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ORNL-TM-3884

THE MIGRATION OF A CLASS OF FISSION PRODUCTS (NOBLE METALS) IN

THE MOLTEN-SALT REACTOR EXPERIMENT R. J. Ked1

This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that i t s use would not infringe privately owned' rights. I

___

i * 3

thek contractors, subcontractors, or their emplOYeCS, maker MY warranty, ~xprersor implied, or assume3 m y legal UaMwty or responsibility for the accuracy, completeners or usefulness of MY Information. rppamtur, product or process disclosed, or represents that its use would not infringe privately owned rights.

ORN L-TM- 388 4

C o n t r a c t No. W-7405-eng-26

R e a c t o r Division

THE MIGRATION

OF A CLASS OF FISSION PRODUCTS (NOBLE METALS) I N THE MOLTEN-SALT REACTOR EXPERIMENT

R. J. K e d l

M o l t e n - S a l t R e a c t o r Program

D e c e m b e r 1972

OAK FUDGE NATIONAL LABORATORY Oak R i d g e , Tennessee 37830 operated by UNION CARBIDE COFPORATION For t h e U.S. ATOMIC ENERGY COMMISSION

DlSTRlBUTlON OF THIS DOCUMENT IS UNUMlfED

Page

FOREWORD

ABSTRACT

INTRODUCTION

DESCRIPTION OF THE MSRE

2.1

General Description

2.2

Description of t h e Reactor

2.3

Description of t h e Fuel Pump

FISSION PRODUCT EXPERIENCE I N THE MSRE 3.1

General Fission Product Disposition

3.2

Fission Product Disposition Measurements

3.2.1

Fuel S a l t and Gas Phase Samples

3.2.2

Gamma Spectrometry of t h e Primary He at Exchanger

3.2.3

Core Surveillance Samples

3.3

t 4.

L-

The Difference Between t h e 235U

17 19 21

and 233U Runs

ANALYTICAL MODEL

23 29

4.1 Physical Basis of Model

4.2

Analytical Modei

4.2.1

Generation from Fission

4.2.2

Generation from Decay o f Precursor

4.2.3

Decay Rate

4.2.4

Deposition Rate on Heat Exchanger

4.2.5

Deposition Rate on Graphite

4.2.6

Deposition Rate on Rest of Fuel Loop

4.2.7

Deposition Rate on Liquid-Gas I n t e r f a c e s

4.2.8

Equation f o r Cs

4.2.9

Noble Metals on S o l i d Surfaces

4.2.10 Noble Metals on Liquid-Gas I n t e r f a c e s

Page

RESULTS FROM THE MSRE

38 38

5.1

Introduction

5.2

Comparison of Measured Deposition on Heat Exchanger t o Theoretical

5.3

Comparison of Measured Deposition on Core Surveillance Samples t o Theoretical

5.4 5.5

Comparison of Fuel S a l t Samples with Cs

48 61

5.6 5.7

Time Constant f o r Noble Metals on I n t e r f a c e s

Comparison of Gas Samples with C

Miscellaneous Noble Metal Observations Laminar Flow Core Surveillance Sample

5.7.2

Noble Metal Distribution i n t h e MSRE

70 73

Con clus i on s

Recommendations

REFERENCES

65 68

5.7.1

CONCLUSIONS AND RECOMMENDATIONS

+!F

APPENDIX A

APTENDIX B

FORENORD The behavior of t h e "noble metal" f i s s i o n products i n t h e f u e l s a l t

i s a subject of major importance t o t h e design of molten-salt reactors. A considerable amount of data bearing on t h e subject was obtained from

operation of t h e Molten-Salt Reactor Experiment.

All o r p a r t of t h e data

has been studied w i t h various degrees of thoroughness by many people w i t h

d i f f e r e n t backgrounds and d i f f e r e n t viewpoints.

Unfortunately, t h e chemi-

c a l and physical s i t u a t i o n s i n t h e r e a c t o r were complex and t h e data obtained are not very accurate o r consistent.

Consequently, no one has de-

veloped an explanation of the d e t a i l e d behavior of noble metals t h a t i s acceptable t o a majority of t h e knowledgeable observers.

It i s generally

agreed t h a t most of t h e f i s s i o n products from niobium through tellurium are reduced t o metals i n t h e f u e l s a l t , t h a t they migrate t o metal and

graphite surfaces and t o s a l t - g a s i n t e r f a c e s , and t h a t they adhere t o t h e surfaces w i t h varying degrees of tenacity. involved

and t h e manner i n which t h e noble metal p a r t i c l e s may be a f f e c t e d

by o t h e r processes i n t h e r e a c t o r

The d e t a i l s of t h e processes

opinions vary widely.

are subjects of frequent debate i n which

This r e p o r t describes t h e a u t h o r ' s i n t e r p r e t a t i o n of

t h e data and explanation of some aspects of t h e operation of t h e reactor. Although o t h e r s would analyze t h e data d i f f e r e n t l y and would reach d i f f e r e n t conclusions concerning some of t h e mechanics, we b e l i e v e t h a t publicat i o n of t h i s r e p o r t w i l l provide information h e l p f u l t o t h e design of molten-salt r e a c t o r systems and t o t h e development of a b e t t e r understandi n g of t h e behavior of f i s s i o n products i n those systems.

ABSTRACT The Molten-Salt Reactor Experiment (MSRE) i s a fluid-fueled experimental nuclear reactor; consequently, f i s s i o n products are dispersed throughout t h e e n t i r e f u e l c i r c u l a t i o n system.

One group of f i s s i o n

products, referred t o as noble metals, e x i s t s i n t h e f u e l s a l t i n t h e reduced m e t a l l i c state.

They are i n s o l u b l e and unwet by salt.

They de-

p o s i t on surfaces exposed t o s a l t such as t h e Hastelloy N piping and the moderator graphite.

They apparently accumulate i n a stable form on the

liquid-gas i n t e r f a c e i n t h e f u e l pump.

The amounts of noble metals on

t h e s e surfaces and on o t h e r deposition sites have been measured.

These

measxrements have been analyzed within t h e framework of mass t r a n s f e r theory.

The a n a l y s i s has been found t o c o r r e l a t e t h e d a t a fram these

sources i n a unified manner.

It i s t h e r e f o r e concluded t h a t t h e noble

metals do migrate i n accordance w i t h mass t r a n s f e r theory, although some parameters s t i l l remain unevaluated.

A hypothesis i s presented t o ex-

p l e i n some of t h e dramatic d i f f e r e n c e s i n r e a c t o r operating c h a r a c t e r i s t i c s between t h e 235U and 233U runs.

It i s recommended t h a t a d d i t i o n a l

noble metal deposition experiments be conducted i n a c i r c u l a t i n g s a l t loop.

Keywords:

F i s s i o n Products

Hoble Metals + Mass Transfer

+ MSRE + Experience + Bubbles + Foaming + M i s t + Physical P r o p e r t i e s + Entrainment + O f f - G a s System + Void Fractions + Corrosion Products f

Fluid Flow

1. INTRODUCTION

The goal of t h e Molten-Salt Reactor Program a t Oak Ridge National Laboratory i s t o develop t h e technology f o r an e f f i c i e n t power producing, thermal-breeding (based on t h e Th-"'U c i a l use.

cycle) nuclear r e a c t o r f o r commer-

The f u e l i n a molten-salt breeder r e a c t o r i s f l u i d , and con-

s i s t s or' UF,: and ThF,: dissolved i n a c a r r i e r s a l t mixture of LiF and BeF2. The l i q u i d u s temperature ranges from 800 t o 940째F, depending on t h e exact

c a r r i e r s a l t composition, with t h e nominal r e a c t o r operating temperature being -120OOF.

The f u e l s a l t i s pumped through a graphite-moderated core

and then through a heat exchanger where it t r a n s f e r s h e a t t o a secondary s a l t system, which then generates steam i n another h e a t exchanger.

One of

t h e unique f e a t u r e s of t h i s concept i s t h a t t h e f u e l i s i n t h e l i q u i d state.

This gives r i s e t o many advantages, t h e p r i n c i p a l one being poten-

t i a l l y very low f u e l cycle cost.

The f l u i d f u e l a l s o c r e a t e s a few l i a -

b i l i t i e s , one of which i s t h a t t h e f i s s i o n products a r e spread throughout

t h e e n t i r e f u e l loop and o t h e r hydraulically connected regions (e.g.,

off-

gas system, dump tanks, etc.).

A 7.3-MW(t) experimental r e a c t o r based on t h i s concept was b u i l t and operated a t Oak Ridge National Laboratory.

This r e a c t o r , t h e Molten-Salt

Reactor Experiment ( h e r e a f t e r c a l l e d t h e MSm), f i r s t went c r i t i c a l i n Zune 1965.

Nuclear operations were terminated i n December 1969.

Being

an experimental r e a c t o r , it was subjected t o a good deal of t e s t i n g and observation.

One of t h e p r i n c i p a l e f f o r t s was t o determine t h e d i s t r i b u -

t i o n of t h e various f i s s i o n products i n t h e f u e l loop and connected regions.

This information i s c r i t i c a l i n t h e design of l a r g e c e n t r a l power

s t a t i o n s [lo00 MW(e)] where t h e heat generated by f i s s i o n products i s substantial. Fission products i n molten f u e l s a l t can be grouped i n t o t h r e e p r i n c i p a l types where t h e mechanics of migration i s t h e distinguishing fea-

ture '

(1)s a l t seekers, ( 2 ) noble gases, and ( 3 ) noble metals.

The s a l t -

seeking f i s s i o n products (which include Sr, Y, Zr, I, C s , Ea, and C e ) are t h e best behaved.

They are soluble i n a f u e l s a l t and remain with t h e

f u e l s a l t i n inventory amounts.

The noble gases a r e K r and Xe.

A great

d e a l of work has been done t o understand noble gas migration, p a r t i c u l a r l y 135Xe because of i t s thermal neutron cross s e c t i o n of over lo6 barns.

The t h i r d group, t h e so-called "noble metals,tt Nb, Mo, Ru, Sb,

and Te, i s t h e subject of t h i s report.

The noble metals a r e reduced by

t h e UF3 i n t h e f u e l s a l t , and t h e r e f o r e e x i s t i n s a l t i n t h e m e t a l l i c

state. .

They a r e insoluble i n f u e l s a l t and a r e unwet by it.

Because of

t h e i r incompatibility with s a l t they migrate t o various surfaces (graph-

i t e and Hastelloy N ) and adhere t o them.

They apparently a l s o migrate t o

gas-liquid i n t e r f a c e s and adhere t o t h e s e i n a s t a b l e manner.

Noble met-

a l s have been found and measured i n f u e l s a l t samples and gas phase samp l e s on t h e surfaces of Hastelloy N and graphite s u r v e i l l a n c e specimens i n t h e core, and on t h e f u e l loop and heat exchanger surfaces.

In this

r e p o r t we s h a l l present a theory of noble metal migration based on convent i o n a l m a s s t r a n s f e r concepts.

We s h a l l then analyze d a t a from t h e above

mentioned samples and measurements i n t h e framework of t h i s theory, and show t h a t noble metals apparently do migrate from t h e f u e l s a l t t o t h e i r various d e p o s i t o r i e s i n accordance w i t h t h e theory.

A major t h e s i s of

t h i s a n a l y s i s i s t h a t noble metals migrate and adhere t o liquid-gas i n t e r -

faces.

A s such, they apparently have p r o p e r t i e s s i m i l a r t o i n s o l u b l e sur-

f a c e a c t i v e agents.

This idea w i l l be used t o explain many of t h e obser-

vations on f i s s i o n product behavior i n t h e r e a c t o r .

It w i l l a l s o be used

t o suggest an explanation f o r t h e r a t h e r dramatic d i f f e r e n c e i n r e a c t o r operating c h a r a c t e r i s t i c s between runs made w i t h 235U and 233U f u e l s .

DESCRIPTION OF THE MSRE

2.1

General Description

The purpose of t h e MSIiE was t o demonstrate, on a p i l o t p l a n t scale, t h e safety, r e l i a b i l i t y , and m a i n t a i n a b i l i t y of a molten-salt r e a c t o r . The operating power l e v e l was 7.3 MW(t).

Because of i t s small s i z e and

o t h e r considerations, it was not intended t o be a breeder, and no thorium was added t o t h e f u e l .

The f u e l consisted of UFh and UF3 dissolved i n a

mixture of LiF, BeF2, and ZrF4. a r e given i n Table 2.1.

I t s composition and physical p r o p e r t i e s

A l l f u e l loop components a r e constructed from

3 Table 2.1. E

MSKE Fuel S a l t Composition and Physical Properties

Composition

- LiF,

Liquid type

BeF2, ZrF4, UF4 (65.0, 29.1, 5.0, 0.9 mole Newtonian English Units

Liquidus temperature

8) Metric Units

813°F

434°C

141 l b / f t 3 0.47 Btu/lb *OF 0.83 Btu/hr a f t 0°F 19 l b / f t * h r <o. 1m-Hg

2.3 g/m3 2.0 X lo3 J/kg*"C 1.43 W/m "C 28 kg/h *m <1 X bar

Properties a t 1200°F (650°C) Density S p e c i f i c heat Thermal conductivity Viscosity Vapor pressure a

0.3 mole $ 235U and 0.6 mole $ 238U.

Hastelloy N, which i s e s s e n t i a l l y unwet by f u e l s a l t under normal operatI

i n g conditions.

The nominal operating temperature was 1200°F with a 40°F

temperature change across t h e core and primary heat exchanger. Figure 2.1 i s a schematic flow diagram of t h e MSRE, which w i l l be described b r i e f l y here.

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

t h e concept a r e a v a i l a b l e i n Refs. 1, 2, and 3 .

The f u e l loop consisted

e s s e n t i a l l y of a c e n t r i f u g a l pump, a heat exchanger, and t h e r e a c t o r vessel.

The nominal flow r a t e was 1200 gpm.

The heat exchanger was a

conventional U-tube type w i t h the fuel salt on the shell side.

Heat w a s

t r a n s f e r r e d t o a secondary coolant s a l t t h a t i n t u r n dumped it t o t h e atmosphere v i a a l a r g e r a d i a t o r .

During periods of shutdown, t h e f u e l

s a l t was drained i n t o e i t h e r of two d r a i n tanks.

I n addition, a t h i r d

d r a i n tank contained a load of f l u s h s a l t f o r r i n s i n g t h e f u e l loop p r i o r t o any maintenance t h a t was required.

Note t h e use of f r e e z e flanges i n

t h e primary and secondary s a l t loops f o r easy disconnection of main components should they need replacement, and a l s o t h e use of f r e e z e valves i n :

the drain lines.

Off-gas from t h e pump bowl passed through a volume holdup,

charcoal beds, and then absolute f i l t e r s before it was discharged up t h e stack.

C h a r a c t e r i s t i c s of t h e r e a c t o r core and f u e l pump influenced t h e

S mia

J+-*-,

FUEL STCRAG€ TANK

FIGURE 2.1.

SCHEMATIC FLOW DIAGRAM OF THE MSRE

SmlUM

FLUORIDE BED

......51.,.;

ABSaCUTE FILTERS

8 +4

mechanics of noble metal f i s s i o n product migration, s o they w i l l be desc r i b e d i n somewhat more d e t a i l i n t h e following sections.

The MSRE went c r i t i c a l June 1, 1965, and nuclear operations were

* 0

terminated December 12, 1969.

The r e a c t o r was c r i t i c a l f o r a t o t a l of

17,655 h r .

Figure 2.2 i s a b r i e f h i s t o r i c a l o u t l i n e of t h e MSREâ&#x20AC;&#x2122;s power

operation.

Note p a r t i c u l a r l y t h a t f o r t h e f i r s t 2 1/2 years t h e r e a c t o r

w a s operated with 235U f u e l .

The f u e l w a s chemically processed and t h e

235U replaced with 233U. Then f o r t h e l a s t 1 1 / 2 years, t h e r e a c t o r w a s operated with 233U f u e l .

Most of t h e results presented i n t h i s report

w i l l be from d a t a obtained during t h e 233U runs. 2.2

Description of t h e Reactor

A d e t a i l e d view of t h e MSRE core and r e a c t o r vessel i s shown i n Fuel s a l t entered t h e r e a c t o r v e s s e l through a flow d i s t r i b u -

Figure 2.3.

t i o n volute near t h e t o p of t h e vessel. 1-in.-thick

It then flowed down through a

annular passage bounded by t h e r e a c t o r v e s s e l and r e a c t o r

core can and i n t o t h e lower v e s s e l plenum.

The fuel then passed up

through t h e graphite moderator region and out t h e t o p o u t l e t pipe.

The

moderator assembly w a s composed of graphite s t r i n g e r s about 5 ft long and 1

2 i n . square.

The s t r i n g e r s had grooves c u t longitudinally i n t h e four

f a c e s , s o t h a t when t h e s t r i n g e r s were stacked together v e r t i c a l l y , t h e grooves formed t h e f u e l channels. .

of Union Carbide Corporation).

The graphite w a s grade CGB ( t r a d e name

It w a s unclad, i n intimate contact with

t h e f u e l , and unwet by fuel s a l t under normal MSRE operating conditions.

In t h e bulk of t h e fuel channels (95 percent of them), t h e f u e l s a l t v e l o c i t y w a s about 0.7 fâ&#x20AC;&#x2122;t/sec, y i e l d i n g a Reynolds number of about 1000. The entrance t o t h e f u e l channels through t h e moderator support g r i d s t r u c t u r e w a s r a t h e r tortuous and turbulence w a s generated t h a t p e r s i s t e d f o r some distance up i n t o t h e f i e 1 channels.

Nevertheless, t h e flow i s

thought t o have been e s s e n t i a l l y laminar i n most of t h e length of t h e fuel channel. 2

Located near the c e n t e r l i n e of t h e core i n a square array w a s an arrangement of t h r e e c o n t r o l rods and one surveillance specimen holder.

Details of t h e specimen holder are shown i n Figure 2.4.

It was positioned

v e r t i c a l l y i n t h e r e a c t o r and extended t h e e n t i r e height of t h e moderator region.

A t times when t h e r e a c t o r w a s shut down and drained, t h e

SALT hl FUEL Low

POWER

Run No.

SALT U FUEL LU3

1.4

MNAMKS TESTS XENCtd STRIPPING

EYPERlMENTS

miGATE CfFGAs RUGWNG

REPLACE UILVES AND FKTERS

RAISE WWER REPAR SAMPLER

R66 J

7-MAIN

ATTAN M L POWER CHECK CONTAINMENT

FULL-POWER RUN BLOWER FALURE pRocEss FLUSH

REPLACE MAIN KOWER

MELT SALT FROM GAS L W S RERACE UY1E SAMPLES TEST CONTAINMENT

SALT

PROCESS F U U SALT

RUN WlTH ONE BLOWER

N W L L SECOND BUNYER N

CnEM CONTAINMENT

LOA0 URANKM-233 REMOVE LOIUWNG D E W

ZERO-PMR EXPERIMENTS

mnus

F '

} REPLACE WARES TEST CONTAMMENT

CORE

J I

(967

A00 PLUTONIUM

RRADIATE ENc4psuITED U

=Go

MAP EP. DEp0SITy)N WITH GAMMA SPECTRMETER ~ P L NEL MEASURE ETRITIUM, REMOVE CORE ARRAY PUT REACTOR IN sTmmy

4 6 6 Y ) M R (Mw)

FIGURE 2.2.

HISTORICAL OUTLINE OF MSRE'S OPERATION

ORNL-LR-DWG 64097RIA

FLEXIBLE CONDUIT TO CONTROL ROD DRIVES

-. .

GRAPHITE SAMPLE ACCESS - - - - Pnwr . ,

COOLING AIR LINES

ACCESS PORT COOLING JACKETS

FUEL OUTLET

/ REACTOR ACCESS PORT

CORE ROD THIMBLES

M A L L GRAPHITE SAMPLES OLD-DOWN ROD OUTLET STRAINER

LARGE GRAPHITE SAMPLES CORE CENTERING GRID

/I\

FLOW DISTRIBUTOR VOLUTE GRAPHITE -MODE STRINGER

CORE WALL COOLING ANNULUS REACTOR CORE CAN REACTOR VESSEL

ANTI-SWIRL VANES VESSEL DRAIN LINE

FIGURE 2.3.

e 'I

SUPPORT GRID

MSRE CORE AND REACTOR VESSEL

ORNL-OWG 68-1214611 TOP GUIDE

* I

,-HASTELLOY

N TENSILE SPECIMENS ROD ENTIRE SPECIMEN ASSEMBLY IS ENCLOSED I N A PERFORATED BASKET AS SHOWN I N PLAN

TOR PERFORATED BASKET 3/8 in. DIAMETER HOLES ON 9/16 i n . CENTERS TRIANGULAR PITCH

OUTLINE OF GRAPHITE CHANNEL CONTAINING CORE SURVEILLANCE SAMPLER G-GRAPHITE SPECIMENS N-HASTELLOY N TENSILE SPECIMENS ROD SECTION A-A INCLUDING BASKET AND SHOWING FLOW AREA

FIGURE 2.4.

MSRE CORE SURVEILLANCE SPECIMENS

AND CROSS SECTION OF FLOW GEOMETRY

9 s u r v e i l l a n c e specimen holder w a s removed from t h e core and taken t o a hot c e l l where t h e samples could be removed.

Samples consisted of

graphite specimens, Hastelloy-N rods f o r t e n s i l e strength t e s t s , and Hastelloy-N flux monitor tubes.

The specimen assembly was made so t h a t

only p a r t of t h e samples need t o be removed, and t h e remaining samples along with new replacements could be r e i n s e r t e d i n t o t h e r e a c t o r f o r further irradiation.

Fission product deposition measurements were then

made on t h e graphite and Hastelloy-N

specimens, which had been removed

from t h e sample holder.

2.3

Description of t h e Fuel Pump

The f u e l pump turned out t o be a very important component i n understanding noble metal migration i n t h e MSRE. i s shown i n Figure 2.5.

48.5 f t of head.

A d e t a i l drawing of t h e pump

I t s rated capacity i s 1200 g p m of f u e l salt a t

The volute i s completely enclosed i n another v e s s e l

r e f e r r e d t o as t h e pump bowl, which served a v a r i e t y of purposes.

contained t h e only f r e e l i q u i d surface i n t h e system and t h e r e f o r e 5

served as an expansion volume f o r s a l t .

It a l s o contained t h e 135Xe

s t r i p p i n g system, a s a l t l e v e l i n d i c a t o r , and t h e f u e l s a l t sampling

â&#x20AC;&#x2DC; t :

facility.

The f u e l expansion capacity of t h e pump bowl was more than

adequate f o r t h e normal operating ranges of t h e MSRE, but w a s n o t adequate for some postulated accident conditions.

Therefore, t h e pump bowl w a s

provided with an overflow tank t h a t would fill if t h e s a l t l e v e l i n t h e pump bowl reached t h e l e v e l of t h e overflow pipe.

The normal operating

helium pressure i n t h e pump bowl, which w a s a l s o t h e pump suction pressure, w a s about 5 psig. The xenon s t r i p p e r w a s a gas-liquid contacting device. spray r i n g containing many small holes (146

- 1/8 i n .

A toroidal

h o l e s , and 145

1/16 i n . h o l e s ) , sprayed t h e s a l t through t h e gas phase. The s a l t flow, which r e s u l t e d i n a mean j e t v e l o c i t y of 7.2 f t / s e c , w a s estimated (not measured) t o be at a rate of about 50 gpm.

The xenon r i c h cover gas w a s

purged from t h e pump bowl by a continuous flow of clean helium from (1)t h e pump s h a f t purge, ( 2 ) two bubbler l e v e l i n d i c a t o r s , and (3) t h e

reference pressure l i n e purge f o r t h e bubblers. t o t h e off-gas system described e a r l i e r .

W I

The purge gas then went

The overflow tank a l s o had a

bubbler l e v e l i n d i c a t o r and i t s purge a l s o went t o t h e off-gas system.

ORNL-DWG 69-1 0172R

SHAFT

SUCTION

k RADIOACTIVE CLEAY CAS

TANK

FIGURE 2 . 5 .

MSRE FUEL PUMP AND OVERFLOW TANK

GAS

aiid -

The spray j e t s impinging on t h e s a l t surface generated l a r g e amounts of

helium bubbles and f l u i d turbulence i n t h e pump bowl.

These bubble9

manifested themselves i n s e v e r a l ways i n t h e operation of t h e MSRE.

one sense t h e i r presence was unfortunate because it complicated t h e understanding of many observations i n t h e r e a c t o r , but i n another sense it w a s f o r t u n a t e because it l e d t o t h e suggestion of e f f i c i e n t ways of removing f i s s i o n products ( p a r t i c u l a r l y noble gases, but possibly also noble

metals ) from future molten-salt r e a c t o r systems. Let us examine t h e behavior of gas bubbles i n t h e s a l t i n t h e pump bowl by first considering t h e carryunder of bubbles.

Note from Figure 2.5

t h a t t h e r e are two bubblers and t h e y a r e at d i f f e r e n t depths.

By d i f f e r -

ence i n reading between t h e bubblers, one can deduce information on t h e average f l u i d density between them,

Void f r a c t i o n s as high as 18 percent

were measured i n t h i s region.'4)Certainly most of t h e bubbles w e r e large and would r i s e t o t h e s u r f a c e , but some of them were small and would be c a r r i e d down i n t o t h e pump suction t o c i r c u l a t e with t h e f u e l s a l t . Estimates of t h e s i z e of t h e s m a l l bubbles i n d i c a t e they were l e s s than

0.010 i n . i n diameter. L

The amounts of bubbles c i r c u l a t i n g with fuel s a l t

could be estimated by a n a l y s i s of (1)l e v e l changes i n t h e pump bowl,

( 2 ) nuclear r e a c t i v i t y balances i n t h e core, (3) sudden pressure release t e s t s , ( 4 ) s m a l l induced pressure p e r t u r b a t i o n s , and ( 5 ) other less d i r e c t observations.

The general conclusion from t h e s e analyses i s t h a t

t h e volume f r a c t i o n of c i r c u l a t i n g bubbles i n t h e f u e l loop during t h e 235U runs was 0.0002 t o 0.00045, (4'5)and t h e void f r a c t i o n during the

233U runs w a s 0.005 and 0 . 0 0 6 . ( ~ )hypothesis ~ i s presented i n Section 3.3 t o explain t h i s r a t h e r l a r g e change i n c i r c u l a t i n g void f r a c t i o n between t h e 235U and 233U runs.

The same hypothesis i s used t o explain t h e d i f -

ference i n overflow rates discussed next. Now consider t h e bubbles t h a t rise t o t h e surface i n t h e pump bowl. There are s t r o n g i n d i c a t i o n s t h a t they produced a f r o t h with a high l i q u i d content on t h e s a l t surface.

f u e l s a l t from t h e pump bowl t o t h e overflow tank even though t h e indicated salt l e v e l i n t h e pump bowl was w e l l below t h e overflow pipe.

G l

For instance, there was a constant flow of

P e r i o d i c a l l y t h i s salt w a s forced back i n t o t h e pump bowl by pressurizi n g t h e overflow t a n k with helium.

This t r a n s f e r must have been due t o

12 f r o t h flowing over t h e l i p of t h e overflow pipe, or t o a m i s t of salt d r i f t i n g i n t o t h e overflow pipe, or t o both.

235U runs ranged from 0.4 t o 1.5 l b / h r .

Overflow rates during t h e

During t h e 233U runs t h e y ranged

from 4 t o 10 l b / h r with excursions up t o 70 l b / h r . ( 4 ) A simple comparison i n d i c a t e s t h a t a r a i n of "more than t h e hardest t o r r e n t i a l downpour" ( 4 1 would have been required t o match t h e lowest overflow rate during t h e

233U runs.

A reasonable conclusion i s t h a t f r o t h with a high l i q u i d con-

t e n t flowing dom t h e overflow pipe accounted f o r t h e high overflow rates. There may a l s o have been m i s t i n t h e pump bowl.

It i s w e l l known t h a t t h e

mechanical action of a high v e l o c i t y j e t impinging on a l i q u i d surface w i l l generate a m i s t .

Bursting bubbles are a l s o known t o produce m i s t .

There are physical i n d i c a t i o n s from t h e r e a c t o r t h a t a m i s t w a s present i n t h e pump bowl.

For example, t h e off-gas l i n e plugged p e r i o d i c a l l y

which w a s probably due t o f r e e z i n g of salt from a m i s t .

S t r i p s of metal

suspended i n t h e gas phase of t h e pump bowl w e r e covered w i t h s m a l l dropl e t s when r e t r i e v e d (see Figure 4 of R e f . 4 ) .

Later when we discuss t h e

a n a l y s i s of gas samples taken from t h e pump bowl, we w i l l explain t h e r e s u l t s by t h e o r i z i n g t h a t a salt m i s t was probably being sampled.

A f a c i l i t y w a s provided f o r t a k i n g s a l t samples from t h e bowl of t h e

f u e l pump.

This sampling f a c i l i t y was used for a v a r i e t y of purposes

including (1)t a k i n g f u e l s a l t samples, ( 2 ) t a k i n g gas phase samples,

( 3 ) adding uranium t o t h e f u e l salt, ( 4 ) adding chemicals t o t h e f u e l s a l t t o c o n t r o l i t s oxidation-reduction s t a t e , and ( 5 ) exposing materials

t o f u e l salt f o r s h o r t periods of t i m e .

The sampling f a c i l i t y w a s q u i t e

complex, and included a dry box, i s o l a t i o n valves, remote handling gear, s h i e l d i n g , instrumentation, e t c .

A d e t a i l e d d e s c r i p t i o n of t h e e n t i r e

f a c i l i t y i s not necessary f o r t h e purposes of t h i s r e p o r t , and only t h e sample s t a t i o n i n t h e pump bowl i s shom i n Figure 2.6.

Note t h a t t h e

sample s t a t i o n i s enclosed by an overlapping s h i e l d arrangement.

It w a s

d e s i r e d t o take t h e s a l t and gas samples under r a t h e r quiescent cond i t i o n s , and t h i s s h i e l d w a s intended t o prevent t h e m i s t , foam, and f l u i d turbulence generated by t h e spray r i n g from p e n e t r a t i n g t o o aggress i v e l y i n t o t h e sampling region.

The s h i e l d does n o t , however, prevent

f r e e movement of s a l t through t h e sample region.

The overlapping portion

of t h e s h i e l d i s n o t closed, and a l s o , t h e s h i e l d i s elevated o f f t h e

ORNL-DWG 6 7 - 1 0 7 6 6 R

-SAMPLE

TRANSPORT CABLE

/-TRANSPORT

PIPE

LATCH ASSEMBLY NORMAL P O S I T I O N \TOP OF FUEL PUMP

-SHIELD L)

WELD,

CAGE FOR SAMPLE CAPSULE

. .

.:'i

ir A 'SAMPLE

CAPSULE (LADLE TYPE)

1 3 / 1 6 in. MAX 1 -~ 01

--. .. T FIGURE 2.6.

2. -3

INCHES

PUMP BOWL SAMPLE STATION

14 I

bottom of t h e pump bowl. The sample capsule shown i s t y p i c a l of t h e first kind of capsules used i n t h e MSRX. capsule."

It i s r e f e r r e d t o as a "ladle

It i s simply a container with open p o r t s i n t h e side.

During

normal sampling, it was lowered with t h e sample t r a n s p o r t cable u n t i l it

w a s w e l l below t h e salt surface.

After f i l l i n g , it w a s l i f t e d out of t h e

s a l t and suspended f o r a period of time a short distance up i n t h e trans,

p o r t pipe i n order t o freeze t h e s a l t .

It was then withdrawn, i s o l a t e d

from t h e f u e l system, and taken t o a hot c e l l f o r analysis.

During t h e

sampling period, a s m a l l purge of helium w a s maintained down,t h e transp o r t pipe. Fig. 2.7 shows o t h e r sample capsules used which w i l l be des-

cribed l a t e r .

3. FISSION PRODUCT EXPERIENCE I N THE MSRE 3.1 General Fission Product Disposition I n t h i s section we w i l l discuss i n a q u a l i t a t i v e way t h e general d i s p o s i t i o n of f i s s i o n products i n t h e MSFG3, and t h e techniques used t o

measure t h i s disposition.

The emphasis'will be on noble metals.

We w i l l

also discuss t h e r a t h e r dramatic differences i n t h e r e a c t o r operating c h a r a c t e r i s t i c s when fueled with 235U and 233U,

and propose a reason f o r

t h i s difference, I 1

A systematic way of c l a s s i f y i n g f i s s i o n products i n t h e MSRE, based

on t h e i r migrational c h a r a c t e r i s t i c s , i s as follows: (1) s a l t seekers, (2)

noble gases, and

( 3 ) noble m e t a l s . A s a group and under normal MSRE operating conditions, t h e s a l t

I I !

seekers are t h e best behaved of a l l f i s s i o n products. seekers are S r , Y, Z r , I, Cs, Ba, C e and Nd.

Examples of s a l t

Unless a f f e c t e d by

migrational c h a r a c t e r i s t i c s of t h e i r precursors, they remain dissolved I !

i n t h e fuel s a l t i n inventory q u a n t i t i e s .

Consider t h e following gen-

e r a l i z e d b e t a decay scheme of f i s s i o n products:

Ir! ORNL-DWG 67-4704

ORNL-OWG 68-6079

t STEEL

(-STAINLESS

VOLUME

20cc

CABLE

314-in.

-%-in.

NICKEL TUBE 0.020 -in. WALL

OD NICKEL

'/e-in.

Li2BeF4

- OD

/ NICKEL

Ni TUBING

CAPILLAW y'/~6-in.

WELD ROD

Lip BeF,

(a) Tu0 Modifications of Freeze Valve Capsules

DRNL-DWG 70-6758

CUT FOR SAMPLE REMOVAL

NICKEL OUTER TUBE

COPPER INNER CAPSULE ALL RETAINER ALL WITH SOFT SOLDER SEAL

OPPER NOZZLETUBE ABRADE FOR SAMPLE REMOVA

(b) Double-Wall Freeze Valve Capsule

FIGURE 2.7.

FREEZE VALVE SAMPLE CAPSULES

cj t

(1) La +

(1) (1) (1) C e + Pr + Nd -t

- salt

where (1)

(3) (4) (2)

seeker,

noble gas, noble metal, and noble metal or s a l t seeker depending on oxidation-reduction state of f u e l salt.

Note t h a t Kr ( a noble gas) i s a precursor of Rb, Sr, Y and Zr ( s a l t seekers) and a l s o t h a t Xe ( a noble g a s ) i s a precursor of C s , Bay L a and Ce ( s a l t seekers).

Also note t h a t Sb and Te (noble m e t a l s ) are precursors

of I ( a s a l t ' s e e k e r ) .

The behavior of precursors of s a l t seekers may

dramatically a f f e c t t h e ultimate d i s p o s i t i o n o f t h e salt seekers themselves.

For instance 9 5 Z r has a y i e l d of about

6.2 percent but i t s pre-

cursor, 9 5 K r , has a y i e l d of only 0.007 percent and i s very short l i v e d . Therefore any migrational tendency of t h e 95Kr w i l l have l i t t l e e f f e c t on t h e 9 5 Z r .

Indeed t h e complete 9 5 Z r inventory was found i n t h e f u e l

salt. ( 6 ) As opposed t o t h i s , consider t h e salt seeker 137Cs with a

cumulative y i e l d of 6.15 percent.

Most of i t s y i e l d comes from t h e decay

of 137Xe t h a t has a y i e l d of about 6.0 percent and a h a l f l i f e of

3.9 min.

Xenon can be t r a n s f e r r e d t o t h e off-gas system and also can

d i f f u s e i n t o t h e porous s t r u c t u r e of t h e g r a p h i t e .

Accordingly, only

80 t o 90 percent of t h e 137Cs inventory w a s found i n t h e fuel s a l t (6 1 and s i g n i f i c a n t q u a n t i t i e s were found deep i n s i d e t h e g r a p h i t e where .it deposited upon decay of i t s precursor." ( 7 y 8 )

In conclusion, t h e s a l t

seeking f i s s i o n products are w e l l behaved and remain dissolved i n t h e c i r c u l a t i n g f u e l s a l t i n inventory q u a n t i t i e s except when influenced by t h e behavior of t h e i r precursors. There are only two noble gas fissicn products, Kr and Xe.

They

appear, however, i n over 30 mass number decay chains and t h e r e f o r e s i g n i f i c a n t l y a f f e c t t h e general f i s s i o n product d i s p o s i t i o n .

Notable

among these f i s s i o n products i s 135Xe with i t s l a r g e thermal neutron *The amounts and concentration p r o f i l e s of salt seekers i n s i d e t h e graphite have been c o r r e l a t e d w i t h theory q u i t e well i n R e f . ( 9 ) f o r t h e case where t h e noble gas precursor i s s h o r t l i v e d .

cross section of 3 x 10 barns.

Because of 135Xe,

a great d e a l of work

has gone i n t o q u a n t i t a t i v e l y understanding noble gas migration i n t h e MSRX.

This work has been reported i n t h e semiannual r e p o r t s and other

documents.

The noble gases, p a r t i c u l a r l y xenon, a r e very insoluble i n

f u e l s a l t ; t h e r e f o r e , they a r e r e a d i l y transported i n t o any a v a i l a b l e gas phase such as t h e c i r c u l a t i n g bubbles, t h e gas space i n t h e pump bowl, and t h e pores of t h e moderator graphite. The t h i r d group of f i s s i o n products i s t h e so-called "noble metals." They a r e reduced by UF3 i n t h e f u e l salt and e x i s t i n t h e r e a c t o r 1

environment in t h e m e t a l l i c s t a t e , hence t h e name. Ag, Sb, Te and sometimes Nb.

Examples are Mo, Ru,

If t h e f u e l s a l t i s i n a well reduced

s t a t e , then t h e Nb e x i s t s as a noble metal, but i f t h e f u e l i s more oxidized, it e x i s t s as a s a l t seeker.

Noble m e t a l s have been found through-

out t h e e n t i r e r e a c t o r f u e l s a l t loop.

They have been found i n l a r g e

q u a n t i t i e s i n f u e l s a l t and gas phase samples from t h e pump bowl.

They have been found i n l a r g e q u a n t i t i e s on t h e Hastelloy-N and graphite-core a

surveillance samples, and on t h e primary heat exchanger tube surfaces and loop piping surfaces. off-gas system.

They have been found at various locations i n t h e

I n t h i s report we w i l l look q u a n t i t a t i v e l y at t h e noble

metals ih t h e s e depositories, within t h e framework of mass t r a n s f e r theory, and t r y t o develop a u n i f i e d model of noble metal migration i n t h e MSRE

3.2

Fission Product Disposition Measurements

Fuel S a l t and G a s Phase Samples

3.2.1

Three types of capsules were used t o obtain f u e l s a l t samples from t h e MSRE pump bowl sample s t a t i o n , and t h e s e are shown i n Figures.2.6

2.7.

Numerous other sampling devices were used f o r s p e c i a l t e s t s but only

those i l l u s t r a t e d were used on a routine b a s i s .

The l a d l e capsule i s t h e

simplest and w a s t h e f i r s t used f o r sampling f u e l s a l t .

This capsule i s

i l l u s t r a t e d i n Figure 2.6 i n t h e process of taking a salt sample. s

and

It i s

simply a s m a l l container with open p o r t s on t h e s i d e t o allow s a l t t o enter.

During t h e course of f'uel s a l t sampling from t h e r e a c t o r , it w a s

found t h a t a m i s t of s a l t , which was heavily contaminated with noble metals, e x i s t e d over t h e s a l t pool.

Presumably t h i s m i s t w a s generated by

18 t h e mechanical action of t h e spray impinging on t h e s a l t surface and by b u r s t i n g bubbles.

When t h e ladles were lowered t o take a s a l t sample,

t h e m i s t adhered t o t h e capsule and g r o s s l y contaminated t h e sample. 11

Freeze valve" capsules , which had previously been developed f o r taking

gas samples, were t h e r e f o r e used t o take s a l t samples i n t h e hope of a l l e v i a t i n g t h i s problem.

Two modifications of t h e s e f r e e z e valve

capsules are shown i n Figure 2.7a. Each capsule contains a vacuum which i s held by a frozen s a l t seal around t h e c a p i l l a r y entrance tube.

The

s a l t seal i s designed t o m e l t a f t e r a delay t i m e long enough t o allow t h e capsule t o be completely submerged i n f u e l s a l t before it opens.

The

i n s i d e surface of t h e capsule i s t h e r e f o r e not contaminated with t h e salt

m i s t when t a k i n g a f u e l s a l t sample.

Freeze valve capsules f o r t a k i n g

salt samples were used first i n run 14.

The noble = t a l concentration

measured i n s a l t from t h e s e capsules w a s generally about two orders of magnitude less than i n s a l t from t h e ladle capsules.

Note t h a t i n both

modifications of t h e freeze valve capsule, t h e s e a l i n g s a l t remains i n s i d e t h e capsule, and puddles over t h e c a p i l l a r y nozzle.

When t a k i n g gas phase

samples, t h e capsule i s lowered i n t o t h e pump bowl u n t i l t h e valve thaws and t h e sanrple i s taken, then it i s withdrawn t o a cooler region so t h e

s a l t puddle may again f r e e z e and contain t h e sample.

These freeze valve

capsules d i d n o t exclude mist and scum on t h e s a l t surface from adhering t o the outside surfaces of t h e capsule.

During capsule processing t h e

outside surfaces w e r e always w e l l leached t o remove t h i s material.

Never-

t h e l e s s , t h e question of t r a n s f e r of contamination from the outside surf a c e t o the i n s i d e materials during chemical processing always remained. This worry l e d t o t h e development of t h e double walled, freeze valve capsule shown i n Figure 2.7%. It i s b a s i c a l l y a freeze valve capsule but

it i s doubly contained, and can be used f o r salt or gas phase sampling. During processing, t h e nozzle and t o p of t h e o u t e r container are cut through.

The inner capsule containing t h e sample then f a l l s away from

t h e o u t e r container and i s free from m i s t and scum.

There was l i t t l e

difference i n t h e measured noble m e t a l concentration between samples taken with freeze valve and with double-walled freeze valve capsules.

Li These t h r e e kinds of capsules were r o u t i n e l y used f o r t a k i n g f u e l D

In t h e a n a l y t i c a l work presented i n t h i s

s a l t and gas phase samples.

r e p o r t , only d a t a f r o m t h e freeze valve and double-walled freeze valve capsules w i l l be used, and they w i l l be used with equal weight.

Ladle

samples w i l l not be used because of t h e problem of m i s t and scum contamiOn a l l p l o t s t h a t follow, t h e data p o i n t s w i l l specify whether

nation.

the capsule was a f r e e z e valve or a double-walled f r e e z e valve type. Another point t o note i s the difference i n volume of t h e two capsule types.

The f r e e z e valve capsule when full holds 50 t o 60 grams of f u e l

s a l t , but t h e double-walled capsule when f u l l holds only 14 t o 15 grams. During t h e sampling procedure, a purge of helium was maintained down t h e sample t r a n s p o r t pipe.

This should make l i t t l e difference t o the s a l t

samples but could be a s i g n i f i c a n t parameter f o r t h e gas samples.

The

helium purge v a r i e d from 575 t o 75 standard cm3/min depending on t h e A l l samples were taken with the higher purge r a t e before sample

sample.

19-16; a f t e r t h a t t h e s a l t samples had t h e higher purge r a t e and t h e gas t

samples t h e lower purge r a t e . Only four f r e e z e valve samples of f u e l s a l t were taken during t h e

235U runs (during run 1 4 ) but many freeze valve and double-walled f r e e z e valve samples were taken during t h e 233U runs. f o r e be discussed most extensively.

The 233U runs w i l l there-

Many l a d l e samples of s a l t were taken

during t h e 233U runs because they continued t o give good r e s u l t s on s a l t seeking f i s s i o n products. radiochemically.

Both the s a l t and gas samples were analyzed

This technique was used t o determine both t h e i d e n t i f i -

c a t i o n and t h e amount of t h e isotopes present.

3.2.2

Gamma Spectrometry of t h e Primary Heat Exchanger The amounts of c e r t a i n noble metal f i s s i m products deposited on t h e

primary heat exchanger tube surfaces were measured by gamma ray spectrometry.

The technique consisted of looking a t a spot on t h e heat ex-

changer with a collimated d e t e c t o r and measuring t h e gamma energy spectrum 5

emitted *om

t h a t spot.

The r e s u l t i n g s p e c t r a were used t o i d e n t i f y and

measure q u a n t i t a t i v e l y t h e f i s s i o n products present.

The first gamma scans were made a f t e r run

1 4 ( l a s t 235U run) and were,

t o an e x t e n t , exploratory i n n a t u r e ; i . e . , an attempt was made t o see i f

s p e c i f i c f i s s i o n product information could be e x t r a c t e d q u a n t i t a t i v e l y from t h e tremendous amount of background gamma r a d i a t i o n emitted by an operating r e a c t o r system. B r i e f l y , t h e equipment consisted of a highly collimated lithium-drifted germanium diode d e t e c t o r t h a t w a s coupled t o a 400 channel analyzer t o determine t h e energy s p e c t r a .

iJ - r

A l l scans were

taken with t h e r e a c t o r shut down and e i t h e r drained or f i l l e d with f l u s h The equipment was mounted on a p o r t a b l e maintenance s h i e l d , s o

salt.

gamma scgns could be taken at many l o c a t i o n s on t h e heat exchanger. together about 100 s p e c t r a were determined.

All

Most of t h e s e were from t h e

h e a t exchanger but a few were from o t h e r components.

The equipment w a s

c a l i b r a t e d s o t h a t t h e measured count rate could be reduced t o atoms of f i s s i o n product per u n i t area of tube surface. by hand and w a s q u i t e tedious.

Data processing was done

It w a s p o s s i b l e t o i s o l a t e q u a n t i t a t i v e l y

four noble metals from a t y p i c a l spectrum (99Mo, lo3Ru, 132Te, and 95Nb). More d e t a i l s of t h e equipment and procedures can be obtained from R e f . lo. A t t h e end of t h e experiment it was concluded t h a t t h e q u a l i t y o f t h e

d a t a and i t s p o t e n t i a l a p p l i c a t i o n s were s u f f i c i e n t l y promising t o w a r r a n t improving t h e system and repeating t h e experiments during l a t e r runs. A f t e r considerable equipment and c a l i b r a t i o n procedure improvement,

t h e gamma spectrometry measurements w e r e repeated after run 19 (233U). An improved G e ( L i ) d e t e c t o r and a 4096 channel analyzer were used.

Data

processing was done with a computer program developed f o r t h i s purpose. Precise alignment w a s achieved by use of a l a s e r beam and surveyor's transit.

Altogether some 1000 s p e c t r a were measured, many of which were

taken with t h e r e a c t o r at d i f f e r e n t power l e v e l s (a f e w w a t t s t o full power).

Another 400 s p e c t r a were taken f o r c a l i b r a t i o n purposes.

Details

of t h e equipment, c a l i b r a t i o n procedures, and d a t a a n a l y s i s can be

obtained from R e f . 11. Gamma s p e c t r a were obtained from o t h e r f u e l loop components besides t h e heat exchanger, such as t h e pump bowl, off-gas l h e , and main loop piping.

The p r i n c i p a l i n t e n t of t h e experiment, how-

e v e r , was t o measure f i s s i o n product deposition q u a n t i t a t i v e l y i n t h e primary heat exchanger.

This w a s t h e only component f o r which an

absolute c a l i b r a t i o n of t h e d e t e c t o r w a s made.

Therefore, t h e s e are

t h e only d a t a t h a t w i l l be analyzed i n t h i s r e p o r t .

The p r i n c i p a l s p e c t r a

of t h e primary h e a t exchanger were obtained a f t e r run 19, although some I

u 2

preliminary data were taken arter run 18. r e a c t o r shut down and drained.

The d a t a were taken with t h e

Altogether, q u a n t i t a t i v e information w a s

obtained f o r 10 noble m e t a l isotopes from t h e s e data.

Figure 3.1 i s

t y p i c a l of t h e kind of d a t a obtained from t h i s experiment.

It shows t h e

amount of 132Te p e r square centimeter of tube surface p l o t t e d against a l o n g i t u d i n a l representation of t h e heat exchanger.

The t o t a l range i n

concentration i s from 0.3 x 1 0 l 2 t o 0.75 x 10l2 disintegrations/min-cm2. The higher value seems t o be associated with t h e b a f f l e p l a t e s and t h e i r windows, whereas t h e lower value i s associated with t h e crossflow p a r t of t h e heat exchanger.

In t h i s report t h e lower value w a s considered t o be

most representative of t h e heat exchanger and, as an example, 0.3 x 10l2 disintegrations/min-Cm2

w a s chosen f o r 132Te.

The q u a l i t y of d a t a from

t h e gamma scans a f t e r run 1 4 were not nearly as good as shown i n Figure

3.1, and a s o r t of weighted average of a l l t h e data w a s used. 3.2.3

Core Surveillance Samples The core surveillance specimens were b r i e f l y described ( s e e Sect. 2.2)

and i l l u s t r a t e d (see Figure 2.4) e a r l i e r .

P e r i o d i c a l l y , when t h e r e a c t o r

w a s shut down and drained, a l l graphite and Hastelloy-N

specimens were

removed and t h e amounts o f f i s s i o n products deposited on t h e surfaces and t h e i n t e r i o r of some were measured.

Radiochemical techniques were used

t o determine t h e i d e n t i t y and amount o f t h e isotopes present. Unfortunate-

l y , t h e f l u i d dynamic conditions i n t h e surveillance specimen holder a r e not well known because the f l o w passages are so complicated.

These fluid

dynamic d i f f i c u l t i e s are discussed i n d e t a i l i n Appendix B where w e

estimate t h e mass t r a n s f e r c o e f f i c i e n t ,

B r i e f l y , l e t us j u s t say t h a t t h e

s u r v e i l l a n c e specimens f e a t u r e i n s i d e corners, outside corners, f l u i d entrance and e x i t regions, and possible stagnation areas.

Superimposed

on t h e s e i s a flow t h a t i s only marginally t u r b u l e n t ( R e 2 3000).

These

d i f f i c u l t i e s were a r e s u l t of t h e many d i f f e r e n t kinds and geometries of specimens t h a t had t o be incorporated i n t o a very confined space. i

The

measured noble metal deposition data on graphite t h a t w i l l be analyzed i n t h i s r e p o r t came from t h e surfaces exposed t o f u e l s a l t , although some

c II

noble metals were a l s o found on t h e i n s i d e surfaces t h a t presumably w e r e

ORNL-DWG 70-7365

YTR. PLUG .

HTR. PLUG

Low-

I I

POSITION HOLE IN SHIELD PLUGS

FIGURE 3.1.

MSRE HEAT EXCHANGER

F I S S I O N PRODUCT DEPOSITION;

13*Te( 1 )

23 The measured noble metal deposition d a t a on

not exposed t o s a l t .

Hastelloy N came from t h e dosimetry wires.

Noble m e t a l deposition measure-

ments were a l s o made on t h e perforated Hastelloy N cage but these w i l l not be analyzed because of t h e almost impossible job of estimating t h e"- m a s s In Appendix B , t h e mass t r a n s f e r c o e f f i c i e n t t o

transfer coefficient.

both t h e graphite and t h e Hastelloy N i s estimated t o be about 0.25 f t / h r . There i s a s i g n i f i c a n t uncertainty associated with t h i s value f o r t h e reasons described above.

The mass t r a n s f e r c o e f f i c i e n t from t h e fuel s a l t

t o t h e h e a t exchanger surfaces i s a much b e t t e r known number than t o t h e surveillance specimens.

Figures 3.2 and 3.3 show t y p i c a l data obtained

from t h e s e specimens. 3.3

The Difference Between t h e 235U and 233U Runs

Following run 14 t h e 235U fuel was removed from t h e carrier s a l t (and f l u s h s a l t ) and t h e r e a c t o r was refueled with 233U. Chemical processing of t h e salt was done at t h e MSRE s i t e , and t h e process i s discussed i n The b a s i c process w a s f l u o r i n a t i o n of t h e f u e l salt

d e t a i l i n R e f . 12.

ivld removal of the uranium as a v o l a t i l e f l u o r i d e .

The corrosion rate on

t h e process tank during processing w a s high and has been estimated t o be about 0 . 1 m i l / h r .

The tank w a s constructed of Hastelloy N and t h e corro-

sion products were NiF2, FeF2, and CrF2.

Following t h e f l u o r i n a t i o n s t e p ,

it w a s t h e r e f o r e necessary t o remove t h e corrosion products from t h e s a l t . This w a s done by reduction with hydrogen and zirconium powder and subsequent f i l t r a t i o n .

Removal of 235U from t h e s a l t w a s e s s e n t i a l l y complete.

The c a r r i e r s a l t w a s then returned t o t h e r e a c t o r system and loaded w i t h 233U.

R u n 15, t h e f i r s t 233U run, was concerned w i t h t h e zero-power

physics experiments w i t h t h i s new f u e l . During run 15 a s i g n i f i c a n t change i n operating c h a r a c t e r i s t i c s of t h e r e a c t o r occurred and p e r s i s t e d u n t i l t h e r e a c t o r was permanently shut

dawn.

During t h e 235U runs, t h e volume f r a c t i o n of c i r c u l a t i n g bubbles i n

t h e f u e l loop was determined t o be between 0.0002 and 0.00045 (see R e f . 5 ) , and t h e overflow rate from t h e pump bowl t o t h e overflow tank ranged from

0.4 t o 1.5 l b / h r , with e s s e n t l a l l y no overflow excursions following beryllium additions.

Beryllium was p e r i o d i c a l l y added t o t h e fuel s a l t ,

primarily t o reduce some UFI, t o UF3 and c o n t r o l t h e oxidation state of t h e

fuel.

During t h e 233U runs t h e volume f r a c t i o n of c i r c u l a t i n g bubbles

ORNL-DWG 70-15013 10’2

CGB IMPREGNATED

C CGB P = POCO

PYI I = PYROLYTIC PARALLEL TO PLANES PY I = PYROLYTIC PERPENDICULAR TO PLANES A = WIDE FACE N = NARROW FACE 2X = DOUBLY EXPOSED

10”

2 10’0

* 9Te 10‘0

0 PN

I I

0 PW

0 CWEX 2

0 CWLX

I &N2X

0 cw2x 1o9

-dN2X

10 I W 5

I O P Y l

2 108

’

DISTANCE FROM BOTTOM OF CORE (in.)

FIGURE 3.2.

DEPOSITION OF ”MO

AND I2’TE

ON GRAPHITE CORE SURVEILLANCE SPECIMENS

ORNL-DWG 70-15014 I

10’’

0 \

‘E -0

10’0

n W n

109 r

/a-

v-v-

FIGURE 3.3.

-v\

106Ru I

I 30

40 DISTANCE FROM BOTTOM OF CORE ( i n . )

DEPOSITION OF 99M0,

‘

32TE,

I2’TE,

I 50

I 03RU,

I . Io6RU,

AND 95ZR

ON HASTELLOY N CORE SURVE LLANCE SPECIMENS AFTER 32,000 MW HRS

i n t h e f u e l loop was 0.005 t o 0.006, and t h e overflow rate ranged from 4 t o 10 l b / h r with excursions up t o 70 l b / h r following a beryllium addition. The mean c i r c u l a t i n g void Traction had gone up by a f a c t o r of about 20 and t h e overflow rate had gone up by a f a c t o r of about 10 i n t h e 233U runs as compared t o t h e 235U runs.

Reference 4 provides a d e t a i l e d discussion

of t h e s e operating v a r i a b l e s and o t h e r s .

The composition of t h e f u e l s a l t

remained nominally t h e same f o r both f u e l s , although t h e uranium concentray t i o n w a s reduced from 0.9 mole percent t o about 0.2 mole percent primarily because t h e 233U was not d i l u t e d with 238U. of s a l t d e n s i t y by 3 t o 4 percent.

This r e s u l t e d i n a lowering

Other physical p r o p e r t i e s ( v i s c o s i t y ,

surface t e n s i o n , e t c . ) might be expected t o change an equivalently s m a l l amount.

The rate of bubble i n j e c t i o n from t h e pump bowl t o t h e loop, and

t h e overflow rate are c e r t a i n l y functions of t h e s e v a r i a b l e s .

In my

opinion, t h e change i n r e a c t o r o p e r a t i o n a l parameters i s much t o o g r e a t t o be explained by such s m a l l changes i n p h y s i c a l p r o p e r t i e s .

Actually,

t h e r e i s evidence t h a t t h e bubble ingestion phenomenon w a s n e a r a thresh-

old region.

This was i n d i c a t e d by a s t e e p change i n void f r a c t i o n when

t h e pump speed w a s changed a s m a l l amount."

It has been speculated t h a t

t h e s m a l l changes i n physical p r o p e r t i e s were coupled i n some way t o t h e ingestion t h r e s h o l d t o y i e l d t h e high void f r a c t i o n s during t h e 233U runs.( 4 )

T h i s suggestion may account f o r p a r t of t h e increased void f r a c -

t i o n , however, I b e l i e v e t h a t it accounts f o r only a s m a l l amount of t h e increase. The question then i s

- Why

t h e difference i n t h e above parameters

when fueled w i t h 235U and 233U. of t h e 233U runs.

P r i o r t o t h e s t a r t of run 15, f l u s h salt was c i r c u l a t e d

i n t h e f u e l loop f o r about 40 h r . t h i s period.

A clue i s given during t h e i n i t i a l h i s t o r y There was no abnormal behavior during

Both t h e c i r c u l a t i n g void f r a c t i o n and t h e overflow rate

were c o n s i s t e n t with what had been expected from p a s t operating h i s t o r y . The f l u s h s a l t w a s drained and t h e fuel s a l t w a s added. started and a g a h no abnormal behavior w a s noted.

Circulation w a s

A f t e r about

h r of

*The fuel pump was pawered by a v a r i a b l e frequency u n i t for a period of time during t h e 233U runs t o i n v e s t i g a t e t h e e f f e c t s of c i r c u l a t i n g voids on 35Xe behavior.

. I ~

c i r c u l a t i o n , a beryllium rod w a s added f o r t h e purpose of reducing p a r t of t h e U4+ t o U3+.

About 2 h r l a t e r , t h e salt l e v e l i n t h e pump bowl

began t o rise i n d i c a t i n g increased bubble

ingestion i n t o t h e fuel loop.

When it peaked o u t , t h e c i r c u l a t i n g void f r a c t i o n w a s about 0.5 percent. A s h o r t t i m e l a t e r t h e overflow r a t e apparently went up t o a higher than

normal value

4 l b / h r ) . Detailed records are not a v a i l a b l e because t h e

data logger w a s n o t f u l l y operable during t h i s period.

When t h e beryllium

rod w a s removed after 1 2 h r of exposure t o salt , 1 0 . 1 grams of beryllium had been dissolved.

The c i r c u l a t i n g void f r a c t i o n and overflow r a t e

remained high f o r 10 h r of circulation u n t i l t h e fuel s a l t w a s drained i n t o t h e dump tanks.

The above occurred before the r e a c t o r went c r i t i c a l

w i t h 233U.

A suggested explanation f o r t h i s behavior i s as follows.

During t h e

e a r l y operational h i s t o r y of run 1 5 , t h e f u e l salt was i n a more oxidized s t a t e than expected. (I3) The d i r e c t evidence t h a t these f l u o r i d e s were there and t h a t t h e f u e l oxidized i s as follows:

(1) The corrosion r a t e on t h e fuel loop w a s high during run 15. (2)

During the 235U runs, 95Nb behaved as a noble metal.

During

t h e i n i t i a l 233U runs , it behaved as a s a l t seeking f i s s i o n product eating t h e f u e l w a s much more oxidized. (14)

, indi-

(3) When t h e beryllium capsule was removed from t h e s a l t , a t h i c k c r u s t w a s found on t h e n i c k e l cage enclosing t h e capsule.

The c r u s t w a s

predominantly s a l t , but t h e residue after e x t r a c t i n g t h e s a l t w a s pre' 3 ) dominantly iron w i t h s m a l l amounts of n i c k e l and chromium. (

Similar

c r u s t s w e r e found on other beryllium capsules added during run 15.

The

perforated capsule f o r beryllium additions during t h e 235U runs usually came out of t h e pump bowl r e l a t i v e l y clean and d i d not have i r o n deposits. The beryllium t h e n , rather than reducing t h e uranium i n t h e fuel,

apparently reduced t h e i r o n and n i c k e l corrosion product f l u o r i d e s t o t h e m e t a l l i c state and they formed a scum on t h e surface of t h e f u e l salt i n t h e pump bowl.

A hypothesis of t h i s analysis i s t h a t t h e f l o a t i n g scum

possesses many p r o p e r t i e s of insoluble surface a c t i v e agents.

One

c h a r a c t e r i s t i c of such surface a c t i v e materials i s t h a t they enhance f r o t h s t a b i l i t y , and an e f f e c t would be produced i n t h e pump bowl much l i k e i n a f r o t h f l o a t a t i o n chamber.

It was suggested earlier t h a t a

mechanism involving heavy f r o t h must be r e s o r t e d t o t o explain t h e high overflow r a t e s experienced during t h e 233U runs.

Another e f f e c t of sur-

face a c t i v e agents i s i n t h e s i z e of t h e bubbles generated.

Development

work with bubble generating devices (15'16) i n t h e form of a v e n t u r i or j e t pump has shown t h a t considerably smaller bubbles a r e generated when a surface a c t i v e m a t e r i a l i s present than when absent.

It i s unclear

whether they a r e a e t u a l l y generated smaller or whether t h e presence of surface a c t i v e agents prevents t h e i r coalescence i n t h e immediate v i c i n i t y of t h e i r generation. smaller.

The r e s u l t , however, i s t h a t they a r e considerably

Now t o extrapolate t o t h e MSRE, one would say t h a t with reduced

corrosion products acting as surface a c t i v e m a t e r i a l s i n t h e pump bowl, more s m a l l bubbles were generated during t h e 233U runs than t h e 235U

runs. '

The smaller t h e bubbles, t h e b e t t e r t h e i r chance of being swept i n t o

the f u e l loop by t h e under flow.

This then would be a mechanistic hy-pothe-

s i s t o explain t h e higher c i r c u l a t i n g void f r a c t i o n s and pump bowl overflow

r a t e s during t h e 233U runs.

O f course, it must be shown that surface

a c t i v e materials e x i s t e d i n t h e pump bowl and f u e l loop f o r a l l t h e 233U

runs.

Although r e g u l a r s a l t sampling capsules came out of t h e r e a c t o r

f a i r l y clean, t h e beryllium addition capsules continued t o show deposits

* I r

of Fe and N i i n varying amounts during t h e remainder of t h e 233U runs. In a d d i t i o n , magnets were p e r i o d i c a l l y lowered i n t o t h e pump bowl i n an

e f f o r t t o recover f r e e m e t a l l i c p a r t i c l e s .

They did recover t h e s e

materials although t h e q u a n t i t i e s were small, l e s s than a gram; however,

it doesn't take a l a r g e amount of surface a c t i v e materials t o have a dramatic e f f e c t on surface behavior. The physical s t a t e or s t a b i l i t y of t h e f r o t h i n t h e pump bowl w i l l be r e f e r r e d t o s e v e r a l times i n t h i s r e p o r t , p a r t i c u l a r l y with regard t o the d i f f e r e n c e s i n i t s s t a t e between t h e 235U and 233U runs.

In common

usage a f r o t h o r foam, such as t h e foam on a g l a s s of beer, often implies a high degree o f s t a b i l i t y .

In terms of bubble l i f e t i m e , t h e mean l i f e

of a bubble i n a head of beer i s orders of magnitude g r e a t e r than i n pure water.

I do not intend t o suggest an increase i n bubble s t a b i l i t y

i n t h e pump bowl anything l i k e t h i s , r a t h e r t h a t t h e i n c r e a s e i n bubble s t a b i l i t y between t h e 235U and 233U runs was r e l a t i v e l y minor.

Let us

see i f we can e x t r a c t something meaningful regarding bubble l i f e t i m e s i n

i I

'kd :

t h e pump bowl.

A t steady s t a t e t h e bubble generation r a t e must equal t h e

The bubble generation r a t e w a s determined by t h e

bubble d e s t r u c t i o n r a t e .

r a t e of gas carryunder from t h e xenon s t r i p p e r s a l t spray.

One might

expect t h a t t h e carryunder w a s more or l e s s constant f o r t h e 235W and 233U runs; at l e a s t we w i l l assume t h i s t o be t h e case.

We w i l l . a l s o

assume t h a t t h e bubble b u r s t i n g r a t e i s proportional t o t h e t o t a l volume of gas bubbles i n t h e s a l t .

This neglects many v a r i a b l e s which a r e

c e r t a i n l y important, such as bubble s i z e , depth of f r o t h , s a l t drainage from t h e bubble swarm, e t c . , nevertheless, i n t h e i n t e r e s t s o f continuing with t h i s discussion, t h e assumption probably i s n ' t t o o bad.

I f it i s

t r u e then at steady s t a t e , we have Bubble Generation Rate = XV where V i s t h e t o t a l volume of gas entrained and X i s t h e "bursting The bubble h a l f l i f e would be 0.693/~. Note

constant" f o r t h e bubbles.

t h a t within these assumptions a t a constant bubble generation r a t e , if t h e bubble h a l f l i f e i s doubled, then t h e t o t a l volume of gas entrained by t h e s a l t would a l s o be doubled.

The f r o t h height i n t h e pump bowl would

then increase a corresponding amount.

A mechanism l i k e t h i s

,with

increase i n bubble l i f e t i m e of t h i s magnitude, could e a s i l y account f o r t h e higher overflow r a t e s experienced during t h e 233U runs. Note part i c u l a r l y t h a t doubling or even t r i p l i n g bubble l i f e t i m e s represents a r a t h e r mild increase i n bubble s t a b i l i t y compared t o more common notions. This, then i s a description of t h e differences i n operating char-

a c t e r i s t i c s between t h e 235U and 233U runs i n t h e MSRE, and a suggested hypothesis t o explain t h e differences.

The e s s e n t i a l s of t h i s hypothesis,

t h a t noble metals and reduced corrosion products w i l l adhere t o liquidgas i n t e r f a c e s , w i l l be r e f e r r e d t o many times i n t h e following sections.

4. 4.1

ANALYTICAL MODEL

Physical Basis of Model

For reasons already discussed, and for o t h e r s which w i l l become apparent i n t h e s e c t i o n on N3SULTS FROM THE MSRE, one might expect t h a t t h e t r a n s p o r t of noble metals from t h e fuel s a l t t o the various surfaces

where they deposit i s controlled by t h e laws of mass t r a n s f e r .

Let us

review b r i e f l y t h e behavior of noble metal f i s s i o n products and f u e l

s a l t i n t h e MSRE as follows: 1. Noble metals a r e born as ions from f i s s i o n and decay of t h e i r

precursor, but become atoms very quickly.

They a r e homogeneously dis-

persed i n t h e s a l t . 2.

Noble metals a r e unstable i n f u e l s a l t ; i . e .

, they

i n t h e reduced m e t a l l i c s t a t e and a r e q u i t e insoluble.

a r e present

They may even

Massive metal objects display contact angles i n t h e (17) range of 90-150".

be unwet by s a l t .

Noble metals deposit on Hastelloy N and graphite surfaces, and

l a r g e amounts a r e found t h e r e .

They a l s o deposit on liquid-gas i n t e r f a c e s , and we i n f e r t h a t

they display some of t h e p r o p e r t i e s of surface a c t i v e agents.

This con-

cept has been used t o explain t h e differences i n 235U and 233U operation of t h e MSRE, and w i l l be used t o explain many of t h e r e s u l t s observed later

. 5.

Fuel salt i s known t o behave as a conventional Newtonian f l u i d .

For instance, t h e primary heat exchanger was designed using conventional heat t r a n s f e r c o r r e l a t i o n s and t h e measured o v e r a l l h e a t t r a n s f e r coeff i c i e n t w a s i n good agreement with t h e design value.

One would expect

t h e same degree of success i n estimating mass t r a n s f e r c o e f f i c i e n t s since t h e t r a n s p o r t phenomenon i s t h e same i n both cases.

One must always be

wary, of course, because sometimes physical and chemical phenomena come i n t o p l a y t h a t complicate t h e simple approach. The above points c o n s t i t u t e t h e e s s e n t i a l requirements f o r a m a t e r i a l t o t r a n s p o r t through t h e f l u i d boundary l a y e r and deposit on t h e surfaces according t o t h e l a w s of mass t r a n s f e r .

Development work from other

r e a c t o r systems has s h a m t h a t , i f t h e chemistry of t h e f i s s i o n products permits-, they w i l l be transported according t o t h e s e l a w s . example, Refs. 18 through 21.

See, f o r

It would t h e r e f o r e seem f r u i t f u l t o a t t a c k

t h e noble metal migration question i n t h e MSRE within t h e framework of

mass t r a n s f e r theory i n i t s simplest form and l e t t h e r e s u l t s speak f o r themselves.

' W

4.2

Analytical Model

F i r s t , we must w r i t e a r a t e balance on t h e noble metals i n f u e l s a l t where they are born.

The conditions of t h i s rate balance a r e as follows.

Note from t h e generalized b e t a decay chain from Sect. 3.1 t h a t noble metals appear i n groups and we have a s i t u a t i o n where noble metals can decay i n t o noble m e t a l s .

Note a l s o from t h e generalized decay scheme t h a t each

noble metal grouping may s t a r t o f f with a s a l t seeking precursor.

The

a n a l y t i c a l model w i l l t h e r e f o r e consider t h e l a s t s a l t seeking precursor of t h e noble metal chain.

W e must do t h i s primarily because of 95Nb whose

precursor i s 9 5 Z r with a h a l f l i f e of 65 days. be f o r t h e unsteady s t a t e .

The a n a l y t i c a l model must

This i s because t h e MSRE, being an experimental

r e a c t o r , had a q u i t e e r r a t i c power h i s t o r y . l i v e d isotopes reach steady state.

Seldom did any of t h e longer

The model w i l l consider t h e e n t i r e

fuel loop t o be a "we1.1 s t i r r e d pot."

One might expect t h i s assumption

t o be adequate because t h e fuel c i r c u i t t i m e around t h e loop i s about

25 seconds and t h e isotopes w e w i l l be dealing with have h a l f l i v e s ranging from hours t o years.

A b e t t e r indication of t h e adequacy of t h i s

assumption would be t o compare t h e f i e 1 c i r c u i t t i m e t o t h e computed residence t i m e of a noble m e t a l atom i n salt before it deposits on a surface.

In Appendix B, it i s shown t h a t t h e longest residence t i m e

(expressed as t h e h a l f l i f e of noble m e t a l s i n f u e l s a l t ) i s twice t h e loop c i r c u i t time.

Lastly, it i s assumed t h a t a l l noble metals migrate

independently of each o t h e r , even when of t h e same chemical species. Consistent with a l l t h e considerations discussed above, we can now write a r a t e balance on t h e noble metals a s s o c i a t e d with f u e l s a l t where they are born.

The equation i n words i s as follows where t h e u n i t s of

each term i s atoms/time. V

dCS dt

= generation r a t e from f i s s i o n decay of precursor

decay r a t e

- deposition - deposition c

- deposition

+ generation rate from

r a t e on graphite

r a t e on heat exchanger rate on r e s t of f u e l loop

deposition r a t e on liquid-gas i n t e r f a c e s (bubbles).

(1)

. 32 See Appendix A f o r t h e Nomenclature.

The two generation terms a r e func-

t i o n s of t h e r e a c t o r power h i s t o r y and y i e l d s of t h e s p e c i f i c isotopes involved.

The decay rate i s a function of t h e concentration of each

s p e c i f i c noble m e t a l and i t s decay constant.

A l l t h e o t h e r terms are

deposition r a t e s and a r e functions of t h e surface area, mass t r a n s f e r c o e f f i c i e n t and t h e concentration p o t e n t i a l .

The deposition r a t e s on t h e

graphite i n t h e core and on t h e heat exchanger have been l i s t e d separately. The rest of t h e loop i s lumped i n t o one term.

Deposition on bubbles has

a l s o been l i s t e d as a separate term because of i t s importance.

Each term

w i l l now be evaluated separately and t h e equation w i l l be i n t e g r a t e d over .time.

4.2.1

Generation from Fission The generation rate d i r e c t from f i s s i o n i s simply as follows: Generation from f i s s i o n = yP

4.2.2

(2 1

Generation from Decay of Precursor A rate balance on t h e last soluble precursor before decaying i n t o

a noble metal w i l l be

I n t e g r a t i n g and evaluating at t h e boundary condition cPS =

Ps a t t = 0 0

we get

The generation rate from decay of t h e precursor will be Generation from decay of presursor = X PVCPS

33 4.2.3 l

Decay Rate The decay r a t e of each noble metal isotope i s simply as follows:

Decay r a t e = XVC 4.2.4

--.

(7)

Deposition Rate on Heat Exchanger Written i n t h e framework of mass t r a n s f e r theory, t h e deposition rate

of noble metals on t h e heat exchanger surfaces can be expressed as follows : Deposition rate on heat exchanger =

hhe

(CS

(8)

CSi)

A t t h i s point it w i l l be necessary t o make an assumption concerning C

( t h e concentration of noble metal i n t h e fuel s a l t at t h e f l u i d - s o l i d interface).

W e w i l l assume t h a t a l l s o l i d surfaces involved (Hastelloy N

and g r a p h i t e ) behave as an i n f i n i t e s i n k ; i . e . , i f a noble metal atom migrates through t h e f l u i d boundary l a y e r and contacts t h e surface, it w i l l s t i c k t h e r e forever.

t h i s analysis.

This i s , of course, one of t h e unknowns i n

The r e s u l t of t h i s assumption i s t h a t Csi

effectively

becomes zero, and t h e equation reduces t o he he cs Deposition rate on heat exchanger = h A 4.2.5

(91

Deposition Rate on Graphite By reasoning s i m i l a r t o t h a t above we can a r r i v e at t h e equation f o r

deposition of noble metals on t h e core graphite as follows: Deposition rate on graphite = h

gra

W e have again assumed a s t i c k i n g f r a c t i o n of 1.0.

4.2.6

Deposition Rate on Rest of Fuel Loop Again by reasoning s i m i l a r t o t h a t above, we a r r i v e at t h e equation

f o r t h e deposition of noble metals on t h e rest of t h e f u e l loop ( a l l Hastelloy N). rest of f u e l loop s Deposition r a t e on r e s t of fuel loop = c ( h A ) c (11)

34 4.2.7

Deposition Rate on Liquid-Gas I n t e r f a c e s As noted before, t h e r e are small bubbles c i r c u l a t i n g with t h e f u e l

Their amount i s small but t h e product of t h e i r surface area and

salt.

mass t r a n s f e r coefficient i s higher than t h e same product f o r a l l t h e r e s t Even i f t h e s t i c k i n g f r a c t i o n t o a

of t h e s o l i d loop surfaces combined.

I f we assume

bubble i s low, t h e i r e f f e c t s w i l l s t i l l be q u i t e strong.

t h a t noble metals deposit on liquid-gas i n t e r f a c e s i n accordance with t h e mass t r a n s f e r theory and t h a t t h e s t i c k i n g f r a c t i o n i s 1 . 0 , w e again a r r i v e at a similar equation f o r t h e r a t e of migration t o bubbles. ,, ,

Deposition r a t e on liquid-gas i n t e r f a c e s (bubbles) = hbubAbubCs (12)

Later w e w i l l deduce t h a t t h e e f f e c t i v e s t i c k i n g f r a c t i o n t o bubbles i s less than unity.

4.2.8

Equation f o r Cs Now t h e individual generation rate terms and decay and deposition

I !

II ,

r a t e terms are s u b s t i t u t e d i n t o t h e o r i g i n a l rate balance around t h e f u e l The constant of i n t e g r a t i o n

s a l t ; Eq. (l), and t h e equation i s integrated. i s evaluated a t t h e boundary condition

cS = c0S a t t = o

(13) S

This w i l l y i e l d t h e equation for t h e concentration ( C ) of a noble metal i n t h e f u e l s a l t a t any time ( t ) . This equation i s as follows:

xp cpos - (

- ypP/v)] -xt e AP

where gra gra X = A + h A

hhe V

1 (hA)rest

loop

bub Abub h V

This equation can be c a r r i e d through t h e power h i s t o r y of t h e r e a c t o r . t h e power l e v e l i s changed, then t h e value of C

w i l l be t h e i n i t i a l condition ( C s ) 0

(15)

preceding t h e power change

at t h e new power l e v e l .

Also note t h a t

35 X has u n i t s of time-’, 5

and i s t h e t h e o r e t i c a l r a t e constant f o r migration

of noble metals from t h e f u e l s a l t t o t h e i r sinks.

4.2.9

Noble Metals on Solid Surfaces Now t h a t we have an expression f o r t h e concentration of noble metals

i n fuel salt as a function of time and t h e power h i s t o r y of t h e r e a c t o r , we can compute t h e amount of noble metals deposited on any surface i n t h e

To do t h i s we must again s e t up an unsteady s t a t e r a t e balance

reactor.

f o r t h a t surface as we did f o r t h e f u e l s a l t .

The same assumptions and

considerations hold t r u e here as they did f o r t h e f u e l s a l t .

In addition,

we w i l l assume t h e r e i s no i n t e r a c t i o n between t h e bubbles and s o l i d surfaces; i . e .

, the

only source of noble metals f o r t h e surfaces i s d i r e c t -

The r a t e balance i s as follows where t h e units of each

l y fromthe salt.

t e r m i s atoms/time-unit area.

dCm = Deposition

Rate

Decay Rate

dt “he individual term a r e as follows: Deposition Rate = hm Cs Decay Rate = ACm Subst itu t i n g we h ave

Now, s u b s t i t u t i n g t h e value of Cs determined i n t h e previous s e c t i o n

(Eq. 14), i n t e g r a t i n g over time, and evaluating t h e constant of i n t e g r a t i o n

cm = cm 0

at t =

we g e t

(YP + Y)

Cm =

hm (ApCzs

xvx A

-q

A-X

- ypp’v]

c s 0

e-xt

AP)

+{ct-

- p(yP + y ) vx

- yPP/V)e ( A - AP)

-A%

Avx +

h%(yp

x p q s - ppP/v] x-A

+ - [hm Ac--x

P( P+y)

hm(APCPs 0

- ypP/V)

(x-XP) ( A 2 )

-At

This then i s t h e equation f o r t h e concentration of a noble metal isotope

on a surface at any time as a function of power.

It can be c a r r i e d

through t h e power h i s t o r y of t h e r e a c t o r j u s t as t h e equation for C

The f

equation i s applicable t o any noble metal isotope and t o any s o l i d surface i n t h e r e a c t o r (Heat Exchanger, Graphite i n t h e Core, Core Surveillance Specimens, e t c . ) by proper choice of t h e mass t r a n s f e r c o e f f i c i e n t .

4.2.10

Noble Metals on Liquid-Gas I n t e r f a c e s

The p r i n c i p a l difference between deposition on a s o l i d surface t r e a t e d above and t h e liquid-gas surface t r e a t e d i n t h i s section i s t h e sink

terms once t h e noble metal has reached t h e surface.

I n t h e case of t h e

s o l i d surface, t h e only sink term considered w a s decay.

I n t h e case of a

liquid-gas i n t e r f a c e , migration of noble metals t o t h e off-gas system repres e n t s another sink term t h a t must be considered. L a t e r on i n t h e analysis of results from t h e MSRE w e w i l l want t o cons i d e r t h e e n t i r e gas phase i n t h e f u e l loop as a w e l l mixed pot.

The

e n t i r e gas phase w i l l consist of bubbles c i r c u l a t i n g i n t h e loop, t h e gas phase i n t h e pump bowl and a l s o p a r t of t h e gas phase i n t h e overflow tank. The gas phase i s defined t o include t h e liquid-gas i n t e r f a c e , so t h a t noble

n e t a l s attached t o t h e i n t e r f a c e w i l l be considered as p a r t of t h e gas phase.

The reasons will become more apparent l a t e r on, but f o r now l e t

just say t h a t it w i l l be necessary t o f i n d a process by which noble metals are removed from t h e r e a c t o r system gas phase as defined above with a r a t h e r low rate constant.

For example, noble metals transported t o the

off-gas system or t h e drain tanks would be considered as removed from t h e gas phase as defined above.

A t any r a t e we w i l l again w r i t e an unsteady

s t a t e rate balance around t h e gas phase.

The same assumptions and con-

s i d e r a t i o n s are s t i l l applicable as i n t h e rate balance around t h e f u e l salt.

This time , however ,instead of using concentration u n i t s (atoms/vol)

we w i l l use t o t a l inventory u n i t s (atoms) i n t h e gas phase.

The rate

balance i s as follows:

-d1- at

Migration rate t o bubbles

- migration

decay rate

rate t o off-gas system.

(22)

37 The individual terms a r e as follows. Migration r a t e t o bubbles

- hbub

bub cs A

Decay r a t e = A I

(24)

Migration r a t e t o off-gas system

(25 1

The transfer of noble metals from t h e pump bowl t o t h e off-gas system i s d e a l t with i n a very generaly way.

We simply defined a r a t e constant

( Z ) which says t h a t t h e rate of t r a n s f e r i s proportional t o t h e t o t a l amount of a noble metal isotope.

Now, s u b s t i t u t i n g t h e individual terms

i n t o t h e rate balance, then s u b s t i t u t i n g the value of Cs from t h e f u e l

s a l t analysis (Eq. 1 4 ) , i n t e g r a t i n g over t i m e and evaluating t h e constant of i n t e g r a t i o n at I = Io a t t = O ,

(26)

we get

- hbAbP[yp + ); I vx

A + X

hb Ab (A+ X x)

11C.s

+ X

- P(Y>

- ~ P P / v e- P 1 x - AP ps - yPP/V -xt co x - AP

P Ps

hb Ab

(

A co

AP) Y)

This then i s t h e equation for t h e t o t a l amount of a noble m e t a l isotope i n t h e gas phase of t h e f u e l loop as a function of time and t h e power h i s t o r y of t h e r e a c t o r , and t h e rate constant ( Z ) f o r noble metal

t r a n s f e r t o t h e off-gas system or dump tanks.

(27)

RESULTS FROM THE MSRE Introduction

5.1

The general approach i n t h e analysis of noble metal migration i n t h e

MSRE w i l l be as follows.

F i r s t , we w i l l compare t h e measured t o t h e o r e t i -

c a l concentration of noble metals found on t h e primary heat exchanger by gamma spectrometry.

This measurement technique gave q u a n t i t a t i v e results

f o r t h e g r e a t e s t number of noble metal isotopes.

The measurements were

made i n s i t u , s o t h e r e can be no question of contamination o r problems associated w i t h hot c e l l processing.

Also t h e f l u i d dynamic conditions i n

t h i s conventional U tube h e a t exchanger a r e f a i r l y w e l l known.

This com-

parison then i s q u i t e good and seems t o put t h e a n a l y t i c a l model on a firm foundation.

We w i l l then look at t h e core s u r v e i l l a n c e samples and show

how they generally confirm t h e results from t h e h e a t exchanger. w i l l look a t t h e f u e l s a l t and gas phase samples.

Then we

A f t e r each one of these

comparisons, observations w i l l be made on t h e nature of noble metal migration.

We w i l l then make a r a t h e r crude attempt t o quantify t h e hypothesis

t h a t noble metals accumulate i n t h e pump bowl and determine rate constants f o r removal of noble metals from t h e pump bowl t o t h e off-gas system t o s e e i f they are physically reasonable.

As noted previously, most of t h e

analysis w i l l be f o r measurements made during t h e 233U

operation (runs 15-

2 0 ) , although some data are a v a i l a b l e f o r t h e 235U operation (run 1-14). 5.2

Comparison of Measured Deposition on Heat Exchanger t o Theoretical The t h e o r e t i c a l amount of each noble m e t a l isotope deposited on t h e -

surface o f t h e heat exchanger tubes w a s computed with Eq. 21, and compared t o t h e measured amount as determined by gamma spectrometry.

The measured

values following Run 1 4 were obtained from Ref. 10 and a f t e r runs 18 and

19 from R e f . 11. Recall t h a t t h e equipment and techniques used t o determine f i s s i o n product deposition a f t e r run 14 were less s o p h i s t i c a t e d than used after runs 18 and 19, t h e r e f o r e , t h e d a t a following run 14 are

less c e r t a i n than t h e newer data. t h e o r e t i c a l amount (e.g.

, mass

The parameters used t o c a l c u l a t e t h e

t r a n s f e r c o e f f i c i e n t s , isotope p a r m e t e r s ,

e t c . ) a r e evaluated and t a b u l a t e d i n Appendix B.

The computed concentra-

t i o n on t h e heat exchanger takes i n t o account t h e e n t i r e power h i s t o r y of the reactor.

The only sink term f o r noble metals attached t o t h e heat

It should be noted t h a t p e r i o d i c a l l y t h e f u e l loop

exchanger i s decay. c

i s cleaned out by c i r c u l a t i n g f l u s h s a l t f o r a s h o r t period of t i m e .

The

computed concentration assumes t h a t t h i s process does not leach noble 3

metals from t h e surfaces.

Some of t h e gamma s p e c t r a following run 14 were

made with and without f l u s h s a l t i n t h e loop and they i n d i c a t e t h i s assumpt i o n t o be t r u e .

The t h e o r e t i c a l amounts of noble metals on t h e heat

exchanger were computed on t h e basis t h a t they do not adhere t o liquid-gas interfaces.

Then, by comparison of t h e computed amounts w i t h t h e measured

amounts, conclusions of a q u a l i t a t i v e nature w i l l be drawn t o t h e e f f e c t t h a t noble metals apparently adhere t o liquid-gas i n t e r f a c e s . The results of t h i s comparison a r e shown i n Figure 5 . l w h e r e t h e r a t i o of measured t o computed amounts on t h e h e a t exchanger i s p l o t t e d against t h e noble metal h a l f l i f e . 23%J

operation.

F i r s t l e t us look at t h e curve measured during

These measurements were made with t h e improved gamma

spectrometry equipment and a r e judged t o be t h e b e s t .

The following

observations can be made. 1. The curve i s made up from 10 isotopes; 3

telluriums, 2

rutheniums,

antimonies, 1 - molybdenum and 1 - niobium, some of

which are duplicated measurements.

They seem t o be rather t i g h t l y grouped

around t h e l i n e , except perhaps 132Te and lo3Ru.

One would conclude

t h e r e f o r e t h a t each noble metal isotope migrates as a function of i t s

own concentration i n s a l t and i s not influenced by other elemental species of noble metals o r even i s o t o p i c species of t h e same element. 2.

Note t h a t t h e curve i s a s t r a i g h t l i n e and has a slope very close

t o zero, i . e . , noble metal migration i s not an unaccounted-for function of i t s own half l i f e .

It i s important t o note at t h i s t i m e , because

l a t e r we w i l l see t h a t t h i s i s not t r u e f o r t h e f u e l s a l t and gas phase samples.

~ 3 . Because t h e slope of t h e l i n e i s almost zero and because of t h e t i g h t grouping of d a t a around t h i s l i n e , some credence must be given t o t h e hypothesis t h a t noble metals migrate according t o t h e simplest form of mass t r a n s f e r theory.

The good c o r r e l a t i o n a l s o speaks w e l l f o r t h e

q u a l i t y of t h e gamma spectrometry data.

Something i s s t i l l missing,

however, because t h e measured t o t h e o r e t i c a l concentration r a t i o i s considerably less than u n i t f o r t h e 23% run determinations, and t h i s w i l l be discussed s h o r t l y .

hri b

ORNL-DWG 70-15021

5.0

. 0.02 0.01

102

103

104

NOBLE METAL HALF L I F E ( h r )

FIGURE 5.1.

COMPARISON OF THE MEASURED TO THEORETICAL AMOUNTS

OF NOBLE METALS ON THE PRIMARY HEAT EXCHANGER

105

b.,

Note t h e good agreement between 95Nb and t h e other noble metals.

It w a s pointed out previously t h a t 95Nb sometimes behaves as a s a l t seeking f i s s i o n product and sometimes as a noble metal, depending on t h e oxidation-

reduction state of t h e f u e l s a l t .

The f u e l w a s i n a reduced s t a t e at t h e

end of runs 18 and 19 s o t h i s 95Nb behavior i s as expected. Now consider t h e d a t a measured during t h e 235U operation. The same observatiops and conclusions can, i n general, be made f o r t h i s curve, b u t on a l e s s e r s c a l e because t h e r e a r e fewer isotopes involved.

The magnitude

of t h e measured t o calculated r a t i o i s d i f f e r e n t and i s i n t h e v i c i n i t y of 1.0.

Again note t h e good agreement between 95Nb and t h e other noble metals.

The f u e l s a l t w a s generally i n a more reduced s t a t e during t h e 235U oper-

a t i o n and Nb behaved as a noble metal. Now

- why

i s t h e magnitude of t h e measured t o t h e o r e t i c a l concentra-

t i o n r a t i o on t h e tube surface close t o unity when measured a f t e r run

( l a s t 235U r u n ) and 0.15 0.20 when measured during runs 18 and 19 233U runs)? A s noted i n section 3.3, t h e r e was a dramatic difference i n ( operational c h a r a c t e r i s t i c s of t h e MSRE when operating w i t h 235U and 233u

The difference was most apparent i n t h e amount of bubbles c i r c u l a t i n g w i t h the fuel salt.

. i

A s noted e a r l i e r t h e void f r a c t i o n of bubbles during t h e

235U runs w a s 0.02

- 0.045%,

while during t h e 233U runs, it was 0.5 t o

0.6%, up by a f a c t o r of about 20.

The t h e o r e t i c a l amount i n t h e denomina-

t o r of t h e ordinate of Figure 5 . 1 i s computed assuming t h a t noble metals do not adhere t o liquid-gas i n t e r f a c e s ( c i r c u l a t i n g bubbles).

It has been

hypothesized t h a t noble metals do deposit on t h e s e i n t e r f a c e s , so t h e bubbles w i l l compete with t h e h e a t exchanger as a noble metal depository. If t h i s concept i s included i n t h e c a l c u l a t i o n , then t h e computed amount

on t h e heat exchanger w i l l diminish and t h e value of t h e ordinate w i l l increase.

It i s a l s o assumed t h a t t h e r e i s no i n t e r a c t i o n between t h e

bubbles and t h e heat exchanger s u r f a c e s , s o i f a noble metal atom migrates

. . hB

t o a bubble, it i s no longer available t o deposit on t h e heat exchanger. 233, Since t h e f u e l s a l t contained many times more bubbles during t h e '

runs than t h e 235U runs

, including

bubbles i n t h e c a l c u l a t i o n ,

t h e 23% curve w i l l move up much more than t h e 235U curve.

Qualitatively

then t h i s w i l l explain t h e difference i n elevation of t h e two curves. Q u a n t i t a t i v e l y , it i s a b i t more d i f f i c u l t , because using t h e b e s t

estimated values of bubble surface a r e a and mass t r a n s f e r c o e f f i c i e n t ( s e e Appendix B ) , both curves a r e pushed upward p a r a l l e l t o themselves t o

a value considerably g r e a t e r than unity.

If t h e empirical b u t nevertheless

mechanistic "sticking fraction" i s brought i n t o t h e c a l c u l a t i o n , we can again b r i n g both curves down t o a value very close t o unity.

The s t i c k i n g

f r a c t i o n i s defined as t h a t f r a c t i o n of atoms t h a t contact t h e i n t e r f a c e and adhere t o it.

If t h e s t i c k i n g f r a c t i o n of noble metals t o s o l i d

surfaces i s maintained a t u n i t y , then a s t i c k i n g f r a c t i o n t o liquid-gas i n t e r f a c e s of 0 . 1

0.2 i s required t o b r i n g t h e value at t h e ordinate

of both curves close t o unity. A s t i c k i n g f r a c t i o n of noble metals t o a liquid-gas i n t e r f a c e con-

siderably less than 1.0 w a s not expected.

For i n s t a n c e ( 2 2 ) , noble metals

w e r e found i n s i g n i f i c a n t q u a n t i t i e s i n helium passed over t h e surface of

quiescent molten f u e l s a l t samples i n a hot c e l l experiment.

The vapor

pressure of noble metals i s diminishingly s m a l l a t t h e s e temperatures and cannot possibly account f o r t h e concentrations observed.

The indivi-

dual noble metal atoms or very s m a l l c l u s t e r s of them, a r e apparently spontaneously expelled from t h e surface.

Considerations of t h e i n t e r f a c i a l

energies involved i n d i c a t e t h i s t o be possible. (23) Therefore , one would expect a s t i c k i n g f r a c t i o n t o bubbles of unity. t h i s apparent paradox i s as follows.

A r a t i o n a l i z a t i o n of

An observation from t h e r e a c t o r i s

t h a t many of t h e smaller bubbles c i r c u l a t i n g with t h e f u e l s a l t completely dissolve i n t h e higher pressure p a r t of t h e f u e l loop. ( 2 4 2 5 ) observation i s a r e s u l t of an analysis t o explain t h e 135Xe e f f e c t s observed i n t h e reactor.

Note t h a t t h e surface tension of molten

s a l t i s q u i t e high, about 200 dynes/cm.

When a bubble i s pressurized by

t h e pump and begins t o dissolve, i t s diameter decreases. pressure above ambient

, as

poisoning

The i n t e r n a l

generated by t h e surface tension (4a/d) becomes

q u i t e high (0.8 p s i f o r 0.005 i n . bubble) and f u r t h e r enhances t h e dissolution rate.

This continues t o t h e l i m i t and t h e bubble dissolves.

The process i s very r a p i d 'and analysis i n d i c a t e s only a few seconds are required.

If t h i s bubble contained some noble metals before it dissolved,

we would end up with a noble metal c l u s t e r associated with t h e s a l t again, r a t h e r than a bubble.

Qualitatively t h e n , here i s a mechanism where t h e

I , -

t h e s t i c k i n g f r a c t i o n of noble metal atoms t o a liquid-gas i n t e r f a c e can be unity but where it a c t u a l l y appears t o be l e s s than unity. Possibly other mechanisms could a l s o be devised.

Conclusions.

The conclusion from examination of noble metal deposi-

t i o n on t h e primary heat exchanger as determined by gamma spectrometry, i s t h a t noble metals apparently do migrate and deposit on these surfaces i n

accordance with t h e l a w s of mass t r a n s f e r i n t h e simplest form.

Deposi-

t i o n on liquid-gas i n t e r f a c e s (bubbles) m u s t be included i n t h e c a l c u l a t i o n t o force t h e calculated concentration t o equal t h e measured concentration on t h e tubes.

The exact mechanism of noble metal deposition on l i q u i d -

gas i n t e r f a c e s i s not known and had t o be handled i n a semiquantitative Essentially these

way involving apparent s t i c k i n g f r a c t i o n s t o bubbles,

same conclusions w i l l be reached a f t e r each analysis section i n t h i s report.

5.3

Comparison of Measured Deposition on Core Surveillance Samples t o Theoretical The t h e o r e t i c a l amount of each noble metal isotope deposited on

t h e surface of t h e graphite and t h e Hastellpy-N core surveillance samples

was computed and compared t o t h e measured amount.

The measured values were

obtained from semiannual r e p o r t s . (26s 279 28 and 29)

m e Same introduce

t o r y remarks a r e applicable here as i n t h e f i r s t paragraph of t h e l a s t section.

Most important, r e c a l l t h a t t h e t h e o r e t i c a l amount w i l l be com-

puted on t h e b a s i s t h a t noble metals do not deposit on liquid-gas i n t e r faces.

The mass t r a n s f e r c o e f f i c i e n t was estimated i n Appendix B t o be

about 0.25 f t / h r f o r both t h e graphite and Hastelloy-N.

The uncertainty

i n t h i s n m b e r i s considerable. The r e s u l t s of t h i s comparison are shown i n Figure 5.2 for t h e graphite specimens and Figure 5.3 f o r t h e Hastelloy-N specimens.

Again,

t h e r a t i o of t h e measured t o t h e o r e t i c a l amount i s p l o t t e d against t h e noble metal h a l f l i f e .

I n drawing l i n e s through t h e d a t a , no weight

was given t o t h e 95Nb p o i n t s f o r reasons t o be discussed l a t e r . following observations can be made:

The

ORNL-DWG 70-15015 IU

5 SAMPLES REMOVE0 FROM CORE AFTER 2

RUN RUN RUN RUN RUN

1 .o

7 11 14 18 18

INTEGRATED EXPOSURE

FUEL -

7,800 Mw 24,000 Mw 32,000 Mw 20,000 Mw 76,000 Mw

hrs hrs hrs

233U 235U AND 233U

102

hrs

235U

hrs

235U 235U

0.5

0.2

0.1 0.05

0.02

0.01

103

NOBLE METAL HALF LIFE ( h r )

FIGURE 5.2.

COMPARISON OF THE MEASURED TO THEORETICAL AMOUNTS

OF NOBLE METALS ON THE GRAPHITE CORE SURVEILLANCE SPECIMENS

104

ORNL-DWG 70-15016 10

5 k

1 .o 0.5

0.2 0.1

0.05

0.02

0.01

102

103

NOBLE METAL HALF L I F E (hr) .

FIGURE 5.3..

COMPARISON OF THE MEASURED TO THEORETICAL AMOUNTS

OF NOBLE METALS ON THE HASTELLOY-N CORE SURVE I LLANCE SPEC1 MENS

(DOSIMETER TUBES)

104

46 P

1. Three d i f f e r e n t sets of graphite samples (Figure 5.2) were exposed t o f u e l s a l t during only t h e 235U operations.

All t h r e e are con-

s i s t e n t with each other and t h e d a t a points a r e r a t h e r t i g h t l y grouped around a s i n g l e l i n e .

An exception i s 95Nb which l i e s w e l l above t h e

Recall t h a t t h e f u e l s a l t w a s i n a reduced s t a t e during t h e 2 35,

line.

operations and 95Rb behaved as a noble metal and would t h e r e f o r e be expected t o f a l l on t h e l i n e . 2.

More w i l l be s a i d about t h i s below.

One s e t of graphite samples was exposed t o f u e l s a l t during only

233U operations. -

This curve f a l l s below and p a r a l l e l t o t h e previous This i s consistent with observations of noble

curve f o r 235U operation.

metals i n t h e heat exchanger i n t h e l a s t section and t h e same conclusions can be drawn.

The curves for graphite m a y have a s i g n i f i c a n t slope at t h e low

h a l f l i f e end which i s not consistent with t h e gamma spectrometry d a t a from t h e heat exchanger.

The slope seems t o be zero however f o r h a l f

l i v e s over about 500 hours.

This w i l l be discussed f u r t h e r below.

The r e l a t i v e elevation of t h e curves through t h e deposition data

on t h e Hastelloy-N surveillance samples (Figure 5.3) do not confirm t h e previous observations from t h e h e a t exchanger and graphite.

There i s

however more s c a t t e r i n t h e data.

Concerning t h e absolute value of t h e measured t o t h e o r e t i c a l r a t i o

of t h e noble metal concentration.

An o v e r a l l average of t h e 23%J

line for

Hastelloy-N (Figure 5.3) seems t o agree with t h e previous value from t h e heat exchanger, however, t h e d a t a from t h e 235U runs do not agree.

The measured t o t h e o r e t i c a l concentration r a t i o f o r a l l t h e

graphite samples seems t o f a l l somewhat below t h e data from t h e heat exchanger and Hastelloy-N samples.

There a r e two explanations f o r t h i s .

F i r s t , inadequate knowledge of t h e m a s s t r a n s f e r c o e f f i c i e n t , although t h i s probably i s n ' t enough t o account f o r t h e e n t i r e discrepancy.

Second,

t h e more p l a u s i b l e explanation i s t h a t t h e s t i c k i n g f r a c t i o n f o r graphite i s less than unity.

More w i l l be s a i d about t h i s below.

Let us consider some of t h e above discrepancies with graphite and see i f we can b r i n g them inâ&#x201A;Źo l i n e with previous observations and conclusions from t h e heat exchanger analysis.

F i r s t l e t us hypothesize t h a t t h e

noble metal s t i c k i n g f r a c t i o n t o graphite i s l e s s than unity.

We have

47 noted t h a t t h i s i s l i k e l y i n observation number 6 above.

This w i l l be con-

firmed more d i r e c t l y i n Section 5.7 when w e look at t h e r e s u l t s of a s p e c i a l laminar flow core surveillance t e s t during t h e l a s t MSRE operationNow, t h e r e i s evidence (30) t h a t Nb forms a s t a b l e carbide w i t h

a1 period.

graphite under MSRE operating conditions. as an i n f i n i t e sink f o r Nb, i . e . ,

The graphite could t h e r e f o r e a c t

i t s s t i c k i n g f r a c t i o n could be unity.

An e f f e c t i v e s t i c k i n g f r a c t i o n of 95Nb t o graphite of u n i t y , and a s t i c k i n g f r a c t i o n of t h e other noble metals of l e s s than u n i t y , would explain t h e 95Nb p o i n t s being s o much higher than t h e other noble metals i n Figure

5.2.

In f a c t , t h e r a t i o between t h e curves and t h e 'g5Nb data c l u s t e r could be a d i r e c t measure of t h e s t i c k i n g f r a c t i o n of t h e other noble metals r e l a t i v e W e can carry t h i s discussion one s t e p f u r t h e r .

t o 95Nb.

2.4 min) i s a precursor of "Mo.

If t h e n , amounts

migrate t o t h e graphite w i t h a s t i c k i n g f r a c t i o n of unity and decay t h e measured amounts o f "Mo

i n t o "Mo,

than t h e o t h e r noble metals. .

(half l i f e =

I t s h a l f l i f e i s high enough so t h a t it

i s a s i g n i f i c a n t migrating species i n t h i s decay chain. of "Nb

"N%

would a l s o be r e l a t i v e l y higher

Therefore, t h e l e f t s i d e of t h e curves i n

Figure 5.2, which are controlled t o an extent by "Mo,

might t e n d t o be

Another e f f e c t t h a t m u s t be considered

higher than t h e other noble metals.

i s t h a t during t h e f i n a l week of run 18, t h e f u e l pump was operated a t a s l i g h t l y reduced speed.

The p r i n c i p a l e f f e c t w a s t h a t during t h i s period

t h e c i r c u l a t i n g void f r a c t i o n w a s more equivalent t o t h e 235U t h e 233U

runs.

runs than

Since t h e r e were fewer bubbles, one m i g h t expect t h a t

during t h i s period t h e nobie metal deposition rate on s o l i d surfaces would be g r e a t e r ,

Since t h e time period was r e l a t i v e l y s h o r t , it would e l e v a t e

only t h e low h a l f l i f e end of t h e curve.

This l a s t point may a l s o h e l p

explain why t h e s h o r t h a l f l i f e end of t h e 233U

curve f o r deposition on

Hastelloy-N (Figure 5.3) i s higher than expected. Conclusion.

The conclusions t h e n , from examination of noble metal

deposition d a t a from t h e core surveillance samples, i s t h a t they generally T

confirm t h e conclusions a r r i v e d a t a f t e r examination of t h e heat exchanger i n Section 5.2.

It appears t h a t t h e sticking; f r a c t i o n of noble metals t o

graphite i s less than unity. t h e behavior of Nb.

We have had t o form a hypothesis t o explain

48 5.4

Comparison of Fuel S a l t Samples with Cs

The t h e o r e t i c a l amount of each noble metal isotope contained i n t h e f u e l s a l t (Eq. ( 1 4 ) i n Section 4.2) w a s computed and compared t o t h e measured amount i n t h e f u e l s a l t samples.

Again t h e t h e o r e t i c a l amount

w a s computed on t h e basis t h a t noble metals do not deposit on l i q u i d gas i n t e r f a c e s .

The measured and computed concentration d i f f e r e d by

1-4 orders of magnitude , t h e measured concentration always being higher. I n an attempt t o resolve t h i s discrepancy , t h e measured-to-theoretical concentration r a t i o w a s p l o t t e d against t h e f r a c t i o n of i t s t o t a l capacity t h a t each sample capsule w a s f i l l e d .

These p l o t s seem t o be unique and

are shown i n Figures 5.4 through 5.9 f o r Io3Ru, '06Ru, 132Te and 95Nb,

respectively.

"MO,

12g%e,

The kind of sample,freeze valve o r double-

w a l l freeze v a l v e , i s distinguished on t h e p l o t s .

Recall t h a t a freeze

valve capsule contains 50-60 grams of s a l t then f u l l and a double w a l l freeze valve capsule holds about 1 5 grams of s a l t when f u l l .

Ladle sample

data a r e not included on t h e s e p l o t s f o r reasons pointed out i n Section

3.2.

A l l reported freeze valve capsule d a t a are on t h e s e p l o t s except

those taken a t zero power.

It i s a c h a r a c t e r i s t i c of t h e a n a l y t i c a l

model t h a t t h e noble metal concentration i n s a l t goes t o zero s h o r t l y after t h e r e a c t o r power goes t o zero.

The measured t o t h e o r e t i c a l con-

centration r a t i o would therefore approach i n f i n i t y i f any noble metals at a l l were measured i n t h e sample.

The following observations can be made

about t h e s e curves.

1. All curves, except t h a t f o r g5Nb, behave i n a similar manner. For samples t h a t were mostly empty, t h e measured t o t h e o r e t i c a l concentrat i o n r a t i o i s orders of magnitude higher than f o r those samples t h a t were mostly f u l l . 2.

95Nb i s being c a r r i e d along f o r comparison, s i n c e it can behave

e i t h e r as a noble metal o r a s a l t seeking f i s s i o n product, depending on Many of t h e reported 95Nb

t h e oxidation-reduction state of t h e s a l t . concentrations i n f u e l s a l t a r e negative.

This i s because t h e reported

95Nb concentration w a s extrapolated back from t h e t i m e of measurement t o

t h e time of sampling.

9 5 Z r with a 65-day h a l f l i f e i s a precursor of

g5Nb, and t h e r e f o r e it m u s t a l s o be counted. ,

The back c a l c u l a t i o n i n time

i s s e n s i t i v e and a s m a l l e r r o r i n t h e 9 5 Z r concentration measurement

ORN L-DWG 71-1876A

104 L

= CONCENTRATION COMPUTED FOR CASE WHERE NOBLE - THEORETICAL METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE a

17-31

FREEZE VALVE CAPSULE

17-32

DOUBLE-WALL FREEZE VALVE CAPSULE

%AMPLE NUMBER TAKEN DURING THIS RUN NUMBER

103

102

101

100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY

F I GURE 5 .'4.

COMPAR I SON OF THE MEASURED

03RU CONCENTRAT I ON

I N FUEL SALT TO THE THEORETICAL CONCENTRATION

, 50

l i ORNL-DWG 71-1877A 105

I 5

THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE

0 2

17-31

O 17-32

FREEZE VALVE CAPSULE DOUBLE-WALL FREEZE VALVE CAPSULE SAMPLE NUMBER TAKEN DURING THIS RUN NUMBER

I I

104

103

102

101

-5 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FUELSALT I N CAPSULE AS FRACTION OF OTAL CAPSULE CAPACITY

FIGURE 5.5.

COMPARISON OF MEASURED

06RU CONCENTRAT I ON

I N FUEL SALT TO THE THEORETICAL CONCENTRAT I ON

ORNL-DWG 7 1 -1878A 104

THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE

FREEZE VALVE CAPSULE

17-31 17-32

DOUBLE-WALL FREEZE VALVE CAPSULE SAMPLE NUMBER TAKEN DURING T H I S RUN NUMBER

103

102

1 .o 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FUEL SALT I N CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY

F I GURE 5.6.

COMPAR I SON OF THE MEASURED "MO

CONCENTRAT I ON

I N FUEL SALT TO THE THEORETICAL CONCENTRATION f

1 -

52 ORNL-DWG 71 -1879A

2 '

LJ "

THEORECTICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE

51F

104

0 17-31 O 17-32

e 18-6

FREEZE VALVE CAPSULE DOUBLE-WALL FREEZE VALVE CAPSULE

LSAMPLE NUMBER TAKEN DURING

0 c (

5z IW

0 V

. F I GURE 5.7.

COMPAR I SON OF THE MEASURED I *'?E

CON.CENTRATI ON

I N FUEL SALT TO THE THEORETICAL CONCENTRATION

6.I

ORNL-DWG 71 -1880A 2

THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE e 17-31 FREEZE VALVE CAPSULE 0 17-32 DOUBLE-WALL FREEZE VALVE CAPSULE ISAMPLE NUMBER TAKEN DURING

104

1o3 5

102

2 10

2 1 .o 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

U.LJ

0.9

1.0

FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY

F I GURE 5.8.

COMPARI SON OF THE MEASURED I 32TE CONCENTRATI ON

I N FUEL SALT TO THE THEORETICAL CONCENTRATION

54 I

ORNL-DWG 71 -1 882A

104 5

2 103

5 THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE 2

O 17-31

FREEZE VALVE CAPSULE

O 17-32

DOUSLE-WALL FREEZE VALVE CAPSULE SAMPLE NUMBER TAKEN DURING T H I S RUN NUMBER

0 5

1.0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FUEL SALT I N CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY F I GURE 5.9.

COMPARI SON OF THE MEASURED 95NB CONCENTRAT I ON

I N FUEL SALT TO THE THEORETICAL CONCENTRATION

can r e s u l t i n a negative 95Nb concentration.

The shotgun p a t t e r n and

a l s o t h e disappearance and reappearance of 95Nb i n Figure 5.9 i s apparently r e l a t e d i n some way t o t h e fuel s a l t f l u c t u a t i n g between t h e oxidized and reduced state. 3.

The sample capsules showing t h e highest r a t i o of measured t o

t h e o r e t i c a l concentration a r e mostly empty.

The c i r c l e d c l u s t e r of

samples labeled as Sample Grouping No. 1 are representative of t h i s end of t h e curve.

They are a l l double-wall freeze valve samples.

They con-

t a i n 2-5 grams of s a l t b u t i f they were f u l l they would hold 14-15 grams.

Conversely, t h e samples showing t h e lowest measured t o t h e o r e t i c a l

concentration r a t i o are f u l l .

The c i r c l e d c l u s t e r labeled Sample Grouping

No. 2 i s used t o represent t h i s end of t h e curve.

This group represents

t h e only freeze valve sample taken during t h e 235U runs.

In t a k i n g

samples, these capsules were f i l l e d from 75% t o 90% of capacity.

The samples are i n t e r n a l l y consistent; i . e . , t h e concentration

of each noble metal isotopes i s p r a c t i c a l l y always e i t h e r high or low i n any individual sample.

This i s i l l u s t r a t e d by t h e c i r c l e d c l u s t e r s of

data p o i n t s r e f e r r e d t o as Sample Grouping No. 1 and Sample Grouping No.

Each grouping on each f i g u r e encompasses t h e same samples.

how c o n s i s t e n t l y t i g h t each grouping is.

Notice

Sample Grouping No. 1 repre-

s e n t s t h e high and of t h e curve and Sample Grouping No. 2 represents t h e lower end of t h e curve.

One might make t h e observatioq t h a t t h e curves seem t o be

approaching a low value of t h e measured t o t h e o r e t i c a l concentration r a t i o

at high sample weights.

P a r t i c u l a r l y t h e s h o r t e r l i v e d isotopes

99 ( Mo and 132Te) almost seem t o be approaching a value of u n i t y (measured concentration = t h e o r e t i c a l concentration). This observation i s not warranted.

Recall t h a t t h e t h e o r e t i c a l concentration i s computed f o r

t h e case t h a t noble metals do not deposit on liquid-gas i n t e r f a c e s . We have hypothesized i n previous s e c t i o n s and l a t e r i n t h i s s e c t i o n t h a t

they must deposlt on t h e s e i n t e r f a c e s .

If t h i s depository i s included

i n t h e c a l c u l a t i o n then t h e computed concentration w i l l be reduced and t h e measured t o t h e o r e t i c a l concentration r a t i o w i l l be increased E

accordingly.

The u n c e r t a i n t i e s associated with t h i s term a r e s o g r e a t

( p a r t i c u l a r l y t h e s t i c k i n g f r a c t i o n ) t h a t w e cannot go i n t o any d e t a i l e d

Let us only say t h a t i f we did include t h i s term i n

analysis of it.

t h e c a l c u l a t i o n , then t h e d a t a points i n Figures 5.4

5.8 could r i s e more

o r less uniformly by as much as a f a c t o r of 5 during t h e 235U operation

( s t i c k f r a c t i o n t o bubbles = 1.0) and a good d e a l more for t h e 233u operation. How can w e explain these curves i n a manner consistent with previous observations and conclusions on noble metal migration?

We have already

hypothesized t h a t noble metals deposit on liquid-gas i n t e r f a c e s (circulat i n g bubbles), and d a t a from t h e heat exchanger and core surveillance samples seem t o confirm t h i s .

Many q u a l i t a t i v e observations discussed

i n s e c t i o n 3 a l s o confirm t h i s hypothesis.

Presumably t h e bubbles would

carry t h e noble metal atoms or s m a l l c l u s t e r s of them t o t h e pump where they would c o l l e c t on t h e l a r g e amount of liquid-gas i n t e r f a c e a v a i l a b l e i n t h i s region.

The small c l u s t e r s could then coalesce i n t o l a r g e r

c l u s t e r s and eventually form a scum f l o a t i n g on t h e s a l t .

L e t us hypothe-

s i z e t h a t t h e s e noble metal c l u s t e r s have p r o p e r t i e s similar t o insoluble surface a c t i v e agents and t h a t t h e pump bowl i s behaving i n a manner

similar t o a f r o t h f l o t a t i o n chamber. before. i31â&#x20AC;&#x2122;

This suggestion has been made

It has beensuggested i n Section 3.3 of t h i s report t h a t

32)

noble corrosion products and f i s s i o n products accumulating i n t h e pump bowl and displaying t h e p r o p e r t i e s of insoluble surface a c t i v e agents

are responsible f o r t h e differences between t h e 235U c h a r a c t e r i s t i c s of t h e E R E .

and 233U

operating

A s a matter of f a c t , f l o t a t i o n of reduced

Fe, N i and C r from f u e l s a l t has been demonstrated i n a t e s t tube s c a l e laboratory experiment. (36) Froth f l o t a t i o n i s a common u n i t operation i n t h e ore processing i n d u s t r i e s .

The fundamental requirements f o r a

f l o t a t i o n process t o work are: (33)

The s o l i d phase must be insoluble i n t h e l i q u i d phase.

The gas-liquid-solid

contact angle m u s t be g r e a t e r than zero

( t h e s o l i d only p a r t i a l l y wet by t h e l i q u i d ) .

3. The s o l i d p a r t i c l e s must be small enough s o t h a t t h e i r sheer mass w i l l not sink them below t h e surface ( g r a v i t y forces < surface tension forces )

â&#x20AC;&#x2122;

57 j

The noble metals (and noble corrosion products from t h e chemical reprocessing) s a t i s f y t h e s e conditions.

They a r e insoluble i n fuel s a l t .

Contact angles are reported i n t h e range of 90-105O.(17) They a r e mostly unwet and i n a range f o r good f l o t a t i o n .

P a r t i c l e diameters of materials

obtained from t h e gas phase over s a l t samples have been estimated by various techniques.

These diameters, thought t o be i n d i c a t i v e of noble metal

p a r t i c l e diameters, range from a few angstroms t o a f e w microns.

(34,35,36) )

The lower end of t h e range possibly represents t h e diameter of individual p a r t i c l e s where t h e upper end i s associated with flocks of p a r t i c l e s .

The

uncertainty of e f f e c t i v e p a r t i c l e diameter i s unimportant since t h e e n t i r e

As noted i n Section 3.3, t h e

range f a l l s i n a region of good f l o t a t i o n .

property of s u r f a c t a n t s t h a t i s s i g n i f i c a n t t o t h i s analysis i s t h a t they enhance bubble s t a b i l i t y and a l s o r e s u l t i n smaller bubbles being generated i n t h e pump bowl.

It would not seem unreasonable t h e n , t h a t small bubbles

a r e present i n t h e s a l t sampling pool e i t h e r as a f a i r l y s t a b l e f r o t h f l o a t i n g on t h e s a l t o r as very s m a l l bubbles d r i f t i n g w i t h t h e s a l t o r both.

These bubbles would be high i n noble metal content (on t h e i r s u r f a c e ) .

When a freeze valve capsule thaws and draws i t s sample, it would be g e t t i n g

a g r e a t e r o r l e s s e r amount of gas with t h e s a l t , depending on t h e s i t u a t i o n i n t h e s a l t pool at t h a t time.

I f it took i n a l a r g e amount of gas , t h e

capsule would be mostly empty but have a high noble metal content (as f o r instance Sample Grouping No. 1 on Figures 5.4

5.8). If it took i n a s m a l l amount of gas, it would be f u l l and r e l a t i v e l y low i n noble metal content (as Sample Grouping No. 2 on t h e same figures),.\ This then i s t h e suggested 548. Note t h a t i f t h i s hypotheexplanation f o r &he curves i n Figures 5.4

sis i s t r u e , then even t h e least contaminated samples , represented by Sample Grouping No. 2 , are s t i l l not representative of t h e fuel s a l t . One might expect t o be able t o c o r r e l a t e t h e condition of t h e s a l t sample (high o r low noble metal concentration) with some other measurable operating v a r i a b l e s of t h e r e a c t o r , f o r instance, overflow rate, c i r c u l a t i n g void f r a c t i o n , e t c .

An attempt w a s made t o do t h i s , however, t h e

MSRE being an experimental r e a c t o r , w a s subjected t o a great many per-

t u r b a t i o n s (power l e v e l changes, reductant and oxidant additions, pump speed changes, draining and flushing t h e f u e l loop, cover gas changes, e t c ) ,

most of which would be expected t o e f f e c t t h e condition of t h e sample, and w e were unable t o e x t r a c t any obvious c o r r e l a t i o n s .

58 The i n t e r a l consistency of t h e samples allows us t o carry t h e analysis one s t e p f u r t h e r .

As noted i n Observation No. 5 , t h e concentrations of a l l

noble metal isotopes a r e usually consistent i n an i n d i v i d u a l sample ( e i t h e r high o r low). Figures 5.4

This i s i l l u s t r a t e d by Sample Grouping No. 1 and No. 2 i n

5.8.

Each grouping on each f i g u r e contains t h e same samples,

and each grouping i s r a t h e r t i g h t .

The geometric m e a n of each group w a s

determined and p l o t t e d against t h e noble metal h a l f l i f e .

The r e s u l t i n g

c o r r e l a t i o n i s shown i n Figure 5.10 and represents a p l o t similar t o t h a t used i n analysis of noble metals on t h e h e a t exchanger and core s u r v e i l Recall t h a t t h e equivalent p l o t s f o r t h e heat exchanger and

lance samples.

core surveillance sample yielded a l i n e with a slope of zero, whereas t h e slopes of t h e l i n e s i n Figure 5.10 are q u i t e high.

If t h e a n a l y t i c a l model

i s correct and t h e samples a r e t r u l y representative of t h e f u e l s a l t , then

t h e slope of t h i s l i n e should a l s o be zero.

We have shown however t h a t t h e

s a l t samples a r e not representative of t h e s a l t , but r a t h e r are more representative of t h e noble metals accumulated on t h e liquid-gas i n t e r f a c e s i n t h e pump bowl.

It i s not s u r p r i s i n g then, t h a t t h e slope i s not zero.

I n Section 5.6 of t h i s report we w i l l a t t a c h some physical s i g n i f i c a n c e t o t h e slope of t h e s e l i n e s . Figure 5.11 i s t h e same kind of p l o t as Figure 5.10, but i n s t e a d of picking grouped data p o i n t s , we averaged geometrically a l l t h e freeze valve and double walled freeze valve capsule data f o r each MSRE run.

Beside each

data point i n parenthesis i s t h e number of samples included i n t h e average.

Note t h a t t h e curves move up t h e ordinate with run number.

This observation

w i l l be made more dramatic when we compare it t o t h e same kind of p l o t f o r

t h e gas phase samples. with run number. Conclusion.

The gas samples do t h e reverse, t h a t i s , decrease

We w i l l discuss t h i s observation at t h a t t i m e .

The emphasis i n t h i s section from examination of f u e l

s a l t samples , concerns t h e attachment of noble metals and reduced corrosion products t o liquid-gas i n t e r f a c e s , and how they apparently accumulate i n t h e MSRE pump bowl and enhance t h e f r o t h i n g and bubble making capacity of i

t h i s region.

This i s consistent with conclusions reached from examination

of data from t h e primary h e a t exchanger and core surveillance samples.

ORNL-OWG 73-1549 105 I

5 OATA'POINTS REPRESENT GEOMETRIC AVERAGE OF CIRCLED GROUPS ON FIGURES 5.4 - 5.8 2

104 0 SAMPLE GROUPING NO. 2

2 103

2 102

r-y I

I I I I I\

fllll

I I

lllllII

llllllI

2 / !

1 5

104

103

102

: &i

NOBLE METAL HALFLIFE

FIGURE 5. IO.

(hr)'

COMPARISON OF MEASURED TO THEORETICAL

CONCENTRATION

IN FUEL SALT TO THE NOBLE METAL HALF LI FE BY GROUPS

Lid

ORNL-DWG 70-15022 104

I I 111

I I I I I

DATA POINTS REPRESENT GEOMETRIC AVERAGE OF CONCENTRATION RATIO FROM ALL SALT SAMPLES TAKEN DURING INDICATED RUN 5

2 103 5

2 102 5

2 10

FIGURE 5.11.

COMPARISON OF MEASURED TO THEORETICAL CONCENTRATION

I N FUEL SALT TO NOBLE METAL HALF L I F E BY RUN NUMBER

. L, i

u 5.5

Comparison of Gas Samples with Cs

The f a c t t h a t t h e noble metals a r e found i n l a r g e q u a n t i t i e s i n t h e z

gas samples from t h e pump bowl may be surprising.

They have a vanishingly

s m a l l vapor pressure s o t h a t c e r t a i n l y i s n ' t t h e mechanism.

I n Section 5.2

it w a s noted t h a t noble m e t a l atoms or very s m a l l c l u s t e r s of them can be

spontaneously expelled from t h e surface of quiescent f u e l s a l t .

This in-

formation w a s determined i n a c a r e f u l l y controlled laboratory t e s t .

Under

conditions t h a t e x i s t i n t h e pump bowl, one may reasonably expect t h e bulk of t h e s e s i n g l e atoms and very small c l u s t e r s t o be trapped and coalesce with t h e l a r g e r amounts of noble metal f i s s i o n products (and noble corrosion products from t h e chemical reprocessing) i n t h e form of scum and deposits on t h e bubbles, r a t h e r than t o break through and migrate t o t h e off-gas system themselves.

A more l i k e l y mechanism, which i s consistent

w i t h t h e conclusions reached s o f a r i n t h i s r e p o r t , i s t h a t most of t h e

noble metals measured i n t h e gas samples are c a r r i e d by a s a l t m i s t t h a t i s high i n noble metal content.

The impingement of t h e j e t s from t h e spray

r i n g generates l a r g e q u a n t i t i e s of bubbles. way back t o t h e surface and become f r o t h . f r o t h i s high i n noble metal content. pure l i q u i d s w i l l not foam.

These bubbles then work t h e i r We have theorized t h a t t h i s

A w e l l accepted observation i s t h a t

If t h e r e i s foam present i n t h e pump bowl,

it i s s u r e l y heavy i n noble metal f i s s i o n products and reduced corrosion products.

Presumably t h e pump bowl reaches a steady s t a t e foaming condi-

t i o n i n which bubbles b u r s t as f a s t as they a r e formed.

They b u r s t spon-

taneously or because of mechanical action of $he spray.

A t any r a t e t h i s

process i s accompanied by t h e generation of a s a l t mist which is highly concentrated i n noble metals. (37)

Presumably t h i s mist w i l l d r i f t i n t o t h e

gas phase of t h e sampling volume and influence t h e gas samples.

Small

amounts of salt-seeking f i s s i o n products a r e always found i n gas phase samples i n d i c a t i n g f u e l s a l t i s present.

i n t o t h e off-gas system m e r e noble metals have been found i n s i g n i f i c a n t concentrations.

This then i s t h e suggested mechanism f o r t h e appearance

of noble metals i n gas phase samples. t

"his mist would a l s o migrate

62 The a n a l y t i c a l model developed i n Section 4 does not contain a term

f o r t h e noble metal concentration i n t h e gas phases. mental b a s i s f o r using concentration u n i t s .

There i s no funda-

The best we can do i s t o com-

pare t h e measured noble metal concentration i n t h e gas sample t o t h e t h e o r e t i c a l concentration i n t h e f u e l s a l t , at t h e t i m e of sampling.

The

t h e o r e t i c a l concentration i n s a l t then becomes a normalizing f a c t o r .

The

same conditions apply t o t h e calculated Concentration as before. The p r i n c i p l e being t h a t t h e t h e o r e t i c a l concentration i s computed f o r t h e case t h a t noble metals do not migrate t o liquid-gas

shows t h e r e s u l t s of t h i s calculation.

interfaces.

Figure 5.12

The ordinate normalizes t h e t o t a l

noble m e t a l count i n t h e gas sample t o t h e t h e o r e t i c a l concentration i n The abscissa normalizes t h e t o t a l measured amount of

one gram of s a l t .

salt seeking f i s s i o n products i n t h e sample t o t h e t h e o r e t i c a l amount i n one gram of s a l t .

Since t h e s a l t seekers are uniformly d i s t r i b u t e d i n

t h e f u e l s a l t , t h e abscissa i s measured i n t h e amount of s a l t i n t h e sample. I n order t o decrease t h e s c a t t e r and t h e t o t a l number of data p o i n t s , t h e ordinate value Of each d a t a point represents t h e geometric mean of t h e f i v e noble metals isotopes indicated i n t h e p l o t and t h e a b s c i s s a value represents t h e geometric mean of four salt-seeking f i s s i o n product isotopes indicated.

There does seem t o be a c o r r e l a t i o n between t h e

amount of noble metal f i s s i o n products i n a sample and t h e amount of saltweeking f i s s i o n products.

Although t h e s c a t t e r i s high, t h e noble metal

content of a sample i s i n proportion t o t h e s a l t content.

This i s con-

s i s t e n t with t h e idea t h a t t h e f u e l s a l t i s t h e c a r r i e r of noble metals. Figure 5.13 i s a p l o t of gas sample d a t a equivalent t o Figure 5.11 f o r s a l t samples. during each 233U

A l l t h e reported d a t a from routine gas samples taken

run were geometrically averaged and p l o t t e d against t h e

noble metal h a l f l i f e ,

The number i n parenthesis beside each d a t a point

i n d i c a t e s t h e number of samples used i n t h e geometric average.

Note t h a t

t h e curves move down t h e ordinate w i t h time as indicated by t h e run number. As noted before t h i s behavior i s p a r t i c u l a r l y s t r i k i n g when these curves 1

are compared with t h e curves i n Figure 5.11 f o r s a l t samples.

The s a l t

sample curves move up t h e ordinate w i t h time, b u t less uniformly than t h e gas sample curves.

This behavior i s not understood but probably repre-

s e n t s a change i n f r o t h i n g c h a r a c t e r i s t i c s i n t h e pump bowl w i t h t i m e .

ORNL-DWG 70-15018

ABSCISSA

EACH DATA POINT IS GEOMETRIC AVERAGE OF RATIO FOR 91Y,

5 10-5

10-4

10-3

MEASURED AMOUNT OF SALT SOLUBLE FISSION PRODUCTS I N GAS SAMPLE (dpm/sample) THEORETICAL AMOUNT OF SALT SOLUBLE FISSION PRODUCTS PER GRAM OF SALT (dpm/gm-salt)

FIGURE 5.12. COMPARISON OF THE AVERAGE AMOUNT OF NOBLE METALS I N GAS SAMPLES TO THE AVERAGE AMOUNT OF SOLUBLE F I SS I ON PRODUCTS IN GAS SAMPLES

10'2

LJ ORNL-DWG 70-14019

102 2 5 103 NOBLE METAL HALF L I F E ( h r )

104

FIGURE 5.13. COMPARISON TO THE MEASURED (GAS SAMPLE) TO THEORETICAL ( I N SALT) CONCENTRATION TO NOBLE METAL HALF L I F E BY MSRE RUN NUMBER

b, t

Recall t h a t at t h e s t a r t of 233U

corrosion products (which a c t as noble metals) suddenly appeared i n t h e

Over a period of time they would eventually work t h e i r

r e a c t o r system. i

operation, a l a r g e amount of reduced

way out of t h e r e a c t o r system and i n t o t h e off-gas system o r possibly t h e overflow tank o r dump tank.

A t any rate t h e foaming c h a r a c t e r i s t i c s

of t h e pump bowl might be expected t o change as t h e amount decreased. Conclusions.

The only conclusion t h a t can be reached from analysis

of t h e gas samples i s t h a t t h e noble metals i n t h e gas samples owe t h e i r existence t o a s a l t m i s t generated by b u r s t i n g bubbles t h a t a r e high i n noble metal content.

This confirms previous conclusions t h a t noble metals

adhere t o liquid-gas i n t e r f a c e s .

5.6

Time Constant f o r Noble Metals on I n t e r f a c e s

W e have concluded t h a t noble metals deposit on liquid-gas i n t e r f a c e s

and remain t h e r e with a high degree of s t a b i l i t y .

This conclusion w a s

reached from q u a l i t a t i v e observations of t h e r e a c t o r behavior, and from noble metal migration analysis i n t h i s report.

I n f a c t , it appears t h a t

t h e measured noble metal content of both, t h e f u e l s a l t and gas phase

samples, i s primarily a measure of t h e concentration associated w i t h t h e s e interfaces.

This w a s t h e r a t i o n a l i z a t i o n t o explain t h e slopes of t h e

l i n e s i n Figures 5.10, 5.11 (salt samples) and 5.13 (gas samples) being other than zero.

I f w e could write an equation f o r t h e amount of noble

metals associated with t h e liquid-gas i n t e r f a c e , we could compare t h e measured concentrations from f u e l s a l t and gas phase samples t o t h i s

An equation t o do (Eq. [27)).This equation i s t h e r e s u l t of

quantity' and possibly make a s i g n i f i c a n t observation. t h i s was derived i n Sect. 4.2.10

a rate balance around t h e e n t i r e gas phase of t h e f u e l loop where t h e g a s , phase i s defined t o include t h e liquid-gas i n t e r f a c e s .

It considers t h e

c i r c d a t i n g bubbles and t h e gas phase of t h e pump bowl and a l l t h e i r respective i n t e r f a c e s t o be w e l l mixed.

There i s no fundamental b a s i s

f o r using concentration u n i t s i n t h e gas phase, s o we w i l l compute t h e t o t a l m o u n t of noble metals i n t h i s mixture.

The sink terms f o r noble

metals i n t h e gas phase involve decay and an a r b i t r a r y rate constant which P

L;d

i s a measure of t h e r a t e at which they migrate out of t h e gas phase as defined above.

I n application w e w i l l evaluate t h e undefined rate

66 constant by forcing t h e slope of curves equivalent t o those r e f e r r e d t o above t o equal zero.

Then we w i l l s e e i f any reasonable physical s i g -

nificance can be assigned t o t h e s e rate constants.

For t h i s analysis

w e w i l l show only t h e gas sample data from Run 19.

These d a t a w i l l be

used, first , because they represent t h e l a r g e s t number of samples (15) taken during a s i n g l e run, second

- because

t h e d a t a points are t i g h t l y

grouped around t h e l i n e (Figure 5.13) and t h i r d

- because

t h e slope of

t h e curve (Fig, 5.13) i s intermediate between t h e slopes of a l l t h e

curves i n Figures 5.10, 5.11 and 5.13. The r e s u l t s of t h e calculation appear i n Figure

5.14, where t h e

measured noble metal i n t h e gas sample (dpm/sample) normalized t o t h e t o t a l amount t h e o r e t i c a l l y associated with t h e liquid-gas i n t e r f a c e and gas phase phase (dpm) i s p l o t t e d against t h e noble metal half l i f e f o r t h r e e d i f f e r e n t values of t h e r a t e constant.

Each d a t a point i s t h e

geometric average of t h e indicated r a t i o f o r a l l gas samples taken during Run 19.

The absolute value of t h e ordinate i s of l i t t l e o r no s i g n i f i -

cance i n t h i s analysis since we a r e only looking at t h e slopes of t h e lines.

It i s apparent t h a t no s i n g l e r a t e constant w i l l f l a t t e n out

t h e e n t i r e curve.

How then can we r a t i o n a l i z e these curves?

t h a t t h e curve between t h e 'O%u

and l2'%e

Note

points and t h e I3*Te and "Mo

points has a zero slope when t h e rate constant of noble m e t a l s i n t h e pump bowl i s about 0.01 h r s - l (equivalent h a l f l i f e approximately 3 days). If t h e h o r i z o n t a l l i n e i s extended when one would observe t h a t more

lo6Ru w a s measured than t h i s analysis would i n d i c a t e .

If a mechanism

were present whereby noble metals could be s t o r e d out of t h e r e a c t o r

system f o r a few hundred days and then introduced i n t o t h e r e a c t o r system, t h e measured Io6Ru concentration would be high.

I n a f e w hundred

days t h e s h o r t e r l i v e d isotopes would have decayed a w a y ; t h e r e f o r e , t h e slope of t h e r i g h t hand s l i d e of t h e curve would not be s i g n i f i c a n t l y affected.

Likely suspects f o r t h i s mechanism would be t h e overflow tank

and t h e dump tank.

The other groups of gas and fuel s a l t samples have

been analyzed i n t h i s framework but t h e r e s u l t s w i l l not be presented. me summarize by saying t h a t they are more o r less consistent.

Let

That i s , t h e

curves i n Figures 5.10, 5.11 and 5.13 can generally be r e d u c e d t o a l i n e with slope zero i f a h a l f l i f e i n t h e pump bowl of a few days i s imposed

67 i t

ORNL-OWG 70-1 5020 10-6

2 10-7

10-8

10-9

lO”0

102 2 5 103 NOGLE METAL HALF LIFE ( h r )

104

FIGURE 5.14. COMPARISON OF MEASURED AMOUNT I N GAS SAMPLE TO TOTAL THEORETICAL AMOUNT IN GAS PHASE INCLUDING INTERFACE WITH PARAMETERS OF AN ARBITRARY RATE CONSTANT FOR REMOVAL FROM SYSTEM

68 on them plus a long decay time of a few hundred t o s e v e r a l hundred days af'ter which they are re-introduced i n t o t h e r e a c t o r system.

The f e w

days h a l f l i f e may be associated w i t h t h e t i m e required f o r t h e spray r i n g t o splash h a l f t h e noble metals i n t o t h e off-gas system as a

mist.

The f e w hundred t o s e v e r a l hundred day decay time may be associa-

t e d w i t h t h e overflow tank o r w i t h t h e frequency of draining and r e f i l l i n g f u e l s a l t i n t h e fuel loop. Conclusion.

The slopes of t h e curves i n Figures 5.10,

5.11 and 5.13

can be explained i n terms of reasonable physical phenomena i n t h e E R E . The physical phenomena have as t h e i r foundation t h e t h e s i s t h a t noble metals migrate and adhere t o liquid-gas

5.7

5.7.1

interfaces.

Miscellaneous Noble Metal Observations

Laminar Flow Core Surveillance Sample Following R u n 18, t h e regular core surveillance specimen fixutre

(Fig. 2 . 4 ) w a s removed and a s p e c i a l t e s t f i x t u r e w a s i n s t a l l e d .

Since

t h i s w a s t o be t h e l a s t chance t o expose material t o an operating molten-

s a l t r e a c t o r core, a l a r g e number of s p e c i a l experiments w a s incorporated

i n t h i s assembly, many of which were not d i r e c t l y r e l a t e d t o f i s s i o n product deposition.

The e n t i r e assembly i s described i n d e t a i l i n

38. One of t h e elements t h a t w a s devoted t o f i s s i o n product deposit i o n i s i l l u s t r a t e d i n Figure 5.15. It w a s designed t o generate laminar

Ref.

flow i n a t h i n annular flow region, t h e outside surface being graphite and t h e i n s i d e surface being Hastelloy-N.

Different surface roughnesses

were a l s o incorporated as shown i n t h e figure. w a s twofold.

The i n t e n t of t h i s t e s t

F i r s t , s i n c e t h e graphite and Hastelloy-N annular surfaces

w e r e exposed t o e s s e n t i a l l y i d e n t i c a l f l u i d dynamic conditions, any

difference i n noble metal deposition would be a d i r e c t measurement of t h e s t i c k i n g f r a c t i o n of one surface r e l a t i v e t o t h e o t h e r , and second, t o note any e f f e c t s of surface roughness.

This sample fixture was removed

after R u n 20 when t h e MSRE w a s permanently shut down, and t h e noble metal deposition w a s measured,

ORNL-OWG 71-1883

INSIDE GRAPHITE SURFACE ROUGHNESS

i n . THICK

nd OUTSIDE GRAPHITE

SURFACE ROUGHNESS

ANNULAR FLOW PASSAGE

APPROXIMATELY TO SCALE

FIGURE 5.15.

SCHEMAT C OF THE LAMINAR FLOW CORE SURVE LLANCE SAMPLE

I n determining t h e measured amount of noble metals on t h e s e s u r f a c e s , t h e Hastelloy-N core w a s sectioned at t h e surface roughness l i n e s of Two f i s s i o n product determinations were t h e r e f o r e made,

demarkation.

one f o r each end.

Similarly, t h e graphite outer body w a s cut i n t o t h r e e

s e c t i o n s , one f o r each surface roughness on t h e outside. i n s i d e has two surface roughnesses.

Note t h a t t h e

Theoretical amounts of noble metals

were computed and compared t o t h e measured amounts i n Figure 5.16.

Again

t h e computed amount i s f o r t h e case where noble metals do not deposit on The estimated s a l t v e l o c i t y through t h e annulus

liquid-gas i n t e r f a c e s . i s 0.7 f t / s e c .

This w i l l y i e l d a Reynolds number of 206 and a mean mass

t r a n s f e r c o e f f i c i e n t throughout t h e annulus of about 0.23 fâ&#x20AC;&#x2122;t/hr.

The

data p o i n t s on Figure 5.16 represent t h e geometric averages of all determi n a t i o n s on t h e Hastelloy-N and graphite annular surfaces. i n d i c a t e t h e maximum and minimum d a t a points. s c a t t e r i n t h e data.

The wings

There w a s considerable

It w a s not possible t o d i s t i n g u i s h f l u i d dynamic

entrance e f f e c t s and surface roughness e f f e c t s from t h e data.

For laminar

o r marginally turbulent f l o w , t h e surface roughness e f f e c t s should have been minimal.

Data from t h e outside surface of t h e graphite body indi-

cated t h i s t o be t r u e .

The outside f l u i d dynamic conditions are not

known b u t t h e f l o w should have been e i t h e r laminar o r just barely turbulent.

The curves i n Figure 5.16 are generally consistent i n magni-

tude of t h e ordinate with those from t h e primary heat exchanger (Fig. 5.1) and t h e Hastelloy-N and graphite core s u r v e i l l a n c e specimens (Figures

5.3 and 5.2, r e s p e c t i v e l y ) . The p r i n c i p a l observation i s t h a t t h e s t i c k i n g f r a c t i o n t o graphite i s apparently l e s s than t h e s t i c k i n g f r a c t i o n t o Hastelloy-N.

Because of s c a t t e r i n t h e data, it i s d i f f i c u l t t o draw

a firm value, but something i n t h e range of 0 . 1 5.7.2

- 0.6

would be i n order.

Noble Metal Distribution i n t h e MSRE It would be informative t o determine t h e noble metal d i s t r i b u t i o n

i n t h e r e a c t o r f u e l system.

&e shown i n Table 5.1.

The r e s u l t s of t h i s inventory c a l c u l a t i o n

ORNL-OWG 71-1881 10

5.0

GRAPHITE SURFACE N

2.0

E n V

1.0

W L

E n.

0.5

0 z

0.02 0.01

FIGURE 5.16.

COMPARISON OF THE MEASURED TO THEORETICAL AMOUNTS OF NOBLE

METALS ON THE LAMINAR FLOW CORE SURVEILLANCE SPECIMENS

72 Table 5.1 Noble Metal Distribution i n t h e MSRE"

Noble Metals on Heat Exchanger Surf aces

40%

Noble Metals on Other Hastelloy-N Surfaces i n Fuel Loop

Noble Metals on Graphite Surfaces i n Core

Noble Metals i n Pump Bowl, Overflow Tank, O f f - G a s System, e t c . (by Difference)

100%

0.4

*Reported as percent of t o t a l inventory of noble metals i n t h e r e a c t o r system as time measurements were made.

I n determining t h i s d i s t r i b u t i o n , t h e measured amounts of noble metals on t h e h e a t exchanger were t h e same values used t o determine t h e ordinate of Figure 5.1.

The percent of inventory of noble metals on o t h e r

Hastelloy-N surfaces i n t a b l e , w a s determined by multiplying t h e percent of inventory on t h e heat exchanger by t h e appropriate r a t i o of products of mass transfer c o e f f i c i e n t s and surface area as t a b u l a t e d i n Table B-1.

The measured amounts of noble metals on t h e graphite surfaces w e r e

t h e same values used t o determine t h e ordinate of Figure 5.2.

Specifical-

l y , t h e d a t a were obtained from t h e core surveillance samples removed a f t e r run 14 (235U)

and run 18 ( 2 3 ~ ) . The measured concentrations from

t h e core surveillance samples w e r e then multiplied by a r a t i o of mass t r a n s f e r c o e f f i c i e n t s (0.063/0.25) t o convert them t o concentrations on t h e bulk graphite (see Appendix B ) .

The difference between 100 percent

and a l l t h e above sinks w a s assumed t o have deposited on bubbles and i s t h e r e f o r e d i s t r i b u t e d i n an unknown manner between t h e pump bowl, overflow tank, off-gas system and dump tank.

The difference i n d i s t r i b u t i o n

between t h e 235U and 233U runs i s q u i t e dramatic, but may not be as l a r g e or as s i g n i f i c a n t as it appears.

Recall from Section 3.2 where t h e

73 gamma spectrometry of t h e heat exchanger i s discussed, it w a s pointed out t h a t t h e lower measured values were used f o r noble metal deposition on t h e h e a t exchanger following run 19 (233U

b u t more of an average value

w a s used f o r noble metal deposition following run 14 (235U).

If t h e

measured values following run 1 4 were weighted lower, then t h e percent of inventory on t h e heat exchanger and other Hastelloy-N surfaces i n Table 5.1 f o r t h e 235U runs would be l e s s , and t h e noble metals i n t h e pump bowl would be higher.

CONCLUSIONS AND RECOMMENDATIONS Conclusions

1. Noble metal f i s s i o n products migrate from t h e f u e l s a l t , where

they are born, t o t h e i r various depositories i n accordance w i t h t h e l a w s of mass t r a n s f e r theory. 2.

Each noble metal isotope migrates as a function of i t s own con-

centration i n f u e l s a l t , and i s not influenced by o t h e r elemental species of noble metals, or even i s o t o p i c species of t h e same element.

There are

however second order effects(probab1y chemical) which must be considered, f o r i n s t a n c e , a chemical i n t e r a c t i o n between Nb and graphite had t o be r e s o r t e d t o , i n order t o explain t h e high 95Nb concentration found on t h e MSRE core surveillance samples.

3. The liquid-gas i n t e r f a c e ( c i r c u l a t i n g bubbles and t h e f r e e surface i n t h e pump bowl) apparently present a s t a b l e surface f o r noble m e t a l deposition , however, t h e e f f e c t i v e s t i c k i n g f r a c t i o n t o t h i s i n t e r f a c e i s considerably less than unity.

This observation should not be regarded as

absolutely conclusive, r a t h e r it i s primarily a deduced phenomenon used t o explain many of t h e results i n t h i s analysis and many of t h e qualitat i v e observations from t h e MSRE.

Associated w i t h t h e l a s t conclusion i s t h e i d e a t h a t noble metal

f i s s i o n products and also reduced corrosion products attached t o a liquidgas i n t e r f a c e have many of t h e p r o p e r t i e s of an insoluble surface a c t i v e

material.

Again , t h i s i d e a should not be regarded as conclusive.

It was ,

however, used t o suggest an explanation f o r t h e dramatic differences i n MSRE operating c h a r a c t e r i s t i c s between the'235U and 233U

runs.

Lid 5.

The s t i c k i n g f r a c t i o n of noble metal f i s s i o n products t o graphite

i s apparently less than t h e s t i c k i n g f r a c t i o n t o Hastelloy-N.

Recommendations The fundamental questions a r e

- how

does one extrapolate t h e s e results

from t h e MSRE t o a l a r g e molten salt r e a c t o r i n a q u a n t i t a t i v e manner,and can one use noble metal migration t o h i s advantage i n t h e design of a reactor?

The conceptual design of a s i n g l e f l u i d 1000 MW(e) MSBR ( 3 9 ) c a l l s

f o r c i r c u l a t i n g helium bubbles t o remove 135Xe.

I n addition, any molten

s a l t c i r c u l a t i n g pump wduld probably have a free l i q u i d surface associated

with it t o eliminate t h e need f o r a packing gland.

There may be other

liquid-gas i n t e r f a c e s associated with a l a r g e r e a c t o r . noble metals would migrate t o these surfaces.

Presumably then,

The amounts would be extreme-

l y d i f f i c u l t t o estimate because of u n c e r t a i n t i e s i n t h e s t i c k i n g f r a c t i o n and other parameters.

Then t h e r e i s t h e question of w h a t happens t o noble

metals a f t e r they reach t h e s e i n t e r f a c e s . quiescent

, noble

If t h e pump bowl i s r a t h e r

metals would accumulate and develop i n t o a heat source.

On t h e other hand, noble metals associated w i t h c i r c u l a t i n g bubbles may

migrate by some mechanism i n t o t h e off-gas system and s e t t l e out on various components t h e r e .

Noble metal sinks associated w i t h liquid-gas

i n t e r f a c e s cannot be handled q u a n t i t a t i v e l y at t h i s time. It i s recommended then, t h a t noble metal migration be studied i n a

c i r c u l a t i n g s a l t loop.

A s p e c i a l loop may not have t o be b u i l t , b u t

r a t h e r a loop b u i l t f o r some o t h e r purpose could be used.

Noble metals

(probably t h e i r precursors) could be added t o t h e s a l t i n t r a c e r amounts and t h e following s t u d i e s made. 1. Deposition on s o l i d surfaces and c o r r e l a t i o n of r e s u l t s with

mass t r a n s f e r theory. 2.

Deposition on liquid-gas i n t e r f a c e s and c o r r e l a t i o n of results

with mass t r a n s f e r theory.

S t a b i l i z i n g e f f e c t s of noble metals ( f i s s i o n products and cor-

rosion products) on bubble and foam i n t e r f a c e s . ,

4. 5.

Effects of t h e noble metal chemical species. Effects of t h e chemical s t a t e of t h e s a l t .

V -.

Various aspects of c i r c u l a t i n g bubble mechanics when noble metals

a r e attached t o t h e i r surfaces, such as

I n t e r a c t i o n of bubbles with s o l i d surfaces.

Bubble dissolution

This probably r e s u l t s i n a

s m a l l packet of noble metals associated again with t h e salt. C.

Bubble renucleation

The s m a l l packet from above w i l l

then probably serve as an e x c e l l e n t bubble renucleation

site.

Prototype t e s t i n g of bubble s e n s i t i v e components proposed f o r t h e

next generation molten s a l t r e a c t o r t o estimate t h e deposition and f u r t h e r migration of noble metals i n t h e s e areas. These s t u d i e s would be accomplished w i t h t r a c e r q u a n t i t i e s of noble metals.

The behavior of t h e next generation r e a c t o r would undoubtedly be

a function of t h e gross amounts of noble metals and reduced corrosion

products p r e s e n t , and we couldn't hope t o properly simulate t h i s i n a c i r crrlating loop.

Therefore, t h e results of t h e proposed study wodd not

answer a l l t h e questions pertainiiig 20 noble metal migration i n t h e -i

r e a c t o r , but it would c e r t a i n l y l a y a good foundation.

The complete s t o r y

of noble m e t a l migration would ultimately have t o come from t h e operation of t h e r e a c t o r i t s e l f .

REFERENCES

1. R. C. Robertson, MSRE Design and Operations Report: P a r t 1, Descript i o n of Reactor Design, USAEC Report O R N L - T M - ~ ~ ~ORNL, , January 1965. 2.

Nuclear Applications and Technology, V. 8 , N. 2 , February 1970.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending J u l y 31, 1964, USAEC Report ORNL-3708.

J. R. Engel et. a l . , Spray, M i s t , Bubbles and Foam i n t h e Molten S a l t Reactor Experiment , USAEC Report ORNL-TM-3027 , ORNL , June 1970.

J. C. Robinson and D. N. Fry, Determination of t h e Void Fraction of t h e MSRE Using Small Induced Pressure Perturbations, USEAC Report ORNETM-2318, ORNL, Feb. 6 , 1969.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending February 28, 1970, ORNL-4548, p 115.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31, 1966, ORNL-4037, p 181.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31 , 1968 , ORNL-4344 , pp 144-145.

R. J. K e a , A Model f o r Computing t h e Migration of V e r y Short Lived Noble Gases i n t o MSRE Graphite, USAEC Report O R N L - T M - ~ ~ ~ O ,ORNL, J u l y 1967.

10. R. Blmiberg e t . a l . , Measurements of Fission Product Deposition i n t h e MSRE with Ge(Li) Gamma Ray Spectroscopy, ORNL Report ORNL-CF68-11-20 , November 19 , 1968, I n t e r n a l D i s t r i b u t i o n Only. 11. A. Houtzeel and F. F. Dyer, A Study of Fission Products i n t h e MoltenS a l t Reactor Experiment by Gamma Spectrometry , USAEC Report ORNL-TM-3151 , ORNL, August 1972. 12.

R. B. Lindauer, Processing of t h e MSRE Flush and Fuel S a l t s , USAEC Report ORNL-TM-2578 , ORNL , August 1969.

13.

R. E. Thoma, Chemical Aspects of MSRE Operations, USAEC Report ORNL-4658, ORNL, t o be published.

14.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending February 28, 1969, ORNL-4396, PP 139-

Ibid.

, PP 95-97. t

16. Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31, 1969, ORNL-4449, p 73.

77 17 1

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending January 31, 1964, 0RN~-3626, pp 132-137.

18. T. S. Kress and F. H. N e i l l , Calculating Convective Transport and Deposition of Fission Products, USAEC Report O R N L - T M - ~ ~ ORNL, ~~, September 1968. M. N. Ozisik, "A Heat/Mass Analogy f o r Fission Product Deposition from Gas Streams ,"Nuclear Science and Engineering, 1 9 , 164-171, 1964.

20.

G. E. Raines e t . a l . , Experimental and Theoretical Studies of FissionProduct Deposition i n Flowing Helium, USAEC Report BMI-1688, BMI ,

1964.

21.

F. H. N e i l l , D. L. Grey and T. S. Kress, Iodine Transport and Deposition i n a High-Temperature H e l i u m Loop, USAEC Report ORNLTM-1386, ORNL, June 1966.

22.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending February 29, 1968, ORNL-4254, pp 100-108. Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31, 1970, ORNL-4622, pp 70-71.

24.

Molten-Salt Reactor Program Semiannual Progress Report for Period Ending August 31, 1969, OWL-4449, pp 8-10. Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending February 28, 1970 , ORNL-4548, pp 6-7.

26.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31 , 1966, ORNL-4037. Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31, 1967, ORNL-4191.

28.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31, 1968, ORNE4344.

Molten-Salt Reactor Program S bruary 28, 1970, ORN

30.

S. S. Kirslis, Personnel Communication, Reactor Chemistry Division, O a k Ridge National Laboratory, December 1970.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending February 28, 1967, ORNL-4119, p 131.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending February 29, 1968, ORNL-4254, pp 125-128.

33.

E. J. Roberts e t . a l . June 29, 1970.

, "Solids

nual Progress Report f o r period

Concentration ," Chemical Engineering,

78 34.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending February 29, 1968, ORNL-4254, pp 100-113.

35.

Molten-Salt Reactor Program Semiannual Progress Report f o r Period Ending August 31, 1968, ORNL-4344 , pp 150-151.

36.

Molten-Salt Reactor Program Semiannual Progress Report for Period Ending February 28, 1969, ORNL-4396, pp 148-150.

37.

H. W. Kohn, Bubbles, Drops and Entrainment of Molten S a l t s , USAEC Report ORNL-TM-2373 , ORNL, December 1968.

38.

C. H. Gabbard, Design and Construction of Core Irradiation-Specimen Array f o r MSRE Runs 19 and 20, USAEC Report ORNL-TM-2743, ORNL, December 22, 1969.

39.

Molten-Salt Reactor Program Staff , Single-Fluid Molten-Salt Breeder Reactor Design Study, USAEC Report ORNL-4541, ORNL ( t o be published).

40.

R. B. Bird e t . a l . , Transport Phenomena, John Wiley and Sons, Inc., 1960.

41.

F. N. Peebles, Removal of Xenon-135 from Circulating Fuel S a l t of t h e MSBR by Mass Transfer t o H e l i u m Bubbles, USAEC Report ORNL-TM-2245 , ORNL, J u l y 23 , 1968.

Id t

79 APPENDIX A NOMENCLATURE

Concentration of noble metal isotope i n f u e l s a l t (atoms/vol)

Value of C

csi

at time zero

Concentration of noble metal isotope i n f u e l s a l t a t l i q u i d gas o r l i q u i d - s o l i d i n t e r f a c e (atoms/vol)

CP"

Concentration of t h e soluble precursor of t h e noble metal i n f u e l s a l t ( a t oms /vel)

CPS

Value of cP" at t i m e zero

Concentration of noble metal isotope on s o l i d s surfaces (atoms / a r e a )

Value of C"

Total amount of noble metal isotope i n gas phase of MSRE f u e l loop including liquid-gas i n t e r f a c e s (atoms )

I O

at t i m e zero

Value of I a t time zero

Volume of f u e l s a l t i n f u e l loop

Time

Reactor power l e v e l (MW)

Cumulative f i s s i o n y i e l d of soluble precursor of noble metal isotope ( f r a c t i o n ) '

Direct f i s s i o n y i e l d of noble metal isotope ( f r a c t i o n )

Decay constant of soluble precursor of noble metal isotope (hrs-1)

Decay constant of noble metal isotope (hrs'l)

hera

Mass t r a n s f e r c o e f f i c i e n t t o graphite i n MSRE core ( f t / h r )

hhe

Mass t r a n s f e r c o e f f i c i e n t t o primary h e a t exchanger ( f t / h r )

hbub

Mass t r a n s f e r c o e f f i c i e n t t o c i r c u l a t i n g bubbles ( f t / h r )

Mass t r a n s f e r c o e f f i c i e n t t o s o l i d surface of i n t e r e s t ( f t / h r )

Agra

Surface area of graphite exposed t o f u e l s a l t (ft

LJ Ah"

Total surface a r e a of Hastelloy-N i n h e a t exchanger exposed t o fuel salt ( f t 2 )

Abub

Surface a r e a of c i r c u l a t i n g bubbles ( f t2 )

Theoretical r a t e constant f o r migration of noble metal isotope from f u e l s a l t t o a l l surfaces exposed t o f u e l saltsee text ( h r s - l )

Generalized rate constant f o r t r a n s p o r t of noble metal isotope from gas phase of f u e l loop (including liquid-gas i n t e r f a c e s ) t o off-gas system (hrs-1)

hheat hmas s

Heat t r a n s f e r c o e f f i c i e n t (Btu/hr-ft2-OF ) Mass t r a n s f e r c o e f f i c i e n t ( f t / h r )

Equivalent diameter (ft)

Thermal conductivity (Btu/hr-ft2-OF)

Diffusion c o e f f i c i e n t (ft2 / h r )

Heat capacity (Btu/lb-'F)

V i s cos i t y ( lb / f t - h r )

Density ( l b / f t 3 )

Velocity ( f t / s e c )

81 APPENDIX B EVALUATION OF MIGRATION PARAMETERS FOR THE MSRE L

Mass Transfer Parameters t o Fuel Loop Surfaces

It has been hypothesized i n t h i s report t h a t noble metals migrate i n t h e MSRE i n accordance with mass t r a n s f e r theory.

The physical and

chemical basis of t h i s hypotehsis a r e presented i n Section 4.1.

This

being t h e case, conventional mass t r a n s f e r c o e f f i c i e n t s should c o n t r o l t h e migration from bulk s a l t t o t h e various surfaces.

These c o e f f i c i e n t s w i l l

be evaluated by t h e heat transfer-mass t r a n s f e r analogy.

For an excellent

derivation of t h i s analogy and assumptions involved, see Reference 40, Chapter 21.

The e s s e n t i a l s of t h e analogy s t a t e t h a t mass t r a n s f e r coef-

f i c i e n t s may be computed from conventional c o r r e l a t i o n s f o r heat t r a n s f e r c o e f f i c i e n t s a f t e r t h e following s u b s t i t u t i o n s have been made.

Mass Transfer

Heat Transfer hhe at For

Nu =

For

Pr =

For

Re = PdV

substitute

mass Nu

substitute

Sc =

substitute

Re =

-- hmass

ClJ

kPD

The analogy ,,olds t r u e f o r any flow geometry and f o r laminar o r t u i d u l e n t

flow.

Ey way of example we may apply t h e s e s u b s t i t u t i o n s t o . t h e Dittus-

Boelter equation f o r turbulent flow as follows :

-gp( d v ) For h e a t t r a n s f e r hheat = 0.023 k

0.8

C l~ 0.4 (%)

For mass t r a n s f e r hmass = 0.023

0.8

0.4

(e) (k) lJ

The MSRE f u e l s a l t loop was divided i n t o major regions. 1c

The mass

t r a n s f e r c o e f f i c i e n t f o r each of t h e s e regions was estimated using accepted h e a t t r a n s f e r c o e f f i c i e n t c o r r e l a t i o n s and t h e analogy above. Results are l i s t e d i n Table B-1.

82 Table B-1

Mass Transfer Parameters f o r MSRE Fuel Loop Mass Transfer" Coefficient (ft/hr)

Surface Area (ft2)

Heat Exchanger ( S h e l l s i d e )

0.55

315

173

Fuel Loop Piping, Core and Pump Volute

1.23

Core Wall Cooling Annulus

0.51

Core Graphite (Exposed t o s a l t )

0.063

154 1465

Miscellaneous (Pump Impeller , Core Support Grid, e t c . ) 10 Percent of Summation of Above Product

Summation of Products

473

*Physical p r o p e r t i e s from Table 2.1. Diffusion c o e f f i c i e n t of noble metals i n f u e l s a l t estimated t o be 5.1 x 10-5 f t z / h r with no d i s t i n c t i o n between chemical species. Mass Transfer Parameters t o Circulating Bubbles

I n Section 3.3 it w a s noted t h a t t h e void volume of bubbles c i r c u l a t i n g with t h e fuel s a l t w a s estimated t o be i n t h e range of 0.02 percent t o 0.045 percent during t h e 235U operation and i n t h e range of 0.5 percent t o 0.6 percent during t h e 23%J

operation.

The bubble diameter i s

not known, but estimates from various i n d i r e c t observations would put t h e diameter somewhere between 0.001 and 0.010 i n . of 0.005 i n .

W e W i l l use a mean value

A program i s underway a t Oak Ridge National Laboratory t o

determine t h e mass t r a n s f e r c o e f f i c i e n t t o bubbles c i r c u l a t i n g with a t u r b u l e n t l y flowing f l u i d . (41)

This program has not y e t been completed,

but i n i t i a l information i n d i c a t e s a mass t r a n s f e r c o e f f i c i e n t t o c i r c u l a t i n g bubbles of about 5.0 f%/hr i s a reasonable value.

Using mean values of

t h e above ranges of void f r a c t i o n s and a f u e l s a l t volume i n t h e f u e l loop of 70.5 f t 3 , w e compute t h e mass t r a n s f e r parameters i n Table B-2.

83 Table B-2

Mass Transfer Parameters t o Circulating Bubbles Mass Transfer Coefficient (ft/hr)

Surface Area (ft2)

During 235U Runs

5 .o

345

During 233U Runs

5.0

5581

Product (ft3/hr 1725 27,900

Mass Transfer Parameters t o t h e Core Surveillance Specimens

The complexity of t h e f l u i d dynamic conditions i n t h i s region has been discussed b r i e f l y i n Section 3.2.

As t h e cross s e c t i o n a l v i e w i n Figure

2.4 shows, t h e rectangular graphite specimens are packed together t o form

a r a t h e r complicated f l u i d dyanmic arrangement f e a t u r i n g both i n s i d e and outside corners.

Adjacent t o each major graphite surface and almost

touching it are t h e Hastelloy-N t e n s i l e specimens and dosimeter tube. The t e n s i l e specimens are p a r t i c u l a r l y d i f f i c u l t t o analyze because they LI

form a l a r g e number of f l u i d entrance and e x i t regions f o r themselves and t h e adjacent graphite.

transfer coefficient.

This e f f e c t would be expected t o increase t h e mass On t h e other hand, t h e close proximity of t h e wide

p a r t of t h e t e n s i l e specmens t o t h e graphite (about 0.040 i n . ) might be expected t o l o c a l l y stagnate t h e f u e l s a l t .

This e f f e c t would tend t o

decrease t h e mass t r a n s f e r c o e f f i c i e n t i n t h e a f f e c t e d region.

Finally,

t h e e n t i r e sample assembly i s enclosed i n a perforated basket.

The

i n t e n t of t h e basket i s t o allow f o r cross flow, but it a l s o contributes t o t h e uncertainty of f u e l s a l t v e l o c i t y p a s t t h e sample.

The perforations

a l s o generate an unknown amount of turbulence i n t h e s a l t ,

All t h e s e f l u i d

dynamic complications were a necessary result of incorporating a l a r g e number of d i f f e r e n t kinds of samples (both graphite and metal) f o r d i f f e r e n t purposes i n t o a r a t h e r confined volume.

I should a l s o point out t h a t

when t h e sample s t a t i o n w a s b u i l t , t h e necessity of being able t o accurate-

l y estimate mass t r a n s f e r c o e f f i c i e n t s w a s not f u l l y realized.

v e l o c i t y i n t h e sample s t a t i o n has been estimated t o be about 2 f t / s e c , p a r t l y by i n d i r e c t measurements and p a r t l y by estimate. could be as much as 25 percent.

The s a l t

The uncertainty

The equivalent diameter of t h e annular

region between t h e samples and perforated basket, and based on a l l sample

84 components exposed t o s a l t i s about 0.62 i n . based on t h e s e values marginally turbulent.

, is

about 3000.

The Reynolds Number then,

This implies t h a t t h e flow i s only

This being t h e case, one might expect t h a t t h e

confining nature of t h e i n s i d e corners formed .by t h e g r a p h i t e , and t h e zones between t h e Hastelloy-N samples and graphite surfaces would tend t o make t h e flow laminar.

Using t h e usual r e l a t i o n s h i p s f o r h e a t t r a n s f e r coef-

f i c i e n t s and applying t h e heat-mass t r a n s f e r analogy, w e would compute an o v e r a l l mass t r a n s f e r c o e f f i c i e n t of 0.31 ft/hr f o r turbulent flow and

0.077 f t / h r f o r laminar flow i n t h i s region.

I n t h i s analysis then, we

w i l l pick an intermediate value of 0.25 f t / h r f o r t h e mass t r a n s f e r coef-

f i c i e n t t o both t h e graphite and Hastelloy-N specimens.

The uncertainty

i n t h i s number i s high, probably more s o f o r t h e Hastelloy-N than f o r t h e graphite. Noble Metal Fission Product Parameters T a b l e B-3 lists t h e noble metal (and precursor) y i e l d and h a l f l i f e

q u a n t i t i e s during t h e 23% runs, and Table B-4 l i s t s those parameters during t h e 235U

runs , used i n t h i s analysis. Table B-3

Noble Metal Fission Product Parameters (233U R u n s ) Noble Metal 9 9 ~ l o3Ru

5Ru lo6Ru

125a 127%

12 9mTe 13 l y e

32Te 9 5Nb

Cumulative Yield* of Precursor

Direct Yield of Noble Metal

% 4.89

2.00 0.706 0.438 0.0839 0.58

0.71 0.441 4.43 6.00

0.0 0.0 0.0 0.0 0 .o 0.0 0.0 0.0 0.0 0.0

H a l f Life

of Precursor 2.4 min 1.2 min 9 min <1 min %9 days 1.9 h r s 4.6 h r s 23 min 2.1 min 65 days

H a l f Life of Noble Metal 66.5 h r s 39.7 days 4.45 h r s 1.01 y r s 2.0 y r s 91 h r s 37 days 30 h r s 77 h r s 35 days

*Based on t h e following Fission Distribution Component Percent of Fissions

233u

235u

239Pu

93.2

2.3

4.5

Table B-4

Noble Metal Fission Product Parameters (235U

Noble Metal

Cumulative Yield of Precursor

Direct Yield of Noble Metal

Runs)

Half L i f e of Precursor

Half L i f e of Noble Metal

6.06

0.0

2.4 min

3.00

0.0

1.2 min

66.5 h r s 39.7 days

lo6Ru

0.39

0.0

c1 min.

1.01 y r s

9mTe

0.71

0.0

4.6 h r s

37 days

4.71

0.0

2 . 1 min

6.22

0.0

65 days

77 h r s 35 days

' 3Ru

'32Te 9 5Nb

Time Constant f o r Noble Metals i n Fuel S a l t

I n Section 4.2 w e derived an expression f o r Cs ( t h e noble metal concentration i n f u e l s a l t ) where t h e f u e l loop i s considered t o be a well '

s t i r r e d pot.

For t h i s assumption t o be adequate, t h e reside: -e t i m e

of noble metals i n f u e l salt must be g r e a t e r than t h e c i r c u i t time of s a l t I

ai*ound t h e f u e l loop (%25 seconds).

The r a t e constant i s defined as

With t h e parameters l i s t e d i n Tables B-1 and B-2,

w e can compute t h e t i m e

The value of X i n

constants involved and they are l i s t e d i n Table B-5.

a l l cases i s n e g l i g i b l e s o t h e value of X i s t h e same f o r a l l noble metal

isotopes.

In t h e case of f u e l s a l t with no bubbles and f u e l salt during

t h e 235U runs , t h e w e l l s t i r r e d pot assumption i s adequate. apparently not adequate during t h e 23%J runs.

It i s

However, valves i n t h i s

t a b l e assume t h e s t i c k i n g f r a c t i o n of noble metals t o bubbles of unity.

I n Section 5.2 w e deduced t h a t t h e e f f e c t i v e s t i c k i n g f r a c t i o n t o bubbles m

i s considerably less than unity 'and a value of 0 . 1

- 0.2

i s suggested.

If a s t i c k i n g f r a c t i o n t o bubbles of 0.1 i s assumed, then t h e noble metal

h a l f l i f e i n f u e l s a l t during t h e 23% runs becomes

seconds.

This

i n d i c a t e s t h e w e l l stirred pot assumption i s adequate i n a l l cases.

86 Table B-5 Time Constants of Noble Metals i n Fuel S a l t X (hrs-1)

Fuel S a l t with no Bubbles Fuel S a l t During t h e 235U Runs Fuel S a l t During t h e 233U Runs

H a l f Life i n S a l t

(set)

6.7

370

31.2

402

6.2

' b

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