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Evaluation of Alternate Secondary (and Tertiary) Coolants for the Molten-Salt Breeder Reactor A. D. Kelmers C. F. Baes

E. S. Bettis J. Brynestad S. Cantor J. R. Engel W. R. Grimes H. E. McCoy A. S. Meyer

National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road, Springfield, Virginia 22161 Price: Printed Copy $5.00; Microfiche $2.25 ~~

This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the Energy Research and Development AdministrationKJnited States Nuclear Regulatory Commission, nor any of their employees, nor any of rheir 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 its use would not infringe privately owned rights.

ORNL/TM- 5325 UC-76

(!ontract No. W-740 5-eng- 26 Chemistry Division

EVALUATION OF ALTERNATE SECONDARY (AND TERTIARY) COOLANTS FOR THE MOLTEN-SALT BREEDER REACTOR

A. C. E. J. S. J.

W. H. A.

D. K e l m e r s F. B a e s S. B e t t i s Brynestad Cantor R. E n g e l R. G r i m e s E. McCoy S. Meyer

APRIL 1976

OAK RIDGE NATIONAL LABORATORY Oak R i d g e , T e n n e s s e e operated by

37830

U N I O N CARBIDE CORPORATION for t h e ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

iii

PREFACE Much o f t h i s r e p o r t w a s w r i t t e n d u r i n g t h e p e r i o d September 1974 through March 1975.

S i n c e t h e n , a d d i t i o n a l i n f o r m a t i o n h a s been developed

which b e a r s upon a number o f t h e a l t e r n a t e c o o l a n t c o n s i d e r a t i o n s .

Prog-

ress r e l a t i n g t o s e v e r a l p e r t i n e n t t o p i c s i s summarized i n t h e f o l l o w i n g paragraphs. The d i s c u s s i o n o f NaBF4-NaF (92-8 mole % ) as a p o t e n t i a l secondary coolant (Section 5 ) i n d i c a t e s t h a t , a t t h e t i m e t h i s r e p o r t w a s d r a f t e d , no d a t a were a v a i l a b l e for e s t i m a t i n g t h e tritium r e t e n t i o n c h a r a c t e r i s t i c s of t h i s s a l t , and .that t h e absence o f tritium t r a p p i n g could b e a disadvantage f o r a sing:Le c o o l a n t .

( T r i t i u m trapping i n t h e coolant salt

i s expected t o b e one o:f t h e b e s t p o t e n t i a l methods f o r l i m i t i n g tritium t r a n s p o r t i n t o t h e steam system and t h e n i n t o t h e environment.)

Since

t h a t t i m e some p r e l i m i n a r y experiments have been performed i n engineerings c a l e equipment a t ORNL which i n d i c a t e t h a t t h i s s a l t mixture does have s u b s t a n t i a l tritium t r a p p i n g c a p a b i l i t y . started in July,

These experiments, which were

1975, fin the Coolant-Salt Technology F a c i l i t y , i n v o l v e

t h e a d d i t i o n o f t r i t i a t e d hydrogen t o h i g h - p u r i t y NaBF4-NaF e u t e c t i c t h a t c o n t a i n s no d e l i b e r a t e a d d i t i v e s t o enhance t h e tritium r e t e n t i o n .

The

tritium-hydrogen mixture i s added by d i f f u s i o n through a metal t u b e t o s i m u l a t e t h e d i f f u s i o n through MSBR heat-exchanger t u b e s , b u t a t flow

rates ( p e r u n i t o f t u b e a r e a ) t h a t are 1 04 t o 1 05 t i m e s t h o s e t o be expected i n a r e a c t o r .

Results o f a r e c e n t s t e a d y - s t a t e experiment , i n which t r i t i a t e d hydrogen w a s added t o t h e system f o r more t h a n

4 weeks,

with t h e salt at

811'~ (lOOO째F), i n d i c a t e d t h a t up t o 96% o f t h e tritium w a s t r a p p e d i n t h e s a l t and subsequently r e l e a s e d t o t h e l o o p o f f - g a s .

The apparent

r a t i o o f t h e c o n c e n t r a t i o n s o f combined and e l e m e n t a l tritium i n t h e c i r c u l a t i n g s a l t w a s about 4000.

T h i s r a t i o i s important i n determining

t h e rate a t which tritium can be removed i n a s t r i p p i n g system as opposed t o t h e r a t e a t which t h e e l e m e n t a l form can escape i n t o t h e steam system. While t h e i n f o r m a t i o n t h a t i s c u r r e n t l y a v a i l a b l e i s i n a d e q u a t e f o r

a c c u r a t e e x t r a p o l a t i o n t o t h e r a t e of tritium release t o t h e steam system o f an MSBR, it appears t h a t t h e sodium f l u o r o b o r a t e s a l t mixture would have a s u b s t a n t i a l i n h i b i t i n g e f f e c t on such release and t h a t environm e n t a l l y a c c e p t a b l e r a t e s (<lo C i / d ) could be achieved w i t h r e a s o n a b l e effort.

A l s o , w i t h r e g a r d t o t h e use o f NaBF4-NaF (92-8 mole % ) as a second-

ary c o o l a n t ( S e c t i o n 5 ) , concern w a s expressed about p o t e n t i a l r e a c t i o n s between c o o l a n t and f u e l s a l t LiF-BeF2-ThF -UF

% ) i n t h e event o f mixing. r a t e o f e v a l u t i o n o f BF

4 (71.7-16.0-12.0-0.3

Laboratory experiments have shown:

mole

( i )The

gas on mixing w a s low, about 30 minutes were

r e q u i r e d t o complete t h e r e a c t i o n

Presumably t h e r a t e - l i m i t i n g s t e p was t r a n s f e r o f NaF a c r o s s t h e salt-salt i n t e r f a c e , t h u s , i n a r e a c t o r system w i t h t u r b u l e n t flow, t h e release o f BF

might b e more r a p i d .

However, t h e r e s u l t s are encouraging r e l a t i v e

t o MSBRs i n t h a t very r a p i d gas r e l e a s e r e s u l t i n g i n s i g n i f i c a n t p r e s s u r e s u r g e s w a s n o t experienced.

( i i )No tendency w a s observed f o r t h e f i s s i l e

f u e l s a l t c o n s t i t u e n t s thorium o r uranium t o r e d i s t r i b u t e or t o form more

v c o n c e n t r a t e d s o l u t i o n s ,, o r t o p r e c i p i t a t e f o l l o w i n g mixing o f c o o l a n t

salt i n t o fuel salt.

( i i i )Apparently a n oxide s p e c i e s forms i n t h e

c o o l a n t s a l t phase which i s more s t a b l e t h a n U02, s i n c e no UO

precipi-

t a t i o n was observed even when molten c o o l a n t - f u e l s a l t mixtures were a g i t a t e d w h i l e exposed t o a i r f o r several hours.

Thus, l a r g e amounts

of oltygenated compounds could b e added t o t h e f l u o r o b o r a t e c o o l a n t s a l t for t h e purpose of s e q u e s t e r i n g tritium, s i n c e l e a k a g e o f such a c o o l a n t

s a l t i n t o t h e f u e l salt,would not l e a d t o p r e c i p i t a t i o n o f uranium or thorium. Investigation

o f NaBF4 melts by x-ray powder d i f f r a c t i o n , i n f r a r e d

s p e c t r o s c o p y and Raman spectroscopy have i d e n t i f i e d t h e s t a b l e r i n g compound N a B F 0

3 3 6 3

melts.

as t h e probable oxygen-containing s p e c i e s i n c o o l a n t

Measurements of condensates t r a p p e d from t h e CSTF l o o p show a

tritium c o n c e n t r a t i o n o f 1 05 r e l a t i v e t o t h e s a l t , s u g g e s t i n g t h a t a v o l a t i v e s p e c i e s may be s e l e c t i v e l y t r a n s p o r t i n g tritium from t h e l o o p through t h e vapor.

may e x i s t as a

Recent r e s u l t s i n d i c a t e t h a t BF;2H,O J

molecular compound i n t h e vapor and could be r e s p o n s i b l e f o r t h e tritium trapping. A l l the above resuLts are f a v o r a b l e for t h e u s e o f NaBF4-NaF (92-8

mole % ) melts as an MSBI3 secondary c o o l a n t .

S a t i s f a c t o r y tritium t r a p p i n g ,

an important c o o l a n t c r i t e r i o n , appears h i g h l y p r o b a b l e , vigorous chemical r e a c t i o n s or p r e s s u r e s u r g e s were n o t encountered on mixing c o o l a n t and f u e l s a l t , and p r e c i p i t a t i o n or s e g r e g a t i o n o f f i s s i l e components w a s not encountered. A t e r n a r y s a l t NaF-LiF-BeF

as an a l t e r n a t e coolant ( S e c t i o n

(45-22-33 mole % ) , has been c o n s i d e r e d

6 ) . Two p u b l i s h e d r e f e r e n c e s gave

f r e e z i n g p o i n t s o f 29OoC and 34OoC f o r a m e l t o f t h i s composition.

Since

t h e f r e e z i n g p o i n t i s an i m p o r t a n t c o o l a n t c r i t e r i o n , d i f f e r e n t i a l t h e r m a l a n a l y s i s t e c h n i q u e s were used t o r e i n v e s t i g a t e t h e p o r t i o n of t h e phase diagram near t h i s composition. t i o n NaF-LiF-BeF

The results confirmed t h a t t h e composi-

(45-22-33 mole % ) had t h e lowest l i q u i d u s t e m p e r a t u r e

i n t h i s r e g i o n o f t h e phase diagram.

A f r e e z i n g t e m p e r a t u r e o f about

3 3 5 O C would be a p r a c t i c a l v a l u e f o r e n g i n e e r i n g c o n s i d e r a t i o n s .

vi i

CONTENTS Page

................ 1.1 Comparison of B e s t MSBR Coolants . . . . . . . . . . . 1 . 2 Recommended F u t u r e Work . . . . . . . . . . . . . . . . INTFODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 2 . 1 Purpose and Organization of t h e Report . . . . . . . .

SUMMARY AND RECOMMENDATIONS

...................... 3 . COOLANT CRITERIA . . . . . . . . . . . . . . . . . . . . . . 3 . 1 S a f e t y S i g n i f i c a n t Events . . . . . . . . . . . . . . . 3.2 A n t i c i p a t e d Off-Design T r a n s i e n t s . . . . . . . . . . . 3.3 Design C h a r a c t e r i s t i c s . . . . . . . . . . . . . . . . 4 REJECTED COOLANT CANDIDATES . .. . . .. 4.1 S i n g l e Seconda.ry Coolants . . . . . . . . . . . . . . . 4.2 Dual Coolant Configurations . . . . . . . . . . . . . . 2.2

Terminology

STATUS OF FLUOROBORATE COOLANT 5.1

5.2 5.3 6

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

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

Safety Significant Criteria Off-Design Trainsients Design F a c t o r s

ALTERNATE SINGLE COOLANT:

7 NaF- LiF-BeF

2"""""

.............. ................. .................... 7 . ALTERNATE DUAL COOLANT CANDIDATES . . . . . . . . . . . . . 6.1 6.2 6.3

Safety Significant Criteria Off-Design T r a n s i e n t s Design F a c t o r s

.................. .......... ............. ............ ............ 8 . ACKNOWLEDGEMENTS . .................... 9 . FEFEmNCES . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 7.2 7.3 7.4 7.5

Secondary Coolant Compressed H e l i u m T e r t i a r y Coolant Molten S a l t T e r t i a r y Coolant L i q u i d Metal T e r t i a r y Coolants Fluidized-Bed T e r t i a r y System

1 1 9 13 13

14 15 17 18

23 27 27 36 38 39 40 45 48 48 49 51 56 57 59

65 69 73 74 75

viii

DEDICATION We d e d i c a t e t h i s r e p o r t t o o u r l a t e f r i e n d and c o l l e a g u e ,

A. S.

(All

Meyer, whose u n t i r i n g e f f o r t s i n developing m o l t e n - s a l t

r e a c t o r technology and u n f a i l i n g good humor w i l l always be remembered.

1 1.

SUMMARY AND RECOMMENDATIONS

1.1 Comparison of Best MSBR Coolants The t h r e e most promising c o o l a n t s e l e c t i o n s f o r an MSBR have been i d e n t i f i e d and e v a l u a t e d i n d e t a i l from t h e many c o o l a n t s c o n s i d e r e d i n t h i s r e p o r t for a p p l i c a t i o n e i t h e r as a secondary c o o l a n t i n 1000-MW(e) MSBR c o n f i g u r a t i o n s u s i n g o n l y one c o o l a n t , or as secondary and t e r t i a r y c o o l a n t s i n an MSBR d u a l c o o l a n t c o n f i g u r a t i o n employing two d i f f e r e n t coolants.

These a r e :

as s i n g l e secondary c o o l a n t s ,

(1) a t e r n a r y sodium-lithium-berylliunfluoride melt (Na1.34Li0.66BeF4), (2)

t h e sodium fluoroborate-sodium f l u o r i d e e u t e c t i c m e l t , t h e p r e s e n t r e f e r e n c e design secondary c o o l a n t ,

and, i n t h e case o f t h e d u a l c o o l a n t c o n f i g u r a t i o n ,

(1) molten l i t h i u m - b e r y l l i m f l u o r i d e ( L i BeF 2

as t h e secondary

c o o l a n t and helium gas as t h e t e r t i a r y c o o l a n t .

A s t r a i g h t f o r w a r d comparison o f advantages and d i s a d v a n t a g e s has been made i n t h e case of t h e two s i n g l e secondary c o o l a n t c a n d i d a t e s and t h i s comparison f a v o r s t h e t e r n a r y f l u o r i d e melt by a s l i g h t margin, p r i m a r i l y because o f problems t o be encountered i f t h e f l u o r o b o r a t e secondary c o o l a n t and

f u e l s a l t were t o become mixed.

The a p p l i c a t i o n of sodium f l u o r o b o r a t e

m e l t as t h e s i n g l e secondary c o o l a n t may o f f e r some p o t e n t i a l , y e t unproven, advantage i n t r i t i u m t r a p p i n g which could o f f s e t o t h e r l e s s d e s i r a b l e characteristics.

The coo:Lants s e l e c t e d f o r t h e d u a l c o o l a n t c o n f i g u r a t i o n

a p p e a r , on t h e b a s i s of pi?esent knowledge, t o r e s o l v e a l l t e c h n o l o g i c a l problems and, i n a d d i t i o n , t o o f f e r o p e r a t i o n a l advantages; however, t h e a d d i t i o n o f a n o t h e r h e a t - t r a n s f e r loop t o t h e MSBR would d e c r e a s e t h e r m a l

e f f i c i e n c y and e n t a i l an i n c r e a s e i n both c a p i t a l and o p e r a t i n g c o s t s .

d i r e c t comparison between t h e more a t t r a c t i v e s i n g l e secondary c o o l a n t and t h e d u a l , secondary and t e r t i a r y , c o o l a n t concepts cannot now be made s i n c e t e c h n o l o g i c a l f a c t o r s on t h e one hand must be e v a l u a t e d r e l a t i v e t o economic c o n s i d e r a t i o n s on t h e o t h e r .

The f u t u r e work n e c e s s a r y t o r e a c h such a

comparison has been i d e n t i f i e d and i s d e t a i l e d i n S e c t i o n 1 . 2 .

1.1.1 S i n g l e c o o l a n t s

I n t h i s MSBR c o n f i g u r a t i o n , h e a t i s t r a n s f e r r e d from t h e f u e l s a l t ( t h e primary c o o l a n t ) to t h e c o o l a n t ( t h e secondary c o o l a n t ) i n s e v e r a l primary h e a t exchangers.

The h e a t e d c o o l a n t i s t h e n c i r c u l a t e d i n independent loops

t o t h e steam g e n e r a t o r s .

A s i d e stream could be withdrawn and processed t o

maintain t h e d e s i r e d c o o l a n t redox p o t e n t i a l . Many p o t e n t i a l secondary c o o l a n t c a n d i d a t e s were c o n s i d e r e d and r e j e c t e d ( S e c t i o n 41, p r i m a r i l y due t o ( i )p o t e n t i a l l y s a f e t y s i g n i f i c a n t i n c o m p a t i b i l i t y w i t h t h e f u e l s a l t or ( i i ) unacceptable c o r r o s i v i t y .

metals or compressed g a s e s appeared unacceptable f o r t h e s e r e a s o n s .

Liquid Of t h e

numerous molten s a l t s c o n s i d e r e d , two appeared s u b s t a n t i a l l y b e t t e r and were e v a l u a t e d i n d e t a i l ,

These are sodium f l u o r o b o r a t e [NaBF -NaF (92-8 4

mole %I, t h e c u r r e n t r e f e r e n c e d e s i g n c o o l a n t ] which i s c o n s i d e r e d i n S e c t i o n 5 , and a t e r n a r y sodium-lithium-beryllium f l u o r i d e (Na1.34Li0.66 BeF4) which i s reviewed i n S e c t i o n 6. A one-on-one

comparison of t h e s e two c a n d i d a t e s w a s c a r r i e d o u t i n

terms o f s e l e c t e d c o o l a n t c r i t e r i a ( S e c t i o n 3 ) and i s p r e s e n t e d i n Table 1.1.

Consideration of p o t e n t i a l l y s a f e t y s i g n i f i c a n t e v e n t s , o f f - d e s i g n

t r a n s i e n t s , and design f a c t o r s are covered i n t h r e e groups o f items.

t h e case o f s a f e t y s i g n i f i c a n t c r i t e r i a , items l a and l b i n Table 1.1,

Change i n nuclear r e a c t i v i t y i n case of l e a k i n t o primary system.

Presence of 'OB precludes any increase i n r e a c t i v i t y due t o bubbles o r voids i n t h e core from BF gas. 3

None.

Chemical r e a c t i o n s i n case of f u e l salt coolant interleakage.

Pressure surges c a w e d by t h e release of BF might 3 t h r e a t e n primary system boundary.

None

Released BF w i l l dissolve i n t h e f u e l salt, may penetrate t J e core graphite, and could harm t h e charcoal beds. BF i n the f u e l salt can be mostly removed by i n e r t - g L sparge; lesser amounts can be burned out neutronically.

No evolution o f gas t o a f f e c t g r a p h i t e o r charcoal. NaF i n t h e f u e l salt w i l l reduce breeding gain by a minor amount.

Off-3esign Transients a.

Leak of coolant i n t o primary system.

Leak o f f u e l salt i n t o coolant.

BeF2, UF4, ThF4, di-and t r i v a l e n t f i s s i o n product f l u o r i d e s , noble-metal f i s s i o n products a r e all insoluble.

Only noble-metal f i s s i o n products are insoluble, all f i s s i l e materials a r e completely soluble.

Leak o f steam i n t o coolant.

Corrosion of m e t a l s , formation of s o l u b l e oxides and low p a r t i a l pressure (<1a t m ) of HF. Heat exchange surfaces could b e fouled by insoluble corrosion product f l u o r i d e s , Na CrF6 and perhaps NaNiF 3 3'

Corrosion of metals, formation of insoluble Be0 and low p a r t i a l pressure of HF. Heat exchange surfaces could be fouled by BeO, and perhaps by N a CrF6 and NaNiF3. 3

Leak of coolant i n t o steam system.

Very l i t t l e i s known about t h e e f f e c t s (such as stress corrosion cracking) of f l u o r i d e s of steam systems.

Leaks t o c e l l atmosphere.

Coolant r e a c t s with moisture t o y i e l d a c i d i c vapors.

Coolant r e a c t s with moisture t o produce Be0 and a c i d i c vapors.

E f f e c t s of mixing f u e l s a l t and coolant a r e l e s s troublesome with Nal. 3hLiO.

66BeF4*

E f f e c t s of mixing steam ,and coolant are roughly t h e same f o r both coolants.

No advantage o r serious problem with e i t h e r coolant.

Table 1.1 (Continued) Criteria

Nal.34Li0.66BeF4

NaBF4-NaF

Comparison

3. Design Factors a.

Corrosivity

Freezing Point

Heat Transfer and Hydrodynamic Properties

d. Vapor Pressure

Radiation Stability

f. Tritium Trapping

Boron in coolant can be reduced by metallic chromium and by some minor alloy constituents.

All properties measured.

Thermal conductivity: k = 0.4 W m-l cp = 0.36 cal g-1 O K - 1 Heat capacity: Viscosity: llb54oc = 1-91 CP; 116210~= 1-07 CP p5500c = 1.86 g cm-3 Density :

Cost and Availability

Less (and possibly no) preheating of feedwater will be necessary if Na .34Li(166 BeF4 is the coolan*.

All properties estimated. k = 0.85 W m-l OK-’ Cp = 0.46 cal g-l Or1 nb54oC = 22 CP; 116210~= 7.4 CP p5500c = 2.1 g cm-3

Overall roughly equivalent. Film coefficient and volumetric heat-capacity better for.Nal,34Li0.66BeF4. Kinematic viscosity more favorable for NaBF4-NaF. Slight advantage for

Sizeable BF decomposition pressure. 3

Insignificant vapor pressure.

No chemical decomposition due to gammas. Neutrons transmute ’OB with no significant increase in corrosivity.

No chemical decomDosition due to ganrmas. No effects on chemical stability due to neutronically induced transmutations.

Both acceptable.

Isotope-exchange and/or oxidizing additives necessary.

Possible advantage for BaBF4-BaF.

8400 ft3 cost $6M.

Clear advantage for NaBFq-NaF.

8400 ft3 cost $0.3’i’M. elements.

Corrosivity less for although %*&~%%~~F& not be significant since corrosion will probably be governed by additives necessary to sequester tritium.

34OoC(554 - 644OF) Two references disagree.

290

High solubility of oxide ion may improve tritium trapping. BF3’H20 vapor species may also be significant. g.

The coolant will not react with alloy constitutents.

Coolant consists of common

Nal. 3hLiO.66BeF4*

t h e t e r n a r y f l u o r i d e is p r e f e r r e d over sodium f l u o r o b o r a t e which would

release BF

gas i n t h e e v e n t of f u e l - s a l t - c o o l a n t

interleakage.

The

p o t e n t i a l l y s a f e t y s i g n i f i c a n t release o f BF3 gas i s t h e most s e r i o u s n e g a t i v e f a c t o r i n c o n s i d e r i n g f l u o r o b o r a t e as a secondary c o o l a n t ( s e e S e c t i o n 5.1.2).

I n t h e case of off-design t r a n s i e n t s , BF

r e l e a s e as a

r e s u l t of minor l e a k s , item 2a, i s a g a i n a problem which is a b s e n t w i t h the ternary fluoride m e l t .

F i s s i l e materials might r e d i s t r i b u t e between

immiscible phases a f t e r l e a k s i n t h e case of f l u o r o b o r a t e w h i l e t h e y would b e completely s o l u b l e i n t h e t e r n a r y f l u o r i d e m e l t , item 2b, a g a i n favori n g t h i s coolant.

I n c o n s i d e r i n g design f a c t o r s , d i f f e r e n c e s i n c o r r o s i -

v i t y o f t h e melts toward H a s t e l l o y N , i t e m 3a, s l i g h t l y f a v o r t h e t e r n a r y f l u o r i d e m e l t b u t t h e d i f f e r e n c e s are minor and i n e i t h e r case c o r r o s i o n

w i l l probably be governed by t h e c o n d i t i o n s s e l e c t e d t o s e q u e s t e r t r i t i u m i n t h e secondary coolant,,

The f r e e z i n g p o i n t c r i t e r i o n , i t e m 3b, c l e a r l y

f a v o r s t h e t e r n a r y fluori.de m e l t s i n c e i t s lower f r e e z i n g p o i n t would req u i r e l e s s p r e h e a t i n g of t h e feedwater.

Heat t r a n s f e r and hydrodynamic

p r o p e r t i e s , item 3c, and r a d i a t i o n s t a b i l i t y , item 3e, are e q u i v a l e n t f o r t h e two c a n d i d a t e s and i n . e i t h e r case q u i t e adequate f o r MSBR a p p l i c a t i o n . The n e c e s s i t y o f m a i n t a i n i n g a f i x e d BF

vapor p r e s s u r e over t h e f l u o r o -

b o r a t e c o o l a n t , i t e m 3d, g i v e s a s l i g h t advantage t o t h e t e r n a r y f l u o r i d e

melt.

The probable n e c e s s i t y o f t r a p p i n g some p o r t i o n of t h e t r i t i u m i n

t h e secondary c o o l a n t , item 3 f , may f a v o r f l u o r o b o r a t e , although a t t h i s

t i m e t r i t i u m t r a p p i n g has n o t been demonstrated e x p e r i m e n t a l l y i n f l u o r o borate m e l t .

F i n a l l y , c o s t and a v a i l a b i l i t y , i t e m 3g, c l e a r l y f a v o r s

f l u o r o b o r a t e s i n c e beryll.im and 99.991- % 7 L i would be r e q u i r e d f o r t h e t e r n a r y f l u o r i d e melt.

The c o s t f o r t h e Na1,34Li0.66

BeF4 c o o l a n t i s l e s s

t h a n t h a t f o r an e q u i v a l e n t volume of Li2BeFq s i n c e t h e l i t h i u m c o n t e n t

i s lower. The r e s u l t of t h i s comparison i s t h a t t h e t e r n a r y f l u o r i d e m e l t i s favored by a m a j o r i t y o f t h e c r i t e r i a , e s p e c i a l l y t h o s e a s s o c i a t e d with p o t e n t i a l l y s a f e t y s i g n i f i c a n t e v e n t s and with t h e a b i l i t y t o cope w i t h off-design t r a n s i e n t s .

The m i n o r i t y of c r i t e r i a t h a t do f a v o r t h e f l u o r o -

b o r a t e c o o l a n t are i n t h e area of d e s i g n f a c t o r s , where v a r i o u s a s p e c t s of t h e t e r n a r y f l u o r i d e c o o l a n t t h a t are l e s s s u i t a b l e could be accommodated by s u i t a b l e e n g i n e e r i n g d e s i g n c o n s i d e r a t i o n s .

While t h e t e r n a r y f l u o r i d e

m e l t appears t o be t h e more s u i t a b l e c o o l a n t f o r an MSBR d e s i g n employing a s i n g l e secondary c o o l a n t , t h e sodium f l u o r o b o r a t e c o o l a n t would l i k e l y

be p r e f e r r e d if it can be shown t o a i d s i g n i f i c a n t l y i n t r i t i u m management i n an MSBR.

1.1.2

Dual c o o l a n t s

I n t h i s MSBR c o n f i g u r a t i o n , h e a t i s t r a n s f e r r e d from t h e f u e l

salt

( t h e primary c o o l a n t ) t o a secondary c o o l a n t i n s e v e r a l primary h e a t exchangers.

The h e a t e d secondary c o o l a n t i s c i r c u l a t e d i n s e v e r a l l o o p s t o

i n t e r m e d i a t e h e a t exchangers where t h e h e a t i s t r a n s f e r r e d t o a t e r t i a r y c o o l a n t which i s t h e n c i r c u l a t e d i n s e v e r a l loops t o t h e s t e a m - r a i s i n g system.

S i d e stream p r o c e s s i n g might b e r e q u i r e d on b o t h t h e secondary

and t e r t i a r y c o o l a n t loops t o remove t r i t i u m , remove c o r r o s i o n p r o d u c t s and/or a d j u s t t h e c o o l a n t redox p o t e n t i a l . Three combinations of d u a l c o o l a n t s were e v a l u a t e d i n d e t a i l f o r t h i s MSBR c o n f i g u r a t i o n ( S e c t i o n 7 ) . lithium-beryllim f l u o r i d e MSRE.

I n each case t h e secondary c o o l a n t was

7Li2BeFq

, the

c o o l a n t used p r e v i o u s l y i n t h e

I t was s e l e c t e d because of i t s complete c o m p a t i b i l i t y with t h e f u e l

s a l t i n t h e e v e n t of mixing due t o l e a k s i n t h e primary h e a t exchanger or o t h e r causes.

I n a d d i t i o n , i t s r e l a t i v e l y high m e l t i n g p o i n t h e l p s de-

crease t h e p o s s i b i l i t y of f u e l salt freezing during thermal t r a n s i e n t s . A l l d e s i g n f a c t o r s a l s o :favored t h i s secondary c o o l a n t with t h e exception

o f c o s t due t o i t s 7 L i c o n t e n t .

Three d i f f e r e n t t e r t i a r y c o o l a n t s were

e v a l u a t e d , a compressed gas (helium, S e c t i o n 7.21,

a d i f f e r e n t molten s a l t

( a t e r n a r y carbonate m e l t , S e c t i o n 7.3) and a l i q u i d metal (molten sodium, S e c t i o n 7.4).

Of t h e s e t h r e e , compressed helium was by f a r t h e most a t -

t r a c t i v e c a n d i d a t e and i s t h e o n l y one recommended f o r f u r t h e r c o n s i d e r a t i o n . Liquid sodium was considered t o be less a t t r a c t i v e due t o s e v e r e chemical i n c o m p a t i b i l i t y w i t h both t h e secondary c o o l a n t and t h e steam i n c a s e of l e a k s , and problems a s s o c i a t e d w i t h t h e r m a l shock t o s t r u c t u r a l components.

T r i t i u m t r a p p i n g methods are being developed for t h e LMFBR b u t may n o t be adequate for MSBRs.

A molten carbonate t e r t i a r y c o o l a n t was more s u i t a b l e

t h a n l i q u i d sodium s i n c e it could r e a d i l y a f f o r d methods of t r a p p i n g l a r g e amounts o f t r i t i u m ; however, it i s chemically r e a c t i v e with t h e secondary c o o l a n t , r e l e a s i n g C02 gas on mixing, and l i t t l e i n f o r m a t i o n i s a v a i l a b l e concerning m a t e r i a l s of adequate c o r r o s i o n r e s i s t a n c e t o c o n s t r u c t t h e t h i r d loop.

Thus t h e c a r b o n a t e t e r t i a r y c o o l a n t concept was n o t f e l t t o

warrant a d d i t i o n a l a t t e n t i o n a t t h i s t i m e . The use of compressed helium (700 p s i a ) as t h e t e r t i a r y c o o l a n t i n an MSBR concept coupled with molten

7 L i BeF as t h e secondary c o o l a n t appears 2 4

t o meet a l l t e c h n o l o g i c a l requirements for an MSBR, b u t only a t some addit i o n a l c o s t f o r c o n s t r u c t i o n and o p e r a t i o n .

The advantages a r e :

(1) t r i t i u m can be r e a d i l y t r a p p e d by t h e a d d i t i o n of 0

and/or H 0 2

a t low c o n c e n t r a t i o n t o t h e helium loop; t h e p r o p o r t i o n s w i l l be

dependent upon t h e r a t e of back d i f f u s i o n o f normal hydrogen from t h e steam system. (2) the

7 Li2BeF4 secondary c o o l a n t i s completely compatible w i t h

t h e f u e l s a l t on i n t e r m i x i n g , t h u s l e a k a g e of secondary c o o l a n t i n t o t h e primary c i r c u i t i s n o t a s e r i o u s matter ( 3 ) steam l e a k s i n t o t h e helium loop from t h e s t e a m - r a i s i n g system would n o t cause a major i n c r e a s e i n c o r r o s i o n o f t h e t e r t i a r y l o o p n o r would helium l e a k i n g i n t o t h e steam system l e a d t o damage ( 4 ) o p e r a t i o n and c o n t r o l of t h e MSBR i s s i m p l i f i e d by t h e " s o f t " coupling i n t r o d u c e d by t h e helium loop ( 5 ) s t a r t - u p of t h e MSBR i s e a s i e r and a much smaller a u x i l i a r y steam

g e n e r a t o r would be r e q u i r e d t h a n i n t h e case o f an a l l molten s a l t MSBR

(6) t h e p o s s i b i l i t y of f u e l f r e e z i n g on t h e r m a l t r a n s i e n t s i s g r e a t l y reduced ( 7 ) steam g e n e r a t o r technology a l r e a d y developed f o r t h e HTGR could

be adapted f o r t h i s MSBR c o n f i g u r a t i o n ( 8 ) p l a n t a v a i l a b i l i t y and m a i n t a i n a b i l i t y would be improved by t h e

added p a s s i v e b a r r i e r i n t r o d u c e d by t h e t e r t i a r y loop. The only apparent d i s a d v a n t a g e s t o t h i s MSBR c o n f i g u r a t i o n are t h e added c o s t and decreased t h e r m a l e f f i c i e n c y .

The c o s t i n c r e a s e comes

p r i m a r i l y from t h e hardware r e q u i r e d f o r t h e t h i r d loop and from t h e decreased t h e r m a l e f f i c i e n c y .

The d e c r e a s e i n t h e r m a l e f f i c i e n c y r e s u l t s

from t h e pumping power n e c e s s a r y t o c i r c u l a t e t h e helium. l i m i n a r y e s t i m a t e s ( S e c t i o n 7.3.2)

Very pre-

i n d i c a t e t h a t t h e added c o s t may be

9 I

r e l a t i v e l y modest, b u t a more d e t a i l e d a n a l y s i s i s needed.

1.2

Recommended Future Work

I n o r d e r t o r e a c h a f i n a l choice of a c o o l a n t or d u a l c o o l a n t s f o r an MSBR, a d d i t i o n a l c o s t information i s r e q u i r e d s o t h a t a q u a n t i t a t i v e comp a r i s o n can b e made amon,g t h e t h r e e c o o l a n t s e l e c t i o n s d e f i n e d i n S e c t i o n 1.1.

The f o l l o w i n g work i s recommended:

(1) p r e l i m i n a r y e n g i n e e r i n g conceptual d e s i g n s and c o s t e s t i m a t e s of a 1000-MW(e) MS:BR with a s i n g l e c o o l a n t c o n f i g u r a t i o n employing e i t h e r t h e t e r n a r y f l u o r i d e melt o r sodium f l u o r o b o r a t e as t h e secondary c o o l a n t .

S i m i l a r i n f o r m a t i o n should be developed simul-

t a n e o u s l y f o r t h e d u a l c o o l a n t c o n f i g u r a t i o n with molten Li2BeF

as t h e secondary c o o l a n t and compressed helium as t h e t e r t i a r y coolant. (2) d e f i n i t i o n of t h e t r i t i u m trapping capability of t h e coolants,

e i t h e r secondary or t e r t i a r y , and e x p e r i m e n t a l demonstration of such t r a p p i n g i n helium, sodium f l u o r o b o r a t e and t h e t e r n a r y f l u o r i d e m e l t , and

( 3 ) e x p e r i m e n t a l e v a l u a t i o n o f t h e f u e l salt-sodium f l u o r o b o r a t e

compatibility questions. The recommended work i t e n i s s h o u l d b e c a r r i e d o u t c o n c u r r e n t l y .

The e x p e r i -

mental work c a l l e d f o r i n recommendations 2 and 3 would h e l p supply accu-

r a t e i n f o r m a t i o n f o r concieptual d e s i g n work and c o s t d a t a .

Simultaneously,

as t h e c o n c e p t u a l d e s i g n s and c o s t e s t i m a t e s advance t h e y w i l l h e l p guide t h e experimental work t o t h e most c r i t i c a l areas.

A d i r e c t comparison of

t h e s i n g l e vs d u a l c o o l a n t MSBR c o n f i g u r a t i o n s and a f i n a l s e l e c t i o n o f an

10 MSBR c o o l a n t ( s ) can be made o n l y a f t e r t h e s e recommendations are c a r r i e d out.

Reduction of t h e comparison of t h e competing concepts t o a comparison

o f c o n s t r u c t i o n and o p e r a t i n g c o s t s p r o v i d e s t h e o n l y means f o r q u a n t i t a t i v e evaluation.

1.2.1

Conceptual d e s i g n s and c o s t estimates Development of c a p i t a l and o p e r a t i n g c o s t s adequate t o make a mean-

i n g f u l comparison among t h e c o o l a n t c h o i c e s and c o n f i g u r a t i o n s i s t h e most important recommendation.

Dollars are t h e o n l y common denominator among

t h e d i s p a r a t e f a c t o r s t h a t must be e v a l u a t e d and a comparison cannot be made u n t i l c o s t estimates are a v a i l a b l e .

Work o f t h i s n a t u r e done d u r i n g

t h e p r e p a r a t i o n of t h i s r e p o r t was, o f n e c e s s i t y , q u i t e l i m i t e d and i s u s e f u l o n l y i n s u g g e s t i n g t h a t c o s t s a s s o c i a t e d w i t h t h e helium t e r t i a r y loop may n o t be u n a t t r a c t i v e . Adequate i n f o r m a t i o n should be developed t o a i d i n t h e s e l e c t i o n of f l u o r o b o r a t e or t h e t e r n a r y f l u o r i d e m e l t i n s i n g l e c o o l a n t c o n f i g u r a t i o n s . I t i s a n t i c i p a t e d t h a t problems a s s o c i a t e d w i t h f u e l s a l t - c o o l a n t i n t e r mising ( S e c t i o n 1.2.3)

may p l a y a dominant r o l e i n t h e s e l e c t i o n and may

favor the ternary fluoride.

The e x t e n t t o which sodium f l u o r o b o r a t e can

assist i n t r i t i u m management i s a l s o q u i t e important.

In evaluating the

d u a l c o o l a n t c o n f i g u r a t i o n , h e a t t r a n s f e r c a l c u l a t i o n s and estimates o f t h e s a l t volumes f o r t h e primary and secondary loops w i l l be important.

Also, s i z i n g of helium-loop components, d u c t s , c i r c u l a t o r s , steam gener a t o r s and t r i t i u m removal systems should b e done w i t h g r e a t e r accuracy. The c o s t and a v a i l a b i l i t y of 7 L i compounds should be b e t t e r d e f i n e d .

Details t o be considered i n t h e conceptual d e s i g n i n c l u d e , f o r example, p r e s s u r e r e l i e f mechanisms i n t h e c o o l a n t loop i n case of major steam

11 i n l e a k a g e , how t o d e a l w i t h or p r e v e n t c o o l i n g t h e f u e l s a l t below i t s l i q u i d u s temperature and s i z i n g t h e a u x i l i a r y steam system.

Also q u e s t i o n s

such as e s t a b l i s h i n g r e l a t i v e l e v e l s o f p l a n t a v a i l a b i l i t y and maintaina b i l i t y i n d i f f e r e n t c o n f i g u r a t i o n s should be considered. 1.2.2

Tritium trapping

I t i s recommended t h a t adequate e x p e r i m e n t a l d a t a b e developed t o d e f i n e t h e c a p a b i l i t y of t r i t i u m t r a p p i n g i n t h e t h r e e c o o l a n t s : t h e t e r n a r y f l u o r i d e melt and t h e f l u o r o b o r a t e m e l t .

helium,

Approximately 2400 C i

of t r i t i u m p e r day w i l l be g e n e r a t e d i n t h e MSBR f u e l s a l t .

Only about

0.1% o f t h i s material can be p e r m i t t e d t o d i f f u s e through t h e steam gene-

r a t o r s t o t h e steam system, from which it would b e d i s c h a r g e d t o t h e environment ( S e c t i o n 3.3.5). t r i t i u m i n t h e MSBR.

(1) t h e U4'/U3+

Many f a c t o r s a f f e c t t h e d i s t r i b u t i o n of

These i n c l u d e : r a t i o i n t h e fuel s a l t , which c o n t r o l s t h e r a t i o of

TH/ ( T ,H ) F ( 2 ) a b i l i t y o f t h e (core g r a p h i t e t o s o r b t r i t i u m and/or (T,H)F

( 3 ) t r i t i u m d i f f u s i o n through t h e primary system p r e s s u r e boundary

t o t h e c e l l atmosphere ( 4 ) t r i t i u m t r a p p i n g i n t h e secondary

or t e r t i a r y c o o l a n t

( 5 ) decreased p e r m e a b i l i t y t o t r i t i u m i n t h e steam g e n e r a t o r tube

walls due t o oxide f o r m a t i o n on t h e steam s i d e . C u r r e n t l y , none o f t h e s e f a c t o r s has been a d e q u a t e l y q u a n t i f i e d . mental work i s under way t o i n v e s t i g a t e items (11, ( 4 ) and ( 5 ) .

ExperiPara-

metric s t u d i e s i n d i c a t e t h a t perhaps h a l f or more of t h e t r i t i u m must be t r a p p e d i n t h e c o o l a n t i n o r d e r t o l i m i t t h e environmental r e l e a s e t o no more t h a n 1 t o 2 C i p e r day.

12 Since t r i t i u m t r a p p i n g i n t h e c o o l a n t has been e s t a b l i s h e d as a s i g n i f i c a n t c r i t e r i o n and t h e a b i l i t y , o r l a c k o f a b i l i t y , t o s e q u e s t e r t r i t i u m has been considered a major f a c t o r i n f a v o r i n g some c o o l a n t c a n d i d a t e s , e x p e r i m e n t a l work on each of t h e f i v e f a c t o r s d e f i n e d i n t h e preceding paragraph w i l l be needed, i n c l u d i n g l a b o r a t o r y experiments and c i r c u l a t i n g loops with p r o v i s i o n s f o r removal o f t r a p p e d t r i t i u m from t h e respective coolant.

P o t e n t i a l environmental impacts due t o r e a c t o r

o p e r a t i o n are c u r r e n t l y r e c e i v i n g i n c r e a s e d a t t e n t i o n and e s t a b l i s h m e n t of an a c c e p t a b l e l e v e l o f t r i t i u m release and e x p e r i m e n t a l demonstration t h a t t h i s could be achieved i n an MSBR are important a s p e c t s i n t h e u l t i -

mate s e l e c t i o n of a p r a c t i c a l and a c c e p t a b l e c o o l a n t or c o o l a n t s f o r t h e

MSBR. 1.2.3

Fuel-salt f l u o r o b o r a t e mixing problems Problems a s s o c i a t e d w i t h t h e i n t e r m i x i n g o f sodium f l u o r o b o r a t e and

f u e l s a l t need t o be more c a r e f u l l y d e f i n e d . t h e primary c i r c u i t w i l l g e n e r a t e BF

gas.

Leakage o f f l u o r o b o r a t e i n t o Over a wide r a n g e of e q u i l i -

brium c o n d i t i o n s t h e r e s u l t i n g p r e s s u r e may n o t b e l a r g e ; however, under dynamic c o n d i t i o n s much g r e a t e r p r e s s u r e t r a n s i e n t s c o u l d b e developed. Such s i t u a t i o n s may be s a f e t y s i g n i f i c a n t and s h o u l d b e a s s e s s e d c a r e f u l l y . If f u e l s a l t l e a k s i n t o t h e secondary c i r c u i t , t h e f i s s i l e materials would

be r e l a t i v e l y i n s o l u b l e i n t h e f l u o r o b o r a t e and would p r e c i p i t a t e .

Addi-

t i o n a l i n f o r m a t i o n i s needed t o understand t h e complicated s a l t system formed a f t e r mixing and t o d e f i n e t h e c o n c e n t r a t i o n o f t h e v a r i o u s components i n t h e phase o r phases which r e s u l t .

Information o f t h i s t y p e w i l l

h e l p e s t a b l i s h t h e s e r i o u s n e s s of f u e l s a l t - c o o l a n t i n t e r m i x i n g , an i m portant evaluation criterion.

13 2.

2.1

INTRODUCTION

Purpose and Organization o f t h e Report

The purpose o f t h i s r e p o r t i s t o e v a l u a t e a l t e r n a t e secondary (and While

t e r t i a r y ) c o o l a n t s f o r t h e Molten-Salt Breeder Reactor (MSBR).

e x t e n s i v e e x p e r i e n c e has been accumulated f o r many y e a r s w i t h molten f u e l

s a l t s ,l i n c l u d i n g o p e r a t i o n of t h e Molten-Salt Reactor Experiment , t h e s e l e c t i o n and e v a l u a t i o n of an MSBR secondary c o o l a n t has r e c e i v e d l e s s attention.

A sodium f l u o r o b o r a t e melt, a c t u a l l y t h e e u t e c t i c composition

NaBF4-NaF (92-8 mole %), was proposed design coolant.

i n 1965 and i s t h e c u r r e n t r e f e r e n c e

It has been recognized, however, t h a t t h i s sodium f l u o r o -

b o r a t e m e l t i s less t h a n i d e a l i n some

and, t h e r e f o r e , an

e v a l u a t i o n o f f l u o r o b o r a t e and a l t e r n a t e c o o l a n t s was c a r r i e d o u t .

This

r e p o r t comprises t h e f i n d i n g o f t h a t e v a l u a t i o n . F i r s t , a s e t o f c o o l a n t c r i t e r i a was e s t a b l i s h e d t h a t would be p e r t i -

n e n t r e g a r d l e s s of what t h e c o o l a n t c h o i c e s might be.

The c r i t e r i a were

divided i n t o three categories: ( i ) safety significant events, ( i i ) antic i p a t e d o f f - d e s i g n t r a n s i e n t s , and ( i i i ) d e s i g n c h a r a c t e r i s t i c s .

These

c r i t e r i a , p r e s e n t e d i n S e c t i o n 3 , were t h e n used t o e v a l u a t e v a r i o u s al-

t e r n a t e c o o l a n t c a n d i d a t e s as w e l l as t o r e e v a l u a t e t h e sodium f l u o r o b o r a t e e u t e c t i c mixture.

A s t h e c r i t e r i a were developed and a p p l i e d , t h e f i r s t

two c a t e g o r i e s dominated many c o n s i d e r a t i o n s and r e s u l t e d i n t h e r e j e c t i o n

of a number of c o o l a n t c a n d i d a t e s ( S e c t i o n 4).

The s t a t u s o f f l u o r o b o r a t e

c o o l a n t , r e l a t i v e t o t h e c r i t e r i a , is p r e s e n t e d i n S e c t i o n 5.

In Sections

6 and 7 , e v a l u a t i o n s o f s e v e r a l p o t e n t i a l a l t e r n a t e c o o l a n t concepts f o r t h e MSBR are p r e s e n t e d .

14 Based on t h e a v a i l a b l e information and r e a s o n a b l e estimates, t h e c o o l a n t s ( f l u o r o b o r a t e and a l t e r n a t e s ) are e v a l u a t e d by degree o f compliance with t h e c o o l a n t c r i t e r i a and p r o b a b i l i t y of s u c c e s s f u l development and a p p l i c a t i o n t o an MSBR.

Also, recommendations are made r e l a t i v e t o

t h e work needed, e i t h e r experimental or conceptual d e s i g n , t o r e s o l v e unknown areas t o permit a f i n a l s e l e c t i o n of an MSBR c o o l a n t or c o o l a n t s . Terminology

2.2

D e f i n i t i o n s f o r some of t h e terms used f r e q u e n t l y i n t h i s r e p o r t follows. Primary Loop

The f i r s t c i r c u l a t i n g loop i s r e f e r r e d t o as t h e p r i -

mary or f i r s t c o o l a n t loop, s i n c e it a c c e p t s t h e h e a t g e n e r a t e d by n u c l e a r This loop c o n t a i n s t h e f u e l s a l t , or primary c o o l a n t ,

f i s s i o n i n t h e core.

which i s c i r c u l a t e d t o t h e primary h e a t exchangers where t h e h e a t i s t r a n s f e r r e d t o t h e f l u i d i n t h e n e x t loop. Secondary Loop

The second c i r c u l a t i n g loop c o n t a i n s t h e secondary

c o o l a n t , which i s t h e f i r s t c o o l a n t o t h e r t h a n t h e f u e l s a l t .

I n t h e con-

c e p t u a l design t h i s c o o l a n t i s a f l u o r o b o r a t e e u t e c t i c m e l t and i s used t o t r a n s f e r h e a t from t h e primary h e a t exchangers t o t h e s t e a m - r a i s i n g system.

Tertiary Loop ( o p t i o n a l )

I n some conceptual MSBR c o n f i g u r a t i o n s a

t h i r d c o o l a n t loop is employed which c o n t a i n s t h e t e r t i a r y c o o l a n t , or t h e second c o o l a n t t r a n s f e r s i t s h e a t v i a i n t e r m e d i a t e h e a t exchangers t o t h e t e r t i a r y c o o l a n t which then c i r c u l a t e s between t h e i n t e r m e d i a t e h e a t exchangers and t h e steam-raising system. Coolant

When used without a d e s c r i b i n g a d j e c t i v e , t h e term "coolant"

r e f e r s t o t h e f l u i d (molten s a l t , l i q u i d metal or compressed g a s ) , w i t h i n

15 t h e secondary or t e r t i a r y loop under c o n s i d e r a t i o n . Steam-Raising System

This r e f e r s t o t h e h e a t exchangers which

t r a n s f e r h e a t from e i t h e r t h e secondary or t e r t i a r y c o o l a n t t o t h e water

or s u p e r c r i t i c a l steam. Steam System

This term d e s c r i b e s t h e e n t i r e system i n c o n t a c t with

steam or water and t h u s i n c l u d e s t h e s t e a m - r a i s i n g system, s u p e r h e a t e r s , p r e h e a t e r s , t u r b i n e s , condensers, e t c . MSBR

A complete m o l t e n - s a l t b r e e d e r r e a c t o r f a c i l i t y o f a nominal

1000-MW(e) c a p a c i t y .

The f u e l i s considered t o be 233U i n t h e nominal

f u e l - c a r r i e r s a l t mixturbe LiF-BeF2-ThF,+ (72-16-12 mole %).

COOLANT CRITERIA

Any secondary coola.nt (or combination of c o o l a n t s i n secondary and

tertiary l o o p s ) used t o t r a n s f e r t h e h e a t generated i n t h e f u e l s a l t i n t h e primary loop t o t h e s t e a m - r a i s i n g system must, o b v i o u s l y , s a t i s f y a number of requirements which w i l l l e a d t o a p r a c t i c a l , safe and economical d e s i g n f o r an MSBR.

C e r t a i n r e s t r i c t i o n s on t h e choice o f c o o l a n t are

a l s o imposed; t h e s e stem p r i n c i p a l l y from s a f e t y requirements and a n t i c i p a t e d e v e n t s r e l a t e d t o off-design c o n d i t i o n s .

Molten s a l t s , l i q u i d

metals, or g a s e s could be s e l e c t e d as c o o l a n t s and, t o some e x t e n t , v a r i o u s

c r i t e r i a would be more or less r e l e v a n t t o one or t h e o t h e r . I n t h i s s e c t i o n , t h e coolant c r i t e r i a

are d e t a i l e d under t h r e e c a t e g o r i e s :

requirements and r e s t r i c t i o n s

(1) s a f e t y s i g n i f i c a n t e v e n t s ,

( 2 ) a n t i c i p a t e d o f f - d e s i g n t r a n s i e n t s and, ( 3 ) d e s i g n c h a r a c t e r i s t i c s .

The s a f e t y c r i t e r i a a r e t h e most r e s t r i c t i v e and a b s o l u t e i n n a t u r e and are r e l a t e d t o e v e n t s which could l e a d t o unacceptable consequences.

The

c r i t e r i a d e t a i l e d under a n t i c i p a t e d o f f - d e s i g n t r a n s i e n t s are a l s o somewhat r e s t r i c t i v e b u t , i n a d d i t i o n , i n v o l v e " t r a d e - o f f "

f e a t u r e s or a s p e c t s

of a c o o l a n t which can be accommodated by a p p r o p r i a t e design.

In general,

t h e s e c r i t e r i a a r e formulated t o p r e s e r v e t h e i n t e g r i t y of r e a c t o r systems d u r i n g t r a n s i e n t s which can be a n t i c i p a t e d .

The t h i r d c a t e g o r y , d e s i g n

c h a r a c t e r i s t i c s , i n c l u d e s c r i t e r i a r e l a t e d t o o p t i m i z i n g t h e MSBR d e s i g n with r e g a r d t o c o s t , thermal e f f i c i e n c y , e t c . , and are t h e l e a s t r e s t r i c t i v e s i n c e d e s i r a b l e or l e s s - d e s i r a b l e f e a t u r e s of c o o l a n t s and t h e assoc i a t e d p l a n t design must be e v a l u a t e d and compared t o r e a c h a workable and economically a t t r a c t i v e d e s i g n of an MSBR. These c r i t e r i a were developed, as much as p o s s i b l e , w i t h o u t conside r i n g any s p e c i f i c c o o l a n t or combination of c o o l a n t s f o r an MSBR, b u t

r a t h e r by e s t a b l i s h i n g a s c r e e n i n g mechanism f o r e v a l u a t i n g c o o l a n t candidates.

Some of t h e c r i t e r i a are a b s o l u t e i n n a t u r e ; a c o o l a n t must meet

t h e s e c r i t e r i a or it cannot be considered a p p l i c a b l e i r r e s p e c t i v e of how a t t r a c t i v e i t might appear i n o t h e r a s p e c t s .

Most of t h e c r i t e r i a , how-

e v e r , a r e r e l a t i v e i n n a t u r e and more or less f a v o r a b l e a s p e c t s o f v a r i o u s c o o l a n t s can be adapted f o r use i n an MSBR through s u i t a b l e design.

The

p r o c e s s o f e v a l u a t i n g p o t e n t i a l c o o l a n t s i s complicated somewhat by t h e p o s s i b i l i t y of u s i n g two c o o l a n t c i r c u i t s (secondary and t e r t i a r y ) cont a i n i n g d i f f e r e n t c o o l a n t s t o t r a n s f e r h e a t from t h e f u e l s a l t t o t h e s t e a m - r a i s i n g system.

Thus, a p a i r o f c o o l a n t s could be s e l e c t e d , n e i t h e r

of which meet a l l t h e c r i t e r i a .

A f i n a l comparison and s e l e c t i o n can be

achieved only when t h e v a r i o u s c r i t e r i a can be reduced t o c a p i t a l and o p e r a t i n g c o s t s ; d o l l a r s are t h e common denominator i n such a comparison.

3.1

S a f e t y S i g n i f i c a n t Events

The c r i t e r i a d e t a i l e d i n t h i s s u b s e c t i o n are l i m i t e d t o t h o s e r e l a t e d t o s a f e t y s i g n i f i c a n t e v e n t s which could l e a d t o temperature and/or p r e s s u r e e x c u r s i o n s o f s u f f i c i e n t magnitude t o b r e a c h , or t h r e a t e n t o b r e a c h , t h e primary system boundary.

The c h a r a c t e r i s t i c s o f an MSBR are

i n h e r e n t l y q u i t e s t a b l e and s a f e ; however, s e l e c t i o n o f an u n s u i t a b l e secondary c o o l a n t could compromise t h e s e c h a r a c t e r i s t i c s i n t h e e v e n t of secondary c o o l a n t e n t e r i n g t h e f u e l c i r c u i t .

Thus, t h e s e c r i t e r i a are

most u s e f u l i n r e j e c t i n g e n t i r e c l a s s e s o f p o t e n t i a l c a n d i d a t e c o o l a n t s from f u r t h e r c o n s i d e r a t i o n . 3.1.1

Increase i n r e a c t i v i t y 3.1.1.1

P r e c i p i t a t i o n of f i s s i l e material.

The c o o l a n t must n o t be

capable o f c a u s i n g s i g n i f i c a n t p r e c i p i t a t i o n and/or s e g r e g a t i o n of f i s s i l e

material by t h e formation of compounds i n s o l u b l e i n f u e l s a l t under any e v e n t ; e.g.,

l e a k s i n t h e primary h e a t exchangers.

For example, t h e t e m -

p e r a t u r e and p r e s s u r e s u r g e s i n t h e c o r e caused by t h e r e t u r n o f a few (probably <lo) kg o f uranium which had p r e c i p i t a t e d o u t s i d e t h e c o r e , might cause damage beforie b e i n g checked by i n h e r e n t shutdown mechanisms. Use o f a c o o l a n t p o t e n t i a l l y capable o f c a u s i n g such e v e n t s would, t h e r e f o r e , l i k e l y r e p r e s e n t an unsafe design.

Precipitation of f i s s i l e material

i n t h e c o o l a n t c i r c u i t probably would n o t r e p r e s e n t a s a f e t y problem, a l though s h u t down and clean-up o f t h e secondary c i r c u i t would be r e q u i r e d t o remove f i s s i o n p r o d u c t contamination.

Hidden p r e c i p i t a t i o n o f f i s s i l e

material would probably v i o l a t e s a f e g u a r d a c c o u n t a b i l i t y requirements. 3.1.1.2

Gas i n j e c t i o n i n t o core.

Another r e s t r i c t i o n r e s u l t s from

t h e f a c t t h a t t h e MSBR c o r e has a small p o s i t i v e r e a c t i v i t y response t o

18 t h e i n t r o d u c t i o n of voids or gas bubbles of m a t e r i a l s w i t h low neutroncapture cross-section.

Thus i n l e a k a g e o f a gaseous c o o l a n t or production

of g a s e s r e s u l t i n g i n a v o i d volume greater t h a n about 1 0 % over t h e e n t i r e core could l e a d t o unacceptable s u r g e s i n f i s s i o n r a t e and must be avoided. Gas g e n e r a t i o n or in-leakage

3.1.2

The g e n e r a t i o n of l a r g e volumes of g a s e s or r e l e a s e of a high-pressure gaseous c o o l a n t as a r e s u l t of a primary h e a t exchanger l e a k cannot be p e r m i t t e d t o o v e r - p r e s s u r i z e and damage or r u p t u r e t h e f u e l s a l t system

or i n a c t i v a t e or poison t h e f i s s i o n - p r o d u c t gas a b s o r b e r beds of t h e o f f gas system.

Such e v e n t s would l e a d t o r e l e a s e of r a d i o a c t i v i t y w i t h i n t h e

MSBR containment and could t r e a t e n t h e primary system boundary.

Minor i n -

leakage or g e n e r a t i o n of gas could probably be accommodated by s u i t a b l e design.

3.1.3

Chemical r e a c t i o n s Vigorous chemical r e a c t i o n s should n o t occur between f u e l and c o o l a n t

as a r e s u l t of a l e a k or o t h e r mixing event.

In addition, coolant leaking

i n t o t h e primary system should n o t r e a c t v i g o r o u s l y with t h e g r a p h i t e moderator.

Chemical r e a c t i o n s which could l e a d t o s e r i o u s damage or des-

t r u c t i o n o f r e a c t o r s t r u c t u r a l components could be considered s a f e t y s i g n i f i c a n t e v e n t s due t o t h e p o t e n t i a l for r e l e a s e o f r a d i o a c t i v i t y .

Minor

chemical r e a c t i v i t y can probably be accommodated by s u i t a b l e design i f t h e r e a c t i o n products a r e innocuous or r e a d i l y removable.

3.2

A n t i c i p a t e d Off-Design T r a n s i e n t s

A v a r i e t y of off-design

c o n d i t i o n s and t r a n s i e n t e v e n t s can be a n t i -

c i p a t e d t o occur d u r i n g t h e p r o j e c t e d 30-year l i f e of an MSBR.

Examples

19 I

o f such e v e n t s are leakls i n h e a t exchangers or t h e r m a l e x c u r s i o n s on r e a c t o r scram or t u r b i n e t r i p .

The design of an MSBR and t h e s e l e c t i o n

of a c o o l a n t o r c o o l a n t s must be a b l e t o accommodate such e v e n t s w i t h a

minimum of r e s u l t i n g down time or damage.

These c r i t e r i a are l e s s r e -

s t r i c t i v e t h a n t h e s a f e t y - r e l a t e d c r i t e r i a and i n v o l v e , i n many c a s e s , v a r i o u s f e a t u r e s which can be accommodated by a p p r o p r i a t e design.

Primary h e a t exchanger leaks

3.2.1

The c o n c e p t u a l design of a 1000-MW(e) MSBR power s t a t i o n f o u r shell-and-tube tubes.

specifies

t y p e primary h e a t exchangers each c o n t a i n i n g 5896

I t i s assumed t h a t some t u b e f a i l u r e s w i l l occur d u r i n g t h e l i f e

o f t h e power s t a t i o n ; t h e r e f o r e , t h e h e a t exchanger d e s i g n i n c o r p o r a t e s p r o v i s i o n s f o r tube-bundle replacement or t u b e plugging by remote operation.

Tube f a i l u r e s w i i l , of c o u r s e , l e a d t o mixing o f f u e l and c o o l a n t .

Both massive and minor l e a k s must be considered. r e s u l t s must b e t o l e r a b l e and r e p a i r a b l e .

Minor l e a k s and t h e i r

Massive l e a k s , such as t h e

c o l l a p s e o f an e n t i r e h e a t exchanger assembly, would n o t be expected t o occur; n o n e t h e l e s s , t h e p o s s i b i l i t y o f such e v e n t s must be recognized and accommodated i n t h e desi.gn.although r e p a i r might be d i f f i c u l t .

It is

f e l t t h a t adequate assuriance of u n i d i r e c t i o n a l l e a k a g e cannot be achieved

by s u i t a b l e adjustment of t h e f u e l and c o o l a n t p r e s s u r e s ; t h u s , leakage i n b o t h d i r e c t i o n s through t h e primary h e a t exchanger must b e considered. S i m i l a r l y , leakage of t h e c o o l a n t i n t o t h e r e a c t o r containment c e l l c o u l d mix c o o l a n t and f u e l i n t h e d r a i n t a n k , and t h e same c r i t e r i a apply t o such an event. The t h r e e most s e r i o u s p o s s i b l e e v e n t s as a r e s u l t o f primary h e a t exchanger l e a k s

i n c r e a s e s i n r e a c t i v i t y , gas g e n e r a t i o n or in-leakage,

and vigorous chemical r e a c t i o n s have been considered above, S e c t i o n s 3.1.1, 3.1.2

and 3.1.3. 3.2.1.1

Reduction i n breeding gain.

I t i s d e s i r a b l e t h a t t h e breeding

g a i n n o t be s u b s t a n t i a l l y reduced as a r e s u l t of t h e i n t r o d u c t i o n of n u c l e a r poisons i n t h e f u e l which cannot r e a d i l y be removed.

This i s a

l e s s s e r i o u s s i t u a t i o n since power g e n e r a t i o n and o p e r a t i o n as a conv e r t e r could be maintained by t h e a d d i t i o n of more uranium.

I n any c a s e ,

t h e v a l u a b l e n u c l e a r f u e l should n o t be r e n d e r e d u s e l e s s or r e q u i r e r e moval from t h e MSBR and expensive processing. 3.2.1.2

Clean-up o f coolant.

Leakage of f u e l s a l t i n t o t h e c o o l a n t

would i n t r o d u c e f i s s i o n products i n t h e c o o l a n t s a l t .

If t h e c o o l a n t i s

r e l a t i v e l y inexpensive it could be d i s c a r d e d a f t e r recovery o f f i s s i l e material.

If t h e c o o l a n t i s more expensive and t h u s cannot be r e a d i l y

d i s c a r d e d , methods f o r p u r i f y i n g t h e c o o l a n t should be a v a i l a b l e . 3.2.1.3

Fuel p r o c e s s i n g chemical p l a n t .

A side-stream o f f u e l i s

continuously withdrawn from t h e r e a c t o r and processed by f l u o r i n a t i o n for t h e recovery of uranium, by r e d u c t i v e e x t r a c t i o n for t h e removal and r e t e n t i o n of p r o t a c t i n i u m and by metal t r a n s f e r f o r t h e removal of f i s s i o n products.

Leakage of c o o l a n t i n t o f u e l could r e s u l t i n t h e t r a n s p o r t of

d i s s o l v e d or suspended c o o l a n t i n t o t h e s e systems.

It would be d e s i r a b l e

i f s i g n i f i c a n t chemical r e a c t i o n s or d i s r u p t i o n of t h e f u n c t i o n o f t h e chemical p r o c e s s i n g p l a n t d i d n o t occur.

3.2.2

Steam-raising system l e a k s The s t e a m - r a i s i n g system of a 1000-MW(e) MSBR c o n t a i n s a l a r g e h e a t -

exchange area having c o o l a n t on one s i d e and s u p e r c r i t i c a l steam on t h e other.

Leaks must be assumed t o occur p e r i o d i c a l l y d u r i n g t h e 30-year

21 l i f e of t h e MSBR.

Althclugh t h e predominant leakage would occur from t h e

high p r e s s u r e steam i n t o t h e lower p r e s s u r e c o o l a n t , it must be assumed t h a t some c o o l a n t could l e a k i n t o t h e s t e a m - r a i s i n g system and be c i r c u l a t e d through t h e power p l a n t and t u r b i n e s .

Leakage i n e i t h e r d i r e c t i o n

must n o t r e s u l t i n vigorous chemical r e a c t i o n s which y i e l d l a r g e q u a n t i -

t i e s of h e a t or gaseous products.

Such e v e n t s could l e a d t o wastage o f

t h e s t r u c t u r a l material and/or d i s r u p t i o n o f t h e i n t e g r i t y o f t h e system, Mild chemical r e a c t i o n could be a c c e p t a b l e i f t h e r e a c t i o n p r o d u c t s are t o l e r a b l e or e a s i l y removable.

Also, leakage should n o t l e a d t o unac-

c e p t a b l y high c o r r o s i o n rates i n e i t h e r t h e c o o l a n t loop or t h e steam system.

I t i s n e c e s s a r y t h a t t h e c o o l a n t n o t b e c a p a b l e o f causing

s t r e s s - c o r r o s i o n c r a c k i n g of t h e steam system f o l l o w i n g a mixing event. Even ppb q u a n t i t i t i e s of some i o n s can l e a d t o s t r e s s - c o r r o s i o n c r a c k i n g of c e r t a i n steam system a l l o y s and, i f such a l l o y s are used, t h e c o o l a n t cannot c o n t a i n such i o n s .

3.2.3

Fuel-salt freezing Events can be expected t o occur which l e a d t o r a p i d t h e r m a l e x c u r s i o n s

and impose o f f - d e s i g n c o n d i t i o n s .

The c o o l a n t or c o o l a n t s and accompanying

system design must be able t o accommodate such e v e n t s .

I n t h e case o f a

r e a c t o r scram, h e a t gene:ration i n t h e f u e l s a l t v i a f i s s i o n w i l l be a b r u p t l y decreased b u t h e a t removal w i l l c o n t i n u e because o f t h e f i n i t e

t i m e involved i n s t o p p i n g t h e f u e l and c o o l a n t c i r c u l a t i o n pumps and t h e

steam t u r b i n e .

Damage t o t h e primary h e a t exchangers caused by stresses

a s s o c i a t e d w i t h f r e e z i n g and thawing o f t h e f u e l s a l t would be d i f f i c u l t and c o s t l y t o r e p a i r and could l e a d t o a r a d i o l o g i c a l hazard.

Thus, an

MSBR d e s i g n should i n c l u d e f e a t u r e s t h a t p r e c l u d e t h e p o s s i b i l i t y o f f u e l

salt freezing.

Two conceivable methods o f p r e v e n t i n g f r e e z i n g would be

t o use (1) a molten s a l t c o o l a n t â&#x20AC;&#x2122; w i t h a f r e e z i n g p o i n t h i g h e r t h a n t h a t of t h e f u e l s a l t 502OC (935OF) or ( 2 ) a c o o l a n t with s u i t a b l e h e a t - t r a n s p o r t properties.

For a s i n g l e c o o l a n t system t h e first method i s u n t e n a b l e s i n c e conv e n t i o n a l steam c y c l e s demand a c o o l a n t with a c o n s i d e r a b l y lower f r e e z i n g p o i n t ( S e c t i o n 3.3.2).

The second method i s c o n c e i v a b l e i f t h e r e a c t o r

design can accommodate a c o o l a n t with poor h e a t - t r a n s p o r t p r o p e r t i e s .

n o t , e n g i n e e r i n g p r e c a u t i o n s , p o s s i b l y i n t h e form of a dependable system t o by-pass c o o l a n t around t h e steam g e n e r a t o r s , would be r e q u i r e d . F o r a d u a l c o o l a n t d e s i g n , t h i s c r i t e r i o n could b e s a t i s f i e d by t h e s e l e c t i o n of a secondary c o o l a n t w i t h a f r e e z i n g p o i n t h i g h e r t h a n 5OOOC (932OF) w h i l e t h e t e r t i a r y c o o l a n t could have a lower f r e e z i n g p o i n t com-

I n t h i s c o n f i g u r a t i o n , freeze-up

p a t i b l e with t h e s t e a m - r a i s i n g system.

would o c c u r f i r s t i n t h e i n t e r m e d i a t e h e a t exchanger.

Volume change on

f r e e z i n g and thawing must be accommodated by s u i t a b l e d e s i g n t o p r e v e n t r u p t u r i n g p o r t i o n s of t h e i n t e r m e d i a t e h e a t exchangers or a g a i n a f a s t a c t i n g by-pass system may be needed.

A gaseous c o o l a n t i n a t e r t i a r y

loop could p o s s i b l y meet t h i s requirement and might h e l p circumvent t h e problem of freeze-up i n t h e i n t e r m e d i a t e h e a t exchange.

3.2.4

Leaks t o or from c e l l atmosphere Leakage o f c o o l a n t i n t o t h e c e l l atmosphere

and traces of water vapor

- nitrogen

p l u s oxygen

should n o t r e s u l t i n v i o l e n t chemical re-

a c t i o n s or l a r g e volumes of gaseous p r o d u c t s .

S i m i l a r l y , l e a k a g e of t h e

c e l l atmosphere i n t o t h e c o o l a n t system a g a i n should n o t l e a d t o vigorous chemical r e a c t i o n s , and t h e r e a c t i o n p r o d u c t s should b e r e a d i l y removable.

An i n c r e a s e d oxygen conlzentration i n t h e c o o l a n t could l e a d t o i n c r e a s e d c o r r o s i v i t y and could conceivably be unacceptable i n t h e e v e n t o f subsequent mixing o f contaminated c o o l a n t and f u e l due t o t h e very low s o l u b i l i t y o f uranium, thorium and p r o t a c t i n i u m oxides i n t h e f u e l s a l t . The p r o d u c t s o f minor i n l e a k a g e s h o u l d be innocuous or r e a d i l y removable.

3.3

Design C h a r a c t e r i s t i c s

The c r i t e r i a r e l a t e d t o normal o p e r a t i n g c o n d i t i o n s a r e p r e s e n t e d i n t h i s subsection.

For s u c c e s s f u l a d a p t a t i o n t o an MSBR d e s i g n , a c o o l a n t ,

or combination o f c o o l a n t s i n two l o o p s , must meet a l l o f t h e s e c r i t e r i a t o some degree.

S i n c e T t i s very u n l i k e l y t h a t any one c a n d i d a t e c o o l a n t

w i l l b e optimum i n a l l c a t e g o r i e s , s e l e c t i o n o f a c o o l a n t t o meet t h e s e

c r i t e r i a i n v o l v e s an averaging of advantages of d e s i r a b l e f e a t u r e s and accommodation o f t h e d i s a d v a n t a g e s where t h e c o o l a n t i s less t h a n optimum. P a r t i c u l a r l y for t h e c r i t e r i a i n t h i s s e c t i o n , c o n s i d e r a t i o n o f t h e u l t i -

mate c a p i t a l and o p e r a t i n g c o s t s a r e n e c e s s a r y i n comparing a l t e r n a t e coolants.

3.3.1

Corrosion The r a t e of corrosi.on of t h e c o o l a n t system boundary must b e c o n s i s -

t e n t w i t h t h e 30-year d e s i g n l i f e of t h e p l a n t .

Coolants which, because

of p r e d i c t a b l y h i g h c o r r o s i o n r a t e s , would l e a d t o r a p i d f a i l u r e o f p l a n t components are unacceptable.

With some c o o l a n t c a n d i d a t e s , c o r r o s i o n

c o n s i d e r a t i o n s amount l a r g e l y t o a comparison o f c o o l a n t c o s t and h e a t t r a n s f e r and f l u i d p r o p e r t i e s w i t h t h e i n c r e a s e d c o s t o f p r o v i d i n g g r e a t e r metal t h i c k n e s s t o w i t h s t a n d i n c r e a s e d c o r r o s i o n .

S o l u b l e cor-

r o s i o n p r o d u c t s may accu.mulate i n molten s a l t c o o l a n t s and must e i t h e r

be removed or n o t i n t e r f e r e with o p e r a t i o n of t h e c o o l a n t loop v i a f o u l i n g of h e a t exchange s u r f a c e s . 3.3.2

Freezing p o i n t Any f r e e z i n g p o i n t below 33OOC (626OF) would probably p e r m i t a feed-

water temperature a s low as 304OC (58OOF) which would be compatible with conventional s u p e r c r i t i c a l steam c y c l e s .

Higher c o o l a n t f r e e z i n g p o i n t s

can be t o l e r a t e d , b u t with i n c r e a s i n g c o s t and e f f i c i e n c y p e n a l t i e s as t h e f r e e z i n g temperature i n c r e a s e s .

If t h e f r e e z i n g p o i n t i s such t h a t

t h e feedwater must be h e a t e d t o 427OC (8OOOF) or h i g h e r , t h e i n c r e a s e d c o s t s a r e probably p r o h i b i t i v e .

Considering t h e t h e r m a l e f f i c i e n c y of

an MSBR and t h e e a s e of normal o p e r a t i o n , t h e r e i s no lower l i m i t t o t h e freezing point.

Gaseous c o o l a n t s , of course, o f f e r a n o t h e r means of at-

t a i n i n g t h e e q u i v a l e n t of a very low f r e e z i n g p o i n t . p r e v e n t i o n of f u e l f r e e z i n g ( S e c t i o n 3.2.3)

The c r i t e r i o n f o r

places additional r e s t r i c t i o n s

on t h e c o o l a n t f r e e z i n g p o i n t i n some design c o n f i g u r a t i o n s .

If a t e r -

t i a r y c o o l a n t loop i s employed, t h e n only t h e t e r t i a r y c o o l a n t need have

a low f r e e z i n g p o i n t .

3.3.3

Heat t r a n s f e r and f l u i d t r a n s p o r t p r o p e r t i e s The c o o l a n t must have h e a t t r a n s f e r and f l u i d t r a n s p o r t p r o p e r t i e s

t h a t a r e compatible with a p r a c t i c a l and economical MSBR deisgn.

These

p r o p e r t i e s i n c l u d e high thermal c o n d u c t i v i t y and h e a t c a p a c i t y , and low v i s c o s i t y ; such a combinatios-.of p r o p e r t i e s i m p l i e s high h e a t - t r a n s f e r c o e f f i c i e n t s and low pumping power requirements.

Thus, f o r a gaseous

c o o l a n t t o meet t h e s e c r i t e r i a it would probably have t o o p e r a t e a t p r e s s u r e s of s e v e r a l hundred p s i .

D i f f e r e n t c o o l a n t s , molten s a l t s ,

l i q u i d m e t a l s , or gases, w i l l o f f e r v a r i o u s combinations o f t h e s e properties.

For t h o s e candidate c o o l a n t s which are n o t s o extreme i n some

p r o p e r t y as t o be summarlily r e j e c t e d , an a n a l y s i s r e l a t i n g t h e c o o l a n t p r o p e r t i e s t o c a p i t a l c c ' s t s (e.g., c o s t s (e.g.,

h e a t exchanger a r e a ) and o p e r a t i n g

pumping power requirements) o f MSBRs would be r e q u i r e d t o

make a q u a l i f i e d s e l e c t i o n between competitive c o o l a n t s .

3.3.4

Vapor p r e s s u r e and composition The vapor p r e s s u r e of a molten-salt or l i q u i d - m e t a l c o o l a n t should

be low a t t h e h i g h e s t temperature t o be encountered d u r i n g normal o p e r a t i o n o r off-design excursions.

Vapor p r e s s u r e s g r e a t e r t h a n one atmosphere

would complicate e n g i n e e r i n g d e s i g n , b u t probably could be accommodated. Even low vapor p r e s s u r e s , 0 . 0 1 t o 1 atmosphere, w i l l r e q u i r e r e c o v e r i n g t h e c o o l a n t vapor from t h e cover gas sweep stream and r e t u r n i n g it t o t h e c o o l a n t t o p r e v e n t d e p l e t i o n of v o l a t i l e c o n s t i t u e n t s .

I t would be ad-

vantageous i f t h e c o o l a n t d i d n o t sublime which could r e s u l t i n formation of a s o l i d from t h e vapor i n c o o l e r r e g i o n s with a s s o c i a t e d r e s t r i c t i o n s i n v e n t l i n e s , e r o s i o n of pump s h a f t s and s e a l s , etc. Consideration of a gaseous c o o l a n t i m p l i e s a s u b s t a n t i a l l y d i f f e r e n t design t o accommodate t h e moderate-to-high 3.3.5

p r e s s u r e s involved.

Tritium c o n t r o l

Because t h e f u e l s a l t c o n t a i n s a high atomic d e n s i t y of l i t h i u m , a s i g n i f i c a n t q u a n t i t y of . t r i t i u m (2420 c u r i e s o r about 0.25 g p e r day i n 6

a 1000-MW(e) MSBR ) i s generated i n t h e r e a c t o r core.

Since metals a t

high temperatures are pe:rmeahle t o i s o t o p e s of hydrogen, a p o r t i o n , calc u l a t e d t o be i n t h e ran,ge of 790-1500 Ci/day f o r t h e r e f e r e n c e concept,

26 could r e a c h t h e steam system, 6'7

V i r t u a l l y a l l of t h i s t r i t i u m would be

r e l e a s e d t o t h e environment by normal system blowdown and/or leakage.

The

a s s o c i a t e d release r a t e would b e 50 t o 1 0 0 times t h a t f o r light-water-cooled n u c l e a r power s t a t i o n s and probably would be environmentally unacceptable. If t h e t r i t i u m were allowed t o accumulate i n t h e steam system by r e q u i r i n g

t o t a l r e c y c l e of a l l steam d i s c h a r g e s , t h e steam system probably would become s u f f i c i e n t l y r a d i o a c t i v e ( 2 C i / g a l ) t o r e q u i r e p e r s o n n e l p r o t e c t i o n d u r i n g maintenance and would be expensive t o maintain.

Thus, it i s prob-

able t h a t t h e c o o l a n t system may be r e q u i r e d t o i n t e r d i c t i n some way t h e

flow o f t r i t i u m . Various t r i t i u m t r a p p i n g schemes have been proposed, i n c l u d i n g ?sot o p i c exchange or o x i d a t i o n w i t h i n t h e c o o l a n t and subsequent side-stream removal o f t r i t i u m as THO or TO- compounds.

F u r t h e r , t h e f o r m a t i o n o f an

oxide f i l m on t h e steam s i d e o f t h e s t e a m - r a i s i n g system i s expected t o d e c r e a s e t h e t r i t i u m p e r m e a b i l i t y i n t h i s p o r t i o n of t h e system and t h u s

raise t h e p a r t i a l pressure of t r i t i u m within the coolant, possibly t o a l e v e l where gas s p a r g i n g or s o r p t i o n could be used for t r i t i u m removal. However, none of t h e s e p o t e n t i a l methods f o r t r i t i u m c o n t r o l has been e v a l u a t e d or demonstrated e x p e r i m e n t a l l y nor has t h e environmental impact been c a r e f u l l y a s s e s s e d t o s e t a l i m i t f o r t r i t i u m release; t h u s , it i s d i f f i c u l t t o e s t a b l i s h q u a n t i t a t i v e c r i t e r i a for t h i s a s p e c t of t h e c o o l a n t or c o o l a n t s .

I t is a p p a r e n t t h a t it would b e b e n e f i c i a l i f tri-

tium d i f f u s i n g i n t o t h e c o o l a n t were s e q u e s t e r e d and n o t allowed t o e n t e r t h e steam system.

Depending on f u t u r e e x p e r i m e n t a l r e s u l t s and environ-

mental c o n s i d e r a t i o n s , t h i s could become a mandatory requirement.

This

requirement could p o s s i b l y be m e t by d i f f e r e n t means i n secondary or

t e r t i a r y loops depending on t h e s e l e c t i o n of a molten s a l t or a gaseous c o o l a n t f o r t h a t loop. 3.3.6

Radiation and chemical s t a b i l i t y The c o o l a n t should be h i g h l y r e s i s t a n t t o r a d i a t i o n damage under a l l

operating conditions.

It should n o t evolve g a s e s or form s l u d g e s or s o l i d s

t h a t f o u l heat-exchange s u r f a c e s . c o r r o s i v e with use.

N e i t h e r should it become i n c r e a s i n g l y

Minor chemical changes d u r i n g o p e r a t i o n could be ac-

commodated by side-stream p r o c e s s i n g t o remove decomposition p r o d u c t s ,

3.3.7

Cost and a v a i l a b i l i t y The material chosen for t h e c o o l a n t should be composed o f compounds

or elements which a r e readily a v a i l a b l e i n adequate q u a n t i t y and which p r e f e r a b l y a r e a v a i l a b l e i n high p u r i t y .

While low c o s t i s d e s i r a b l e ,

t h e c a p i t a l and o p e r a t i n g c o s t s a s s o c i a t e d with a given c o o l a n t w i l l be more important t h a n t h e i n i t i a l c o o l a n t c o s t .

I n i t i a l cost is a relative

i t e m and must be compared t o o t h e r a t t r i b u t e s . 4.

4.1

REJECTED COOLANT CANDIDATES

S i n g l e Secondary Coolants

I n t h i s s e c t i o n many p o s s i b l e c o o l a n t s having a wide v a r i e t y o f chemical and p h y s i c a l p r o p e r t i e s are c o n s i d e r e d ,

Most a r e r e j e c t e d be-

cause l e a k a g e of coolant: i n t o f u e l s a l t could cause, or could t h r e a t e n t o cause, s a f e t y - s i g n i f i c a n t e v e n t s , or e l s e because t h e c o o l a n t would be too corrosive.

Three c o o l a n t s used i n o t h e r r e a c t o r s , sodium, helium,

and H 0 , were unacceptable because of s a f e t y - r e l a t e d problems t h a t could 2

a r i s e i f l e a k s occurred i n t h e primary h e a t exchangers.

For t h e same

r e a s o n , o r g a n i c c o o l a n t s and low m e l t i n g o x i d e s ( n i t r a t e s , n i t r i t e s , and c a r b o n a t e s ) were r e j e c t e d .

The molten mixture of L i F and BeF2 used as t h e

MSRE c o o l a n t w a s r e j e c t e d because of t h e p e n a l t y i n thermodynamic e f f i -

ciency a s s o c i a t e d w i t h i t s high f r e e z i n g p o i n t .

The low m e l t i n g m e t a l s ,

l e a d , t i n , and bismuth, were r e j e c t e d because of t h e i r c o r r o s i v i t y toward s t r u c t u r a l a l l o y s ; mercury w a s dismissed on account of i t s s c a r c i t y i n the e a r t h ' s crust. cations ( N i

Hydroxides and s a l t s which c o n t a i n e a s i l y r e d u c i b l e

, Sn

, Pb 2+ ,

and B i

) w e r e r e j e c t e d because they

would n o t be s t a b l e i n n i c k e l - o r iron-base a l l o y s o f c o n s t r u c t i o n .

Al-

though a s i n g l e f a c t o r w a s s u f f i c i e n t f o r r e j e c t i o n , many of t h e s e f l u i d s could have been d i s m i s s e d f r o m f u r t h e r c o n s i d e r a t i o n because of o t h e r s e r i o u s shortcomings.

4.1.1

Coolants used i n o t h e r n u c l e a r r e a c t o r s 4.1.1.1

Sodium.

Sodium and o t h e r a l k a l i m e t a l s a r e capable o f

chemically reducing a l l c a t i o n s i n t h e f u e l s a l t e x c e p t l i t h i u m i n c a s e of t h e i r leakage i n t o f u e l s a l t .

The uranium f l u o r i d e s , UF4 and UF3,

are

t h e most r e a d i l y reduced f u e l s a l t components and t h e reduced forms o f uranium a r e e i t h e r s p a r i n g l y s o l u b l e ( U F 3 ) o r i n s o l u b l e (uranium metal) in fuel salt.

Accordingly, i n l e a k a g e of sodium through a primary h e a t -

exchanger would p r e c i p i t a t e UF

o r uranium metal, e i t h e r of which could

subsequently cause an unacceptable i n c r e a s e i n r e a c t i v i t y . metal, from BeF

could a l l o y w i t h s t r u c t u r a l metals.

Beryllium

Thus, sodium i s

n o t an a c c e p t a b l e c o o l a n t c h o i c e . Sodium has a d d i t i o n a l s e r i o u s drawbacks.

The very high thermal

c o n d u c t i v i t y o f sodium, while advantageous f o r compact h e a t exchanger and steam g e n e r a t o r d e s i g n , i s a d i s t i n c t disadvantage i n d e a l i n g w i t h

t h e r m a l t r a n s i e n t s and avoiding p o s s i b l e f u e l freeze-up.

Another d i s -

advantage of sodium i s i t s vigorous r e a c t i o n with water or steam t o prod u c t hydrogen and sodium hydroxide or sodium o x i d e , depending on whether

water or sodium i s i n excess.

Very r a p i d c o r r o s i o n may accompany leakage

of steam i n t o sodium. Sodium may n o t offei? an e f f e c t i v e means of s e q u e s t e r i n g t r i t i u m .

Al-

though i n t h e LMFBR t h e t r i t i u m problem may be s o l v e d v i a c o l d t r a p p i n g of sodium t r i t i d e , t h i s approach would probably n o t t r a p t r i t i u m i n t h e MSBR a d e q u a t e l y s i n c e it i s p r e s e n t i n q u a n t i t i t e s t h a t are 50 t o 1 0 0 t i m e s g r e a t e r t h a n i n t h e LMFBR. 4.1.1.2

Helium (and o t h e r high-pressure g a s e s ) .

Helium cannot be

used as t h e MSBR s e c o n d a ~ yc o o l a n t because i t s leakage i n t o t h e f u e l - s a l t could l e a d t o unacceptable power s u r g e s a s s o c i a t e d w i t h t h e p o s i t i v e reSimilarly,

a c t i v i t y c o e f f i c i e n t f o r v o i d s or bubbles ( c r i t e r i o n e.1.1.2).

o t h e r high-pressure g a s e s having low n e u t r o n c a p t u r e c r o s s - s e c t i o n s ( C O

H2, N e , Ar) can be r e j e c t e d . Even i f " f a i l - s a f e "

, leak-free

primary h e a t exchangers were developed,

t h e r e are s t r o n g i n c e n t i v e s f o r f a v o r i n g a low-pressure l i q u i d secondary c o o l a n t o v e r a high-pressure gaseous secondary c o o l a n t .

The lower volu-

metric h e a t c a p a c i t y o f g a s e s would r e q u i r e s u b s t a n t i a l i n c r e a s e s i n t h e

f u e l s a l t i n v e n t o r y and .in heat-exchange surface a r e a , and r e s u l t i n g r e a t e r power r e q u i r e d t o c i r c u l a t e t h e c o o l a n t .

Although t h e s e disadvantages can

b e d e a l t w i t h , t h e sum of t h e c o s t s t o do s o c o u l d be s u b s t a n t i a l . 4.1.1.3

7LiF'-BeF2.

These two f l u o r i d e s are major components of t h e

f u e l s a l t and t h e consequences would be minimal i f a 'LiF-BeF

were mixed with t h e f u e l .

coolant

This optimum c o o l a n t - f u e l c o m p a t i b i l i t y was t h e

c h i e f reason for u s i n g a mixture of 'LiF Molten-Salt Reactor Experiment (MSRE).

and BeF2 (66-34 mole % ) i n t h e I n t h i s a p p l i c a t i o n , h e a t was

t r a n s f e r r e d from t h e c o o l a n t a t 546OC (1015OF) t o a low-efficiency, cooled r a d i a t o r .

air-

The c o o l a n t performed i n a completely s a t i s f a c t o r y

manner i n t h e MSRE.

However, i n an MSBR t h e c o o l a n t should b e able t o

d e l i v e r h e a t t o p o r t i o n s of t h e steam system a t much lower temperatures t h a n 546OC.

Molten mixtures of

stantial cost for auxiliaries.

7 LiF-BeF2 could b e used, b u t only a t subThe exact composition t h a t could be used

i n t h e MSBR c o o l a n t c i r c u i t would be a compromise between h i g h f r e e z i n g p o i n t and high v i s c o s i t y ; compositions of i n t e r e s t would have LiF c o n t e n t s of 60 t o 67 mole % r e s u l t i n g i n f r e e z i n g p o i n t s between 440 and 46OOC (825-86OOF).

The need t o p r e v e n t s a l t from f r e e z i n g i n t h e steam-raising

equipment would r e q u i r e an abnormally high feedwater temperature, and r e s u l t i n a d e c r e a s e i n t h e thermal e f f i c i e n c y of t h e r e a c t o r .

Assuming

a s u p e r c r i t i c a l steam c y c l e i n which t h e feedwater would b e p r e h e a t e d t o 426OC (8OO0F), approximately t h e lowest temperature if LiF-BeF2 were t h e c o o l a n t , Robertson3 e s t i m a t e d t h a t t h e n e t p l a n t e f f i c i e n c y would be 41.3%

as compared t o 44.5% i n a system having a feedwater temperature of 371OC (7OOOF).

The unconventional s i z e of t h e p r e h e a t equipment ( e s p e c i a l l y

p r e s s u r e b o o s t e r pumps) would impose a d d i t i o n a l c o s t s .

A second unfavorable f a c t o r a s s o c i a t e d w i t h use o f c o s t of t h e c o o l a n t inventory. 7LiF-BeF2

LiF-BeF2 i s t h e

Assuming an i n v e n t o r y of 8500 f t 3 o f

(66-34 mole % ) and e s t i m a t e d p r i c e s of $120/kg 7 L i and $86/kg

B e , t h e c o o l a n t would c o s t

approximately $13 m i l l i o n .

Although a s i n g l e c o o l a n t MSBR could n o t use a c o o l a n t composed s o l e l y of 7 L i F and BeF2 because of i t s high f r e e z i n g p o i n t and c o s t , t h i s

31 I

c o o l a n t appears very a t t : r a c t i v e i n t h e d u a l c o o l a n t c o n f i g u r a t i o n s desc r i b e d i n S e c t i o n 7. 4.1.1.4

H 0 (water or s u p e r c r i t i c a l steam). -2

Leakage of water i n any

form i n t o t h e f u e l s a l t w i l l cause p r e c i p i t a t i o n o f t h e f i s s i l e m a t e r i a l i n t h e f u e l , and, perhaps, a s u b s t a n t i a l i n c r e a s e i n t h e bubble volume r e a c h i n g t h e c o r e due t o steam formation.

A second s e r i o u s shortcoming o f

water stems from t h e high f r e e z i n g p o i n t , 446OC (835OF), of t h e f u e l s a l t . Both of t h e s e e v e n t s are p o t e n t i a l l y s a f e t y s i g n i f i c a n t .

To p r e v e n t f u e l

s a l t from f r e e z i n g i n primary h e a t exchangers r e q u i r e s t h a t t h e c o o l a n t water be a gas (steam) with a l l t h e economic disadvantages t h a t are thereby i n c u r r e d ( s e e S e c t i o n 4.:1.1.2).

For t h e s e r e a s o n s , as w e l l as t h e need

t o e n s u r e a g a i n s t any p o s s i b l e f i s s i o n - p r o d u c t contamination of t h e steampower system, i t has never appeared f e a s i b l e t o r a i s e steam d i r e c t l y i n t h e

MSBR primary h e a t exchanger. 4.1.1.5

Organic c o o l a n t s .

This class o f c o o l a n t s i s o c c a s i o n a l l y

proposed as a means of coping w i t h t h e MSBR t r i t i u m problem.5

The rela-

t i v e l y low temperatures (400 t o 45OoC, 752 t o 842OF) a t which p y r o l y s i s occurs i s a s u f f i c i e n t b a s i s f o r r e j e c t i n g t h e s e materials.

In addition,

an o r g a n i c c o o l a n t , i f mixed with molten f u e l s a l t , i s l i k e l y t o chemically reduce and p r e c i p i t a t e t h e uranium. 4.1.2

Low-melting metals and s a l t s 4.1.2.1

Metals (Pb,, Sn, B i , Hg).

These metals, with t h e p o s s i b l e

exception of mercury, would r a p i d l y corrode t h e n i c k e l or iron-base a l l o y s t r u c t u r a l materials l i k e l y t o be used i n t h e c o o l a n t c i r c u i t a t t h e design temperature of t h e MSBR.

Even i f compatible c o n s t r u c t i o n a l materials were

a v a i l a b l e , t h e c o s t s of c i r c u l a t i n g t h e s e dense l i q u i d s would be very high.

A s w i t h sodium, t h e high t h e r m a l c o n d u c t i v i t y o f t h e s e molten metals i s a

disadvantage when t h e r m a l t r a n s i e n t s occur.

Aside from i t s w e l l known

t o x i c i t y , t h e s c a r c i t y o f mercury i n t h e e a r t h ' s c r u s t i s s u f f i c i e n t f o r r e j e c t i n g t h i s metal as an MSBR c o o l a n t .

A similar argument perhaps ap-

p l i e s t o bismuth; i n a d d i t i o n , t h i s metal expands upon s o l i d i f i c a t i o n , a p r o p e r t y which would complicate t h e d e s i g n of c o o l a n t c i r c u i t s .

For t h e s e

f o u r heavy metals t h e shortcomings are s e r i o u s enough t o d i s c o u n t t h e i r use as an MSBR c o o l a n t . 4.1.2.2 c a r b o n a t e s ).

Oxide-containing s a l t s ( n i t r a t e s , n i t r i t e s , hydroxides, and When a c o o l a n t c o n t a i n i n g s u b s t a n t i a l oxide , or oxide w i t h i n

a complex i o n , mixes w i t h t h e f u e l s a l t , p r e c i p i t a t i o n of a s o l i d phase c o n t a i n i n g uranium oxide i s l i k e l y , and t h i s could b e a s a f e t y - s i g n i f i c a n t event. These oxide-containing c o o l a n t c a n d i d a t e s which f r e e z e below 4OOOC (752OF) have o t h e r s e r i o u s disadvantages.

Nitrates, e i t h e r a l o n e or mixed

w i t h n i t r i t e s , could react v i o l e n t l y with t h e moderator g r a p h i t e i f a l e a k occurred i n a primary heat-exchanger.

Carbonates a r e a l s o unacceptable

because of f u e l s a l t - c o o l a n t mixing c o n s i d e r a t i o n s .

The r e a c t i o n of

f l u o r i d e w i t h carbonate could release C 0 2 i n q u a n t i t i e s s u f f i c i e n t t o i n c r e a s e t h e bubble f r a c t i o n i n t h e c o r e t o unacceptable l e v e l s .

Molten

hydroxide c o o l a n t s , although i d e a l for managing t h e MSBR t r i t i u m problem, must be r e j e c t e d because of t h e i r c o r r o s i v e n e s s t o n i c k e l - and iron-based alloys of construction. 4.1.2.3

S a l t s containing reducible cations.

There are a number of

low m e l t i n g h a l i d e s t h a t a r e unacceptable because t h e y would cause i n t o l e r a b l e c o r r o s i o n of t h e s t r u c t u r a l a l l o y H a s t e l l o y N.

One example i s

This s a l t can o x i d i z e any of

SnF2 whose m e l t i n g p o i n t i s 2 1 2 O C (414OF). t h e major components of H a s t e l l o y N.

I n g e n e r a l , m e t a l l i c h a l i d e s con-

t a i n i n g i o n s more oxidizi-ng than Ni2'

w i l l n o t be s t a b l e i n n i c k e l - b a s e

a l l o y s ; t h e c a t i o n s i n t h i s c a t e g o r y i n c l u d e Sn Cu

S a l t m e l t s c o n t a i n i n g s u b s t a n t i a l Ni2'

, Pb2+,

Fe3',

B i n + and

c o n c e n t r a t i o n s are r e j e c t e d

because t h e y are l i k e l y t:o l e a d t o mass t r a n s f e r of n i c k e l metal.

The

same c o n s i d e r a t i o n s apply t o iron-based a l l o y s ; i n a d d i t i o n , f e r r o u s h a l i d e s would be unaccept:able because o f m a s s - t r a n s f e r problems.

4.1.3

Other molten h a l i d e mixtures There a r e s e v e r a l o t h e r molten h a l i d e s mixtures t h a t have f r e e z i n g

p o i n t s below 4OO0C (752OF) which cannot be r e j e c t e d because of a f a i l u r e t o meet an important c r i t e r i o n ; n o n e t h e l e s s t h e s e mixtures have s h o r t comings which r e n d e r thexl l e s s a t t r a c t i v e MSBR secondary c o o l a n t s .

These

molten h a l i d e s are d i s c u s s e d very b r i e f l y below. 4.1.3.1

Mixtures of a l k a l i c h l o r i d e s .

of L i C 1 , N a C 1 , and KC1.

These are t y p i c a l l y mixtures

The L i C l i s r e q u i r e d t o o b t a i n a s u f f i c i e n t l y low

m e l t i n g p o i n t and, because of t h e n e c e s s i t y f o r u s i n g l i t h i u m - 7 , t h e s e c o o l a n t s would b e f a i r l y expensive.

Leakage of c o o l a n t i n t o t h e f u e l s a l t

would cause a r e a c t i v i t y loss which would be p r i m a r i l y remedied by removal of c h l o r i d e from t h e f u e l s a l t ; u n f o r t u n a t e l y , potassium and sodium cannot b e removed from t h e f u e l - s a l t by t h e f u e l p r o c e s s i n g c i r c u i t and t h e s e i o n s would reduce t h e b r e e d i n g g a i n .

A more s e r i o u s s i t u a t i o n could a r i s e

if c o o l a n t l e a k e d i n t o t h e steam system.

F e r r i t i c a l l o y s may be a c c e p t a b l e

although c h l o r i d e s cause s t r e s s - c o r r o s i o n c r a c k i n g i n many steam-system m a t e r i a l s a t very low c o n c e n t r a t i o n s i n water or i n steam.

4.1.3.2

Mixtures of N a C l and A l C l , The main p o t e n t i a l advantages

of t h e s e mixtures are low f r e e z i n g p o i n t , 151OC (303OF) f o r an equimolar mixture, low v i s c o s i t y , and low c o s t s .

If a s u b s t a n t i a l volume of t h i s

melt leaked i n t o t h e f u e l - s a l t , a s o l i d of t h e c r y o l i t e s t r u c t u r e (Na3A1F6) would l i k e l y p r e c i p i t a t e ; i f c a r r i e d i n t o t h e c o r e , t h e s e s o l i d s could

r e s t r i c t flow i n t h e channels through t h e moderator.

Although t h e vapor

p r e s s u r e of t h e s e mixtures i s moderately h i g h , t h e vapor can be e a s i l y prevented from condensing by h e a t i n g c o l d surfaces (e.g., about 18OOC (356OF).

vent l i n e s ) t o

The g e n e r a l c o n s i d e r a t i o n s of s t r e s s - c o r r o s i o n

cracking r e s u l t i n g from c h l o r i d e s i n t h e steam system apply t o t h i s c o o l a n t

as w e l l .

I t i s n o t clear how t h i s c o o l a n t would t r a p t r i t i u m .

4.1.3.3

ZrF,,-KF-NaF (42-48-10 mole%).

This s a l t mixture i s sometimes

considered as a p o s s i b l e inexpensive a l t e r n a t e c o o l a n t because o f i t s chemical c o m p a t i b i l i t y w i t h f u e l - s a l t i n case of a l e a k i n t h e primary h e a t exchanger.

However, t h e b r e e d i n g g a i n would b e permanently decreased

by t h e presence of potassium i n t h e f u e l s a l t .

A disadvantage of t h i s m e l t

i s a s s o c i a t e d w i t h i t s condensible vapor, preponderantly ZrF4.

The llsnowll

t h a t would form could block v e n t l i n e s and cause problems i n pumps t h a t c i r c u l a t e t h e coolant.

4.1.4

Fluidized-bed c o o l a n t s

A fluidized-bed

concept could, i n p r i n c i p l e , be used t o t r a n s p o r t h e a t

d i r e c t l y from t h e f u e l s a l t t o t h e steam system. suggested by an independent design study. s i o n s were reached.

This a l t e r n a t i v e was a l s o

However, no d e f i n i t i v e conclu-

To avoid t h e p o t e n t i a l hazards a s s o c i a t e d w i t h t h e

presence of high p r e s s u r e steam-system p i p i n g i n s i d e t h e r e a c t o r primary containment and a d j a c e n t t o p i p i n g c o n t a i n i n g f u e l s a l t , such a system

would probably n e c e s s i t a t e two f l u i d i z e d beds t o perform t h e f u n c t i o n s of t h e primary h e a t exchanger and t h e steam g e n e r a t o r .

Heat would be

t r a n s f e r r e d t o t h e bed m a t e r i a l i n s i d e t h e primary containment and t h e bed m a t e r i a l would be t r a n s p o r t e d t o steam g e n e r a t o r s l o c a t e d o u t s i d e t h e containment. This approach appeam t o o f f e r a number of advantages over o t h e r s i n g l e - c o o l a n t concepts.

The f a v o r a b l e h e a t - t r a n s f e r and h e a t - t r a n s p o r t

c h a r a c t e r i s t i c s of t h e f l u i d i z e d beds would reduce t h e heat-exchange

surface-area and fluid-pumping requirements t o v a l u e s n e a r t h o s e normally a s s o c i a t e d with l i q u i d c o o l a n t s .

Absolute gas p r e s s u r e s p o t e n t i a l l y could

be as low or lower t h a n fuel-system p r e s s u r e s t o minimize t h e p o s s i b i l i t y o f i n t r o d u c i n g gas i n t o t h e primary loop and r e a c t o r c o r e i n t h e e v e n t of

a fuel-tube failure.

The u s e of an i n e r t f l u i d i z i n g g a s , probably helium,

and an a p p r o p r i a t e l y i n e r t bed m a t e r i a l could s i g n i f i c a n t l y reduce s t r u c -

t u r a l - m a t e r i a l constraint:s.

For example, o r d i n a r y steam-system materials

could probably be used f o r t h e steam g e n e r a t o r s s i n c e r e s i s t a n c e t o corr o s i o n by molten s a l t would n o t be a requirement. m a t e r i a l , a d d i t i o n s of H,,O and/or 0

With an i n e r t bed

might be made t o t h e gas phase t o

i n t e r d i c t t h e flow o f t r i t i u m from t h e f u e l system t o t h e steam system. A l t e r n a t i v e l y , t h e bed m a t e r i a l i t s e l f might perform t h e t r i t i u m t r a p p i n g function. A major requirement of t h i s concept would be c o m p a t i b i l i t y of t h e

bed material with t h e m a t e r i a l s of both t h e f u e l system and t h e steam system.

A s a minimum, t h e r e should b e no vigorous chemical i n t e r a c t i o n

between t h e f u e l s a l t and t h e bed material.

I n a d d i t i o n it should be

p o s s i b l e t o s e p a r a t e t h e bed m a t e r i a l from f u e l s a l t without e x c e s s i v e

36 1

e f f o r t , and small amounts of bed materials should n o t s t r o n g l y a f f e c t t h e chemical, p h y s i c a l , and n e u t r o n i c c h a r a c t e r i s t i c s o f t h e f u e l s a l t .

p a r t i c u l a r , t h e bed material should have minimal neutron-moderating capab i l i t y t o avoid e i t h e r p o s i t i v e n u c l e a r r e a c t i v i t y e x c u r s i o n s if bed

material were t o e n t e r t h e c o r e , or n u c l e a r c r i t i c a l i t y i n t h e f l u i d i z e d bed i f l a r g e amounts of f u e l s a l t were t o l e a k i n t o t h e bed.

Other po-

t e n t i a l effects of s a l t l e a k s , such as l o s s of f l u i d i z a t i o n would a l s o have t o be shown t o be a c c e p t a b l e .

Bed materials t h a t c o u l d react chemi-

c a l l y with high-temperature steam (e.g. able.

g r a p h i t e ) would a l s o be unaccept-

S i n c e t h e bed material would l i k e l y become a c t i v a t e d and/or con-

taminated i n u s e , i t s h o u l d have s u f f i c i e n t mechanical s t a b i l i t y t o prec l u d e t h e replacement and d i s p o s a l of l a r g e amounts of s o l i d s . While an e x t e n s i v e s u r v e y has n o t been made, i t appears l i k e l y t h a t a

combination of f l u i d i z i n g gas and bed material could be i d e n t i f i e d t h a t meets t h e fundamental requirements f o r an MSR a p p l i c a t i o n .

However, s i n c e

f l u i d i z e d - b e d h e a t exchangers have n o t been used i n power r e a c t o r a p p l i c a t i o n s , a s u b s t a n t i a l e f f o r t would be r e q u i r e d t o develop and demonstrate t h e o p e r a b i l i t y , r e l i a b i l i t y , m a i n t a i n a b i l i t y , and s a f e t y o f t h i s concept

a t l e v e l s commensurate w i t h t h e requirements f o r n u c l e a r systems.

I n view

o f t h e f a c t t h a t o t h e r , more c o n v e n t i o n a l systems appear t o b e adequate f o r MSBR a p p l i c a t i o n s , t h e r e appears t o be no i n c e n t i v e f o r f u r t h e r conside r a t i o n of a f l u i d i z e d - b e d s i n g l e c o o l a n t a t t h i s t i m e . 4.2

The optimum, i.e.

Dual Coolant Configurations

s a f e s t , and most economic, r e a c t o r c o n f i g u r a t i o n

may i n v o l v e two c o o l a n t s i n s e r i e s ; a secondary c o o l a n t which t r a n s f e r s h e a t from t h e f u e l s a l t t o a t e r t i a r y c o o l a n t which, i n $ t u r n , produces t h e

steam t o d r i v e t h e t u r b o g e n e r a t o r s .

Thus two c o o l a n t s could be employed,

n e i t h e r of which meet a1.L t h e c o o l a n t c r i t e r i a .

Dual c o o l a n t configura-

t i o n s which appear t o be t h e most a t t r a c t i v e w i l l be d i s c u s s e d i n d e t a i l i n S e c t i o n 7.

The purpose of t h i s s u b s e c t i o n i s t o i d e n t i f y t h e f l u i d s

t h a t are a c c e p t a b l e i n secondary and t e r t i a r y loops.

4.2.1

Secondary c o o l a n t s

O f a l l t h e f l u i d s r e j e c t e d i n t h i s s e c t i o n , o n l y molten LiF-BeF2

m i x t u r e s are c l e a r l y a c c e p t a b l e secondary c o o l a n t s .

The c r i t e r i a f o r r e -

j e c t i n g a l l t h e o t h e r s a:; s i n g l e secondary c o o l a n t s a l s o apply f o r r e j e c t i n g them as secondary c o o l a n t s i n d u a l c o o l a n t c o n f i g u r a t i o n s .

They

e i t h e r would pose s a f e t y problems i f l e a k s o c c u r r e d i n t h e primary h e a t exchangers o r e l s e t h e y are t o o c o r r o s i v e toward s t r u c t u r a l a l l o y s .

4.2.2

T e r t i a r y (steam-raising) coolants

I t was assumed t h a t t h e prime purpose o f an a d d i t i o n a l c o o l a n t loop i s t o p r o v i d e t h e means for s e q u e s t e r i n g t r i t i u m .

Hence, any f l u i d t h a t

cannot do t h i s a t r e a s o n a b l e c o s t while m a i n t a i n i n g a t o l e r a b l e r a t e o f c o r r o s i o n was n o t considered a c c e p t a b l e .

N i t r a t e s and n i t r i t e s decompose

a t t o o low a temperature t o w a r r a n t s e r i o u s c o n s i d e r a t i o n .

Heavy metals

as w e l l as t h e s a l t s w i t h r e d u c i b l e i o n s mentioned i n S e c t i o n 4.1.2 e l i m i n a t e d from f u r t h e r c o n s i d e r a t i o n on t h e same b a s i s ( c o r r o s i v i t y which t h e y were d i s c o u n t e d as secondary c o o l a n t s .

are by

Sodium i s n o t l i k e l y

t o s e q u e s t e r enough of t h e t r i t i u m t o be an a c c e p t a b l e t e r t i a r y c o o l a n t . Helium or t h e t e r n a r y e u t e c t i c (Na, L i , K12C03 are t h e two most a t t r a c t i v e t e r t i a r y coolants i d e n t i f i e d i n t h i s study. The use of n i t r a t e - n i t r i t e mixtures as a s t e a m - r a i s i n g c o o l a n t has

38 been considered

due t o t h e e a s e o f t r a p p i n g t r i t i u m by o x i d a t i o n t o THO.

I t a p p e a r s , however, t h a t t h e thermal s t a b i l i t y o f such mixtures i s n o t adequate t o accommodate t h e design temperature of 621OC (1150OF).

The

maximum temperature would probably have t o be lowered t o 482-538OC (900100OF) t o p r e v e n t s u b s t a n t i a l d e g r a d a t i o n of t h e n i t r a t e - n i t r i t e mixture w i t h a concomitant decrease i n o v e r a l l p l a n t t h e r m a l e f f i c i e n c y .

Also,

very l i t t l e information e x i s t s concerning p o t e n t i a l materials o f containment a t t h e s e temperatures.

For t h e s e reasons n i t r a t e - n i t r i t e mixtures

are n o t f e l t t o be a t t r a c t i v e t e r t i a r y c o o l a n t s and were n o t considered further.

STATUS OF FLUOROBORATE COOLANT

A s a l t mixture composed of sodium f l u o r o b o r a t e and sodium f l u o r i d e

was f i r s t proposed2 as t h e MSBR secondary c o o l a n t i n 1965 a f t e r it was recognized t h a t t h e MSRE c o o l a n t , 7LiF-BeF2 (66-34 mole % ), would be und e s i r a b l e i n a b r e e d e r r e a c t o r due t o i t s high c o s t and h i g h f r e e z i n g point.

On t h e b a s i s o f a reported''

e u t e c t i c temperature o f 304OC (579OF1,

low c o s t , and estimates of p h y s i c a l and chemical properties'' a c c e p t a b l e , molten NaBF4-NaF

t h a t seemed

(92-8 mole % ) appeared t o be an a t t r a c t i v e

secondary c o o l a n t f l u i d f o r t h e MSBR. The p r i n c i p a l advantages of t h e f l u o r o b o r a t e c o o l a n t a r e low c o s t and low v i s c o s i t y .

Its a c t u a l f r e e z i n g p o i n t 384OC (7230F)12 i s somewhat

h i g h e r t h a n d e s i r e d , b u t t h e p e n a l t i e s a s s o c i a t e d with t h i s f r e e z i n g p o i n t

are n o t g r e a t .

The c h i e f disadvantages o f t h i s s a l t a r i s e from e v e n t s

a s s o c i a t e d w i t h f u e l - s a l t - c o o l a n t mixing.

These i n c l u d e g e n e r a t i o n of

BF3 g a s and probable r e d i s t r i b u t i o n of f i s s i l e material between immiscible

Most o t h e r p r o p e r t i e s of f l u o r o b o r a t e are a c c e p t a b l e w i t h ade-

phases.

q u a t e design c o n s i d e r a t i o n s . trapping tritium.

A p o s s i b l e advantage i s i t s p o t e n t i a l f o r

I n t h e following s e c t i o n t h e NaBF -NaF secondary c o o l a n t 4

i s e v a l u a t e d i n terms of c r i t e r i a d e t a i l e d i n S e c t i o n 3 . 5.1

5.1.1

Safety Significant C r i t e r i a

I n c r e a s e of r e a c t i v i t y Any mixing between c o o l a n t and f u e l s a l t i s very u n l i k e l y t o l e a d t o

an i n c r e a s e i n n u c l e a r r e a c t i v i t y . t h e very high c r o s s s e c t i o n of 'OB

This c o o l a n t i s i n h e r e n t l y s a f e due t o which v i r t u a l l y g u a r a n t e e s t h a t f u e l -

s a l t - c o o l a n t mixing would cause an immediate d e c r e a s e i n r e a c t i v i t y as soon

as c o o l a n t i s swept i n t o t h e core.

5.1.2

Chemical r e a c t i o n s

I n t h e e v e n t t h a t f l u o r o b o r a t e mixed w i t h f u e l s a l t due t o l e a k s i n t h e primary h e a t exchangers or secondary c o o l a n t system, t h e r e a c t i o n of s a f e t y s i g n i f i c a n c e i s t h e decomposition of NaBF 4' NaBF4(l)

NaF(d)

BF3(g).

The r e a c t i o n would be d i s p l a c e d t o t h e r i g h t by d i s s o l u t i o n of NaF i n t h e f u e l s a l t mixture.

From measurements of t h e s o l u b i l i t y o f BF3 i n v a r i o u s

molten f l u o r i d e s 1 3 and from t h e p r e l i m i n a r y study14 of phase e q u i l i b r i a of c o o l a n t and f u e l salt:;, t h e f o l l o w i n g w i l l occur when c o o l a n t and f u e l

s a l t are mixed i n v a r i o u s p r o p o r t i o n s : (a)

an i n c r e a s e i n BF 3 p r e s s u r e (except f o r q u i t e low c o n c e n t r a t i o n s of c o o l a n t i n :fuel s a l t )

(b)

a b s o r p t i o n of h e a t

(c)

p a r t i a l i m m i s c i b i l i t y of t h e r e s u l t i n g phases.

To b e t t e r d e f i n e t h e p r e s s u r e s g e n e r a t e d by mixing f u e l s a l t and c o o l a n t

more measurements would b e r e q u i r e d , p a r t i c u l a r l y dynamic mixing e x p e r i ments t o i n v e s t i g a t e non-equilibrium c o n d i t i o n s . A BF

p r e s s u r e s u r g e caused by a t u b i n g f a i l u r e i n a primary h e a t

exchanger might l e a d t o f u r t h e r damage o f t h e h e a t exchanger.

It is

l i k e l y t h a t a p r e s s u r e r e l i e f device would b e a c t i v a t e d which would rel e a s e gaseous f i s s i o n products i n t o t h e c e l l .

If a break o c c u r r e d i n a

c o o l a n t c i r c u l a t i o n l i n e such t h a t a l a r g e volume ($500 f t

s p i l l e d out

o n t o t h e c a t c h pan on t h e f l o o r of t h e c e l l and d r a i n e d i n t o t h e f u e l d r a i n t a n k through t h e t h e r m a l l y a c t i v a t e d r u p t u r e d i s c a t t h e t o p o f t h e t a n k , BF3 g e n e r a t e d i n t h e t a n k could c a r r y gaseous f i s s i o n p r o d u c t s o u t i n t o t h e c e l l through t h e open d r a i n . The p r o b a b i l i t y of such a c c i d e n t s a r e a d m i t t e d l y very small.

The

c h i e f v a l u e i n c o n s i d e r i n g them l i e s i n d e v i s i n g t h e n e c e s s a r y e n g i n e e r i n g s a f e g u a r d s t o c o u n t e r such p o s s i b l e dangers and i n comparing f l u o r o b o r a t e with o t h e r p o t e n t i a l secondary c o o l a n t c a n d i d a t e s .

5.2

5.2.1

Off-Design T r a n s i e n t s

Leaks i n t h e primary h e a t exchanger The e f f e c t s and consequences of l e a k s o u t l i n e d i n t h i s s u b s e c t i o n

are caused by t r a n s i e n t s which can b e a n t i c i p a t e d , and are d i f f e r e n t i a t e d from s a f e t y s i g n i f i c a n t e v e n t s .

I n t h i s d i s c u s s i o n , BF

pressure surges

a r e n o t considered s e r i o u s enough t o cause b r e a k s i n t h e primary containment boundary.

I n t h e c a s e of a small h o l e i n a t u b e w a l l , c o o l a n t w i l l

l e a k i n t o t h e f u e l s a l t and most of t h e BF3 and a l l of t h e NaF w i l l d i s s o l v e i n t h e f u e l s a l t mixture.

I n case of a l a r g e r l e a k such as a t u b e

b r e a k , c o o l a n t w i l l flow i n t o f u e l s a l t and f u e l s a l t may flow i n t o t h e For s t i l l 1arg:er l e a k s two immiscible l i q u i d phases may form.

coolant.

Whenever BF

i s released i n t h e primary c i r c u i t , some w i l l be swept i n t o

t h e off-gas system where it may i n t e r a c t w i t h t h e c h a r c o a l beds and a s m a l l e r amount may d i f f u s e i n t o t h e core g r a p h i t e , b u t most of t h e BF3

is likely t o dissolve i n the f u e l salt. 5.2.1.1

Gas g e n e r a t i o n or inleakage.

(a)

Dissolution i n f u e l salt.

The s o l u b i l i t y of BF3 i s r e l a t i v e l y high i n f u e l s a l t and even h i g h e r i n f u e l s a l t c o n t a i n i n g a s u b s t a n t i a l c o n c e n t r a t i o n o f NaF.13

For example,

a t 704OC (13OOOF) 4 moles o f BF3 confined t o a volume h a l f - f i l l e d w i t h f u e l s a l t w i l l b e p a r t i t i o n e d a t e q u i l i b r i u m , 3 moles BF3 i n t h e s a l t and one mole i n t h e vapor space.

If 1 f t

of c o o l a n t e q u i l i b r a t e d w i t h t h e

3 inventory of f u e l s a l t (1720 f t ) and i f t h e only vapor space i n t h e f u e l

bubble f r a c t i o n i n t h e s a l t , t h e BF c i r c u i t were a 1% would be only 0.23 a t m ( 3 p s i ) . ft

saturation pressure

A f a r smaller amount of c o o l a n t ( 0 . 1 2 7

1, i f c a r r i e d i n t o t h e c o r e , c o n t a i n s enough boron

the reactor subcritical.

( 0 . 6 kg) t o r e n d e r

The 1 0 . 9 kg of sodium i n 1 f t 3 o f c o o l a n t would

have a minor e f f e c t on r e a c t i v i t y ; about 2 , 0 0 0 kg of sodium i n t h e c o r e would be r e q u i r e d t o cause t h e same n e g a t i v e r e a c t i v i t y e f f e c t as 0.6 kg o f boron.

The consequences of a much l a r g e r l e a k are d i s c u s s e d i n s u b s e c t i o n

5.2.1.2.

(b)

I n t e r a c t i o n of BF, with c h a r c o a l .

Some of t h e BF

r e l e a s e d or

g e n e r a t e d i n t h e primary c i r c u i t and f u e l d r a i n t a n k w i l l be swept i n t o t h e off-gas system.

Adverse consequences might be i n a c t i v a t i o n o f t h e

c h a r c o a l beds, or worse, s e l e c t i v e s o r p t i o n of BF xenon and krypton.

is not available.

and d e s o r p t i o n of t h e

Data f o r p r e d i c t i n g t h e e f f e c t o f BF3 on c h a r c o a l beds

(c)

I n t e r a c t i o n of BF, w i t h c o r e g r a p h i t e .

No chemical r e a c t i o n s ,

i n c l u d i n g i n t e r c a l a t i o n , a r e known between BF3 and g r a p h i t e and chemical

effects on g r a p h i t e should b e a b s e n t . 5.2.1.2

Reduction i n breeding gain.

The breeding r a t i o o f an MSBR,

nominally 1.07, i s reduced when f l u o r o b o r a t e c o o l a n t or any o t h e r s o u r c e o f boron i s p r e s e n t i n t h e core.

The amount o f boron i n t h e c o r e t h a t

changes t h e r e a c t o r i n t o a c o n v e r t e r , i.e.,

reduces t h e b r e e d i n g r a t i o t o

1 . 0 0 , i s q u i t e small; 3 kg i n t h e s a l t o r 2 kg d i s p e r s e d i n t h e g r a p h i t e .

The s i t u a t i o n with sodium i s q u i t e d i f f e r e n t .

I t t a k e s approximately

9000 kg of sod-ium i n t h e c o r e t o reduce t h e breeding r a t i o t o 1.00 and

t h u s sodium p r e s e n t s much less of a problem.

The sodium i n sodium f l u o r o -

b o r a t e , t h a t e n t e r s t h e primary c i r c u i t would n o t p e n e t r a t e t h e g r a p h i t e .

(a) Boron and sodium i n t h e f u e l s a l t . 1f t

I n a "small" l e a k case,

o f c o o l a n t i s assumed t o l e a k i n t o t h e f u e l s a l t .

c o o l a n t c o n t a i n s 4.7 kg of boron. w i t h i n t h e 1074 f t

This amount of

If t h e boron i s d i s p e r s e d homogeneously

of f u e l s a l t i n t h e c o r e , it would now c o n t a i n 2.95 kg

of boron.

More t h a n enough w i l l e n t e r t h e c o r e t o b r i n g t h e r e a c t o r sub-

critical.

The boron c o n t e n t of t h e f u e l s a l t can be g r e a t l y reduced by

allowing t h e f u e l s a l t temperature t o r i s e or by s p a r g i n g with an i n e r t gas t o e f f e c t BF3 removal.

These l a s t two o p e r a t i o n s might be done i n t h e

f u e l d r a i n tank or i n t h e f u e l s t o r a g e tank.

The BF3 v o l a t i l i z e d from t h e

s a l t could be t r a p p e d i n d i s p o s a b l e NaF a b s o r b e r s .

Any 'OB

t h a t cannot be

removed from t h e f u e l s a l t by chemical o r p h y s i c a l methods could be neut r o n i c a l l y burned o u t . Larger i n l e a k a g e of f l u o r o b o r a t e p r e s e n t s g r e a t e r d i f f i c u l t i e s .

For

example, i f a double-ended t u b i n g r u p t u r e occurred i n t h e h e a t exchanger

and a volume of f l u o r o b o r a t e e q u a l t o t h e heat-exchanger s h e l l (416 f t

were t o mix w i t h t h e f u e l s a l t , t h e mixture would c o n t a i n approximately 2500 kg of boron and 57510 kg of sodium.

Most of t h e boron and a l l of t h e

sodium would be i n t h e f u e l d r a i n tank.

A s with a small l e a k , t h e boron

i n t h e f u e l s a l t could be reduced t o an a c c e p t a b l y low l e v e l by some comb i n a t i o n of s p a r g i n g and h e a t i n g .

Even i f a l l t h e boron were removed

from t h e f u e l s a l t , t h e amount o f sodium d i s s o l v e d i n t h e f u e l s a l t i s such t h a t t h e breeding g a i n of t h e r e a c t o r would be halved.

Since sodium

cannot be removed by o n - s i t e r e p r o c e s s i n g , continued use of t h e f u e l s a l t would depend on economic c o n s i d e r a t i o n s . (b)

Boron i n t h e g r a p h i t e .

No q u a n t i t a t i v e d a t a e x i s t t o d e f i n e what

BF3 w i l l do t o t h e g r a p h i t e t h a t w i l l be used i n t h e MSBR.

r e a c t o r c o n t a i n s about 0.3 x 1 0

A 1000-MW(e)

kg of g r a p h i t e and i f about 2 kg of

boron i s p r e s e n t on or i n t h i s g r a p h i t e , t h e breeding r a t i o w i l l d e c r e a s e t o 1.00.

If boron cannot b e desorbed from t h e g r a p h i t e , t h e r e a c t i v i t y

l o s s e s could be overridden by adding more uranium t o t h e f u e l s a l t . 5.2.1.3

Clean-up of c o o l a n t and c o o l a n t c i r c u i t .

Any s i z e a b l e break

i n a heat-exchanger tube i s l i k e l y t o l e a k f u e l s a l t i n t o t h e c o o l a n t .

Since a l l components of t h e f u e l s a l t except L i F are i n s o l u b l e i n NaBF4, t h e uranium i n t h e f u e l : s a l t w i l l most probably be e n t r a i n e d i n t h e e.g., c o o l a n t as a c r y s t a l l i n e complex of UF 4â&#x20AC;&#x2122;

Na2UF6.

But even i f t h e

L i F were n o t leached o u t by t h e c o o l a n t , t h e f u e l would f r e e z e when t h e temperature i n t h e c o o l a n t dropped below 502OC (935OF). c o o l a n t could be discarded.

The contaminated

Discard o f t h e c o o l a n t i n one of f o u r c o o l a n t

c i r c u i t s involves t h e loss of only about $100,000. 5.2.1.4

Contamination of chemical p l a n t .

Entry of boron i n t o t h e

o n - s i t e f u e l - p r o c e s s i n g f a c i l i t i e s w i l l n o t create a s e r i o u s problem. The BF3 may s e p a r a t e from t h e UF6 i n t h e f l u o r i n a t i o n and r e c o n s t i t u t i o n steps.

I n g e n e r a l , diminution of boron i n t h e f u e l s a l t t o some ac-

c e p t a b l e l e v e l by p r i o r s p a r g i n g i s t h e obvious p r o v i s i o n f o r p r o t e c t i n g t h e chemical p l a n t from boron and t h i s should be r e a d i l y a c h i e v a b l e . 5.2.2

Other l e a k s 5.2.2.1

I n l e a k a g e o f steam,

Small l e a k s i n t h e s t e a m - r a i s i n g system,

i f d e t e c t e d and r e p a i r e d w i t h i n a s h o r t t i m e , would n o t be very harmful.

Indeed, small amounts o f water may a i d t r i t i u m c o n t r o l i n t h e c o o l a n t . Large l e a k s , however, w i l l g r e a t l y increase metal c o r r o s i o n and mass t r a n s -

fer.

An a s s o c i a t e d complication i s t h e f o r m a t i o n of t h e r e l a t i v e l y i n -

s o l u b l e c o r r o s i o n p r o d u c t s Na CrF and NaNiF3, which could f o u l steam3 6 g e n e r a t o r s u r f a c e s and/or r e s t r i c t narrow c o o l a n t flow c h a n n e l s .

A method

f o r removing t h e s e s o l i d s may have t o be developed. 5.2.2.2

Leakage of c o o l a n t i n t o steam.

A l a r g e break i n a steam-

g e n e r a t o r t u b e could i n t r o d u c e small amounts of c o o l a n t s a l t i n t o t h e steam-feedwater system c o u n t e r t o t h e p r e s s u r e d i f f e r e n t i a l .

The pH of

t h e water w i l l be changed due t o t h e formation of HF w i t h f l u o r i d e - c o n t a i n i n g s a l t s and t h e water w i l l a l s o c o n t a i n ppm c o n c e n t r a t i o n s of f l u oride,

There i s no evidence t h a t t h i s could l e a d t o s t r e s s - c o r r o s i o n

c r a c k i n g b u t t h i s h a s n o t been s t u d i e d s u f f i c i e n t l y and e x p e r i m e n t a l i n v e s t i g a t i o n s would be needed. 5.2.2.3

Leaks t o t h e c e l l atmosphere.

Small amounts of c o o l a n t

l e a k i n g i n t o t h e c e l l w i l l r e a c t with m o i s t u r e i n t h e atmosphere and g e n e r a t e a c i d i c vapors t h a t could cause minor c o r r o s i o n of t h e metal l i n i n g t h e c o n c r e t e c e l l walls.

5.3

5.3.1

Design F a c t o r s

Corrosion The i n h e r e n t c o r r o s i v i t y of e u t e c t i c NaBF -NaF m e l t toward n i c k e l 4

base a l l o y s i s small.

I n Hastelloy N , t h e minor a l l o y c o n s t i t u e n t s C r y

T i , Mn, S i , and H f w i l l react w i t h t h e s a l t .

For i n s t a n c e , t h e following

r e a c t i o n , a t r e a c t o r temperatures has a n e g a t i v e s t a n d a r d f r e e energy: (1

x)Cr(c)

2NaF(d) i- NaBF4(d)

* Na3CrF6(c)

t CrxB(c)

If t h e c o n c e n t r a t i o n of chromium i n t h e a l l o y i s low (<0.1 atomic f r a c t i o n ) , t h i s r e a c t i o n i s l i k e l y t o be d i f f u s i o n l i m i t e d and o f l i t t l e s i g n i f i c a n c e . I n c r e a s e d c o n c e n t r a t i o n s of chromium i n t h e containment a l l o y could l e a d t o increased corrosion rates.

Na3CrF

i s sparingly soluble i n fluoro-

b o r a t e 1 7 and plugging ofâ&#x20AC;&#x2122; flow channels might be a problem. 5.3.2

Freezing p o i n t The NaBF4-NaF e u t e c t i c f r e e z e s a t 384OC (723OF)I2 which i s t o o high

t o b e compatible with t h e h i g h e s t feedwater temperature (%29O0C, 58OOF) t h a t could be used i n an unmadified s u p e r c r i t i c a l steam c y c l e .

The d e s i g n

m o d i f i c a t i o n s and c o s t s necessary t o p r e h e a t t h e feedwater t o 371OC (700OF) and t h e low p r e s s u r e steam t o 343OC (65OOF) have been e s t i m a t e d and are a very small f r a c t i o n of t h e t o t a l p l a n t c o s t .

Although f u r t h e r

design m o d i f i c a t i o n s may be necessary t o p r e v e n t p o s s i b l e damage caused by

s a l t f r e e z i n g on c o l d s p o t s i n t h e s a l t s i d e of steam g e n e r a t o r s , t h e a s s o c i a t e d c o s t s are probably modest. 5.3.3

H e a t - t r a n s f e r and hydrodynamic p r o p e r t i e s The h e a t - t r a n s f e r p r o p e r t i e s , t h e r m a l c o n d u c t i v i t y and thermal capa-

c i t y of f l u o r o b o r a t e a r e a c c e p t a b l e ; t h e kinematic v i s c o s i t y ( v i s c o s i t y

d i v i d e d by d e n s i t y ) i s very f a v o r a b l e .

Measurement of h e a t t r a n s f e r

i n a t u b u l a r t e s t - s e c t i o n of a pump loop showed t h a t t h e performance of f l u o r o b o r a t e was c o n s i s t e n t with t h e Seider-Tate c o r r e l a t i o n which successf u l l y c o r r e l a t e s d a t a f o r a wide v a r i e t y of non-metallic f l u i d s flowing turbulently i n pipes.

Vapor p r e s s u r e and composition

5.3.4

The e q u i l i b r i u m decomposition p r e s s u r e s of BF

over t h i s c o o l a n t are

moderate; a t t h e h o t - l e g temperature of t h e c o o l a n t l o o p , 621OC (115OOF)

'BF~

i s 0.33 a t m ( 5 p s i ) .

The BF3 cannot condense i n t h e a n n u l a r spaces

i n t h e pumps s i n c e i t s c r i t i c a l temperature i s - 1 2 O C

(10OF).

Present

designs of c o o l a n t pumps c a l l f o r helium o r some o t h e r i n e r t gas t o sweep downshaft toward t h e pump bowl. BF

I n t h e p r o c e s s , t h e sweep gas carries

o u t of t h e pump bowl and i f makeup BF3 i s n o t added, t h e c o o l a n t w i l l

slowly change composition and i n c r e a s e i n f r e e z i n g p o i n t .

The He-BF3 g a s

mixture cannot be continuously d i s c h a r g e d from t h e p l a n t because of t h e t o x i c i t y and chemical r e a c t i v i t y of BF3. volved i s probably t o o expensive t o waste.

Also, t h e amount of helium i n Thus, it would b e n e c e s s a r y

t o develop a BF3 r e c i r c u l a t i o n system. 5.3.5

Radiation and chemical s t a b i l i t y The e f f e c t of t h e i n t e n s e gamma r a d i a t i o n t o which t h e c o o l a n t s a l t

would be exposed i n t h e primary heat-exchangers was i n v e s t i g a t e d by E. L. Compere e t a1.20

S a l t w a s exposed f o r 1460 h r i n a H a s t e l l o y N

c a p s u l e experiment a t 6OOOC (1112OF) and no evidence of chemical decompos i t i o n was d e t e c t e d from vapor p r e s s u r e measurements or from metal weight losses.

Fluoroborate c o o l a n t w i l l a l s o be i r r a d i a t e d by delayed n e u t r o n s i n t h e primary h e a t exchangers.

T h e i r e f f e c t on f l u o r o b o r a t e chemistry has

n o t been s t u d i e d e x p e r i m e n t a l l y .

However, i n v e s t i g a t i o n of BF3 gas

s u g g e s t s t h a t t h e o n l y s i g n i f i c a n t chemical change i n t h e s a l t would be t h e r e l e a s e of f l u o r i n e by t h e r e a c t i o n :

l0BF3

-f

7LiF

a t F

The number of delayed n e u t r o n s i n primary h e a t exchangers w i l l be about n / s e c , t h u s t h e maximum y i e l d of F proximately 5 moles p e r y e a r .

from t h e c o o l a n t w i l l be o n l y ap-

This r a t h e r small amount of o x i d a n t can

probably be handled by t h e p r o c e s s i n g methods t h a t would b e used t o c o n t r o l t h e redox c o n d i t i o n s of t h e c o o l a n t .

5.3.6

Tritium The s e q u e s t e r i n g of t r i t i u m i n t h e f l u o r o b o r a t e c o o l a n t i s as y e t

undemonstrated.

I o n i c s p e c i e s c o n t a i n i n g o x i d i z e d hydrogen which could

exchange with and t r a p t r i t i u m are i d e n t i f i e d (BF30H->, expected (HF2

+ >. There

or s u s p e c t e d (H could b i n d H

or T

a l s o may be polymeric o x y f l u o r o b o r a t e i o n s which

It i s n o t y e t known, however, i f one or more of

t h e s e s p e c i e s can be s t a b i l i z e d a t a s u f f i c i e n t l y high c o n c e n t r a t i o n s t o be e f f e c t i v e for t r i t i u m t r a p p i n g w i t h o u t exceeding t h e l i m i t s of t h e o x i d a t i o n p o t e n t i a l of t h e c o o l a n t beyond which c o r r o s i v e a t t a c k of n i c k e l b a s e a l l o y s would become e x c e s s i v e .

There i s some e x p e r i m e n t a l evidence

t h a t t h e p r o t o n of t h e BF30H- s p e c i e s d i d n o t undergo t h e expected r a p i d exchange w i t h D

Thus, t h e r e are a number of unresolved q u e s t i o n s con-

c e r n i n g t h e p o t e n t i a l of t h i s c o o l a n t f o r t r i t i u m t r a p p i n g , b u t t h i s i s a l s o t r u e f o r o t h e r possible molten-salt coolants.

5.3.7

Cost and a v a i l a b i l i t y The c o o l a n t components, NaBF,+ and NaF, are inexpensive and r e a d i l y

a v a i l a b l e commercially, although t h e p u r i t y necessary f o r r e a c t o r use may r e q u i r e some development e f f o r t s . 8500 f t

I n 1971, t h e e s t i m a t e d c o s t 1 8 o f t h e

inventory was $0.37 M. 6.

ALTERNATE SINGLE COOLANT:

7 NaF- LiF-BeF2

The composition of t h i s c o o l a n t i s NaF-7LiF-BeF

7 1.34 Li0.66BeF4.

(45-22-33 mole % ) or

Thus, t h i s c o o l a n t i s very similar t o t h e MSRE c o o l a n t ,

except t h a t sodium i s s u b s t i t u t e d f o r approximately two-thirds of t h e lithium ions.

A secondary c o o l a n t composed o f t h e s e f l u o r i d e s has a major

advantage of c o m p a t i b i l i t y with f u e l s a l t ; i . e . ,

t h e consequences of i n t e r -

leakage or mixing w i t h t h e f u e l s a l t a r e minor or r e a d i l y r e v e r s i b l e . Another advantage o f t h i s c o o l a n t i s a very low vapor p r e s s u r e .

Disadvan-

t a g e s are r e l a t i v e l y high inventory c o s t and i n c r e a s e d c o r r o s i v i t y when mixed with steam.

This c o o l a n t i s reviewed i n terms of t h e c r i t e r i a de-

t a i l e d i n S e c t i o n 3, as was t h e f l u o r o b o r a t e c o o l a n t i n S e c t i o n 5 . 6.1

Safety Significant C r i t e r i a

Nal, 347Li0.66BeF4 i s a very safe c o o l a n t t h a t i s compatible with f u e l salt.

Mixing of f u e l s a l t and c o o l a n t w i l l n o t p r e c i p i t a t e uranium, w i l l

n o t release or absorb much h e a t , and w i l l n o t g e n e r a t e gases by chemical reactions.

I n case c o o l a n t i s c a r r i e d i n t o t h e c o r e , r e a c t i v i t y w i l l de-

c r e a s e s l i g h t l y because of t h e sodium.

6.2 6.2.1

Off-Design T r a n s i e n t s

Leaks i n primary :heat exchangers 6.2.1.1

Secondary coolant i n t o f u e l s a l t .

The e f f e c t o f g r e a t e s t

s i g n i f i c a n c e w i l l be a rleduction i n t h e b r e e d i n g g a i n a r i s i n g from t h e d i s s o l u t i o n o f NaF i n t h e f u e l s a l t ; however, a c o n s i d e r a b l e volume of coolant is necessary.

F o r instance, approximately 2000 kg of sodium i n

t h e c o r e could b e n e c e s s a r y t o r e n d e r t h e r e a c t o r s u b c r i t i c a l .

Intro-

d u c t i o n of t h i s much sodium i n t o t h e c o r e would r e q u i r e t h e i n l e a k a g e and n

homogeneous d i s t r i b u t i o n of about 6570 l i t e r s ( 2 3 0 f t ' )

of secondary

coolant. The f u e l reprocessi:ng system would n o t be s i g n i f i c a n t l y a f f e c t e d , however, it would n o t remove t h e sodium from t h e f u e l s a l t .

A decision

would have t o b e made between continued o p e r a t i o n with a lower breeding performance by o v e r r i d i n g t h e sodium poisoning v i a a d d i t i o n of more uranium, r e c l a i m i n g t h e 7 L i and B e c o n t e n t by s u i t a b l e p r o c e s s i n g methods,

or replacement of t h e f u e l c a r r i e r s a l t . Other e f f e c t s of i n l e a k a g e would be minimal.

This c o o l a n t i s m i s c i b l e

w i t h f u e l s a l t and would n o t p e n e t r a t e t h e moderator g r a p h i t e .

Also, f u e l

s a l t contaminated w i t h c o o l a n t would have v i r t u a l l y t h e same s o l u b i l i t i e s of i n e r t g a s e s and hydrogen. 6.2.1.2

F u e l s a l t i n t o t h e secondary c o o l a n t c i r c u i t .

The damage t o

t h e c o o l a n t and t h e c o o l a n t c i r c u i t would n o t be very g r e a t i n case of an i n l e a k a g e of f u e l s a l t .

A l l c o n s t i t u e n t s and f i s s i o n products contained

i n t h e f u e l s a l t are s o l u b l e i n t h e c o o l a n t s a l t except f o r %oble" m e t a l l i c f i s s i o n product;; Nb, Mo, T c , Ru, Ag, and Sb.

I n such an e v e n t

it would probably be n e c e s s a r y t o d i s p o s e of t h e contaminated c o o l a n t and

r e p l a c e it.

After t h e l e a k was r e p a i r e d , clean c o o l a n t s a l t c o u l d be

used t o f l u s h t h e loop.

Any r a d i o a c t i v i t y remaining would be due t o t h e

noble metal f i s s i o n p r o d u c t s a d h e r i n g t o metal s u r f a c e s . 6.2.2

Other l e a k s 6.2.2.1

I n l e a k a g e of steam.

The e f f e c t s of steam upon t h e c o o l a n t

and c o o l a n t loop would be c o r r o s i o n o f metal t o form complex c o r r o s i o n product f l u o r i d e s and, if t h e s o l u b i l i t y of oxide i n t h e c o o l a n t were exceeded, p r e c i p i t a t i o n of b e r y l l i u m oxide. follows:

H20(g)

The r e a c t i o n s can b e w r i t t e n as

M(al1oy) t NaF(d) t BeF2(d)

-f

BeO(s)

NaMF3(c)

H2(g),

where M r e p r e s e n t s Fe or N i , and 3/2H20(g) t Cr(al1oy) t 3NaF(d)

3/2BeF2(d)

-f

BeO(c)

Na3CrF6(c) t 3/2H2(g).

These r e a c t i o n s would probably proceed t o completion because t h e c o r r o s i o n product f l u o r i d e s are very s t a b l e , 2 3 and t h e hydrogen would r e a d i l y escape from t h e system.

Thus, f o r every water molecule t h a t l e a k s i n t o t h e cool-

a n t , s l i g h t l y less t h a n one atom of metal w i l l be corroded.

Cleanup o f t h e

c o o l a n t system a f t e r such a l e a k c o u l d be a s i g n i f i c a n t t a s k . 6.2.2.2

Leakage o f c o o l a n t i n t o steam.

t o t h a t of t h e fluoroborate coolant.

The s i t u a t i o n h e r e i s similar

Very l i t t l e i s known about t h e i n f l u -

ence of t h e s e f l u o r i d e s i n r e l a t i o n t o s t r e s s - c o r r o s i o n c r a c k i n g and such i n f o r m a t i o n would have t o be o b t a i n e d p r i o r t o s p e c i f y i n g t h e materials f o r

an MSBR steam system. 6.2.2.3

Leaks t o t h e c e l l atmosphere.

Small amounts of c o o l a n t

flowing i n t o t h e c e l l w i l l r e a c t with moisture t o produce Be0 and HF. HF may cause minor c o r r o s i o n o f t h e m e t a l t h a t covers t h e c e l l w a l l .

The

51 6.3

6.3.1

Design F a c t o r s

Corrosivity Pure Na

BeF4 i s n o t c o r r o s i v e toward a l l major and v i r t u a l l y 1.34Li0. 66

a l l minor c o n s t i t u e n t s of s t r u c t u r a l metal a l l o y s .

Inadvertent intro-

d u c t i o n o f o x i d a n t s (steam l e a k s ) or d e l i b e r a t e a d d i t i o n of o x i d a n t s ( f o r

t r i t i u m c o n t r o l ) w i l l s u b s t a n t i a l l y i n c r e a s e t h e c o r r o s i v i t y o f t h e coolant.

The o x i d i z i n g e f f e c t s of steam l e a k s have a l r e a d y been d i s c u s s e d i n

S e c t i o n 6.2.2.1.

A s with f l u o r o b o r a t e , o x i d i z i n g agents may have t o be

added t o Nal.34Li0.66BeF

secondary c o o l a n t circui.t.

t o s e q u e s t e r t r i t i u m which d i f f u s e s i n t o t h e I t w i l l have t o be determined i f e f f e c t i v e

c o n c e n t r a t i o n s of t h e s e o x i d i z i n g a d d i t i v e s can be a t t a i n e d i n t h i s s a l t and i f t h e y are t o l e r a b l e t o materials of c o n s t r u c t i o n . 6.3.2

Freezing p o i n t Na1.34Li0.66

BeF4 has a l i q u i d u s temperature t h a t may be 29OOC (554OF)

b u t could be as high as 34OOC (644OF).

The lower temperature i s t a k e n

from a published study24 of t h e Na2BeF Li2BeF4 phase diagram, shown i n 4F i g . 6.1.

The higher liquidus temperature is derived from a preliminary

diagram of NaF-LiF-BeF2,

Fig. 6.2,

i n a compilation25 of phase diagrams.

The composition chosen for t h i s (NaF-LiF-BeF2)

c o o l a n t i s based on t h e

l i q u i d u s temperature v a l l e y p r e s e n t e d i n Fig. 6.1. temperature o f Na1.34Li0,66

If t h e l i q u i d u s

BeF4 i s a c t u a l l y 34OOC (644OF) as shown i n

Fig. 6.2, t h e n a s h i f t t o a s l i g h t l y d i f f e r e n t composition NaF-LiF-BeF2 (41-23-36 mole % ) should provide a l i q u i d u s temperature of 328OC (622OF), perhaps s t i l l low enough t o o p e r a t e a s u p e r c r i t i c a l steam c y c l e without freezing t h e coolant salt.

If a NaF- LiF-BeF2 mixture i s t o be chosen as

ORNL-DWG. 7 5 - 1590

600

500

400 a'+a.

300

200

100

+No LiBeG y-Na$eG+ Y-Na3Li( BeF4&

0 0

Mol. %

Na2BeF4 Fig.

6.1.

Phase diagram Na BeF4-Li2BeF4. 2

IO0 Li2BeQ

ORNL-LR-DWQ

DOTTED LINES REPRESENT INCOMPLETELY DEFINED PHASE BOUNDARIES AND ALKEMADE LINES

46424R

TEMPERATURE (OC) COMPOSITION (mole 7.)

Fig. 6.2.

Phase diagram of LiF-NaF-BeF2 system.

t h e secondary MSBR c o o l a n t , t h e r e i s an obvious need t o r e f i n e t h e phase diagram d a t a i n t h e composition r e g i o n around 33 mole % BeF2.

6.3.3

H e a t - t r a n s f e r and hydrodynamic p r o p e r t i e s H e a t - t r a n s f e r p r o p e r t i e s a r e good by comparison with o t h e r molten-

salt coolants.

The thermal c o n d u c t i v i t y i s e s t i m a t e d t o be twice t h a t

of e u t e c t i c NaBF4-NaF and t h e h e a t c a p a c i t y p e r u n i t mass i s about 20% greater.

The v i s c o s i t y of Na1.34Li0.66BeF4

e u t e c t i c NaBF4-NaF,

i s about 1 0 times t h a t of

but certainly s t i l l acceptable.

This p r o p e r t y has

n o t been measured f o r t h i s s p e c i f i c mixture, b u t t h e d a t a f o r t h e NaFand LiF-BeF2 b i n a r y systems26 s u g g e s t t h a t t h e v i s c o s i t y should be almost t h e same as t h a t of t h e MSRE c o o l a n t and t h u s should be a s a t i s f a c t o r y h e a t t r a n s p o r t medium.

6.3.4

Vapor p r e s s u r e and composition The e q u i l i b r i u m vapor p r e s s u r e should b e very low.

d a t a i n t h e NaF-BeF2

and LiF-BeF2

By combining

systems, an e s t i m a t e d vapor p r e s s u r e

of 0.002 t o r r i s obtained a t 621OC (115OoF), t h e o u t l e t temperature o f t h e

primary h e a t exchangers.

The vapor composition h a s n o t been determined,

b u t BeF ( g ) should be t h e dominant vapor s p e c i e s . 2

6.3.5

Radiation and chemical s t a b i l i t y Experience gained i n t h e MSRE

t e s t i n g program

and i n t h e e x t e n s i v e i n - p i l e r a d i a t i o n

of similar molten s a l t s t r o n g l y s u g g e s t t h a t as long as

t h e temperature i s k e p t g r e a t e r t h a n 15OoC, r a d i o l y t i c decomposition of t h i s s a l t w i l l be of no importance t o t h e c o r r o s i o n of s t r u c t u r a l metals.

6.3.6

Tritium The t r a p p i n g of t r i t i u m i n t h i s c o o l a n t w i l l l i k e l y r e q u i r e concen-

t r a t i o n s of o x i d i z i n g i o n s such as OH- or Ni2'

which could a l s o o x i d i z e

metals t h a t w i l l be i n c o n t a c t with t h e c o o l a n t .

I t i s known31 t h a t sub-

s t a n t i a l c o n c e n t r a t i o n s o f OH- can b e maintained i n t h e LiF-BeF2 system

a t 500-700째C (932-1292OF) and s i m i l a r c o n c e n t r a t i o n s can be expected i n molten NaF-LiF-BeF2.

The s o l u b i l i t y of metal i o n o x i d a n t s N i 2 + or Fe

may be l i m i t e d because t:hese i o n s a r e s t a b i l i z e d i n complex, r e l a t i v e l y i n s o l u b l e s o l i d s , as NabliF3 or NaFeF3. 23

The c o n c e n t r a t i o n s o f hydroxide

and oxide i n t h i s c o o l a n t may also be c o n s t r a i n e d by t h e need t o p r e v e n t p r e c i p i t a t i o n of U 0 2 i n case c o o l a n t l e a k s i n t o t h e f u e l s a l t . t h e f l u o r o b o r a t e secondary c o o l a n t , t h e b a s i c q u e s t i o n i s :

A s with

Can t h e i o n i c

s p e c i e s which t r a p s t r i t : i u m be s t a b i l i z e d a t s u f f i c i e n t l y high concentrat i o n without causing i n t : o l e r a b l e c o r r o s i v e a t t a c k ?

Experimental i n v e s t i -

g a t i o n w i l l be r e q u i r e d t o answer t h i s q u e s t i o n . 6.3.7

Cost and a v a i l a b i l i t y The inventory of secondary c o o l a n t given i n t h e conceptual design

would c o s t approximately $6 m i l l i o n , assuming u n i t prices'' for

7 L i and $16.50/kg for BeF2.

of $120/kg

The r e l a t i v e l y high c o s t of t h e c o o l a n t

i n v e n t o r y i s an i n c e n t i v e f o r minimizing t h e volume o f s a l t r e q u i r e d i n t h e c o o l a n t c i r c u i t s through a p p r o p r i a t e design.

Cost as w e l l as v i s -

c o s i t y could perhaps be reduced by s u b s t i t u t i n g MgF2, A1F3, or ZrF4 f o r p a r t of t h e BeF2. Beryllium i s n o t an abundant element i n t h e e a r t h ' s c r u s t and world demand, i f maintained a t t h e r a t e of 1968, would d e p l e t e world r e s e r v e s i n about 25 y e a r s .

Hcwever, b e r y l l i u m m i n e r a l r e s o u r c e s are a t l e a s t

1000 times g r e a t e r t h a n t h e r e s e r v e s .

If t h e s e r e s o u r c e s are developed,

t h e supply o f b e r y l l i u m would be adequate f o r a l l m o l t e n - s a l t r e a c t o r needs.

F o r t u n a t e l y , t h e c o s t of t h e o r e i s a small p a r t of t h e c o s t of

t h e metal and, t h u s , t h e p r i c e o f b e r y l l i u m may n o t r i s e a p p r e c i a b l y as t h e s e r e s o u r c e s are developed.

ALTERNATE DUAL COOLANT CANDIDATES

I n t h e d u a l c o o l a n t MSBR c o n f i g u r a t i o n t h e r e a c t o r and primary loop c o n t a i n i n g t h e f u e l s a l t are considered t o be similar t o t h o s e d e t a i l e d i n t h e c o n c e p t u a l d e ~ i g n . The ~ f u e l s a l t i s c i r c u l a t e d through t h e primary h e a t exchangers where h e a t i s t r a n s f e r r e d t o t h e secondary c o o l a n t .

The

secondary c o o l a n t i s c i r c u l a t e d t o i n t e r m e d i a t e h e a t exchangers where h e a t

i s t r a n s f e r r e d t o a t e r t i a r y c o o l a n t , and t h e t e r t i a r y c o o l a n t i s c i r c u l a t e d t o t h e s t e a m - r a i s i n g system.

The i n c e n t i v e s f o r c o n s i d e r i n g t h e

a d d i t i o n of a t h i r d loop t o an MSBR i n c l u d e a b e t t e r o p p o r t u n i t y f o r tritium t r a p p i n g , a d d i t i o n of a n o t h e r p a s s i v e b a r r i e r between t h e f i s s i o n product i n v e n t o r y i n t h e r e a c t o r and t h e steam system, improved chemical c o m p a t i b i l i t y between a d j a c e n t loop f l u i d s i n t h e e v e n t of l e a k s , and, i n some c a s e s , improved p l a n t o p e r a b i l i t y and m a i n t a i n a b i l i t y .

Financial

p e n a l t i e s a r e , of c o u r s e , a s s o c i a t e d with t h e a d d i t i o n o f a t h i r d loop i n both c a p i t a l and o p e r a t i n g c o s t s . Three d u a l c o o l a n t MSBR c o n f i g u r a t i o n s are c o n s i d e r e d i n t h i s s e c t i o n

as a l t e r n a t e r e a c t o r concepts t h a t could s a t i s f y , t o v a r y i n g d e g r e e s , t h e coolant c r i t e r i a .

Each employs a molten s a l t secondary c o o l a n t and i n each

c a s e Li2BeF4 has been s e l e c t e d as t h e molten s a l t ( S e c t i o n 7.1).

This was

due p r i m a r i l y t o t h e e x c e l l e n t c o m p a t i b i l i t y between t h i s material and t h e f u e l s a l t i f t h e y should become mixed.

Also, L i BeF 2

has an a d e q u a t e l y

57 I

high f r e e z i n g p o i n t s o t h a t t h e problems of f u e l s a l t freeze-up a r e subs t a n t i a l l y diminished. For t h e t e r t i a r y c o o l a n t , a g a s , a d i f f e r e n t molten s a l t , a l i q u i d

metal and a l s o a n o v e l f l u i d i z e d bed h e a t exchanger are considered.

The

r e a c t o r c o n f i g u r a t i o n u t i l i z i n g helium gas ( S e c t i o n 7 . 2 ) appears t o r e s o l v e a l l t e c h n o l o g i c a l problems and t o s a t i s f y a l l c o o l a n t c r i t e r i a ; however, o n l y a t an expense due t o t h e a d d i t i o n a l hardware r e q u i r e d f o r t h e t e r t i a r y l o o p , reduced thermal e f f i c i e n c y and i n c r e a s e d pumping power requirements. Molten c a r b o n a t e s are considered ( S e c t i o n 7.3) as t h e t e r t i a r y c o o l a n t , and l i q u i d sodium a l s o has been e v a l u a t e d ( S e c t i o n 7.41, b u t b o t h seem l e s s a t t r a c t i v e and are u n l i k e l y t o be s u i t a b l e t e r t i a r y c o o l a n t s .

The f l u i d i z e d

bed h e a t exchanger concept ( S e c t i o n 7 . 5 ) would r e q u i r e s u b s t a n t i a l develop-

ment b e f o r e it could be a p p l i e d t o a r e a c t o r system; f o r t h i s r e a s o n , no work on t h i s concept i s recommended a t t h i s time.

7.1

Secondary Coolant

Lithium b e r y l l i u m f l u o r i d e , Li2BeF4, appears t o be n e a r i d e a l as a secondary c o o l a n t i n t h i s c o n f i g u r a t i o n .

No o t h e r f l u i d appeared t o be a

s e r i o u s c a n d i d a t e ( S e c t i o n 4.2.1). 7.1.1

Advantages 7.1.1.1

Compatibility with f u e l salt.

The secondary c o o l a n t ,

L i BeF4, i s completely compatible w i t h f u e l s a l t under any c o n d i t i o n or 2 e v e n t , such as primary h e a t exchanger l e a k s .

No p r e c i p i t a t i o n of f i s s i l e

material can occur n o r are r e a c t i o n p r o d u c t g a s e s evolved.

Indeed, no

r e a c t i o n s occur o t h e r t h a n mixing of two m i s c i b l e l i q u i d s .

Leakage of

c o o l a n t i n t o f u e l s a l t only d i l u t e s t h e uranium and thorium, which could

be e a s i l y c o r r e c t e d by removing f u e l c a r r i e r s a l t i n t h e f u e l p r o c e s s i n g f a c i l i t y or by t h e a d d i t i o n of more f i s s i l e material.

No n u c l e a r poisons

would be added t o t h e f u e l s a l t and no p u r i f i c a t i o n or s p e c i a l p r o c e s s i n g would be r e q u i r e d .

Leakage of f u e l s a l t i n t o t h e secondary c o o l a n t c i r -

c u i t would r e q u i r e s h u t down and clean-up due t o r a d i o l o g i c a l contaminat i o n , b u t no p r e c i p i t a t i o n or o t h e r r e a c t i o n s would occur.

I t might b e

advantageous t o maintain t h e p r e s s u r e d i f f e r e n t i a l so t h a t leakage i n most

cases would be predominantly from t h e secondary c o o l a n t i n t o t h e f u e l . Then, i n t h e case of minor l e a k s it might n o t b e necessary t o s h u t down and r e p a i r immediately. 7.1.1.2

Avoidance of f u e l s a l t f r e e z i n g .

The Li2BeF4 secondary

c o o l a n t f r e e z e s congruently a t 459OC (858OF) while t h e f u e l s a l t f r e e z e s i n c o n g r u e n t l y , t h e f i r s t s o l i d s , a thorium-rich phase, appear a t t h e l i q u i d u s temperature of 502OC (935OF) while t h e f u e l s a l t i s n o t comp l e t e l y f r o z e n u n t i l t h e s o l i d u s temperature o f %445OC (Q833OF) i s reached. Thus t h e s e l e c t i o n o f Li2BeFq as t h e secondary c o o l a n t s u b s t a n t i a l l y diminishes t h e problem of f u e l f r e e z i n g ( c r i t e r i o n 3 . 3 . 2 ) s i n c e it would f r e e z e above t h e f u e l s a l t s o l i d u s temperature.

If i t were n e c e s s a r y , t h e

l i q u i d u s temperature of t h e secondary c o o l a n t could be i n c r e a s e d t o 510OC (95OOF) by using LiF-BeF2 (70-30 mole %). With t h i s composition, t h e h e a t t r a n s f e r p r o p e r t i e s of t h e secondary c o o l a n t should diminish markedly as t h e temperature f a l l s t o n e a r t h e f u e l s a l t f r e e z i n g p o i n t and t h u s f u e l

s a l t f r e e z e up could probably be f u r t h e r delayed or completely prevented. 7.1.2

Disadvantage The l i t h i u m i n t h e secondary c i r c u i t would have t o be compose of

99.99t %

L i t o prevent r u i n i n g t h e f u e l s a l t by t h e a d d i t i o n o f 6 L i i n

59 I

case c o o l a n t leaked i n t o t h e primary c i r c u i t . 7

The added c o s t s due t o t h e

L i would depend on f a c t o r s such as t h e composition and volume o f t h e

secondary c o o l a n t as w e l l as t h e p r i c e o f

7 L i compounds, b u t are l i k e l y

t o be s e v e r a l m i l l i o n d o l l a r s f o r an MSBR. 7.1.3

F u t u r e work The f u t u r e work needed t o more a c c u r a t e l y e v a l u a t e Li2BeF4, or o t h e r

LiF-BeF2 mixtures, as a secondary c o o l a n t involves (1) a b e t t e r under-

s t a n d i n g o f t h e f u e l f r e e z i n g problem, ( 2 ) accurate c o s t e s t i m a t e s f o r 7

L i compounds i n t h e q u a n t i t i e s t h a t would be needed i n an MSBR, and ( 3 )

c o n s i d e r a t i o n of design f a c t o r s t h a t would minimize t h e volume o f s a l t i n t h e secondary c i r c u i t . 7.2

Compressed Helium T e r t i a r y Coolant

O f t h e compressed gases t h a t could be considered as t e r t i a r y c o o l a n t s ,

helium a t about 700 p s i a appeared by f a r t h e most a t t r a c t i v e .

Considerable

experience has been accrued over t h e y e a r s with helium i n v a r i o u s gascooled r e a c t o r s and much development work has been done i n t h e HTGR program.

The primary advantage of helium i s i t s c h e m i c a l i n e r t n e s s .

7.2.1

Advantages 7.2.1.1

Tritium t r a p p i n g .

Generation of t r i t i u m i n r e l a t i v e l y l a r g e

amounts (approximately 2400 C i p e r day) i s unavoidable i n a m o l t e n - s a l t reactor. 6 y 7

V i r t u a l l y a11 of t h e t r i t i u m must be s e q u e s t e r e d and removed

t o p r e v e n t it from e n t e r i n g t h e steam system from which it i s r e l e a s e d t o t h e environment.

The Usie of t h e helium loop may be t h e e a s i e s t and p o s s i -

b l y t h e l e a s t expensive way of removing t r i t i u m from an MSBR p l a n t s i n c e t h e t r i t i u m can be captured i n t h e helium and removed from t h i s helium

loop.

Some p r e l i m i n a r y c a l c u l a t i o n s have been made on t h e q u a n t i t i e s o f

water and oxygen t h a t must be added t o t h e helium t o t r a p t h e t r i t i u m . Assuming t h a t no p r o t e c t i v e f i l m forms on t h e steam s i d e of t h e steam g e n e r a t o r s t o l i m i t hydrogen d i f f u s i o n from t h e steam, approximately 1 0 0 s t a n d a r d l i t e r s of oxygen p e r hour would have t o be added t o t h e helium t e r t i a r y loops t o t r a p t r i t i u m as t r i t i a t e d water and t h e e n t i r e volume of helium would have t o be processed t o remove t h e t r i t i a t e d water once every f o u r hours.

If hydrogen d i f f u s i o n from t h e steam system i s reduced

due t o formation o f an oxide f i l m , less oxygen a d d i t i o n would be r e q u i r e d b u t a l i t t l e water would need t o be added.

This s i t u a t i o n , which r e p r e -

s e n t s t h e most l i k e l y o p e r a t i n g c o n d i t i o n s , would r e s u l t i n a water conc e n t r a t i o n i n t h e helium t e r t i a r y loop of t h e o r d e r of 1 0 ppm and an oxygen c o n c e n t r a t i o n of about 0 . 1 ppm i n o r d e r t o lower t h e t r i t i u m loss t o t h e steam system and environment t o approximately 2 C i p e r day or l e s s . 7.2.1.2

Reactor c o n t r o l .

A n u c l e a r r e a c t o r has a very f a s t r e s p o n s e

t o c o n t r o l parameters because of t h e n a t u r e of t h e f i s s i o n p r o c e s s .

When

t h e r e i s a t i g h t c o u p l i n g o f t h e r e a c t o r t o t h e steam t u r b i n e l o a d , for example, by means of c i r c u l a t i n g s a l t c i r c u i t s , c a r e f u l d e s i g n i s r e q u i r e d t o p r e v e n t c o n t r o l "hunting" when sudden l o a d t r a n s i e n t s are imposed on t h e system.

C o n t r o l i s much e a s i e r on coupled systems when one of t h e sub-

systems has a much l a r g e r t i m e c o n s t a n t t h a n t h e o t h e r .

The time c o n s t a n t

f o r t h e helium loop i s very l a r g e compared t o t h a t of t h e r e a c t o r or t h e secondary c o o l a n t loop because of t h e n a t u r e of t h e h e a t t r a n s f e r i n a gas loop and t h i s eases t h e c o n t r o l problem and f a c i l i t a t e s s t a b l e operation. 7.2.1.3

C o m p a t i b i l i t y with t h e steam system.

S i n c e it i s c e r t a i n

61 I

t h a t t h e r e w i l l be l e a k s i n t h e steam g e n e r a t o r s d u r i n g t h e 30-year l i f e of an MSBR p l a n t , it i s d e s i r a b l e t h a t such l e a k s n o t r e s u l t i n harmful c o r r o s i o n of e i t h e r t h e s a l t loop or t h e steam system.

If t h e steam

g e n e r a t o r i s heated by helium, t h e r e i s no i n c o m p a t i b i l i t y problem and no c o r r o s i o n d i f f i c u l t y aggravated by a steam l e a k .

The a d d i t i o n o f steam

t o t h e moist helium wou1.d n o t r e s u l t i n s i g n i f i c a n t a t t a c k o f t h e materials of c o n s t r u c t i o n of t h a t loop and t h e t r i t i a t e d water could be removed i n

a p r o c e s s i n g s i d e stream. 7.2.1.4

Steam syst:em design.

kind o f steam system d e s , i r e d .

A helium loop can be combined w i t h any

By u s i n g a helium t e r t i a r y loop, steam con-

d i t i o n s can be chosen t o optimize t h e steam system, although it appears t h a t a s u p e r c r i t i c a l steam system i s v e r y d e s i r a b l e i f n o t necessary f o r

a salt-steam g e n e r a t o r hecause of f r e e z i n g p o i n t c o n s i d e r a t i o n s of t h e salt.

Steam system components would l i k e l y have a l o n g e r l i f e expectancy

i n t h e case o f a helium t e r t i a r y loop than with s a l t - h e a t e d steam g e n e r a t o r s s i n c e no a d d i t i o n a l corrlosive m a t e r i a l s a r e i n t r o d u c e d t o t h e steam system by a l e a k . 7.2.1.5

U t i l i z a t i o n o f e x i s t i n g s t e a m g e n e r a t o r technology.

If a

helium t e r t i a r y loop i s used i n t h e MSBR p l a n t , t h e same t y p e of steam g e n e r a t o r s could be used as have a l r e a d y been developed f o r gas-cooled reactors.

The e x a c t u n i t s used for HTGR p l a n t s may n o t be d i r e c t l y a p p l i -

c a b l e t o an MSBR p l a n t , b u t c e r t a i n l y much of t h e development work necessary f o r such a u n i t could be adapted.

The same i s t r u e f o r t h e

helium c i r c u l a t o r s and o t h e r components o f t h e helium loop.

There remains

a c o n s i d e r a b l e amount of development work r e q u i r e d t o design and prove a

steam g e n e r a t o r t o g e n e r a t e steam d i r e c t l y from molten s a l t ; f o r example,

problems r e g a r d i n g thermal stress, e s p e c i a l l y i n tube s h e e t s , w i l l r e q u i r e resolution. 7.2.1.6

S i m p l i f i e d s t a r t u p procedures.

S o l i d - f u e l e d , water-cooled

r e a c t o r s can s t a r t a t room temperature and t h e e n t i r e p l a n t can be brought t o o p e r a t i n g temperature on n u c l e a r power; however, molten-salt systems, p a r t i c u l a r l y systems with salt-steam g e n e r a t o r s , r e q u i r e more e x t e n s i v e considerations.

Because o f t h e high f r e e z i n g p o i n t s of t h e molten s a l t s ,

t h e s a l t as w e l l as t h e s a l t loops must be brought up t o temperatures of approximately 54OOC (%lOOO째F)

b e f o r e t h e loops are f i l l e d w i t h s a l t .

The

steam system must a l s o be brought up t o o p e r a t i n g temperature by an a u x i l l a r y steam source b e f o r e t h e secondary s a l t can be i n t r o d u c e d i n t o t h e steam g e n e r a t o r s .

The f o s s i l f i r e d a u x i l l a r y b o i l e r must b e of t h e o r d e r

of 1 0 % of t h e thermal r a t i n g o f t h e p l a n t , or 225 MW(t), for molten-salth e a t e d steam g e n e r a t o r designs. S t a r t u p of t h e p l a n t would be s i m p l i f i e d i f t h e r e were a helium t e r The

t i a r y loop s i n c e an i n d i r e c t h e a t i n g source would n o t be r e q u i r e d .

helium t e r t i a r y loop could be s t a r t e d up cold u s i n g a much smaller a u x i l l a r y b o i l e r t o b r i n g up t h e steam system and h e a t could be f e d back "up-

stream" i n t o t h e s a l t secondary and primary loops of t h e MSBR p l a n t .

The

b o i l e r would only be of s u f f i c i e n t s i z e t o overcome t h e h e a t l o s s e s i n t h e v a r i o u s systems and t h u s r e p r e s e n t a s i g n i f i c a n t c a p i t a l s a v i n g s .

Auxiliary

e l e c t r i c a l h e a t would s t i l l be r e q u i r e d for t h e s a l t loops b u t t h e e n t i r e s t a r t u p procedure would become much less complicated.

7.2.1.7

P l a n t a v a i l a b i l i t y and m a i n t a i n a b i l i t y .

The a d d i t i o n of a

helium t e r t i a r y loop would i n c r e a s e t h e p l a n t a v a i l a b i l i t y and s i m p l i f y maintenance, p r i m a r i l y due t o e l i m i n a t i o n of most o f t h e c o r r o s i o n problems a s s o c i a t e d w i t h o t h e r MSBR c o n f i g u r a t i o n s .

Steam leaks i n t h e steam r a i s i n g

63 I

system, a common g e n e r a t i n g p l a n t problem, would n o t l e a d t o t h e formation of c o r r o s i v e materials i:n t h e secondary loop nor can t o x i c m a t e r i a l s be i n t r o d u c e d i n t o t h e steam system t h a t would r e q u i r e e x t e n s i v e clean-up b e f o r e r e p a i r s could be s t a r t e d f o l l o w i n g s h u t down a f t e r a steam system leak. 7.2.2

Disadvantages While t h e advantages of adding a helium t e r t i a r y h e a t t r a n s f e r loop

are r e a l , t h e r e a r e a l s o some disadvantages.

These a r e mainly i n t h e a r e a

of c a p i t a l c o s t s and can be a c c u r a t e l y a s s e s s e d from an economic s t a n d p o i n t . I t i s n o t s o easy t o q u a n t i f y t h e o f f s e t t i n g economic advantages, p r i m a r i l y

i n s a f e t y and e a s e of o p e r a t i o n , which r e s u l t from t h e a d d i t i o n o f t h e helium loop. I t i s obvious t h a t a d d i t i o n a l components would be r e q u i r e d i n t h e

helium t e r t i a r y loop.

Helium d u c t i n g , helium c i r c u l a t o r s , and i n t e r m e d i a t e

h e a t exchangers would be r e q u i r e d ,

The c o o l a n t s a l t i n v e n t o r y would be

somewhat g r e a t e r and t h e steam g e n e r a t o r s would have more s u r f a c e a r e a t o t r a n s f e r h e a t from t h e helium t o steam. P r e l i m i n a r y c o n c e p t u a l d e s i g n work gave e s t i m a t e s t h a t seem t o i n d i -

cate t h e c o s t of adding a helium t e r t i a r y loop t o an MSBR i s n o t p r o h i b i tive.

One i n t e r m e d i a t e exchanger w i l l be needed t o t r a n s f e r h e a t from t h e A heat

secondary c o o l a n t s a l t t o t h e helium f o r each secondary s a l t loop. t r a n s f e r c a l c u l a t i o n showed t h a t about 1 0 6 , 8 0 0 f t

of s u r f a c e a r e a would

b e r e q u i r e d for h e a t transfer from t h e secondary s a l t t o helium.

In the

c o n c e p t u a l design3 t h e prlimary h e a t exchangers had a t o t a l s u r f a c e area of 54,252 f t

These exchangers were t o be made o f H a s t e l l o y N.

t o helium exchangers l i k e l y could be made of s t a i n l e s s s t e e l .

The s a l t

This

cheaper material would make t h e c o s t p e r s q u a r e f o o t somewhat less f o r t h e s t a i n l e s s u n i t s , b u t s i n c e f a b r i c a t i o n c o s t s are a l a r g e f r a c t i o n of t o t a l c o s t , t h e s a v i n g would be l e s s t h a n t h e r a t i o of t h e c o s t of t h e

materials.

Thus, t h i s very p r e l i m i n a r y e s t i m a t e s u g g e s t s t h a t t h e i n t e r -

mediate h e a t exchangers for use w i t h t h e helium loop would l i k e l y add about 4% t o t h e t o t a l d i r e c t c o s t of t h e MSBR. An a d d i t i o n a l item of c a p i t a l c o s t are t h e helium c i r c u l a t o r s . on HTGR c o s t f i g u r e s t o the capital cost.

Based

for t h e s e items, approximately 3% would be added

Because of t h e lower temperature of helium approxi-

mately 593OC (1100OF) i n s t e a d of 815OC (15OOOF) as i n t h e HTGR, a l a r g e r

mass flow of helium would be r e q u i r e d so t h a t s i x c i r c u l a t o r s o f HTGR s i z e would b e r e q u i r e d .

For t h e same r e a s o n (lower helium t e m p e r a t u r e ) , more steam g e n e r a t o r s would be r e q u i r e d f o r comparable power i n t h e MSBR t h a n are needed f o r t h e HTGR.33

By i n c r e a s i n g t h e steam g e n e r a t o r c o s t p r o p o r t i o n a l t o t h e i n -

c r e a s e d s u r f a c e area r e q u i r e d , t h e a d d i t i o n a l d i r e c t c a p i t a l c o s t o f t h e helium-steam g e n e r a t o r s would b e about 8%. The

L i BeF i n t h e secondary loop i s more expensive t h a n t h e NaF-NaBF4 2 4

m e l t considered i n t h e conceptual d e ~ i g n . ~ I t i s roughly e s t i m a t e d t h a t t h e Li2BeFq used would add about $13 m i l l i o n t o t h e c o s t over t h a t of fluoroborate. To summarize t h e c a p i t a l c o s t e s t i m a t e s , it appears t h a t t h e a d d i t i o n of t h e helium t e r t i a r y loop would add approximately 1 5 t o 20% t o t h e t o t a l plant capital cost.

From t h i s would be s u b t r a c t e d a c r e d i t for t h e reduced

s i z e o f t h e a u x i l i a r y steam system.

Differences i n p l a n t c o n t r o l and p l a n t

p r o t e c t i v e f a c t o r s have n o t been d e f i n e d and may add d e b i t s or c r e d i t s t o the costs,

I n a d d i t i o n t o t h e c a p i t a l c o s t items, t h e r e i s an o p e r a t i n g

c o s t p e n a l t y i n t h e power r e q u i r e d t o o p e r a t e t h e steam t u r b i n e d r i v e n helium c i r c u l a t o r s which r e q u i r e 73 megawatts; t h i s would reduce t h e overa l l thermal e f f i c i e n c y from 44.4% t o about 42.7%. 7.2.3

Future work I n t h i s preliminary e v a l u a t i o n of a l t e r n a t e c o o l a n t s , it has n o t been

p o s s i b l e t o do a thorough optimized p l a n t design study.

A more d e t a i l e d

conceptual d e s i g n s t u d y would be r e q u i r e d t o a c c u r a t e l y assess t h e o p e r a t i n g advantages and economic p e n a l t i e s a s s o c i a t e d with t h i s MSBR c o n f i g u r a t i o n .

I t is e s s e n t i a l t h a t t h i s be done, because only d o l l a r s o f f e r a d i r e c t method o f comparison of t h e s e c o o l a n t schemes. Although t r i t i u m t r a p p i n g i n moist helium appears c e r t a i n , an e x p e r i mental demonstration of t h e s e q u e s t e r i n g o f t r i t i u m i n t h e helium stream along with a p o s i t i v e method f o r i t s removal from t h e helium should be carried out.

The removal of t r i t i u m must be done a t such a r a t e as t o

l i m i t t h e d i f f u s i o n of t r i t i u m i n t o t h e steam t o a v a l u e of 2 Ci/day or The 240,000 square f e e t of steam g e n e r a t o r s u r f a c e p r e s e n t s a very

less.

l a r g e a r e a through which t r i t i u m could d i f f u s e and t h e d i f f u s i o n process w i l l compete w i t h any removal system employed t o remove t r i t i u m from t h e

helium. 7.3

Molten S a l t T e r t i a r y Coolant

I n s e l e c t i n g an MSBR c o n f i g u r a t i o n u t i l i z i n g two d i f f e r e n t molten s a l t s i n t h e secondary and t e r t i a r y l o o p s , i t i s d e s i r a b l e t o choose a

secondary c o o l a n t t h a t i s compatible with t h e f u e l s a l t and t o choose a t e r t i a r y c o o l a n t t h a t ca:n s e q u e s t e r t r i t i u m and i s chemically compatible w i t h t h e steam system.

If t h e secondary c o o l a n t i s chosen t o be

compatible with t h e f u e l s a l t and t h e t e r t i a r y c o o l a n t i s chosen t o be compatible w i t h steam, an i n c o m p a t i b i l i t y problem between t h e secondary and t e r t i a r y c o o l a n t f r e q u e n t l y e x i s t s a t t h e i n t e r m e d i a t e h e a t exchanger i n case of l e a k s .

This problem e l i m i n a t e d many c o o l a n t combinations from

f u r t h e r c o n s i d e r a t i o n and remains a major disadvantage w i t h t h e combination of two molten salts d e s c r i b e d i n t h i s s e c t i o n . The two molten s a l t c o o l a n t s s e l e c t e d are Li2BeF4 f o r t h e secondary c o o l a n t and a t e r n a r y c a r b o n a t e e u t e c t i c melt, Li2C03-Na2C03-K2C03 (42.5-30.6-26.8

mole %), for t h e t e r t i a r y c o o l a n t .

The chemistry o f car-

bonate melts has been r e c e n t l y reviewed 34-36 and s u g g e s t s s e v e r a l p o t e n t i a l advantages o v e r f l u o r i d e melts f o r a p p l i c a t i o n as a t e r t i a r y MSBR c o o l a n t . Advantages

7.3.1

7.3.1.1

Tritium trapping,

An a p p r e c i a b l e c o n c e n t r a t i o n o f hydroxide

can be d i s s o l v e d i n t h e t e r n a r y c a r b o n a t e e u t e c t i c melt, t h u s

i o n , OH-,

p r o v i d i n g a l a r g e r e s e r v o i r o f hydrogen i n t h e t e r t i a r y l o o p f o r i s o t o p i c exchange with t r i t i u m .

For example, a t a CO /H 0 p a r t i a l p r e s s u r e r a t i o 2 2

of 1 0 i n t h e vapor o v e r t h e m e l t , t h e OH- c o n c e n t r a t i o n i n t h e melt would be 2.8 x

mole f r a c t i o n or 480 ppm.

The OH- c o n c e n t r a t i o n was cal-

culated f o r the equilibrium 20H-(d)

C02(g) = C032- ( d )

H20(g)

where t h e e q u i l i b r i u m c o n s t a n t has been determined36 t o be 31.6 a t 6OOOC. Hydroxide, or t r a p p e d OT-, s i d e stream.

could be removed from t h e melt i n a p r o c e s s i n g

For example, s p a r g i n g w i t h d r y C02 a t 1 a t m p r e s s u r e would

lower t h e OT- c o n c e n t r a t i o n t o 4.8 ppm, assuming an e q u i l i b r i u m p a r t i a l

-5 p r e s s u r e of 1 0 atm of H20.

I n c o n s i d e r i n g t r i t i u m t r a p p i n g , it has been assumed t h a t t h e exchange r e a c t i o n s T2(g) t OH-(d) = TH(g)

-1-

OT-(d)

THO(g)

and T ( g ) t H20(g) = TH(g)

are r a p i d i n t h i s carbonate medium. experimentally. 7.3.1.2

This would have t o be demonstrated

Direct o x i d a t i o n of t r i t i u m t o T 0 or THO seems u n l i k e l y . 2

Compatibiliity with t h e steam system.

t h e t e r t i a r y c o o l a n t (3920C,

The f r e e z i n g p o i n t of

738OF), i s adequate f o r s u p e r c r i t i c a l steam

c y c l e s u t i l i z i n g steam i?eheat of t h e feedwater t o p r e v e n t c o o l a n t f r e e z i n g i n t h e feedwater h e a t e r s . l e a d t o i n i t i a l CO of OH-.

Leaks of steam i n t o t h e carbonate c o o l a n t would

e v o l u t i o n and t r a n s i e n t c o r r o s i v i t y due t o t h e presence

However, t h i s could probably be c o r r e c t e d by C02.

Thus steam

i n l e a k a g e could be considered as h e l p i n g t o reduce t r t i u m d i f f u s i o n t o t h e steam system by isotopic: d i l u t i o n .

Leakage of t h e carbonate t e r t i a r y

c o o l a n t i n t o t h e steam system, however, could have s e r i o u s consequences,

as d i s c u s s e d i n S e c t i o n 7.3.2.2. 7.3.2

Disadvantages

7.3.2.1

I n c o m p a t i b i l i t y of secondary and t e r t i a r y c o o l a n t s .

Leaks

i n t h e i n t e r m e d i a t e heat: exchangers w i l l l e a d t o mixing o f secondary c o o l a n t (Li2BeF,+) and tertiary c o o l a n t ( a t e r n a r y carbonate e u t e c t i c m e l t ) , r e s u l t i n g i n t h e e v o l u t i o n of gaseous C02 and p r e c i p i t a t i o n o f BeO. L i BeF

(0 t M2C03(R) = C02(g)

t BeO(c) t 2 LiF(d)

The e q u i l i b r i u m c o n s t a n t f o r t h e r e a c t i o n BeO(c) t C02(g)

( d ) t C03

(d),

2 MF(d)

e s t i m a t e d t o be about 8 x

loe3,

indicates t h a t dissolution of beryllium

oxide from t h e system a f t e r a l e a k would n o t be p r a c t i c a l s i n c e it would r e q u i r e e x c e s s i v e C 0 2 o v e r p r e s s u r e s , i n t h e o r d e r of 100 a t m (1470 p s i ) . Thus, mechanical removal of p r e c i p i t a t e d Be0 from t h e h e a t exchangers and loop could be necessary i n some s i t u a t i o n s . I n o r d e r t o p r o t e c t t h e primary h e a t exchanger from damage due t o C 0 2 p r e s s u r e and t h u s avoid t h e p o s s i b i l i t y of r e l e a s i n g r a d i o a c t i v i t y ,

t h e secondary c o o l a n t c i r c u i t and probably a l s o t h e t e r t i a r y c i r c u i t would have t o i n c l u d e some p r e s s u r e r e l i e f mechanism t o r e l i e v e t h e C02 evolved on leakage.

Leakage of secondary c o o l a n t i n t o t h e t e r t i a r y loop

would a l s o l e a d t o i n c r e a s e d c o r r o s i o n i n t h e t e r t i a r y loop.

The problems

a s s o c i a t e d w i t h coping w i t h t h e i n c o m p a t i b i l i t y of t h e s e t w o c o o l a n t s r e p r e s e n t s i g n i f i c a n t design c o n s i d e r a t i o n ; however, t h e y a r e , i n p r i n c i p l e , s i m p l e r t h a n t h o s e a s s o c i a t e d w i t h primary h e a t exchanger problems because no r a d i o a c t i v i t y i s involved. 7.3.2.2

Materials of c o n s t r u c t i o n .

I t would b e n e c e s s a r y t o develop

a material o f c o n s t r u c t i o n for t h e t e r t i a r y loop.

It is possible t h a t

a l l o y s high i n n i c k e l and chromium, such as Incoloy 800, could be u t i l i z e d . S t a i n l e s s s t e e l has been shown

t o resist c o r r o s i o n a t 6 O O O C (1112OF)

through t h e mechanism of p a s s i v a t i o n w i t h chromium oxide.

High chromium-

h i g h n i c k e l a l l o y s may a l s o be p o t e n t i a l c a n d i d a t e s f o r development.

A s e r i o u s problem may a l s o e x i s t i n t h e choice of m a t e r i a l s f o r t h e steam

system s i n c e leakage of carbonate i n t o t h e steam system could l e a d t o hydroxide s t r e s s - c o r r o s i o n c r a c k i n g o f many c o n v e n t i o n a l steam system alloys. 7.3.2.3

Vapor p r e s s u r e .

The vapor p r e s s u r e above t h e e u t e c t i c due

t o decomposition of

2C03

( d ) t o C02(g) i s q u i t e low and i n c r e a s e s o n l y

s l i g h t l y w i t h added hydroxide.

A t an OH- c o n c e n t r a t i o n o f 480 ppm, which

would be t y p i c a l f o r systems c o n t a i n i n g OH- f o r i s o t o p i c exchange with t r i t i u m , t h e CO

p a r t i a l p r e s s u r e would be about 0.4 mm Hg (0.008 p s i )

assuming t h a t t h e o n l y source of water i n t h e vapor phase i s due t o 2-

20H-

H20 t 0

7.3.3

Future work

No f u t u r e work seems warranted a t t h i s t i m e t o f u r t h e r e v a l u a t e t h e t e r n a r y carbonate melt ,as a t e r t i a r y c o o l a n t .

The most s e r i o u s n e g a t i v e

f a c t o r s a r e t h e chemical i n c o m p a t i b i l i t y between t h e secondary and t e r t i a r y c o o l a n t s and t h e :?resent l a c k o f a m a t e r i a l capable of w i t h s t a n d i n g t h e c o r r o s i v e carbonate m e l t f o r 30 y e a r s .

Also, p h y s i c a l p r o p e r t i e s o f

t h i s m e l t are n o t w e l l known, p a r t i c u l a r l y h e a t t r a n s p o r t and t r a n s f e r p r o p e r t i e s such as thermal c o n d u c t i v i t y , h e a t c a p a c i t y and v i s c o s i t y .

i s p o s s i b l e t h a t t h e t e r n a r y carbonate e u t e c t i c could be u t i l i z e d as an

MSBR c o o l a n t , b u t c o n s i d e r a b l e work would be r e q u i r e d and it now appears l e s s a t t r a c t i v e than o t h e r p o t e n t i a l coolants. 7.4

Liquid Metal T e r t i a r y Coolants

The use of l i q u i d m e t a l s as a h e a t t r a n s p o r t medium o f f e r s e v e r a l s i g n i f i c a n t advantages when compared t o molten s a l t s or gases.

These

i n c l u d e e x c e l l e n t h e a t t r a n s f e r p r o p e r t i e s which p e r m i t t h e design of

small, very e f f i c i e n t h e a t exchangers, low v i s c o s i t y and, i n t h e c a s e of sodium, a low f r e e z i n g p o i n t which s i m p l i f i e s o p e r a t i o n and s t a r t - u p . A s d i s c u s s e d i n S e c t i o n 4 , no l i q u i d metal appears a p p l i c a b l e as a sec-

ondary c o o l a n t for MSBRa due t o chemical r e a c t i v i t y with t h e f u e l i n t h e case of l e a k s i n t h e pri.mary h e a t exchanger.

Advantage could be t a k e n

70 o f t h e s e d e s i r a b l e p r o p e r t i e s if l i q u i d metals were u t i l i z e d as a t e r t i a r y c o o l a n t , however t h e r e are a l s o s e r i o u s d i s a d v a n t a g e s .

These

i n c l u d e chemical r e a c t i v i t y w i t h t h e steam or w i t h t h e secondary c o o l a n t i n t h e e v e n t of l e a k s i n steam g e n e r a t o r s or t h e i n t e r m e d i a t e h e a t exchanger, and l a c k of a s o l u t i o n t o t h e t r i t i u m management problem.

Sodium

was e v a l u a t e d s i n c e c o n s i d e r a b l e experience i s being accumulated i n t h e LMFBR program w i t h sodium as a h e a t t r a n s p o r t medium.

Considerable e f f o r t

i s c u r r e n t l y involved i n t h e d e s i g n o f steam g e n e r a t o r s .

Two t y p e s of

s o l u t i o n s t o t h e chemical i n c o m p a t i b i l i t y problem are b e i n g considered. The first i n v o l v e s t h e d e s i g n of c a r e f u l l y welded "leak free" h e a t exchangers coupled w i t h t h e c a p a b i l i t y f o r r a p i d d e t e c t i o n of l e a k s and s h u t down followed by e a s y r e p a i r .

Such steam g e n e r a t o r s s h o u l d be r e a d i l y

a p p l i c a b l e t o t h i s MSBR c o n f i g u r a t i o n .

The second approach i n v o l v e s t h e

d e s i g n of a more complex doubled-wall steam g e n e r a t o r w i t h i n e r t g a s purging of a n n u l a r channels t o diminish t h e p o s s i b i l i t y of sodium-water i n t e r m i x i n g i n t h e e v e n t of a l e a k . 7.4.1

Advantages

7.4.1.1

Steam g e n e r a t o r s .

The use o f sodium as t h e t e r t i a r y c o o l a n t

might allow t h e a d o p t i o n of LMFBR-type steam g e n e r a t o r s .

Due t o t h e

f a v o r a b l e h e a t t r a n s f e r p r o p e r t i e s of l i q u i d metals t h e s e steam g e n e r a t o r s would be c o n s i d e r a b l y smaller t h a n t h o s e r e q u i r e d for steam g e n e r a t i o n from molten s a l t s or g a s e s . 7.4.1.2

Smaller i n t e r m e d i a t e h e a t exchangers.

Due t o t h e f a v o r a b l e

h e a t t r a n s f e r p r o p e r t i e s o f sodium, t h e i n t e r m e d i a t e h e a t exchanger could be of s i g n i f i c a n t l y s m a l l e r s i z e , and t h u s lower c o s t , t h a n i n t h e c a s e s with e i t h e r gaseous or m o l t e n - s a l t t e r t i a r y c o o l a n t concepts.

7.4.1.3

O p e r a t i o n a l advantages.

The low m e l t i n g p o i n t and t h e com-

p a c t d e s i g n of t h e steam g e n e r a t o r s allow c o n s i d e r a b l e freedom for o p t i mization o f steam c o n d i t i o n s and f l e x i b i l i t y i n o p e r a t i n g c h a r a c t e r i s t i c s . 7.4.2

Disadvantages

7.4.2.1

I n c o m p a t i b i l i t y of sodium and water.

Sodium and water r e a c t

r a p i d l y w i t h t h e evo1ut:ion of h e a t and p o t e n t i a l l y combustible hydrogen gas.

I n c o n s i d e r i n g sodium-steam g e n e r a t o r s f o r t h e MSBR, i t i s assumed

t h a t adequate safeguards t o prevent s i g n i f i c a n t chemical e v e n t s w i l l have been developed by t h e LIMFBR program.

These could t a k e t h e form of l e a k -

f r e e steam g e n e r a t o r s or more complex double-walled steam g e n e r a t o r s with p r o v i s i o n f o r a n n u l a r gas flow.

Such chemical r e a c t i o n s r e p r e s e n t an

a r e a of concern which i s a b s e n t i n a l l t h e o t h e r MSBR c o n f i g u r a t i o n s cons i d e r e d i n t h e r e p o r t and a r e c l e a r l y a n e g a t i v e f a c t o r i n c o n s i d e r i n g t h e use of sodium. 7.4.2.2

I n c o m p a t i b i l i t y of sodium and Li213e&.

I n t h e event of

leakage i n t h e i n t e r m e d i a t e h e a t exchanger sodium w i l l react r a p i d l y with t h e Li2BeF

t o form i n s o l u b l e 5e metal and NaF.

Clean up and r e p a i r

of t h e secondary c o o l a n t jystem w i l l probably prove expensive i n any c a s e and, i f t h e B e metal a l l o y s s i g n i f i c a n t l y with t h e s t r u c t u r a l metal of t h e secondary c o o l a n t loop, replacement of t h a t loop might b e r e q u i r e d . Since t h e secondary c o o l a n t loop i n c l u d e s t h e primary h e a t exchanger w i t h t h e a s s o c i a t e d r a d i o a c t i v i t y , r e p a i r or replacement could b e very d i f f i c u l t and expensive. 7.4.2.3

No s o l u t i o n t o t h e t r i t i u m management problem.

The use of

sodium as a t e r t i a r y c o o l a n t may n o t o f f e r a u s e f u l s o l u t i o n t o t h e tritium problem i n t h e MSBR.

I n t h e LMFBR, which has a t r i t i u m problem

72 50- t o 100-fold less than t h e MSBR, it i s planned t o remove t r i t i u m by cold-trapping as NaT.

However, most metal t r i t i d e s a r e r e l a t i v e l y un-

s t a b l e a t e l e v a t e d temperatures and t h u s may n o t a f f o r d a means o f seq u e s t e r i n g a s i g n i f i c a n t amount o f t r i t i u m for t h e MSBR. 7.4.2.4

Thermal shock.

The e x c e l l e n t h e a t t r a n s p o r t p r o p e r t i e s of

l i q u i d metals i n t r o d u c e a concomitant problem: exchange s u r f a c e s .

thermal shock t o h e a t

This complicates t h e d e s i g n , l i m i t s t h e o p e r a t i n g

c h a r a c t e r i s t i c s and makes off-design t r a n s i e n t e v e n t s a more s e r i o u s problem t h a n with molten s a l t or gas t e r t i a r y c o o l a n t s . 7.4.2.5

I n c o m p a t i b i l i t y w i t h t h e c e l l atmosphere.

Molten sodium i f

r e l e a s e d from a l e a k i n t h e t e r t i a r y system i n t o t h e containment c e l l , would react with t h e oxygen and moisture i n t h e c e l l atmosphere.

avoid t h i s , t h e t e r t i a r y loop c e l l atmosphere would have t o be an i n e r t A l t e r n a t e l y , a n c i l l a r y systems could be r e q u i r e d t o p r e v e n t or

gas.

c o n t a i n f i r e s due t o sodium leakage a t any p o i n t i n t h e t e r t i a r y loop,

i t s d r a i n t a n k s and o t h e r a u x i l i a r y equipment.

Again, equipment developed

f o r t h e LMFBR may be a d a p t a b l e t o t h e MSBR. 7.4.2.6

Sodium containment.

A t r a n s i t i o n from n i c k e l - b a s e a l l o y s

f o r containment o f t h e L i BeF4 secondary c o o l a n t t o iron-base a l l o y s for 2 containment o f t h e sodium t e r t i a r y c o o l a n t i s e s s e n t i a l a t t h e sodium-

salt interface.

This i s n o t a major disadvantage and it might a l s o be

d e s i r a b l e t o make such a t r a n s i t i o n f o r economic r e a s o n s .

7.4.3

Future work

The disadvantages a s s o c i a t e d w i t h use of sodium i n an MSBR i n t h e manner d i s c u s s e d appear s u f f i c i e n t t o discourage a d d i t i o n a l c o n s i d e r a t i o n . I f t h e LMFBR p r o j e c t f i n d s a t t r a c t i v e s o l u t i o n s t o some o f t h e s e problems

73 I

and i f a l t e r n a t e MSBR concepts appear l e s s a t t r a c t i v e i n t h e f u t u r e , a d d i t i o n a l c o n s i d e r a t i o n could be given t o sodium as a t e r t i a r y c o o l a n t . 7.5

Fluidized-Bed T e r t i a r y System

One p o s s i b l e v a r i a t i o n of t h e helium t e r t i a r y loop might make use of f l u i d i z e d - b e d h e a t exchangers i n t h e s t e a m - r a i s i n g system.

Since t h e

f l u i d i z e d beds would be i s o l a t e d from t h e f u e l s a l t by t h e L i 2 B e F

secondary c o o l a n t s a l t , d e s i g n l i m i t a t i o n s and materials c o m p a t i b i l i t y requirements would be s u b s t a n t i a l l y reduced from t h o s e d i s c u s s e d e a r l i e r (Sec. 4.1.4)

f o r a s i n g l e - c o o l a n t f l u i d i z e d - b e d system,

In t h i s appli-

c a t i o n s i n g l e - v e s s e l f l u i d i z e d beds could be c o n s i d e r e d , w i t h t h e secondary-coolant t u b e bu:ndles and steam t u b e s i n t h e same v e s s e l ,

Poten-

t i a l l y a d v e r s e n u c l e a r e f f e c t s a s s o c i a t e d with i n t e r m i x i n g of t h e f l u i d i z e d bed m a t e r i a l s and f u e l s a l t would r e q u i r e e s s e n t i a l l y no consideration.

The absence of .large q u a n t i t i e s of l o n g - l i v e d r a d i o a c t i v e materials

( e x c e p t p o s s i b l y t r i t i u m ) from t h e secondary c o o l a n t would permit t h e u s e of more c o n v e n t i o n a l maintenance methods f o r t h e f l u i d i z e d beds. However, t e r t i a r y f l u i d i z e d beds would r e t a i n some of t h e p o t e n t i a l disadvantages o f s i n g l e - c o o l a n t f l u i d i z e d beds.

T r i t i u m t r a p p i n g and

removal probably would have t o be accomplished i n t h e f l u i d i z e d - b e d l o o p s ,

so a p p r o p r i a t e c o n s i d e ~ a t i o nof t r i t i u m contamination would be r e q u i r e d . I n a d d i t i o n , any a d v e r s e e f f e c t s on bed performance due t o s a l t l e a k s would have t o be shown t o be managable. The use of f l u i d i z e d beds would g r e a t l y reduce t h e o p e r a t i n g economic p e n a l t y of a gaseous t h i r d - l o o p c o o l a n t a s s o c i a t e d w i t h c o o l a n t pumping power, b u t i t i s n o t c:Lear t h a t major c a p i t a l equipment s a v i n g s would be realized.

While f l u i d ? - z e d beds have c e r t a i n a t t r a c t i v e f e a t u r e s , an

e x t e n s i v e development program would b e r e q u i r e d t o b r i n g them t o t h e p o i n t r e q u i r e d f o r u s e i n f i s s i o n r e a c t o r systems.

P o s s i b l e f u t u r e de-

velopments of f l u i d i z e d beds should be considered f o r a p p l i c a t i o n i n MSBRs b u t no work appears t o be warranted a t t h i s time. 8.

ACKNOWLEDGEMENTS

The committee wishes t o acknowledge c o n t r i b u t i o n s by W. K. Furlong and L. Maya t o many of t h e c a l c u l a t i o n s and L. M. F e r r i s for e d i t o r i a l assistance. appreciated.

Also, t h e s e c r e t a r i a l a s s i s t a n c e of Nancy Pope was g r e a t l y

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