The

chimney i s connected t o t h e l i d of t h e metal j a c k e t v e s s e l i n a manner such t h a t t h e c e n t r a l chamber and t h e annular o u t e r chamber can be maint a i n e d under s e p a r a t e and d i f f e r e n t atmospheres.

A LiF-ThF4 blanket mix-

100

G Table 15.

P r e c i p i t a t i o n of Protactinium from Molten LiF-ThF4

(73-27 Mole %) by Thorium Reduction i n t h e Presence of S t e e l Wool Sample

Concentration (mg/g)

Tot a1

231pa (mg)

S a l t a f t e r HF-H2 treatment

0.0634

20.3

S a l t j u s t before t r a n s f e r

0.081

26.1

S a l t 35 min a f t e r t r a n s f e r

0 079

24.9

S a l t a f t e r 50 min thorium exposure

0.0026

0.69

S a l t a f t e r 45 min thorium exposure

0.0009

0.27

Nonmagnetic f r a c t i o n of material in steel liner

0.20

11.5

Magnetic f r a c t i o n of m a t e r i a l i n steel l i n e r

0.628

10.2

Unfiltered salt after t r a n s f e r t o nickel vessel

0.0076

1-75

Steel l i n e r wall

0.0006

S t a i n l e s s s t e e l dip l e g

0.53

F i l i n g s from thorium rod

0.29

All s a l t samples

1.35

T o t a l protactinium recovered

23Lpa

25-5

101

ture containing protactinium f l u o r i d e i s placed i n t h e annular chamber and a LiF-NaF-KF mixture i s placed i n t h e inner chamber.

An atmosphere of HF

i s used t o sparge t h e LiF-NaF-KF mixture and a reducing metal and (beryll i u m o r thorium) i s added t o t h e s a l t i n t h e o u t e r chamber.

The protac-

tinium f l u o r i d e i n t h e o u t e r chamber i s reduced, dissolved i n and t r a n s f e r r e d through t h e bismuth and i s oxidized by HF and dissolved i n t h e LiFNaF-KF mixture i n t h e i n n e r cylinder.

Additional study i s necessary t o

e s t a b l i s h (1) t h e rate a t which such a system can be made t o work, ( 2 ) t h e q u a n t i t y of reducing metals t r a n s f e r r e d t o t h e recovery s a l t , and t h e c m p l e t e n e s s t o which t h e r e a c t i o n can be e a s i l y driven. which seems t o have s e v e r a l u s e f u l variations--does,

The system--

however, look promis-

ing.

It is also c l e a r t h a t , as i n t h e rare e a r t h reduction process, electrochemical reduction of t h e protactinium f l u o r i d e s should be successful.

I n t h i s case, it might seem e s p e c i a l l y promising i f (as now seems

l i k e l y ) t h e protactinium is being reduced i n t h e presence of metal t o a s t a b l e i n t e r m e t a l l i c compound.

Attempts

t o reduce protactinium e l e c t r o -

chemically w i t h a v a r i e t y o f m e t a l l i c e l e c t r o d e s t o a s c e r t a i n (1)t h e type and composition of t h e i n t e r m e t a l l i c compound, and ( 2 ) whether a simple recovery process with a s o l i d e l e c t r o d e can be achieved are scheduled f o r study.

102

MSBR I N - L I N E ANALYSIS PROGRAM The r a p i d a c q u i s i t i o n of d a t a concerning t h e compositions of t h e f u e l , coolant, and cover gas i s highly d e s i r a b l e i n t h e operation of a f l u i d f u e l reactor.

To be of most value t h e data should be r e p r e s e n t a t i v e of t h e re-

a c t o r at zero time preferably with as l i t t l e time-delay as p o s s i b l e i n order t o evaluate changes i n c a p o s i t i o n from normal conditions and t o

take r e q u i s i t e action.

sis.

This state can only be a t t a i n e d by i n - l i n e analy-

I n v e s t i g a t i o n s are under way t o develop instrumentation capable of

providing instantaneous data.

It i s proposed t o devote considerable ef-

f o r t i n t h i s d i r e c t i o n as p a r t of t h e MSBR program.

The a l t e r n a t i v e i s t o

sample t h e f u e l and coolant a t periodic i n t e r v a l s and remove the sample

f o r a n a l y s i s a t an appropriate a n a l y t i c a l laboratory.

This procedure i s

time-consuming and thereby s u f f e r s obviously from a d e f i n i t e time l a g i n providing information s o t h a t unknown events and information concerning t h e s e events a r e out of phase. Although i n - l i n e instrumentation i s a well-established technique, i t s a p p l i c a t i o n t o molten salt r e a c t o r s i s e s s e n t i a l l y i n i t s infancy

- parti-

c u l a r l y i n regard t o r a d i a t i o n and i t s e f f e c t on maintenance of operating equipment.

The o b j e c t i v e i s thus t o apply t h e successful i n - l i n e techni-

ques t h a t have been used t o c o n t r o l many nonradioactive chemical processes t o c o n t r o l t h e r e a c t o r f u e l , coolant, and cover gas.

H e l i u m Cover Gas I n addition t o t h e a n t i c i p a t e d impurities (atmospheric contaminants , CF4, K r , and Xe) have been found t o represent s i g n i f i c a n t contaminants i n t h e MSRE off-gas system.

While it has not y e t been p o s s i b l e t o measure

hydrocarbons i n t h e MSRE blanket g a s , organic deposits have s e r i o u s l y

> .

i n t e r f e r e d with t h e operation of t h e MSRE off-gas system and hydrocarbons i n concentrations of s e v e r a l hundred p a r t s p e r m i l l i o n have been found i n t h e off-gas from t h e MSRE pump t e s t loop and i n simulated pump leak experiments.

(These experiments i n d i c a t e t h a t most, i f not a l l , of t h e

hydrocarbcm e n t e r s t h e pump bowl through a mechanical j o i n t which can be welded i n f u t u r e models. )

I n t h e s e t e s t s t h e t o t a l hydrocarbon concen-

t r a t i o n w a s measured continuously by a flame i o n i z a t i o n d e t e c t o r and t h e i n d i v i d u a l hydrocarbons

-- p r i n c i p a l l y l i g h t unsaturates -- were i d e n t i -

f i e d by gas chromatography.

Gas chromatography i s a near p e r f e c t technique f o r automated analysis. This technique i s now highly developed and refined, and considerable experience has been gained from research i n other r e a c t o r programs on t h e

analysis of helium by gas chromatographic techniques.

The determination

of permanent gas impurities i n molten s a l t r e a c t o r blanket gases w i l l re-

q u i r e an instrument of improved s e n s i t i v i t y t h a t i s compatible w i t h intense radiation.

A simple ahromatograph has 'been used t o measure ppm

and lower concentrations of H2, 02, N2, CH4, , K r , Xe, and CF4 i n t h e offgas from an MSRE i n - p i l e t e s t .

These contaminants were resolved on a 1 O X

molecular s i e v e column and measured with a helium discharge d e t e c t o r , which has t h e following l i m i t s of detection.

104 Table 16.

S e n s i t i v i t y f o r Detection of Contaminants i n Helium by Gas Chromatography Component

P a r t s per B i l l i o n

H2째 H2 O2 Kr

100

>10 >10

cH4 CF4

>10 20

The determination of H20 and C02 w i l l r e q u i r e a more complex i n s t r u ment w i t h multiple columns; probably a three-column instrument w i l l be required f o r all t h e above components.

Also it w i l l be necessary t o e l i -

minate all organic materials of construction completely i f extended dependable operation i s t o be obtained w i t h highly r a d i o a c t i v e samples.

An all-

metal pneumatically actuated sampling valve i s being developed f o r t h i s application.

This valve w i l l a l s o be operable a t high temperatures t o

minimize t h e adsorption of t r a c e s of moisture.

The e f f e c t s of hydro-

carbons on t h e chromatograph has not been t e s t e d but w i l l probably require some modification of t h e proposed instrument. Gas chromatography i s t h e most highly developed method f o r t h e auto-

matic analysis of hydrocarbon mixtures; however, t h e r e s o l u t i o n of t h e complex mixtures a n t i c i p a t e d i n t h e blanket gas requires columns packed with organic s u b s t r a t e s , which a r e not c m p a t i b l e with t h e highly radioa c t i v e samples.

Also, t h e experience with t h e pump t e s t loop has i n d i c a t c

ed t h a t t h e continuous measurement of t h e total. concentration of hydrocarbons would provide adequate information f o r r e a c t o r operations.

These

measurements, made with a flame i o n i z a t i o n d e t e c t o r , provided data t o d i f f e r e n t i a t e between possible locations of l e a k s ; conversely, t h e complete analyses were of value only i n development s t u d i e s f o r t h e s e l e c t i o n of means of removing t h e hydrocarbons.

The flame i o n i z a t i o n detector would

probably not be s u i t a b l e f o r i n d i n e analysis of t h e r e a c t o r blanket gases because i t s operation would i n j e c t s u b s t a n t i a l q u a n t i t i e s of a i r i n t o t h e off-gas system.

An a l t e r n a t e method which w i l l not introduce contaminants

i s being developed,

I n t h i s technique t h e hydrocarbons a r e oxidized t o

carbon dioxide and water with copper oxide, and t h e thermal conductivity of t h e canbusted stream i s compared with t h a t of t h e same gas a f t e r t h e

C02 and H 0 a r e removed by a s c a r i t e and magnesium perchlorate. 2

This

method has been t e s t e d with a bench t o p apparatus and found t o give a sign a l proportional t o hydrocarbon concentration over t h e range of i n t e r e s t with a l i m i t of detection below 10 ppm.

If p o s s i b l e , a s i m i l a r apparatus

w i l l be t e s t e d on t h e off-gas of t h e MSRE. Spectrophotometry of Molten S a l t s Absorption spectrophotometry and electrochemical a n a l y t i c a l techniques are p o t e n t i a l l y applicable f o r i n - l i n e a n a l y s i s .

The absorption

s p e c t r a of s e p a r a t e s o l u t i o n s of U(1V) and U(II1) i n fluoride-base molten

salts have been obtained.

ased on a consideration of t h e s e s p e c t r a ,

U(II1) could be determined a t a wavelength of 360 mp t o a concentration l e v e l of

E. 300 ppm i n t h e presence of up t o 1 mole % of U(1V) i n molten

LiF-BeF2-ZrF4.

Such a spectrophotometric method, which i s based on a

c h a r a c t e r i s t i c spectrum, would be a s p e c i f i c and d i r e c t method.

Performed

106 11

in-line,"

t h i s determination would provide a d i r e c t , s p e c i f i c , and con-

tinuous monitor of t h e U ( I I 1 ) concentration i n t h e molten h e 1 salt. Similarly a r e l a t i v e l y weak peak a t 1000 mu i n t h e absorption s p e c t r a of t e t r a v a l e n t uranium could be used t o monitor U( I V )

, provided

t h e concen-

Any corrosion products

t r a t i o n of U ( I I 1 ) does not exceed about 1000 ppm. ?

i n the molten salt, even if present a t s e v e r a l times t h e concentration l e v e l t h a t i s expected, w i l l not i n t e r f e r e with t h e proposed determinations. The e f f e c t of t h e s p e c t r a of t h e various f i s s i o n products i s not known primarily because t h e i r equilibrium oxidation s t a t e s are not known w i t h certainty.

It seems reasonable t o assume, however, t h a t l i t t l e i f any ef-

f e c t w i l l be observed.

Perhaps t h e most i n t e r f e r e n c e w i l l be f r o m t h e

r a r e e a r t h s , probably as s o l u b l e f l u o r i d e s .

On t h e b a s i s of experimental

evidence t h e rare-earth s p e c t r a i n molten f l u o r i d e salts should present sharp but i n s e n s i t i v e absorption peaks. Recently a very i n t e n s e absorption peak at 235 mu has been found f o r U ( 1 V ) i n LiF-BeF2 melts.

Preliminary estimates i n d i c a t e t h a t t h i s peak

could be used f o r t h e i n - l i n e measurement of uranium concentrations as l o w as 5 t o 10 ppm.

I f no i n t e r f e r i n g ions a r e p r e s e n t , t h e peak could be

applied as a s e n s i t i v e d e t e c t o r of leaks i n t o coolant s a l t streams and t o measure residual uranium i n depleted reprocessing streams. The design of a spectrophotometer t o be used i n t h e s e proposed appli-

cations is r a t h e r w e l l defined.

Modification of an e x i s t i n g commercial

spectrophotometer, a Cary Model 14-H manufactured by Applied Physics Company, w i l l adequately meet t h e design c r i t e r i a .

I n order t o eliminate

most of t h e r a d i a t i o n which i s present i n t h e salt sample t h e o p t i c a l pathl e n g t h of t h e spectrophotometer w i l l be extended %. t h r e e f e e t ; a t t h e

, -

107

-4d

same time t h e imaging of t h e o p t i c a l system w i l l be modified t o provide more i n t e n s e illumination of t h e sample area. It appears t h a t t h e piping which w i l l d e l i v e r t h e molten salt t o t h e

sample c e l l can be extended a convenient distance from t h e r e a c t o r core s o t h a t environmental r a d i a t i o n may be no problem t o t h e servicing of t h e \

e l e c t r o n i c components of t h e spectrophotometer.

If t h e r a d i a t i o n i s above

t o l e r a n c e , separation of e l e c t r o n i c and o p t i c a l components can best. be handled by building one i

n t housing t h e o p t i c a l components and an-

other instrument containing t h e e l e c t r o n i c s .

Schematic diagrams of t h e

c e l l design and spacing a r e shown i n Figs. 22 and 23. i

If t h e spectrophotometer i s t o monitor t h e spectrum of U ( I I 1 ) con-

tinuously and monitor t h e spectrum of U(1V) occasionally, t h i s type of r e p e t i t i v e analysis i s r e a d i l y adaptable t o an automatic c y c l i c operation

with t h e data recorded by d i g i t i z i n g equipment. Electrochemical Studies

I n p r i n c i p l e electrochemical analyses of molten salts a r e a t t r a c t i v e f o r in-line a n a l y s i s s i n c e t h e technique lends i t s e l f so w e l l t o remote operations.

I n addition

reduced i s determinable '

species an s o l u t i o n s that can be oxidized or rochemistry.

s t be known, however.

s o l u t i o n and t h e r e a c t could e s t a b l i s h t h e no

The chemica3 behavior of t h e I d e a l l y , one

t i a l of t h e f u e l and observe f l u c t u a t i o n s

and deviations from t h i s

s manner t h e norm

havior of t h e f u e l is

ably changes i n t h i s behavior would

-be c o r r e l a t e d with obs

able reference e l e c t r o

ded.

e r a t i n g be-

To accomplish t h i s task a r e l i -

To t h i s end, it i s planned t o in\

v e s t i g a t e various metal-metal ion couples (nickel-nickel fluoride, nickel-

108 ORNL- DWG 65-39f5A

kâ&#x20AC;&#x2122; ;

FREEZE VALVE

Fig. 22.

Molten S a l t Reactor In-Line Spectrophotometric F a c i l i t y

109

ORNL- DWG. 65-3976

INSULATION\

\CELL

Fig. 23.

U c

C e l l Space Optical Design

. 110

n i c k e l oxide, beryllium-beryllium f l u o r i d e , f o r example) as p o s s i b l e r e f e r ence electrodes that a r e compatible w i t h f l u o r i d e melts.

I n p r a c t i c e , the /

metal-metal i o n reference appears t o be t h e b e s t choice from t h e standpoint of i n v e s t i g a t i n g and s e t t i n g up of electropnalyticalmethods f o r anI

a l y s i s of molten f l u o r i d e s .

One model of t h e Ni-NiF2 electrode has been

t e s t e d and found t o be r e v e r s i b l e and reproducible but of l i m i t e d s e r v i c e life.

The u s e f u l l i f e t i m e of t h i s electrode i s l i m i t e d t o a f e w weeks by

t h e d i s s o l u t i o n of a t h i n membrane of boron. n i t r i d e which serves as a " s a l t bridge" between t h e Ni-NiF2 h a l f c e l l and t h e molten sample.

While it may

be mechanically f e a s i b l e t o replace electrodes i n r e a c t o r process streams p e r i o d i c a l l y , a mbch more dependable system could be constructed i f an ins u l a t i n g m a t e r i a l t h a t i s compatible . w i t h molten f l u o r i d e s could be dis,

covered. A three-electrode system,_ an i n d i c a t o r e l e c t r o d e ,67 quasi-reference

electrode,

' been applied t o and an i s o l a t e d counter e l e ~ t r o d e , ~has

molten salts successfully.

Approximate p o t e n t i a l s f o r observed e l e c t r o d e

reactions f o r s e v e r a l e l e c t r o a c t i v e species are shown i n Fig. 24.

Theo-

r e t i c a l l y , it is p o s s i b l e t o measure t h e concentration of t h e metal ions

at t h e i r decomposition p o t e n t i a l s independent of t h e presence of o t h e r

m e t a l ions as long as t h e p o t e n t i a l d i f f e r e n c e i s at l e a s t 0.3 v.

The

presence of gross q u a n t i t i e s of one metal and t r a c e q u a n t i t i e s of another o f t e n r e s u l t s i n swamping of the decomposition p o t e n t i a l . This technique hhs already been applied t o samples from t h e MSRE t o

determine t h e oxidation state of i r o n and n i c k e l which appeared t o be \

present. i n t h e f u e l i n concentrations above t h a t predicted t o e x i s t i n equilibrium with INOR-8at t h e observed concentrations of chromium.

Conr

111

”

ORNL- DWG. 66-6277

CATHODIC

I + 2.0

I + 4.0

-4.0

-2.0

POTENTIA,L VOLTS VERSUS PLATINUM QUASI-REFERENCE ELECTRODE

e P o t e n t i a l s f o r ElectGode

n Molten LiF-BeF2-ZrFh

.O mole $,

ci L

at Temp. 500°C)

112

c e n t r a t i o n s of i o n i c i r o n and' n i c k e l of only about 10 and 1 ppm were determined by voltammetric scans of remelted samples t h a t had been withdrawn from t h e MSRE before it w a s operated at power.

These values compare w i t h 1

t o t a l concentrations (determined chemically) of 125 and 45 ppm, respectivel y , and i n d i c a t e t h a t t h e major f r a c t i o n s of these contaminants are probably present as f i n e l y divided metals.

Thus

concentrations of t h e s e

corrosion products i n t r u e i o n i c s o l u t i o n ar,e more c o n s i s t e n t w i t h thermodynamic predictions. The three-electrode system a l s o o f f e r s s i g n i f i c a n t p o t e n t i a l as a technique f o r i n - l i n e monitoring of uranium i n r e a c t o r f u e l s .

I n MSRE

type melts at 5OO0C U(1V) t o U ( I I 1 ) reduction waves have been found t o be reproducible t o b e t t e r than 1% i n measurements during a two-hour period, and t o about 2 t o 3%f o r i n t e r m i t t e n t measurements taken over a one-month 1

period.

I f t h e r e p r o d u c i b i l i t y could be improved, the technique could a l s o

be used t o measure t r i v a l e n t uranium.

T h e a a t i o of reverse t o forward

scan currents i s u n i t y when only U ( 1 V ) i s p r e s e n t , but t h e r a t i o ,increases \

as UF

i s added t o t h e melt.

One l i m i t t o t h e r e p r o d u c i b i l i t y of t h e

voltammetric measurement i s t h e p r e c i s i o n of d e f i n i t i o n of t h e a r e a of t h e -_

With present instrumentation it 'is necessary t o

i n d i c a t i n g electrode.

l i m i t t h e e l e c t r o d e a r e a by i n s e r t i n g a 20-gauge platinum wire only 5 mm /

i n t o t h e melts t o l i m i t t h e currents t o measurable values.

It i s apparent

t h a t only a s m a l l change i n m e l t l e v e l w i l l produce a s i g n i f i c a n t e r r o r i n

electrode area.

A new voltammeter i s being b u i l t t h a t w i l l measure twentyfold higher

currents s o that^ an electrode w i t h more reproducible area can be used.

This instrument a l s o permits f a s t e r sweep rates which w i l l minimize the*

113 e f f e c t s of s t i r r i n g i n flow c e l l s which w i l l be necessary f o r process an-

alysis.

With t h e s e refinements it i s possible t h a t uranium can be continu-

ously monitored with accuracy t h a t i s comparable t o t h a t of hot c e l l analyses.

An a l t e r n a t e method f o r defining t h e electrode a r e a i s t o use an

i n s u l a t i n g sheath.

Boron n i t r i d e sheaths have been used with some success

but a r e slowly attacked by t h e salts.

The technique would be g r e a t l y

s i m p l i f i e d i f a r e a l l y compatible i n s u l a t o r were a v a i l a b l e , and a m a t e r i a l s development program would appear t o be merited. A new phenomena which m a y o f f e r a combined e l e c t r o l y t i c and gas analy-

sis technique f o r oxide determination has r e c e n t l y been observed.

When

LiF-BeF2 melts are electrolyzed i n vacuo at t h e p o t e n t i a l ( + 1.0 v ) of an anodic wave which has been a t t r i b u t e d t o t h e oxidation of oxide i o n , gas evolution i s noted at t h e i n d i c a t o r electrode.

;

The gas w a s found t o be

predominantly C02 ( r e s u l t i n g from t h e reaction of e l e c t r o l y t i c oxygen with t h e p y r o l y t i c g r a p h i t e e l e c t r o d e o r t h e g r a p h i t e container) w i t h l e s s e r q u a n t i t i e s of CO and 02.

I f 100% current e f f i c i e n c y can be achieved, a

coulometric method would r e s u l t .

Alternately t h e evolved gases could be

purged from t h e e l e c t r o l y t i c c e l l and analyzed gas chromatographically. Determination of Oxide by Hydrofluorination

The q u a n t i t a t i v e evolution of oxide as water by hydrofluorination of molten f l u o r i d e s a l t mixtures has been s u c c e s s f u l l y applied t o t h e determination of oxide i n t h e highly r a d i o a c t i v e MSRE f u e l samples.

The sampl-

i n g ladle, containing about 50 g of salt , i s s e a l e d i n a n i c k e l hydrof l u o r i n a t o r with a d e l i v e r y tube spring-loaded against t h e surface of t h e

salt. i

After t h e system i s purged a t 3OO0C with a hydrofluorinating gas

mixture of anhydrous HF i n hydrogen, t h e s a l t i s melted, t h e delivery tube

. 114

i s driven beneath the s u r f a c e of t h e s a l t , and the melt i s purged with hydrofluorinating gas mixture, t h e oxide being evolved as water.

The ef-

f l u e n t from t h e hydrofluorinator i s passed through a sodium f l u o r i d e column at 70째C t o remove t h e HF, and t h e water i n a f r a c t i o n of t h i s gas

stream i s measured with t h e c e l l of an e l e c t r o l y t i c moisture monitor.

The

i n t e g r a t e d s i g n a l from t h e moisture monitor c e l l is p r o p o r t i o n a l t o t h e concentration of oxide i n t h e sample.

The water i s evolved q u i t e r a p i d l y

w i t h analyses e s s e n t i a l l y camplete within about 30 minutes a f t e r t h e salt

i s melted.

Most of t h i s time i s consumed i n purging t h e water from t h e

sodium f l u o r i d e t r a p and i n "drying down" t h e c e l l . The components required t o c a r r y out t h i s determination i n t h e h o t

c e l l are shown i n Figs. 25 and 26.

Figure 25 shows from l e f t t o r i g h t :

t h e sampling ladle; a n i c k e l l i n e r , which p r o t e c t s t h e hydrofluorinator bottom; t h e hydrofluorinator top, with i t s replaceable d e l i v e r y t i p and b a f f l e s t o r e t a i n t h e s a l t i n t h e l i n e r ; t h e hydrofluorinator bottom and a clamping yoke t o seal the hydrofluorinator v i a a Teflon O-ring.

Figure

26 shows t h e assembled hydrofluorinator i n t h e furnace on t h e r i g h t connected with a pneumatically actuated coupler t o t h e compartment which contains t h e sodium f l u o r i d e column, the moisture c e l l , a c a p i l l a r y gas

stream s p l i t t e r and t h e necessary valving and connections. A t t h e r e a c t o r s t a r t u p samples of flush salt and f u e l were analyzed by both t h e hydrofluorination and KBrF4 methods with s a t i s f a c t o r y agreement.

The KBrF4 r e s u l t s were p o s i t i v e l y biased by about 20 ppm which i s

r e a d i l y explained by atmospheric contamination of t h e pulverized salt. Since the r e a c t o r has been operating a t power t h e r e s u l t s o f t h e samples analyzed have f a l l e n i n t h e range of 50

ppm.

ss embled Hydro fluorinator

116

F i g . 26.

Hot-Cell Apparatus fop Oxide i n MSRE S a l t s .

117

The hydrofluorination method should be equally applicable t o t h e a n a l y s i s of MSBR samples, as no i n t e r f e r e n c e i s a n t i c i p a t e d from thorium. Because t h e r e a c t i o n i s rapid and q u a n t i t a t i v e it o f f e r s promise f o r app l i c a t i o n t o process analysis and might a l s o be combined w i t h a determination of reducing power.

The reactions involved i n t h e process determi-

nation a r e as follows :

+ ~ H F2 mn +

n/2 H ~ +,

(3)

w i t h evolved water and hydrogen measured.

Application of r e a c t i o n (1)could be c a r r i e d out by e i t h e r of two techniques.

I n t h e simplest approach t h e molten s a l t would be subjected

t o a single-stage e q u i l i b r a t i o n with HF i n a hydrogen o r helium c a r r i e r and t h e oxide computed from known equilibrium constants.

This approac9 i s

s u b j e c t t o several. problems, t h e most s e r i o u s of which i s t h a t a c t i v i t i e s of oxide r a t h e r t h a n concentrations a r e measured.

oxides a r e not determined.

Thus p r e c i p i t a t e d

Also, it would be necessary t o maintain accu-

r a t e temperature c o n t r o l because equilibrium constants of t h e r e a c t i o n a r e r e l a t i v e l y dependent on temperature.

An a l t e r n a t e approach which

would circumvent t h e above problem but would r e q u i r e a more complex apparatus i s t o e q u i l i b r a t e a constant stream of t h e f u e l with a countercurrent flow of hydrofluorinating gas.

By proper s e l e c t i o n of parameters

(HF concentration,. temperature, c o n t r a c t o r design and flow r a t e s ) it i s t h e o r e t i c a l l y possible t o approach q u a n t i t a t i v e removal of oxide from t h e

u' L

e f f l u e n t salt s o t h a t a steady s t a t e i s reached i n which t h e water evolved

118 i s equivalent t o t h e oxide introduced i n t h e salt stream.

Rate constants

f o r hydrofluorination a r e not available. Thorium and Protactinium

All of t h e experimental work on t h e proposed methods has been c a r r i e d out on MSRE type s a l t s but should a l s o provide adequate anglyses f o r t h e MSBR f u e l .

I n t h e a n a l y s i s of t h e MSBR blanket t h e presence of thorium

and protactinium must a l s o be considered.

A t t h i s time the i n - l i n e analyr

sis of thorium does not appear e s s e n t i a l t o t h e operation of t h e r e a c t o r

a p o s s i b l e exception i s the monitoring of thorium i n t h e core t o d e t e c t leaks between t h e blanket and t h e core.

Also, on t h e b a s i s of i t s spectro-

photometric, e l e c t r i c a l and thermodynamic p r o p e r t i e s , thorium i s not expected t o i n t e r f e r e s i g n i f i c a n t l y with any of t h e proposed methods.

The

i n - l i n e a n a l y s i s of protactinium m u s t be considered as a p r i o r i t y determin a t i o n because t h e concentration of protactinium must be maintained at a low l e v e l i n t h e blanket f o r e f f i c i e n t breeding.

I n t h e absence of ex-

perimental d a t a , t h e spectrophotometric method appears t o o f f e r t h e most p r o f i t able avenue of i n v e s t i g a t i o n . /

Reprocessing System Monitoring of t h e continuous f u e l reprocessing system w i l l probably be of even more importance than t h e monitoring of t h e main reactof system, because t h e compositions of t h e reprocessing streams a r e more s u b j e c t t o r a p i d operational control.

Moreover, t h e reprocessing system o f f e r s

s e v e r a l avenues f o r t h e temporary o r permanent loss of f i s s i o n a b l e

material.

S a l t streams which w i l l r e q u i r e continuous measurement of t r a c e

concentrations of uranium include t h e e f f l u e n t s from f l u o r i n a t o r s of t h e f u e l and blanket reprocessing streams.

P a r t of t h e r e s i d u a l uranium i n

e i t h e r of t h e s e streams i s subject t o permanent loss e i t h e r i n t h e s t i l l bottoms of t h e f u e l system o r i n t h e waste of t h e blanket f i s s i o n product disposal.

The i n - l i n e analysis of major concentrations of uranium i n t h e

make-up stream from t h e recombiner w i l l a l s o be required f o r inventory control.

With t h e possible exception of a change i n t h e concentration and/

o r nature of t h e corrosion products t h e techniques t h a t a r e developed f o r t h e a n a l y s i s of r e a c t o r salts should be equally applicable t o t h e reprocess i n g system. Gaseous e f f l u e n t s streams from t h e UF6 cold t r a p s and t h e recombiner system could introduce temporary l o s s e s v i a t r a n s f e r of uranium t o t h e offgas system and w i l l r e q u i r e i n - l i n e analysis f o r t r a c e concentrations. G a s streams t h a t contain major concentrations of UF6 (e.g.,

e f f l u e n t s from

t h e f l u o r i n a t o r s ) can probably be adequately monitored by u l t r a v i o l e t spectrophotometers, but no completely s a t i s f a c t o r y methods have y e t been L

found f o r t h e i n - l i n e analysis of t r a c e concentrations of uranium i n gas

streams.

Several techniques a r e being considered t o monitor t h e Fluid

Bed V o l a t i l i t y P i l o t P l a n t , and any methods developed should be i d e a l l y s u i t e d f o r t h e M$BR reprocessing system.

120

PROPOSED PROGRAM OF CHEMICAL DEVELOPMENT The chemical s t a t u s of molten f l u o r i d e s as r e a c t o r materials, presented i n some d e t a i l i n preceding s e c t i o n s , i n d i c a t e s strongly t h a t thermal breeders based upon t h e s e materials a r e f e a s i b l e .

The discussion above,

however, p o i n t s out s e v e r a l problem areas t h a t remain and numerous specif i c d e t a i l s t h a t r e q u i r e examination and experimental i n v e s t i g a t i o n .

b r i e f summary of t h e s e areas and s p e c i f i c plans f o r t h e necessary s t u d i e s

i s presented under t h e s e v e r a l headings below.

An estimate of t h e manpower

and money required, over t h e next 8 y e a r s , t o accomplish t h e s e research and development a c t i v i t i e s i s presented as Table 17. It 'is axiomatic t h a t t h e course of research and development a c t i v i t i e s

i s seldom smooth and i s d i f f i c u l t t o p r e d i c t i n d e t a i l f o r a long period. Researches l e a d t o valuable findings t h a t can be e x p l o i t e d , and unsuspected problems a r i s e and r e q u i r e a d d i t i o n a l e f f o r t s f o r t h e i r s o l u t i o n .

i s very u n l i k e l y , t h e r e f o r e , t h a t t h i s budget breakdown w i l l prove accurate i n d e t a i l , but t h e o v e r a l l sums and manpower, year by y e a r , should be suff i c i e n t f o r t h e purpose. i

Phase Equilibrium Studies Equilibrium phase behavior of t h e proposed f u e l and blanket systems

i s r e l a t i v e l y well established.

A c a r e f u l and d e t a i l e d examination, using

a l l t h e most advanced techniques, should be made of t h e region c l o s e t o t h e proposed compositions i n t h e f u e l and, e s p e c i a l l y , i n t h e blanket system.

I n a d d i t i o n , t h e j o i n from t h e LiF-UF4 e u t e c t i c , through t h e f u e l

composition, t o t h e barren fuel solvent w i l l r e q u i r e some examination. None of t h e s e s t u d i e s i s urgently needed i n t h e next year o r two. havior of t h e LiF-BeF

Be-

UF4 system with moderate f r a c t i o n s of t h e UF4 i

Table 17.

1968

Development Area M Phase Relationships Fuels and Blankets Coolants

Projected Breakdown of Chemical Development f o r MSBR Program

-1969

0.5 2

‘MY

20 40

1971

1972

M Y $

1975 M Y $

0.25 0.25

10 10

0.25 0.25

1 0.25

0.25 0.25

10 10

0.25 0.25

Solution Thennodjrngmi

Physical Properties

Radiation Effects Fission Product Behavior

1974

10 10

40 40

Oxide and Oxyfluo

1973

1 1

0.5 1

1970

10 3

800 150

10 3

800 150

10 3

800 150

10 3

800 150

500

100

400 100

Protactinium Chemistry

120

Fission P r o p c t Separations

120

160

Development of Continuous . Production

100

,100

0.5

2 26.5

70 1260

160

Chemical Services Subtotal Analytical Development Analytical Services I

Total

2 22.5 2

‘

io50 80

3 1 0 0

4 130 -

27.5

34.5

1430,

1550

2 7 0 3 1 0 5

33.0 1760

29.5

33.25 1755

26.0

1465

22.0

170

175

225

-5

225

160

180

42.0 2095

%oes not include a cbnsiderable capital expenditure f o r in-pile f a c i l i t y .

44.25 2160

40.5

4 140 4 140 6 200

1615

2020

19.5

930

170

180

32.0

1425

29.5

1280

6 180 36.0

1815

lo75

Estimate for t h i s might be as high as $600,000.

it-

122

reduced t o UF3, and more d e f i n i t i v e information as t o s o l u b i l i t y of raree a r t h f l u o r i d e s , a l k a l i n e e a r t h f l u o r i d e s , plutonium f l u o r i d e , etc. i n t h e mixtures near t o t h e f u e l mixture are of somewhat more urgency.

At-the

modest research l e v e l shown, t h e s e s t u d i e s should be l a r g e l y concluded i n a four-year period, and very m i n o r ' e f f o r t s a r e projected beyond t h a t i n t e r Val. \

Phase behavior i n t h e fluoroborate systems ( o r t h e a l t e r n a t i v e s preThe items

sented above) proposed as coolants i s much l e s s well understood.

of first p r i o r i t y are t o define t h e phase behavior (including equilibrium BF

pressure) i n t h e NaF-BF

system; such s t u d i e s should include a syste-

matic examination of t h e e f f e c t of B203 on t h e phase equilibrium.' t h e binary system i s e s t a b l i s h e d , t h e NaF-KF-BF examined.

However, should t h e BF

Once

t e r n a r y system should be

system appear u n a t t r a c t i v e ( f o r exI

ample', by reason of incompatibility with Hastelloy N) t h e s e phase s t u d i e s should immediately be s h i f t e d t o examination of t h e most promising a l t e r native.

In addition, a s m a l l exploratory study of systems based upon SnF2

s h o d d be attempted, s o t h a t t h e i n t e r e s t i n g p r o p e r t i e s of t h i s material can be exploited i f i t s compatibility with Hastelloy "can

be demonstrated.

Oxide and Oxyfluoride Behavior

Behavior of oxides, and of oxide and hydroxide ions over p e r t i n e n t regions of t h e LiF-BeF2, LiF-BeF2-UF4,

and LiF-BeF2-UF4-ZrF

reassuring and i s now reasonably w e l l understood.

is 4 systems -

Some a d d i t i o n a l e f f o r t

on such behavior i n t h e proposed MSBR f u e l system (and i t s c l o s e composit i o n a l r e l a t i v e s ) i s s t i l l necessary.

L i t t l e is known of oxid

i n systems with moderate t o high concentrations of thorium.

behavior

Accordingly,

a high p r i o r i t y i n t h e s e s t u d i e s must be given t o examination of oxides t

123 and hydroxides i n LiF-BeF sition.

ThF4 systems at and near t h e blanket compo-

A study of d i s t r i b u t i o n of uranium and thorium between t h e a n t i c i -

pated (U-Th)02 s o l i d s o l u t i o n and t h e molten f l u o r i d e phase as a function of temperature and m e l t composition w i l l follow.

Extension of t h i s study

t o include equilibrium d i s t r i b u t i o n of protactinium w i l l a l s o be done i f oxide processes f o r t h i s element s t i l l appear a t t r a c t i v e .

It i s expected

t h a t a 2-man e f f o r t can answer, during t h e next t h r e e y e a r s , t h e urgent questions t h a t a f f e c t f u e l and blanket production techniques and system c l e a n l i n e s s requirements.

Minor questions and c a r e f u l refinement of some

of t h e d a t a are expected t o j u s t i f y a continuing e f f o r t at a slower pace thereafter. Solution Thermodynamics We f e e l a d i s t i n c t need f o r more information about thermodynamics of

many p o s s i b l e f i s s i o n product o r corrosion product species i n d i l u t e soluâ&#x20AC;&#x2DC;

t i o n i n f l u o r i d e melts.

We need t o augment t h e program now under way

which attempts t o obtain t h i s d a t a by EMF measurements and by d i r e c t measurements of chemical e q u i l i b r i a .

The following s e v e r a l items would be

accorded nearly equal p r i o r i t y :

EMF Study of M/MFx Electrodes Experiments using metal/metal f l u o r i d e e l e c t r o d e s with reference e l e c t r o d e s such as Be/Be

or H

2â&#x20AC;&#x2122;

HF/Pd w i l l attempt t o determine t h e

i d e n t i t y , s o l u b i l i t y , and thermpdynamic s t a b i l i t y of lower valence fluor i d e s of such elements as niobium, molybdenum, ruthenium, technetium, and copper i n LiF-BeF2 melts. ably t h e order l i s t e d .

kd i

The p r i o r i t y order f o r t h e s e elements i s prob-

The method should also be used, as opportunity

124

permits, t o f i r m our present values f o r t h e f l u o r i d e s of i r o n , chromium, and nickel.

Such s t u d i e s should prove of real value i n (1) decisions as

t o s u i t a b i l i t y of improved container metals (such as niobium) and ( 2 ) evaluation of p o s s i b l e f i s s i o n product species and behavior.

These EMF

s t u d i e s would, a t least i n t h e i n i t i a l steges, be coupled w i t h measurements of e q u i l i b r i a such as

Nb + XHF $ NbFx + X/2 H2. ,

Thermodynamics of Rare-Earth Fluorides We w i l l use t h e c e l l

f o r which t h e expected c e l l r e a c t i o n i s

We hope t o measure AGf and AH

f o r c r y s t a l l i n e rare-earth t r i f l u o r i d e s .

These data a r e badly needed i n order t o c a l c u l a t e AGf values f o r t h e dissolved salts from s o l u b i l i t y data. Electrochemical Deposition Studies L i q u i d bismuth s o l u t i o n s of niobium, molybdenum, ruthenium, and

technetium w i l l be s t u d i e d (1)t o i n v e s t i g a t e t h e f e a s i b i l i t y of e l e c t r o chemical deposition and ( 2 ) t o determine a c t i v i t y c o e f f i c i e n t s of metal s o l u t e s i n t h e l i q u i d bismuth.

These s t u d i e s a l s o could support present

s t u d i e s of chemical reduction of rare e a r t h s (and Pa) i n t o B i , by demons t r a t i n g electrochemical reduction of t h e same ions. A c t i v i t i e s of LiF and BeF2 Measurements with t h e c e l l Be

125 should be extended over a wider temperature i n t e r v a l and composition range t o confirm, improve, and extend our knowledge of t h e thermodynamics of t h e LiF-BeF2 system.

[If a s u i t a b l e Tho

ThF4 e l e c t r o d e can be demonstrated,

a s i m i l a r study i n t h e LiF-ThF4 and LiF-BeF2-ThF4 systems w i l l be made.] Other Studies

As t i m e permits and t h e need r e q u i r e s we would attempt (1)study of r e a c t i o n e q u i l i b r i a involving BF

( e s p e c i a l l y w i t h s t r u c t u r a l metal ele-

ments and a l l o y s ) i n mixtures of i n t e r e s t as coolants, and ( 2 ) continued study of t h e r e a c t i o n 2HF + S2- Z H2S

+ H2.

Physical P r o p e r t i e s W e b e l i e v e it unlikely t h a t t h e physical property values l i s t e d i n

t h i s document f o r t h e f u e l , t h e blanket, and t h e s e v e r a l coolants are i n I

e r r o r s u f f i c i e n t t o cause r e j e c t i o n of t h e f l u i d . i

However, t h e state of

knowledge of p h y s i c a l and heat t r a n s f e r p r o p e r t i e s of t h e s e f l u i d s i s uns a t i s f a c t o r y , and a considerable e f f o r t w i l l be required t o e s t a b l i s h t h e values with p r e c i s i o n . Vapor pressures need t o be evaluated f o r t h e s e v e r a l mixtures (through decomposition pressures of t h e BF based coolants w i l l be estab-

l i s h e d with t h e i r phase behavior).

Since t h e Values are known t o be low,

t h e s e measurements do not per se deserve a high p r i o r i t y , b u t s t u d i e s with t h e fâ&#x20AC;&#x2122;uel should be included e a r l y s i n c e they w i l l assist with t h e d i s t i l l a t i o n studies. Density, c o e f f i c i e n t of thermal expansion, and Viscosity d a t a w i l l be required t o confirm t h e present estimates.

S p e c i f i c h e a t and heat of

126 /-

fusion values are a l s o needed.

A l l t h e s e values w i l l , i f s u f f i c i e n t l y

trJ

high accuracy can be achieved and i f a s u f f i c i e n t concentration range i s covered, be helpful i n checking present methods f o r estimating t h e propert i e s and w i l l be u s e f u l i n attempts t o formulate a c o n s i s t e n t theory of high temperature l i q u i d s . Surface tension measurements of t h e s e v e r a l s a l t s , and t h e i r c l o s e compositional r e l a t i v e s , should be e s t a b l i s h e d during t h e next two years as time and resources permit. Thermal conductivity i s t h e property t h a t i s most d i f f i c u l t t o measure f o r molten s a l t s (as f o r o t h e r ' l i q u i d s ) , and t h e one f o r which av a i l a b l e information i s most i n s u f f i c i e n t .

A program f o r measurement of

t h i s property f o r f u e l , blanket, and any of t h e m a t e r i a l s l i k e l y t o be

chosen as coolant i s urgently required.

Thermal conductivity of l i q u i d s

can be estimated w i t h some p r e c i s i o n i f t h e v e l o c i t y of sound i n t h e l i q u i d i s known; measurements of sonic v e l o c i t y i n t h e molten s a l t mixtures

should, t h e r e f o r e , be undertaken as a reasonable backup e f f o r t . Radiation E f f e c t s and F i s s i o n Product Behavior These two i t e m s promise t o be t h e most demanding, and t h e most expensive, i n t h e l i s t of necessary chemical development a c t i v i t i e s .

No ad-

verse e f f e c t s of r a d i a t i o n upon t h e f u e l s , the moderator, o r t h e compatib i l i t y of the fuel-graphite-metal

system have been observed.

However, no

r e a l i s t i c tests of t h e s e combinations have been made a t power d e n s i t i e s so high as those proposed f o r MSBR.

Studies p r e s e n t l y under way, and radi-

a t i o n f a c i l i t i e s p r e s e n t l y a v a i l a b l e , should by e a r l y FY 1968 permit long-

term tests t o high f u e l burnup at power d e n s i t i e s i n t h e 300 k w / l i t e r range.

Such s t u d i e s are done i n i n - p i l e thermal convection loops which

ii *

127

expose a very high f r a c t i o n of t h e t o t a l f u e l t o t h e highest f l u x ; t h e assembly i s equipped s o t h a t samples of gas can be taken a t w i l l , samples of f u e l can be withdrawn, and samples of enriched f u e l can be added as desired.

These tests w i l l be valuable both i n assessment of possible r a d i a t i o n

damage problems and i n evaluation of f i s s i o n product behavior. W e a r e convinced t h a t exposures at even higher power d e n s i t i e s (up at

l e a s t t o 1000 Kw/liter) are necessary i n t h i s program and we w i l l attempt

t o design and operate such f a c i l i t i e s .

Success i n t h i s venture would per-

m i t not only an accelerated t e s t program f o r t h e numerous possible problem areas but would a l s o s a f e l y assess such r e a c t o r accident p o s s i b i l i t i e s as blocked flow channels, pump stoppages, e t c .

We hope t h a t such t e s t s can

be conducted at t h e Oak Ridge Research Reactor i n loops cooled by thermal convection, and t h e considerable sums budgeted f o r t h e e f f o r t are predicated on t h a t hope.

If t h e s t u d i e s must be done elsewhere, o r i f forced

convection loops (with t h e attendant pump development problem) must be used, then t h e estimates of staff and funding required a r e c e r t a i n l y t o o

low. With c a r e f u l analysis o f off-gases from such systems and r a p i d radio-

chemical a n a l y s i s of f u e l samples, we should g e t d e f i n i t i v e data on f i s s i o n product behavior at t r u l y r e a l i s t i c concentrations and production

rates.

Careful checks of g r a p h i t e and metal from such t e s t s immediately

after termination of t h e

should a f f o r d r e a l i s t i c d a t a on d i s t r i b u t i o n

of f i s s i o n products i n t h e s e materials. Radiation l e v e l s , from gamma rays and from t h e delayed neutrons, i n t h e coolant mixture are, c l e a r l y , much lower than t h o s e f o r t h e f u e l , b u t â&#x20AC;&#x2122;

W I

r a d i a t i o n damage t o t h e BF based coolants i s not n e c e s s a r i l y a t r i v i a l

128 ,--

u ~

matter.

When such a coolant mixture i s e s t a b l i s h e d as t o phase behavior,

heat t r a n s f e r c a p a b i l i t y , and compatibility, it should be given a longterm t e s t a t higher-than-realistic

r a d i a t i o n l e v e l s t o see whether such

damage i s a p o s s i b i l i t y . Fission Product Separations The d i s t i l l a t i o n process i s , at p r e s e n t , t h e expected technique f o r reprocessing of t h e f u e l mixture, and development a c t i v i t i e s associated w i t h t h a t process a r e described i n another report i n t h i s s e r i e s .

A small

e f f o r t on vapor-liquid e q u i l i b r i a i n d i r e c t support of t h a t development

will continue, as needed, as a p o r t i o n of t h e present program. Highest p r i o r i t y w i l l , f o r t h e p r e s e n t , be devoted t o t h e study of

reduction of rare-earth f l u o r i d e s from LiF-BeF2 mixtures i n t o d i l u t e a l l o y s of t h e rare-earth metals i n bismuth, o r i n t o s t a b l e i n t e r m e t a l l i c compounds of o t h e r types.

These s t u d i e s can be c a r r i e d by mid-1968 t o a p o i n t where

a sound evaluation of t h e i r p o t e n t i a l can be made.

If t h e process shows

p r m i s e , it w i l l be u s e f u l t o examine electrochemical vs chemical techniques f o r i t s prosecution, and it may prove necessary t o i n v e s t i g a t e means of recovery of l i t h i u m f r m t h e r a r e earth-bismuth alloy. The search f o r insoluble compounds which a r e s t a b l e toward t h e molten f u e l and which are capable of ion exchange r e a c t i o n s with rare-earth cations i n t h e f u e l mixture w i l l be continued.

Attempts w i l l , be made t o use

Zr02 doped with t r a c e s of rare-earth oxides, uranium carbide doped with rare-earch carbide, and o t h e r r e f r a c t o r y compounds, as w e l l as any promising rare-earth i n t e r m e t a l l i c compounds. Exploratory s t u d i e s of reduction of t h e more noble f i s s i o n products w i l l be i n s t i t u t e d as d e f i n i t e information on t h e n a t u r e of t h e species i n

W t

129

solution becomesavailable.

Should~niobium or molybdenum, for example, be

shown to exist in the fuel mixture as=a fluoride, or electrochemical means will'be

their

removal by chemical

attempted.

Study of the equilibrium HF + I- + HI + Fwill

be continued and extended to include effects

temperature.

of melt composition and

This technique msy prove valuable in removing a major frac-

tion of xenon precursors on a short time cycle and may minimize requirements for impermeability

of the graphite moderator and core structure.

these studies continue to appear promising, they should be extended to include possible removal of tellurium

by volatilization

of H2Te.

Protactinium Chemistry e

The surprising very dilute

fact that protactinium

as fluoride

is removed from

solutions in LiF-BeF2-ThFq by reduction with thorium metal (or

with moderately stable intermetallic

compoundsof thorium) represents a

breakthrough which must be exploited.

Accordingly,

first

priority

given to continued and increased study of this reaction. must be paid to determination of the ultimate

element. istic

Primary attention

compound. Success in

will

be applied at both tracer include electrochemical

(ppb) and realreductions with a

variety

of metal electrodes , and chemical reductions in the presence of I selected.metallicâ&#x20AC;&#x2DC; constituents. The process by-which protactinium

systematic study of means for recovery of the

Techniques, which will

(ppm) concentrations,

state of the protactinium;

is presently believed to be a stable intermetallic this venture should.permit

will

is precipitated

by an excess of

130 I

Be0 o r Tho w i l l continue t o be examined. 2

Attempts w i L l be made t o estab-

l i s h t h a t passage of t h e melt through beds of oxide (Zr02 w i l l be included) w i l l remove t h e protactinium without r e a c t i o n with o t h e r constituents.

t h i s i s t r u e , as previous t e s t s have strongly suggested, a c a r e f u l study of t h e e f f e c t of extraneous i o n s , of t h e behavior of uranium, and of t h e e x t e n t of contamination of t h e melt by oxide and hydroxide ion w i l l be made. Methods f o r recovery of t h e protactinium o r of t h e 233U product from whichever of t h e s e processes seems promising w i l l be undertaken as soon as an understanding of t h e removal mechanism permits. Development of Continuous Production Methods

As t h e discussion of production technology above makes c l e a r , t h e present production methods have been adequate f o r m a t e r i a l s f o r MSFE; t h e f u e l , coolant, and f l u s h salt were furnished i n a high and completely s a t i s f a c t o r y s t a t e of p u r i t y .

It seems very l i k e l y t h a t t h e present u n i t

processes w i l l serve t o prepare MSBR f u e l and (perhaps with minor modific a t i o n s ) blanket.

However, t h e 25,000 l b of material f o r MEXE required

nearly a year t o prepare i n t h e e x i s t i n g batch processing f a c i l i t i e s , and provision of a considerably l a r g e r quantity f o r an MSBR would be q u i t e uneconomical i f t h i s equipment were used. The p u r i f i c a t i o n process i s q u i t e a simple one.

It seems c e r t a i n

t h a t it can be engineered i n t o a continuous process with t h e throughput per

u n i t of t i m e and manpower much g r e a t e r than t h a t of t h e present batch operation.

The r e l a t i v e l y small development e f f o r t adjudged necessary f o r t h i s

conversion i s scheduled s o t h a t t h e f i n i s h e d p l a n t could be a v a i l a b l e f o r run-in on l a r g e q u a n t i t i e s of salt needed i n t h e engineering-scale tests.

131

Chemical Services Under t h i s heading a r e lumped t h e many and d i v e r s e ways i n which t h e molten s a l t chemists perform gervices i n d i r e c t support of other portions of t h e development e f f o r t .

These w a y s range from (1)examination and

i d e n t i f i c a t i o n (as by t h e o p t i c a l microscope) of deposits found i n engineering t e s t loops , (2) determination of permeability of g r a p h i t e specimens,

( 3 ) in-place hydrofluorination of batches of s a l t before reuse i n t e s t equipment, ( 4 ) manufacture of small batches of s p e c i a l s a l t compositions f o r corrosion o r physical property t e s t s , and ( 5 ) l i a i s o n among t h e engineers, r e a c t o r o p e r a t o r s , h o t - c e l l operators, and a n a l y t i c a l chemists s o t h a t t h e many s p e c i a l samples receive proper handling and d a t a from them

are reasonably i n t e r p r e t e d .

It i s d i f f i c u l t t o s p e c i f y , long i n advance, t h e d e t a i l s of such s e r v i c e s , but many y e a r s ' experience encourages us i n t h e b e l i e f t h a t t h e i

suggested l e v e l w i l l be needed. Analytical Development I n order t o apply i n - l i n e a n a l y t i c a l techniques t o t h e MSBR, cons i d e r a b l e preliminary information and d a t a must be gathered so t h a t a sound evaluation of p o s s i b l e s u c c e s s f u l r e a c t o r applications can be made. This approach w i l l permit a maximum s h i f t of e f f o r t t o those concepts that appear t r , be most fruitful.

For example, t h e experience gained i n t h e

a n a l y s i s of hydrocarbons i n t h e MSRE off-gas i s being used now i n t h e des i g n of a gas chromatograph

o determine automatically t h e various con-

s t i t u e n t s i n t h e cover gas.

Work on t h i s p r o j e c t i s c u r r e n t l y under way

and w i l l be d i r e c t e d towards t h e MSBR.

. c

The long term i n - l i n e a n a l y s i s program i s planned i n t h i s t e n t a t i v e

132

ib"

order of p r i o r i t y .

Construct a laboratory f a c i l i t y which w i l l provide a flowing

s a l t stream, probably driven by a gas lift.

Provision w i l l be made f o r t h e

addition of contaminants t o t h e salt including oxide, sampling, c a p a b i l i t y f o r hydrofluorination and e l e c t r o l y t i c treatment of t h e salt.

This f a c i l i -

t y w i l l be used t o provide tests of electrochemical methods f o r uranium,and

corrosion products and f o r measuring t h e electrochemical p o t e n t i a l of t h e s a l t vs a standard reference c e l l . b.

I n i t i a t e i n v e s t i g a t i o n of a countercurrent e q u i l i b r a t i o n

method f o r t h e determination of oxide by hydrofluorination. c.

Accurately determine r e p r o d u c i b i l i t y of operation of spectro-

photometric c e l l f o r f u t u r e a p p l i c a t i o n t o determination of uranium and protactinium. 11.

Continue b a s i c i n v e s t i g a t i o n s of electrode processes t o observe

i f chromium, oxide, and t r i v a l e n t uranium can be determined i n t h i s manner.

111. IV.

I n v e s t i g a t e materials as i n s u l a t o r s f o r reference electrode. Conduct i n - p i l e t e s t i n g of any i n - l i n e techniques which prove i

s u c c e s s f u l i n Section I. V.

ty.

Develop gas chromatographic analyses compatible w i t h high a c t i v i -

Includes r a d i a t i o n t e s t i n g of packing materials , t e s t i n g s o l i d ad-

sorbents f o r hydrocarbops. ~

Development of all metal valving.

Testing ef-

f e c t of r a d i a t i o n on detectors.

VI.

I n v e s t i g a t i o n of a l t e r n a t e continuous methods of i n - l i n e gas

analysis, e.g.,

thermal conductivity, referencing gas after chemical sepa-

r a t i o n t o o r i g i n a l gas stream.

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t o r s , ORNL TM-1854 (Aug. 1966) G. Long, Reactor Chem. Div. Ann. Progr. Rept. Jan. 31, 1965, ORNL-

3789, p. 65. 30. :

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32.

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ORNL-3420, pp. 165-7. 36.

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37.

F. F. Blankenship e t al., Reactor Chem. Div. Ann. P r o w . Rept.

Jan. 31, 1963, ORNL-3417, p. 17.

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, Reactor

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p. 38.

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44.

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138

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p. 30. Young, Anal. Chem. Div. Ann. Progr. Rept. Nov. 15,

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2, - 253 (1965).

139 DISTRIBWION 1-50.

51. 52.

53. 54. 55. 56. 57. 58. 59. 60.

61. 62.

63. 64. 65. 66. 67. 68. 69. 70.

71. 72.

73, 74. 75. 76. 77. 78. 79. 80.

81. 82.

83. 84. 85. 86. 87. 88. 89. 90.

91. 92.

93. 94. 95. 96. 97. 98.

. c

MSRP Directorâ&#x20AC;&#x2122;s Office 99 H. E. Goeller R. K. Adams 100-103. W. R. Grimes G. M. Adamson 104. A. G. G r i n d e l l R. G. Affel 105. R. H. Guymon L. G. Alexander 106. B. A. Hannaford R. F. Apple 107. P. H. Harley C. F. Baes 108. D. G. Harman J. M. Baker log. C. S. H a r r i l l S. J. B a l l 110. P. N. Haubenreich W. P. Barthold 111. F. A. Heddleson H. F. Bauman 112. P. G. Herndon S. E. Beall 113. J. R. Hightower M. Bender 114. H. W. Hoffman E. S. B e t t i s 115. R. W. Horton F. F. Blankenship 116. T. L. Hudson R. E. Blanco 117. H. Inouye J. 0. Blomehe 118. W. H. Jordan R. Blumberg 119. P. R. Kasten E. G. Bohlmann 120. R. J. Ked1 C. J. Borkowski 121. M. T. Kelley G. E. Boyd 122. M. J. Kelly J. Braunstein 123. C. R. Kennedy M. A. Bredig 124. T. W. K e r l i n R. B. Briggs 125. H. T. Kerr H. R. Bronstein 126. S. S. Kirslis G. D. Brunton 127. A. I. Krakoviak D. A. Canonico 128. J. W. Krewson S. Cantor 129. C. E. Lamb W. L. Carter 130. J. A. L a n e G. I. Cathers 131. R, B. Lindauer J. M. Chandler 132. A. P. Litman E. L. Compere 133. M. I. Lundin W. H. Cook 134. R. N. Lyon L. T. Corbin 135. H. G. MacPherson J. L. Crowley 136. R. E. MacPherson F. L. Culler 137. C. D. Martin J. M. Dale 138. C. E. Mathews D. G. Davis 139. R. W. McClung S. J. D i t t Q 140. H. E. McCoy A. S. Dworkin 141. H. F. McDuffie J. R. Engel 142. E. P. Epler 143. C. J. McHargue D. E. Ferguson 144. L. E. McNeese L. M. F e r r i s 145. A. S. Meyer A. P. Fraas 146. R. L. Moore H. 4. Friedman 147. J, P. Nichols J. k. Frye, Jr. 148. E. L. Nicholson C. H. Gabbard 149. L. C. Oakes R. B. Gallaher 150. P. P a t r i a r c a

140

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151. A. M. Perry 152. H. B. Piper 153. B. E. Prince 154. J. L. Redford 155 M. Richardson 156. R. C. Robertson 157 H. C. R o l l e r 158-207. M. W. Rosenthal 208. H. C. Savage 209. C. E. S c h i l l i n g 210. Dunlap S c o t t 211. H. E. Seagren 212. W. F. Schaffer , 213. J. H. S h a f f e r 214. M. J. Skinner 215. G. M. Slaughter 216. A. N. Smith 217. F. J. Smith 218. G. P. Smith 219. 0. L. Smith 220. P. G. Smith 221. W. F. Spencer 222. I. Spiewak 223. R. C. S t e f Q 224. H. H. Stone 225. J. R. Tallackson 226. E. H. Taylor 227. R. E. Thoma 228. J. S. Watson 229. C. F. Weaver 230. B. H. Webster 231. A. M. Weinberg 232. J. R. Weir 233. W. J. Werner 234. K. W. West 235. M. E. Whatley 236. J. C. White 237. L. V. Wilson 238. G. Young 239. H. C. Young 240-241. C e n t r a l Research Lib. 242-243. Document Reference Sect. 244-25h. Laboratory Records 255. Laboratory Records RC

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F. Cope, AEC-OR0 J. Larkin, AEC-OR0 L. Matthews , AEC-OR0 W. McIntosh, AECWashington H. M. Roth, AFC-OR0 M. Shaw , AEC-Washingt on W. L. Smalley, AEC-OR0 R. F. Sweek, AEC-Washington

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