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THE EQUILIBRIUM OF DILUTE UF3 SOLUTIONS CONTAINED IN GRAPHITE L. M.

Toth

1. 0. Gilpatrick

i


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


ORNL-TM- 4056 C o n t r a c t No. W-7405-eng-26

REACTOR CHEMISTRY DIVISION THE EQUILIBRIUM OF DILUTE UF3 SOLUTIONS CONTAINED IN GRAPHITE L. M. Toth and L. 0. G i l p a t r i c k

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

DECEMBER 1972

OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37830 o p e r a t e d by

UNION CARBIDE CORPORATION f o r the U. S. ATOMIC ENERGY COMMISSION


1 I

THE EQUILIBRIUM OF DILUTE UF3 SOLUTIONS CONTAINED IN GRAPHITE L. M. Toth and L. 0. Gilpatrick ABSTRACT The equilibrium of dilute UF3-UF4 molten fluoride solutions in contact with graphite and UC2:

has been studied as a function of temperature (37O-70O0C), melt composition and atmospheric contamination. Equilibrium quotients, Q = (UF3)4/ (UF4)3 for the reaction were determined by measuring the UF3 and UF4 concentrations spectrophotometrically. The solvents used were primarily LiF-BeF2 mixtures. Results from this solvent system were related to the reactor solvents LiF-BeF~-ZrFq(65.6-29.4-5 mole X ) and LiF-BeF2-ThF4(72-16-12 mole X ) . It has been found that the equilibrium quotient is very sensitive to both temperature and solvent changes increasing as either the temperature increases or the alkali-metal fluoride content of the solvent decreases.

INTRODUCTION The relative stability of dilute UF3-UF4 molten fluoride solutions contained in graphite is of practical importance to Molten Salt Breeder Reactors, MSBR, in which these solutions are used as nuclear fuels.

Be-

cause the reactors contain a large amount of graphite in the core serving as a neutron moderator, reaction of UF3 with graphite: 4UF3 (d) to form UF

+

2C 2 3UF4(d)

+

*

UC2

(1-1)

1 4 and uranium dicarbide has long been recognized as a major

factor limiting the amount of UF3 which can be maintained in solution. Although typical fuel mixtures consist essentially of 1 mole % U 235

F4

or less in a solution of LiF and BeF

2'

the ease of UF4 reduction to UF

the chromium in the metal containment vessel**

* The subscript "d" indicates that the component is in solution. **Hastelloy N, a nickel-based alloy containing Cry Fe and Mo has been used2 to fabricate the metal containment vessel f o r the Molten Salt Reactor Experiment, XSRE.

3 by


2

n e c e s s i t a t e s t h e c o n s i d e r a t i o n of UF3 c h e m i s t r y as w e l l .

The e f f e c t of t h e

c o r r o s i o n . r e a c t i o n of Eq. 1-2 i s t o l e a c h chromium from t h e s t r u c t u r a l

metal and c a u s e i t t o a p p e a r i n s o l u t i o n as CrF2. I n o r d e r t o minimize t h e c o r r o s i o n , t h e e q u i l i b r i u m of Eq. 1-2 i s s h i f t e d t o t h e l e f t by r e d u c i n g a small p e r c e n t a g e ( a p p r o x i m a t e l y 1%) of 3 t h e UF4 t o UF3 t h r o u g h t h e a d d i t i o n of b e r y l l i u m metal: 2UF4(d)

+ Beo 2

2UF 3 ( d ) iBeF2

(1-3)

Although a s m a l l amount of UF3 i s b e n e f i c i a l i n r e v e r s i n g t h e c o r r o s i o n mechanism, i t p r o d u c e s c o m p l i c a t i o n s due t o p o s s i b l e r e a c t i o n v i a Eq. 1-1 and t h e r e s u l t i n g u n d e s i r a b l e f o r m a t i o n of a n i n s o l u b l e uranium c a r b i d e . R e f e r e n c e t o "UF3 s t a b i l i t y " i n t h i s p a p e r w i l l t h e r e f o r e mean s p e c i f i c a l l y t h e e q u i l i b r i u m c o n c e n t r a t i o n of UF3 r e l a t i v e t o UF4 as d e t e r m i n e d by Eq.

1-1. T h i s e q u i l i b r i u m h a s n e v e r b e e n e x p e r i m e n t a l l y measured d e s p i t e t h e f a c t t h a t i t i s t h e major f a c t o r i n d e t e r m i n i n g UF3 s t a b i l i t y f o r m o l t e n

s a l t r e a c t o r systems.

Although t h e y used i n d i r e c t means, Long and

B l a n k e n s h i p 4 are t h e o n l y i n v e s t i g a t o r s who h a v e a t t e m p t e d t o measure t h e equilibrium.

S i n c e t h e i r work i s t h e b a s i s on which a l l p r e v i o u s estimates

of UF3 s t a b i l i t y h a v e been made, i t w i l l b e reviewed i n d e t a i l , w i t h t h e

e q u i l i b r i u m e x p r e s s i o n s i n f r a c t i o n a l c o e f f i c i e n t s as used by t h e a u t h o r s . They s t u d i e d t h e r e d u c t i o n of UF4 ( b o t h p u r e s o l i d p h a s e UF4 and i n m o l t e n f l u o r i d e s o l u t i o n ) w i t h hydrogen:

1

-E

2 2

+

UF

+ 4

+

UF3

+ HF

(1-4)

and d e t e r m i n e d t h e e q u i l i b r i u m q u o t i e n t s f o r t h e above r e d u c t i o n :

L

by measuring HF and H2 r a t i o s e v o l v e d from a r e a c t i o n v e s s e l c o n t a i n i n g UF4 and UF3.

From t h e s o l i d - p h a s e UF4 r e d u c t i o n t h e y o b t a i n e d e q u i l i b r i u m

c o n s t a n t s , KR, f o r t h e r e d u c t i o n . quotient, Q

R

, for

These, combined w i t h t h e e q u i l i b r i u m

t h e d i l u t e s o l u t i o n s and t h e a c t i v i t y c o e f f i c i e n t f o r

UF3, yw3, o b t a i n e d from s o l u b i l i t y d a t a , e n a b l e d them t o c a l c u l a t e t h e


3 I

a c t i v i t y c o e f f i c i e n t f o r UFq,

YUF4'

i n t h e molten f l u o r i d e s o l u t i o n .

By

combining t h e f r e e e n e r g y e x p r e s s i o n f o r Eq. 1.-4 w i t h one f o r t h e decompos i t i o n of UF3 i n t o UF4 and Uo: UF

3(d)

+ 3 zuF4(d)

+

1

t h e y o b t a i n e d a n e x p r e s s i o n f o r t h e e q u i l i b r i u m q u o t i e n t , QD, of Eq. 1-6, i n terms of t h e e q u i l i b r i u m q u o t i e n t f o r Eq. 1-4, e f f i c i e n t s of UF4 and HF. ( c . f .

QR,

and t h e a c t i v i t y co-

p 18, Ref. 4, p a r t 11).

Using f r e e e n e r -

g i e s of f o r m a t i o n f o r UC2 and UC from Rand and K u b a c h e ~ s k i ,which ~ were a c c e p t a b l e a t t h e t i m e , t h e y e s t i m a t e d uranium. a c t i v i t i e s i n t h e c a r b i d e s and concluded t h a t s o l u t i o n s i n which up t o 60% of t h e i n i t i a l 1 mole % of

UF4 i s c o n v e r t e d t o UF3 are e x p e c t e d t o b e s t a b l e i n t h e p r e s e n c e of graphite.

I n a d d i t i o n t h e y concluded t h a t t e m p e r a t u r e and s o l v e n t changes

s h o u l d have l i t t l e e f f e c t on t h e e q u i l i b r i u m mechanism of Eq. 1-1 s i n c e t h e y found no s i g n i f i c a n t e f f e c t from them on t h e H2 r e d u c t i o n mechanism of Eq.

1-4. I n view of t h e s i g n i f i c a n c e o f Eq. 1-1 t o Molten S a l t R e a c t o r Technol-

ogy, a c l o s e r e x a m i n a t i o n of i t i s c l e a r l y w a r r a n t e d .

The development of

s p e c t r o p h o t o m e t r i c t e c h n i q u e s f o r t h e s t u d y of m o l t e n f l u o r i d e s and t h e r e a l i z a t i o n of s o l v e n t e f f e c t s on m o l t e n f l u o r i d e c h e m i s t r y , have g i v e n 6 impetus t o t h e s t u d y . We have a l r e a d y i d e n t i f i e d UC2, as t h e s t a b l e uran-

i u m c a r b i d e p h a s e i n e q u i l i b r i u m w i t h UF3-UF4 s o l u t i o n s i n g r a p h i t e .

The

o b j e c t of t h i s r e p o r t i s t o d e s c r i b e t h e e f f e c t s of t e m p e r a t u r e , s o l v e n t , and a t m o s p h e r i c c o n t a m i n a t i o n on t h e e q u i l i b r i u m .

Both t h e f o r w a r d and t h e

back r e a c t i o n of Eq. 1-1 i n t h e r e f e r e n c e s o l v e n t system LiF-BeF2 have been followed.

The d a t a i n t h e r e f e r e n c e s o l v e n t s y s t e m have been r e l a t e d t o

p r a c t i c a l r e a c t o r s o l v e n t s such a s t h e Molten S a l t R e a c t o r Experiment, XSRE, s o l v e n t , LiF-BeF2-ZrF4

(65.6-29.4-5

mole %) and t h e proposed Molten S a l t

B r e e d e r R e a c t o r , MSBR, s o l v e n t , LiF-BeFz-ThFq

(72-16-12

mole % ) . Our f i n d -

i n g s are compared w i t h e a r l i e r o b s e r v a t i o n s which have n o t been reviewed before.


4 EXPERIMENTAL Equilibrium q u o t i e n t s f o r t h e back-reaction*

(Eq. 3-2) were d e t e r m i n e d

by measuring UF3 and UF4 c o n c e n t r a t i o n s s p e c t r o p h o t o m e t r i c a l l y w i t h a Cary Model 14-H r e c o r d i n g s p e c t r o m e t e r .

The sample s y s t e m c o n s i s t e d of a c o n t r o l -

l e d t e m p e r a t u r e , i n e r t atmosphere f u r n a c e shown i n F i g . 2-1 which h e l d a diamond-windowed g r a p h i t e s p e c t r o p h o t o m e t r i c c e l l .

Molten f l u o r i d e s a l t

s o l u t i o n s and r e a g e n t uranium c a r b i d e s were c o n t a i n e d i n t h i s c e l l .

Ab-

s o r p t i o n s p e c t r a of t h e m o l t e n s a l t s o l u t i o n were measured a g a i n s t a n a i r and UF were d e t e r m i n e d by s u b t r a c t i n g 3 4 i n d e p e n d e n t l y determined s o l v e n t b l a n k s p e c t r a u s i n g s t a n d a r d d i g i t a l com-

reference.

N e t s p e c t r a due t o UF

S p e c t r a were measured i n t h e n e a r i n f r a - r e d and v i s i b l e

puter techniques.

r e g i o n s from 4000 t o 33000 c m - l .

The a b s o r p t i o n s p e c t r a of UF3 and UF4

s e r v e d as t h e p r i m a r y means o f m o n i t o r i n g t h e s e components i n s o l u t i o n a s a f u n c t i o n of t e m p e r a t u r e , t i m e , and s o l v e n t c o m p o s i t i o n . Materials

-

Molten s a l t s o l v e n t c o m p o s i t i o n s were p r e p a r e d by mixing

c a l c u l a t e d amounts of t h e p u r e component f l u o r i d e salts.

Optical quality

c r y s t a l f r a g m e n t s from t h e Harshaw Chemical Co. w a s the s o u r c e of LiF. 8 B e r y l l i u m f l u o r i d e was p r e p a r e d by vacuum d i s t i l l a t i o n from a l a r g e s p e c i a l p u r c h a s e s u p p l i e d by t h e Brush B e r y l l i u m Co.

The water-clear,

g l a s s - l i k e p r o d u c t c o n t a i n e d no s p e c t r o g r a p h i c a l l y d e t e c t a b l e c a t i o n imp u r i t i e s , b u t w a s e x c e e d i n g l y h y d r o s c o p i c and had t o b e s t o r e d under v e r y anhydrous c o n d i t i o n s .

Uranium t e t r a f l u o r i d e w a s t a k e n from a l a b o r a t o r y

p u r i f i e d s p e c t r o s c o p i c s t a n d a r d which c o n t a i n e d less t h a n 10 ppm of t o t a l cation impurities.

Thorium t e t r a f l u o r i d e w a s p a r t of a s p e c i a l p u r c h a s e

from t h e N a t i o n a l Lead Co. which c o n t a i n e d no g r e a t e r t h a n 100 ppm i n any cation impurities. Storms and coworkers of t h e Las Alamos S c i e n t i f i c L a b o r a t o r y s u p p l i e d e a c h of t h e uranium c a r b i d e s used i n t h i s s t u d y and s u p p l i e d t h e f o l l o w i n g analysis : Uranium d i c a r b i d e

-

UC2

**

w t % C = 8.83 o r 75.74 mole % C 02 = 200 ppm

*See R e s u l t s and D i s c u s s i o n S e c t i o n f o r a n e x p l a n a t i o n of why t h e backr e a c t i o n w a s measured. **Although uranium d i c a r b i d e i s a s u b s t o i c h i o m e t r i c compound ,I3 i t w i l l b e i d e n t i f i e d as UC2 i n t h i s p a p e r .


5

ORNL-DWG 7Z-tO653

High Temperature Furnace System f o r A b s o r p t i o n S p e c t r a of 2-1 Molten F l u o r i d e s .


6

c r y s t a l l a t t i c e by X-ray

-

Uranium s e s q u i c a r b i d e

'2'3

-..-

a = 3.5251 + 0.00051 c = 5.9962 T - 0.00081

w t % C = 6.99 o r 59.83 mole % C 02 = 50 ppm

c r y s t a l l a t t i c e by X-ray

a = 8.0889 + 0.00091

These h i g h p u r i t y c a r b i d e s were r e c e i v e d as l u s t e r o u s gray-black g r a n u l e s which ranged i n s i z e from 1 / 2 t o 1 mm3.

They were s h i p p e d s e a l e d i n g l a s s

ampules and s t o r e d i n a helium f i l l e d d r y box.

Exposure t o even t h e d r y

box c o n d i t i o n s was k e p t t o t h e a b s o l u t e minimum needed f o r weighings and

c e l l loadings. Procedure

-

Even though t h e r e a g e n t s a l t s were q u i t e f r e e of c a t i o n

i m p u r i t i e s , t h e y were n o t f r e e of o x i d e s and H20 t o t h e d e g r e e needed.

All

c o m p o s i t i o n s were t h e r e f o r e t r e a t e d w h i l e m o l t e n a t 600째C f o r o x i d e removal by s p a r g i n g f o r s e v e r a l h o u r s w i t h r e a g e n t HF g a s o r HF-H2 g a s m i x t u r e s . 9 R e s i d u a l H F w a s t h e n s t r i p p e d f r o m t h e m e l t w i t h H e p r i o r t o cooling. Clean p o r t i o n s of t h e r e c o v e r e d s a l t ''button" were t h e n c r u s h e d and used t o c h a r g e t h e s p e c t r o p h o t o m e t e r c e l l , by weighing o u t t h e f l u o r i d e s i n a h e l ium drybox which was m a i n t a i n e d a t a water vapor c o n t e n t < 0 . 1 ppm and a t an 02 c o n t e n t < 2 ppm.

Between 0 . 5 and 0.6 gm of s a l t s o l v e n t made a con-

v e n i e n t c e l l l o a d i n g t o which w a s added from 5 t o as much as 100 mg of the uranium c a r b i d e under s t u d y .

Poco AXF-5Q1 g r a d e g r a p h i t e "

spectrophoto-

m e t r i c c e l l s were used which were p u r i f i e d a f t e r f a b r i c a t i o n by h e a t i n g i n a n H2 g a s stream t o 1000째C and t h e n f l u s h e d f r e e of H2 w i t h H e .

Subsequent

d r y box h a n d l i n g and l o a d i n g t e c h n i q u e s have been d i s c u s s e d earlier.''

A

"dash p o t " s t i r r e r made from platinum-10% rhodium (see F i g . 2-1) w a s used t o h a s t e n t h e a t t a i n m e n t of e q u i l i b r i u m which i s o t h e r w i s e dependent l a r g e l y on d i f f u s i o n .

I t proved t o b e a g r e a t a i d i n s h o r t e n i n g t h e time needed

t o reach equilibrium.

We observed a small b u t temporary l o s s of t r a n s m i s -

s i o n d i r e c t l y a f t e r s t i r r i n g i n some c a s e s which w a s e q u a l t o 0.15 absorba n c e u n i t s a t 4000 cm- 1

.

W e have assumed t h i s r e c o v e r a b l e l o s s t o b e

caused by t h e temporary s u s p e n s i o n of f i n e p a r t i c l e s which l a t e r s e t t l e . Whole g r a i n s of t h e c a r b i d e were used a f t e r e a r l y a t t e m p t s t o increase t h e s u r f a c e area by c r u s h i n g caused t h e c a r b i d e t o c o l l e c t a t t h e window and i n t e r f e r e w i t h t h e o p t i c a l t r a n s p a r e n c y of t h e c e l l . s o l i d c a r b i d e phase w a s always m a i n t a i n e d i n t h e c e l l .

A l a r g e excess of t h e On some o c c a s i o n s ,

-


7

t h e e x p e r i m e n t a l sequence w a s i n t e r r u p t e d and a d d i t i o n a l uranium c a r b i d e

w a s added t o d e m o n s t r a t e t h a t a n e x c e s s was i n d e e d p r e s e n t .

No change was

o b s e r v e d i n t h e c o n c e n t r a t i o n s of UF3 o r UF4 :in t h e homogeneous s o l u t i o n s as a r e s u l t of t h e s e a d d i t i o n s . S p e c t r a l Measurements

-

Molar c o n c e n t r a t i o n s of d i s s o l v e d UF3 and UF4

were d e t e r m i n e d s i m u l t a n e o u s l y i n s o l u t i o n a t a series of t e m p e r a t u r e s above t h e m e l t i n g p o i n t by measuring o p t i c a l d e n s i t i e s a t 9174 and 11360 cm-1. These wave numbers r e p r e s e n t t h e maximum a b s o r b a n c e v a l u e s f o r d i s s o l v e d UF4 and UF3 r e s p e c t i v e l y i n t h e n e a r i n f r a - r e d r e g i o n as shown i n F i g s . 2-2 and 2-3.

The s t r o n g UF3 a b s o r p t i o n i n t h e v i s i b l e r e g i o n from 16000 t o

33000 cm-I w a s i n g e n e r a l t o o i n t e n s e t o b e u s e f u l s i n c e t h e s o l u t i o n s s t u d i e d had i n i t i a l UF4 m o l a r i t i e s i n t h e r a n g e of 0.04 t o 0.10.

Figures

2-2 and 2-3 show t h a t f o r s p e c t r a of p u r e UF4 and UF3 t h e r e i s a c o n t r i b u t i o n from e a c h a t t h e most s e n s i t i v e a b s o r b a n c e r e g i o n of t h e o t h e r member. S t a t e d d i f f e r e n t l y , t h e a b s o r b a n c e a t 9174 c m - l p r i m a r i l y due t o UF4, b u t n o t e n t i r e l y s o .

i n a mixed s o l u t i o n i s

T h i s c o n d i t i o n i s s o l v e d unique-

l y f o r t h e c o n t r i b u r i o n from e a c h s p e c i e s by t h e s o l u t i o n of a set of simult a n e o u s l i n e a r e q u a t i o n s e q u a l t o number t o t h e number of components i n t h e s y s t e m which c o n t r i b u t e t o t h e n e t s p e c t r a , i n our case 2. Using Beer's l a w - l o g l / I o = Av = where

(E v )T

(M)TL

(2-1)

I = measured o p t i c a l i n t e n s i t y of t h e s a m p l e

Io = measured o p t i c a l i n t e n s i t y of t h e r e f e r e n c e s o l v e n t A

=

t o t a l absorbance a t a given frequency,

( E ~ = ) molar ~ absorption coefficient a t

OOT R

v.

v and t e m p e r a t u r e T

= m o l a r i t y of component i n s o l u t i o n

= c e l l p a t h l e n g t h = 0.635 cm

The f o l l o w i n g s e t of e q u a t i o n s are s u f f i c i e n t t o d e t e r m i n e t h e s e p a r a t e molar c o n c e n t r a t i o n s i n a mixed s o l u t i o n a t a p a r t i c u l a r t e m p e r a t u r e .


ORNL-DWG 7 2 - 9634

20

1

I

--I 18

-I 5 W

16 14

E v)

0,

c -

12

v

z

10

w

2

~8 LL

w

0

v z

0 L

LT 0 0

rn

a

6

4

2 0

4

5

6

2-2 (72-16-16

7

8

9

10

11 12 13 14 WAVE NuMBER ( c m-

16

18

20

UF4 Spectrum ( a p p r o x i m a t e l y 1 mole %) i n LiF-BeFz-ThFq mole %) a t 575°C.

22

24

(x103)


9


10 where :

1 = 9174 cm-' 2 = 11360 c m - l

3 = UF3 component 4 = UF4 component Solving Eqs.

(2-2) and (2-3)

d e s i r e d m o l a r i t i e s s i n c e Avl

simultaneously f o r

(M41T

and (M31T g i v e s t h e

and Av2 are measured and t h e

E

v a l u e s are

known from p r e v i o u s c a l i b r a t i o n s .

(M4IT

-

(M3)T

-

4

(â&#x201A;Źv2 A v l

-

&3V 1

Av2 >

3 4 - & 3 & 4 ) R(Ev2EV1 V l v2 A (â&#x201A;ŹV1 v2

v l ~Av l )

(2-4)

E

- E3v l Ev2 4 ) (&v2 v l

(2-5)

E4

Because s p e c t r a were r e c o r d e d v e r s u s a n e u t r a l screen i n the r e f e r e n c e beam ( s e e F i g . 2-1)

i t w a s always n e c e s s a r y t o s u b t r a c t a s o l v e n t spectrum,

o r b l a n k , which w a s i n d e p e n d e n t l y determined f o r e a c h e x p e r i m e n t a l spectrum, t o g e t t h e n e t a b s o r b a n c e due t o s p e c i e s i n s o l u t i o n . Analyzing composite s p e c t r a r e q u i r e d making c a l i b r a t i o n s f o r E w i t h s o l v e n t m e l t s c o n t a i n i n g a known c o n c e n t r a t i o n of p u r e UF4 and UF3. of

E

Values

a r e reduced w i t h i n c r e a s i n g t e m p e r a t u r e b e c a u s e o f two e f f e c t s : t h e

change of m o l a r i t y caused by t h e r m a l e x p a n s i o n

and t e m p e r a t u r e e f f e c t s on

t h e a b s o r p t i o n s p e c t r a themselves. Changes i n m o l a r i t y due t o t e m p e r a t u r e changes were a d j u s t e d by u s i n g S . C a n t o r ' s d a t a l 2 f o r t h e m o l a l volume of v a r i o u s f u s e d f l u o r i d e s a l t s and

assuming t h a t t h e m o l a l volumes a r e a d d i t i v e t o w i t h i n + 3% a c c o r d i n g t o t h e following general r e l a t i o n s :

N T [ x1 (v1 ) T (M1)T

where

N

T

= NT

+ x 2 (v 2 )T

------ ]

=

1000 m l

x1

= moles p e r l i t e r of s o l u t i o n a t t e m p e r a t u r e T

x1 = mol f r a c t i o n of component #I ( v ~ =) molar ~ v o l of 1 a t t e m p e r a t u r e T i n cc/mole

(M l ) T= m o l a r i t y of 1 i n m o l e s / l a t t e m p e r a t u r e T

(2-6) (2-7

1


11 Molar a b s o r p t i o n s were f i r s t measured f o r p u r e UF4 s o l u t i o n s a t v a r i o u s t e m p e r a t u r e s u s i n g a known c o n c e n t r a t i o n and a t m o l a r i t i e s a d j u s t e d 4 f o r e x p a n s i o n . Measured v a l u e s of (c4 ) and ( E ~ a~r e ) r e~c o r d e d i n T a b l e V1 T

2-1. A c o r r e s p o n d i n g c a l i b r a t i o n w a s performed f o r p u r e UF3 under i d e n t i c a l

conditions. ing agent.

T h i s was most e a s i l y a c h i e v e d by a d d i n g a n e x c e s s of a reducBoth z i r c o n i u m and uranium metal were used f o r t h i s p u r p o s e ,

(2-8) and (2-9).

t h e y r e a c t as shown i n E q s .

Zr

+

4UF4 + -+ 4UF3

U

+

3UF4

+ ZrF4

4UF3

The e f f e c t on t h e p r o p e r t i e s of t h e s o l u t i o n s caused by t h e p r o d u c t i o n of ZrF4 i n E q . 2-8 was v e r y small and hence n e g l e c t e d f o r these d i l u t e s o l u tions.

When uranium was used c o n c e n t r a t i o n s had t o b e i n c r e a s e d by

113

i n t h e i n i t i a l s o l u t i o n s as shown i n Eq.(2-9). 4 P u r e UF3 s o l u t i o n s i n c o n t a c t w i t h g r a p h i t e r e s u l t i n t h e l o s s of uranium o v e r t h o s e c a l c u l a t e d f o r UF

a s shown i n Eq. (1-11. F o r t u n a t e l y 2 t h i s r e a c t i o n i s r a t h e r slow under t h e c o n d i t i o n s t h a t w e h a v e s t u d i e d , and

from s o l u t i o n by t h e f o r m a t i o n of UC

i t w a s p o s s i b l e t o c o r r e c t f o r t h i s l o s s by measuring a b s o r b a n c e s a s a

f u n c t i o n of t i m e t o d e t e r m i n e t h e r a t e of l o s s (dA / d T ) , and c o r r e c t i n g f o r V

t h e l o s s by e x t r a p o l a t i n g back t o z e r o t i m e . does n o t r e s u l t i n a l o s s of UF

3

Reducing UF

from s o l u t i o n .

w i t h uranium 4 The a d d i t i o n of Eq. (1-1)

and (2-9) r e s u l t s i n t h e c y c l i c c o n v e r s i o n of 1J and C t o UC

2

w i t h no n e t

change of UF3 c o n c e n t r a t i o n i n s o l u t i o n as shown i n Eq. (2-10). 4UF3

+

2C

3UF4

+

U

-+

4UF3

u+

2c

-+

uc2

-+

3UF4

+ UC2

An a l t e r n a t e approach t o d e t e r m i n i n g t h e m o l a r a b s o r p t i o n c o e f f i c i e n t s ( E ~ )f o r

s o l u t i o n s a r e more 4 s o l u t i o n s under o u r e x p e r i m e n t a l c o n d i t i o n s , the c a l i b r a -

UF3 i n s o l u t i o n h a s a l s o been used.

s t a b l e t h a n UF

3 t i o n r e s u l t s f o r UF

are t h o s e f o r UF

Since UF

4 are more r e l i a b l e and a s s o c i a t e d w i t h l e s s e r r o r t h a n

Using t h i s f a c t t h e u n c e r t a i n t y a s s o c i a t e d w i t h t h e UF3 3' c a l i b r a t i o n c a n b e reduced by measuring t h e a b s o r p t i o n spectrum of a m i x t u r e


T a b l e 2-1 Fiolar A b s o r p t i o n C o e f f i c i e n t s f o r Molten F l u o r i d e S o l u t i o n s of UF4 and UF

L B Solvent:LiF*BeF 2 2 (66.7-33.9)

Solution i n Mole % Spectra

Molar A b s o r p t i o n Coefficient

E11360

LB S o l v e n t : LiF- BeF2

E9170

UF3

€9170

(72- 16 -12) UF4

E11360

€9170

3 70

44.2

400

Temperature

E11360

MSRR S o l v e n t : LiFoBeP2 ThF4

(4 8- 52)

UF4

UF3

3

UF3

E11360

E9170

7.6

3.1

18.4

43.2

7.8

3.1

17.8

€11360

UF4

E9170

€11360

E9170

O C

450

46.0

10.0

3.90

18.7

41.7

8.1

3.1

16.9

500

44.2

10.0

3.85

17.9

40.1

8.4

3.1

16.0

58.5

14.5

2.80

19.2

5 50

41.7

10.0

3.80

17.1

38.5

8.6

3.1

15.1

56.8

14.5

2.70

18.2

6 00

39.0

10.0

3.75

16.3

36.9

8.9

3.1

14.2

55.0

14.5

2.65

17.2

6 50

36.2

10.0

3.70

15.4

35.3

9.2

3.1

13.3

53.3

14.5

2.55

16.2

7 00

33.5

10.0

3.65

14.8

33.7

9.4

3.1

12.4

51.6

14.5

2.50

15.2

7 50

30.7

10.0

3.60

14.1

49.9

14.5

2.40

14.5

48.0

14.5

2.35

13.8

800


13 I

of UF3 and UF4 where t.he UF3 i s g e n e r a t e d by p a r t i a l l y r e d u c i n g a d i l u t e

UF4 s o l u t i o n of known c o n c e n t r a t i o n . r e d u c t i o n was UC2.)

(The r e d u c t a n t chosen f o r p a r t i a l

The spectrum i s t h e n c o n v e r t e d t o d i g i t a l form a l o n g

w i t h a UF4 r e f e r e n c e spectrum.

Using i t e r a t i v e computer t e c h n i q u e s , v a r y i n g

amounts of t h e UF

spectrum ( i . e . , k x (UF4 spectrum) where k i s t h e coef4 f i c i e n t which i s v a r i e d i n t h e i t e r a t i o n p r o c e s s ) a r e s u b t r a c t e d u n t i l t h e

r e s u l t i n g spectrum v i s u a l l y matches t h a t of p r e v i o u s l y measured ( u n c a l i b r a t e d ) UF3 s p e c t r a .

When a match i s found f o r a p a r t i c u l a r v a l u e of k, the

i n s o l u t i o n and t h u s t h e a b s o r p t i o n c o e f f i c i e n t can b e 3 c a l c u l a t e d knowing t h e t o t a l amount of UF b e f o r e r e d u c t i o n . Comparison of 4 E v a l u e s by t h i s method w i t h t h e t o t a l r e d u c t i o n method showed agreement

c o n c e n t r a t i o n of UF

w i t h i n a 5% u n c e r t a i n t y .

I n T a b l e 2-1,

absorption c o e f f i c i e n t s are l i s t e d f o r t h e various solu-

t i o n s and t e m p e r a t u r e r a n g e s t h a t have been s t u d i e d .

Values were t a k e n

from smoothed f u n c t i o n s which w i t h i n t h e l i m i t s of our p r e c i s i o n are a l i n e a r f u n c t i o n of t e m p e r a t u r e . RESULTS AND DISCUSSION

An e q u i l i b r i u m e x p r e s s i o n such as t h e one w r i t t e n i n Eq. 1-1 i m p l i e s

t h a t c e r t a i n c r i t e r i a are v a l i d :

(1) The e q u i l i b r i u m e x p r e s s i o n s h o u l d i n -

c l u d e a l l r e a c t a n t s and p r o d u c t s which a r e i n v o l v e d i n t h e r e a c t i o n and t h e s e components should combine i n t h e s t o i c h i o m e t r y i n d i c a t e d by the expression.

( 2 ) The e n t i r e p r o c e s s must b e r e v e r s i b l e .

Before q u a n t i t a t i v e d a t a f o r t h e e q u i l i b r i u m i n Eq. 1-1 w e r e measured, t h e above c r i t e r i a w e r e examined i n t h e f o l l o w i n g manner:

The e q u a t i o n

r e p r e s e n t s a h e t e r o g e n e o u s e q u i l i b r i u m between a m o l t e n - f l u o r i d e

solution

of UF

and UF and two s o l i d p h a s e s , UC2 and g r a p h i t e . The i d e n t i f i c a t i o n 3 4 of t h e UF and UF w a s made by t h e c h a r a c t e r i s t i c a b s o r p t i o n spectrum of 3 4 each component i n t h e n e a r - i n f r a r e d and v i s i b l e r e g i o n s (4000-33000 cm-')

.

The i d e n t i f i c a t i o n of t h e s e two s o l u t e components i s w e l l e s t a b l i s h e d bec a u s e t h e i r a b s o r p t i o n s p e c t r a have been t h o r o u g h l y documented.

''

I n view

of t h e e x t e n s i v e s p e c t r o s c o p i c work which has p r e c e d e d , there i s no spec-

t r a l e v i d e n c e f o r any c a t i o n s i n t h e s o l u t i o n o t h e r t h a n IJ+3, U+4.


14

The s o l i d p h a s e components, UC2 and g r a p h i t e , e x h i b i t no m e a s u r a b l e These p h a s e s were i d e n t i f i e d by t h e i r

s o l u b i l i t y i n molten f l u o r i d e s .

r e s p e c t i v e X-ray d i f f r a c t i o n p a t t e r n s .

A s e r i o u s anomally a r i s e s as a

phase i d e n t i f i c a t i o n s i n c e i t s formation i s contrary t o 2 t h e e s t a b l i s h e d p h a s e diagram13 f o r t h e U-C s y s t e m which shows UC2 t o b e

r e s u l t of t h e UC

m e t a s t a b l e w i t h r e s p e c t t o U2C3 and g r a p h i t e a t t e m p e r a t u r e s less t h a n 1500째C.

On t h e b a s i s of t h e uranium-carbon

p h a s e diagram and the a c c e p t e d

f r e e e n e r g i e s of f o r m a t i o n f o r t h e uranium c a r b i d e s a t t e m p e r a t u r e s less t h a n 1000"K, U2C3 s h o u l d b e t h e c a r b i d e p h a s e which w a s i d e n t i f i e d .

Never-

h a s been r e p e a t e d l y shown t o form a t these t e m p e r a t u r e s and 2 h a s been e s t a b l i s h e d as t h e s t a b l e c a r b i d e p h a s e i n t h e e q u i l i b r i u m of Eq. t h e l e s s , UC

1-1.

The r e a d e r who i s i n t e r e s t e d i n t h e d e t a i l s of t h i s i d e n t i f i c a t i o n

i s r e f e r r e d t o an earlier paper.

6

I n t h e p r e s e n t p a p e r w e have i n c l u d e d a

series of e q u i l i b r a t i o n e x p e r i m e n t s where excess U C

2 3

was used t o r e d u c e

s o l u t i o n s v i a t h e back r e a c t i o n of Eq. 1-1. R e s u l t s a r e compared w i t h 4 similar e x p e r i m e n t s where UC w a s used as a r e d u c t a n t . 2

UF

One of t h e s i m p l e s t and y e t most c o n v i n c i n g o b s e r v a t i o n s t o o f f e r f o r t h e e q u i l i b r i u m i s t h a t t h e s t o i c h i o m e t r y of t h e s o l u b l e uranium f l u o r i d e species follows the four-to-three

r e l a t i o n s h i p of Eq. 1-1.

When a s o l u t i o n

i n LiF-BeF2 i s a l l o w e d t o r e a c t w i t h graph3 i t e i t i s o b s e r v e d t h a t 4 moles of UF form 3 moles of UF4. F o r example, 3 when a 0.068 molar s o l u t i o n of UF w a s a l l o w e d t o r e a c t v i a Eq. 1-1 t o form 3 UF4, i t w a s o b s e r v e d t h a t under c o n d i t i o n s where r e a c t i o n was more t h a n 99% of a p p r o x i m a t e l y 0 . 1 m o l e % UF

c o m p l e t e , a 0 , 0 4 9 molar UF one of UF

3

4

solution resulted.

o x i d a t i o n , t h e n 4UF3 s h o u l d form 4UF

I f t h e p r o c e s s were m e r e l y 4'

For example:

where M i s a metal such a s N i . F i n a l l y t h e r e v e r s i b i l i t y of t h e r e a c t i o n was d e m o n s t r a t e d by the rev e r s i b l e t e m p e r a t u r e dependence of t h e e q u i l i b r i u m .

From a p a r t i c u l a r tem-

p e r a t u r e a t which t h e s y s t e m had a t t a i n e d e q u i l i b r i u m and c o n c e n t r a t i o n s of UF3 and UF

measured, t h e t e m p e r a t u r e c o u l d b e r e p e a t e d l y r a i s e d o r lowered 4 c a u s i n g t h e r e l a t i v e c o n c e n t r a t i o n s of UF and UF t o s h i f t and a t t a i n e q u i 3 4 l i b r i u m c o n c e n t r a t i o n s a t t h e s e new t e m p e r a t u r e s . When t h e s y s t e m w a s

-


15 1

r e t u r n e d t o t h e i n i t i a l t e m p e r a t u r e , t h e o r i g i n a l c o n c e n t r a t i o n s of UF3 and UF

were r e p r o d u c e d .

Q u a n t i t a t i v e a s p e c t s of t h e t e m p e r a t u r e dependence

4 w i l l be given i n t h e following sections.

The c r i t e r i a t e s t : s e s t a b l i s h e d t h e e q u i l i b r i u m p r o c e s s as w r i t t e n i n Eq. 1-1.

We found i t more p r a c t i c a l t o measure t h e back r e a c t i o n mechanism: 3UF4

+

-f

UC2 + 4UF3

+

2C

(3-2

1

s i n c e , by i n t e n t i o n a l l y a d d i n g excess UC

w e could i n s u r e t h a t t h e molten 2' f l u o r i d e s o l u t i o n w a s always i n c o n t a c t w i t h a l l the reactive s o l i d p h a s e s .

F u r t h e r m o r e , w e c o u l d i n t e r r u p t t h e e q u i l i b r a t i o n and add f r e s h UC

to 2 d e m o n s t r a t e t h a t t h e o r i g i n a l c a r b i d e had n o t been consumed o r a l t e r e d duri n g t h e c o u r s e of t h e r e a c t i o n .

This p r o c e d u r e a l s o i n s u r e d t h a t more ac-

t i v e r e d u c i n g a g e n t s , i n c l u d i n g o t h e r uranium c a r b i d e s , were n o t p r e s e n t . An e q u i l i b r i u m q u o t i e n t f o r Eq. 3-2 c a n b e w r i t t e n : (UF )

4

(3-3)

Q = (uF4)

and UF are e x p r e s s e d i n mole f r a c t i o n s of t h e s o l u t i o n . Q i s 3 4 s i m p l y t h e r e c i p r o c a l of t h e e q u i l i b r i u m q u o t i e n t , Q ' , f o r t h e f o r w a r d

where UF

r e a c t i o n of Eq. 1-1.

'The d a t a i n t h e f o l l o w i n g p a r a g r a p h s w i l l b e p r e s e n -

t e d as Q v a l u e s i n terms of t h e back r e a c t i o n and s h o u l d n o t b e c o n f u s e d w i t h forward a c t i o n . The e f f e c t of v a r i a b l e s s u c h as t e m p e r a t u r e , m e l t c o m p o s i t i o n , c a r b i d e c o m p o s i t i o n and a t m o s p h e r i c c o n t a m i n a t i o n on t h e e q u i l i b r i u m o f Eq. 3-2 t h e s o l v e n t s y s t e m LiF-BeF i n t h e following sections.

2

in

have been measured and are t r e a t e d s e p a r a t e l y S i n c e t h e e q u i l i b r i u m of Eq. 3-2

( a l s o Eq.1-1)

i s t h e c e n t r a l theme of t h i s p a p e r , i t w i l l o f t e n b e c i t e d as s i m p l y " t h e

equilibrium.

"

E f f e c t of Temperature on t h e E q u i l i b r i u m P r e v i o u s r e s u l t s 4 from t h e hydrogen r e d u c t i o n of UF

i n molten f l u o r 4 i d e s o l u t i o n s i n d i c a t e d t h a t t h e t e m p e r a t u r e e f f e c t on t h e e q u i l i b r i u m of Eq. 1-1 s h o u l d b e s m a l l .

However, when w e measured t h e e q u i l i b r i u m by

e i t h e r t h e f o r w a r d o r t.he back r e a c t i o n , w e found i t t o b e v e r y s e n s i t i v e t o temperature.

T h i s c.an b e s e e n q u a l i t a t i v e l y by examining t h e m o l t e n


16 f l u o r i d e a b s o r p t i o n s p e c t r a of F i g . 3-1 f o r e q u i l i b r i u m m i x t u r e s of d i l u t e UF3 and UF4 i n LiF-BeF2

(66-34 mole

X),

i n L B , o v e r e x c e s s UC2 a t v a r i o u s

2

components of

temperatures.

The s p e c t r a a r e due o n l y t o t h e UF3 and UF

the solution.

T h e r e f o r e , by comparing t h e s e s p e c t r a w i t h t h e s p e c t r a of

4

p u r e UF4 and p u r e UF3 ( F i g s . 2-2 and 2-3 r e s p e c t i v e l y ) , i t can b e s e e n t h a t

a t 500"C, most of t h e uranium i n s o l u t i o n i s p r e s e n t a s UF

4 whereas a t

700"C, enough UF3 i s p r e s e n t t o make t h e composite spectrum r e s e m b l e t h a t

of F i g . 2-3.

The composite spectrum a t 600째C r e s e m b l e s n e i t h e r of t h e two

p u r e component s p e c t r a b u t i n s t e a d a n i n t e r m e d i a t e m i x t u r e of t h e two. The q u a n t i t a t i v e a s p e c t s of t h e s e s p e c t r a were c a l c u l a t e d by t h e procedure described i n the experimental section.

From a b s o r p t i o n s p e c t r a such

as t h o s e i n F i g . 3-1 c o n c e n t r a t i o n s of UF3 and UF4 were d e t e r m i n e d i n mole f r a c t i o n s and used i n Eq. 3-3 t o c a l c u l a t e e q u i l i b r i u m q u o t i e n t s , Q , a t various temperatures.

The d a t a a r e p r e s e n t e d i n T a b l e 3-1 a l o n g w i t h Q

Q v e r s u s l/TK (where TK 10 A t t h e t o p of t h e f i g u r e i s shown t h e c e n t i -

v a l u e s which are t h e n p r e s e n t e d i n F i g . 3-2 as l o g

i s t h e Kelvin temperature).

g r a d e s c a l e and a t t h e r i g h t s i d e of t h e f i g u r e , t h e e q u i l i b r i u m r a t i o ,

where [UF ] and [UF 3 a r e t h e c o n c e n t r a t i o n s i n s o l u t i o n . (Note t h a t t h e 3 4 denominator of Eq. 3-4 r e p r e s e n t s t h e t o t a l uranium f l u o r i d e i n s o l u t i o n . ) These R v a l u e s have been t h e customary manner i n which UF -UF c o n c e n t r a 3 4 t i o n s a r e e x p r e s s e d w i t h i n t h e MSRE program. The two l i n e s drawn t h r o u g h t h e d a t a p o i n t s r e p r e s e n t t h e e x p e r i m e n t a l u n c e r t a i n t y of t h e d a t a which

a r i s e s mainly from t h e b a s e l i n e e r r o r i n t h e a b s o r p t i o n spectra.

Equilib-

r i a a t v a r i o u s t e m p e r a t u r e s were approached from b o t h t h e h i g h (open c i r c l e s ) and low ( c l o s e d c i r c l e s ) t e m p e r a t u r e d i r e c t i o n .

The s y s t e m was con3 Then

i n i t i a l l y h e l d a t c a . 50째C above t h e t e m p e r a t u r e d e s i r e d u n t i l t h e UF

r e a c t i n g w i t h UC2 v i a Eq. 3-2). 4 t h e t e m p e r a t u r e w a s dropped 50" and t h e UF3 c o n c e n t r a t i o n was allowed t o c e n t r a t i o n had c e a s e d t o grow (UF

w i t h g r a p h i t e u n t i l no f u r t h e r change c o u l d b e 3 The e q u i l i b r i u m c o u l d b e s h i f t e d r e p e a t e d l y i n t h i s manner by

f a l l by r e a c t i o n of UF detected.

v a r y i n g t h e t e m p e r a t u r e of t h e s y s t e m .

The t r a i n of p o i n t s a t any g i v e n


17

I

5000

I

I

I

I

I

ORNL- DWG 72-9633 I I I

15,000

10,000 WAVENUMBER (crn-’1

3-1 S p e c t r a of D i l u t e UF3-UF4 M i x t u r e s i n LiF-BeF2 (66-34 mole X) Showing Temperature E f f e c t on t h e E q u i l i b r i u m : 4UF3 2C 2 3UF4 UC2.

+

+


18

T a b l e 3-1 T y p i c a l E q u i l i b r i u m d a t a used i n F i g u r e s 3-2 t o 3-5 where Q and R a r e d e f i n e d by

Eqs. 3-3 and 3-4. Run

1 2 3

4 5

6

Solvent

Carbide Phase

L2B

uc2

L2B

uc2

L2B

uc2

L2B

uc2

L2B

uc2

L2B

'2'3

Temp ("C)

Measured Absorbance Mole F r a c t i o n Q 11360 9170 U F( ~1 0 4 ) (104) ~ ~ ~ (x1~8) cm-1 em-1 0.128

R

500

0.130

0.515

0.264

7.25

5 50

0.217

0.505

0.804

7.22

600

0.485

0.517

2.76

6.68

1951.0 0.29

650

0.692

0.527

4.57

6.14

19080.0 0 . 4 3

70 0

1.305

0.780

9.83

7.60

212300.0 0.56

500

0.254

0.538

0.99

7.17

26.0 0.12

11.12

0.035 0.10

7

I1

I1

600

0.409

0.416

2.36

5.29

2070.0 0 . 3 1

8

I1

It

700

1.083

0.567

8.29

4.74

443000.0 0.64

9

LB

370

0.147

0.384

0.53

5.52

10

11

I1

400

0.215

0.330

1.04

4.67

115.0 0.18

11

11

I1

450

0.380

0.328

2.23

4.36

2950.0 0.34

12

11

11

500

0.482

0.274

3.09

3.21

27600.0 0.49

uc2

4.6 0.087

550

0.925

1.317

3.97

18.31

401.0 0.18

14

I1

11

600

1.257

1.165

6.13

15.16

4050.0 0.29

15

11

II

650

2,575

1.433

14.32

129000.0 0.49

13

MSBR

uc2

14.0


19

ORNL- DWG 72- 4 07l9R

TEMPERATURE ("C)

-2

550

500

573 600 622 650

700 0.6

-3

0.5 0.4

-4

L. 3

0.3

+ro

-5 0

LL

z

3

0 0

0.2

-

7 ro LL

-6

3 II

C t 0.1

-7

-8

-9 1.30

0.03 1.25

1.20

4.45 'ooo/T

1.10

1.05

1.00

(OK)

3-2 E q u i l i b r i u m q u o t i e n t s , Q = (UF3) 4/ (UFq) 3, v e r s u s t e m p e r a t u r e f o r U C ~+ 3 ~ ~ 4 ( 2 d )4UP3(d) + 2C in t h e s o l v e n t LiF-BeF2 (66-34 mole %).


20

t e m p e r a t u r e r e p r e s e n t s t h e approach t o e q u i l i b r i u m w i t h o n l y t h e lowermost ( f o r open c i r c l e s ) and uppermost ( f o r c l o s e d c i r c l e s ) b e i n g t h e b e s t measured e q u i l i b r i u m v a l u e . The l a r g e t e m p e r a t u r e e f f e c t on t h e e q u i l i b r i u m i s e x e m p l i f i e d by F i g . 6 3 . 2 where t h e q u o t i e n t , Q , s h i f t s by 10 i n g o i n g from 500 t o 700°C. I n p r a c t i c a l terms, t h i s means t h a t t h e c o n c e n t r a t i o n of UF

r e l a t i v e t o the

3

t o t a l uranium f l u o r i d e i n s o l u t i o n i s i n c r e a s e d from ca. 5% a t 500°C t o ca.

60% a t 700°C. found when U2C3

The same l a r g e t e m p e r a t u r e e f f e c t on t h e e q u i l i b r i u m i s ( i n p l a c e of UC2) i s e q u i l i b r a t e d w i t h UF

4

solutions.

These

d a t a are p r e s e n t e d i n T a b l e 3-1 and t h e r e s u l t i n g Q v a l u e s a r e p l o t t e d i n F i g . 3-3.

Here t h e Q v a l u e s a r e g r e a t e r a t a g i v e n t e m p e r a t u r e t h a n i n F i g .

3-2 and t h e r e f o r e s u p p o r t t h e i d e n t i f i c a t i o n of UC

2

as t h e s t a b l e c a r b i d e

phase of Eq. 1-1.

Furthermore t h e U C e q u i l i b r a t i o n e x p e r i m e n t s demon2 3 s t r a t e t h a t t h e UF3 s t a b i l i t y i n d i l u t e f l u o r i d e s o l u t i o n s as w e l l a s t h e t e m p e r a t u r e e f f e c t on t h e e q u i l i b r i u m would n o t b e f a r d i f f e r e n t from t h a t p r e s e n t e d i n Fig. 3-2,

even i f t h e i d e n t i f i c a t i o n of UC

2

as t h e s t a b l e

c a r b i d e p h a s e of Eq. 1-1 were n o t c o r r e c t . The d a t a of F i g . 3-2 can b e used t o c a l c u l a t e t h e change i n e n t h a l p y f o r t h e equilibrium.

By d e f i n i n g t h e s t a n d a r d s t a t e of t h e s o l u t e s UF

and 3 UF a s one mole p e r c e n t i n L B , t h e i r a c t i v i t y c o e f f i c i e n t s a r e u n i t y and 4 2 The change t h e n t h e e q u i l i b r i u m q u o t i e n t s become e q u i l i b r i u m c o n s t a n t s , K. i n e n t h a l p y , AH, f o r t h e r e a c t i o n i n t h e t e m p e r a t u r e r a n g e of 500-650°C

can

be c a l c u l a t e d from t h e s l o p e of t h e l i n e i n F i g . 3-2 u s i n g t h e e x p r e s s i o n :

where R i s t h e g a s c o n s t a n t .

The v a l u e o b t a i n e d f o r AH of Eq. 3-2 i s 9 9 . 3

Kcal/mole which i s s u r p r i s i n g l y l a r g e i n view of t h e e n t h a l p y change c a l c u l a t e d from e n t h a l p i e s of f o r m a t i o n f o r t h e p u r e , u n d i l u t e d components a t e i t h e r 298" o r 800°K.

These v a l u e s are g i v e n i n T a b l e 3-2 and y i e l d AHo =

-10 Kcal/mole f o r t h e u n d i s s o l v e d components of Eq. 3-2 a t 298°K and AHo -12.90 Kcal/mole a t 800°K.

=

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

c a l c u l a t i o n s i n c e no h e a t s of s o l u t i o n f o r UF

and UF4 are a v a i l a b l e . It 3 should b e n o t e d t h a t even t h e s i g n of t h e AH i s d i f f e r e n t : W e measure an endothermic p r o c e s s whereas a s l i g h t l y e x o t h e r m i c p r o c e s s i s e x p e c t e d .


21

ORNL-DWG 7 2 - 40747

-4

TEMPERATURE ("C) 550 573 600 622 650

500

700

-2 0.6

-3 0.5

*

h

0.4

-4

a s m

5 +

Yr)

LL

3

0.3

0 -

-5

7 LL

3 II

e 0.2

-6

3.1

-7

-0 1.30

1.25

1.20

1.15

1.40

1.05

4-00

'ooo/T (" K 1

3-3 E q u i l i b r i u m q u o t i e n t s , Q = (UF3) 4 / (UF4) 3 , v e r s u s t e m p e r a t u r e 3 m 4 ( d ) $ 4UF3(d) 3/2C i n t h e s o l v e n t LiF-BeF2 (66-34 mole %>. f o r l/2U2C3

+

+


22

Table 3-2 Enthalpy Data in (Kcal/mole) Sources of the Data are from Tabulations referenced as Superscripts _ ~ _ _ ~

~-

UF4

uc2 -20(1) l3

AHi98 0

uF3

-450 (5)5

14.9914

8.7913

H0800-H29 8

~~

C

-345 (10) 15 11.8

0

1.8316

Table 3-3 Equilibrium Quotients, Q, and Ratios, R, for UF3, UF4 Solutions in Atmospheric Contaminated System (Taken from Ref. 24) Solution (Mole W ) LiF-BeF2

675째C

R

66-34

.025

48-52

.13

575째C

Q 2.7~10-lo

R

Q

.004

1.8~10-l~

~ x I O - ~ .03

6.2~10-~'


23

Heats of s o l u t i o n c o u l d p l a u s i b l y a c c o u n t f o r a l a r g e amount of t h e d i s c r e p a n c y s i n c e i t i s o b s e r v e d t h a t t h e h e a t of s o l u t i o n f o r CeF

i n L2B 3 (600-800째C) i s 1 7 Kcal/mole whereas o n l y 10 t o 1 2 Kcal/mole i s p r e d i c t e d . 1 7

From t h e e n t h a l p y d a t a , w i t h o u t t h e h e a t s of s o l u t i o n , w e can o n l y c o n c l u d e t h a t t h e thermodynamic d a t a i s n o t a d e q u a t e t o p r e d i c t t h e change i n enthalpy f o r t h e reaction. E f f e c t of S o l v e n t on t h e E q u i l i b r i u m I n t h e same way t h a t t e m p e r a t u r e s h i f t e d t h e e q u i l i b r i u m , changes i n t h e s o l v e n t composition d i d a l s o .

The o r i g i n a l p u r p o s e of t h i s r e s e a r c h

w a s t o d e m o n s t r a t e t h a t changes i n t h e f l u o r i d e i o n c o n c e n t r a t i o n (which have a l r e a d y been shown t o a f f e c t t h e c o o r d i n a t i o n b e h a v i o r of d i l u t e UF,.

9

s o l u t i o n s 1 8 ) m i g h t b e . r e l a t e d t o s h i f t s i n redox e q u i l i b r i a as w e l l .

The

e f f e c t of changing t h e s o l v e n t c o m p o s i t i o n on the e q u i l i b r i u m i s exemplif i e d by comparing t h e e q u i l i b r i u m q u o t i e n t s , Q, f o r LiF-BeF2

(48-52 mole %),

LB, i n F i g . 3-4 w i t h t h o s e p r e v i o u s l y shown i n F i g . 3-2 f o r t h e L2B compo-

sition.

A t any g i v e n t e m p e r a t u r e ( e . g . ,

5OO0C), Q i s c o n s i d e r a b l y l a r g e r

Therefore t h e i n t h e LB c o m p o s i t i o n t h a n i n t h e c o r r e s p o n d i n g L B m e l t . 2 e q u i l i b r i u m of Eq. 3-2 i s s h i f t e d t o t h e r i g h t by i n c r e a s i n g t h e c o n c e n t r a t i o n of BeF2 i n t h e so:Lvent, i . e . , by making t h e s o l v e n t more F- d e f i c i e n t through t h e a d d i t i o n of a component which c o o r d i n a t e s s t r o n g l y w i t h f l u o r i d e ions.

The r a t i o , R , of Eq. 3-4 a t 500째C h a s been i n c r e a s e d from ca.

0.05 f o r t h e L2B s o l v e n t t o ca. 0.55 f o r t h e LB s o l v e n t ( c . f . F i g s . 3-2 and 3-4 a t 500째C). The magnitude of t h i s change can b e compared w i t h t h a t which i s p r e 4+ d i c t e d from Baes' a c t i v i t y c o e f f i c i e n t s f o r M3+ and M Realcations. i z i n g t h a t t h e o n l y d i f f e r e n c e i n e q u i l i b r i u m q u o t i e n t s between t h e two

m e l t s i s t h e r a t i o of t h e r e s p e c t i v e a c t i v i t y c o e f f i c i e n t s , y, f o r UF3 and UF

4

r a i s e d t o t h e a p p r o p r i a t e powers:


24

ORNL-DWG 72-40748

-2

370

TEMPERATURE ("C) 450 400

500

7

I_

0.6

-3

-

0.5

0.4 -4

I

-5 0 $ 0 0

-

-6

0.3*; 3

t n

LL

3

0.2

7 PI)

LL

3 II

e 0.4

-7

-0

-9

1.55

1.45

1.50

'O*O/T

1.40 (OK

4.35

4.30

1

4 E q u i l i b r i u m q u o t i e n t s , Q = (UF3) /(UF4)3, v e r s u s t e m p e r a t u r e 3-4 4UF3(d) 2C i n t h e s o l v e n t LiF-BeF2 (48-52 mole %). f o r UC2 + 3UF4(d)

+


25

3-6

where L2B and LB d e n o t e t h e s o l v e n t s y s t e m s .

S i n c e Baes d e f i n e s a l l a c t i v -

i t y c o e f f i c i e n t s as u n i t y i n t h e r e f e r e n c e c o m p o s i t i o n , L2B, t h e n :

.QLB

(ki)

=

3- 7

-

Q L B~

(e,," \

The r i g h t - h a n d

/

term c a n b e e s t i m a t e d from Baes' d a t a a t 600"C19 where

= 0.7 and yUF z = 1 0 by e x t r a p o l a t i n g t o LiF-BeF (48-52 2 'UF3 keF3 4 'ThF 3 By t h i s p r o c e d u r e Q ?Q i s e s t i m a t e d t o b e 4 x 10 mole %>. LB L2B

'

.

From o u r d a t a , Q

a t 600째C can b e d e t e r m i n e d by e x t r a p o l a t i n g /Q LB L2B with t h e value, t h e d o u b l e l i n e s t o 600째C and comparing t h i s v a l u e , Q 4LB '

r e a d from Fig. 3-2. We f i n d QL,/QL2, =: 5 x 1 0 a g r e e s r e a s o n a b l y Q L ~ 9B w i t h t h e estimate from Baes' d a t a . Even b e t t e r agreement c o u l d b e o b t a i n e d i f a non-linear of F i g . 3-4)

e x t r a p o l a t i o n (which i s s u g g e s t e d by t h e t r e n d i n t h e d a t a

i s made.

F u r t h e r m o r e , i t s h o u l d b e n o t e d t h a t Baes' a c t i v i t y

c o e f f i c i e n t s a r e o n l y a p p r o x i m a t e f o r UF d a t a f o r CeF

3'

3

s i n c e t h e y are a c t u a l l y b a s e d on

The comparison s e r v e s t o show t h a t the magnitude of t h e

s o l v e n t e f f e c t i s i n r e a s o n a b l e agreement w i t h p r e v i o u s d a t a and consequen-

t l y must b e c o n s i d e r e d when e s t i m a t i n g UF

3

s t a b i l i t i e s i n other molten

f l u o r i d e s o l v e n t systems. T h i s l e a d s t h e n t o t h e p r a c t i c a l q u e s t i o n , What UF p e c t e d i n t h e MSRE and t h e MSBR s o l v e n t s ? " .

3

s t a b i l i t y i s ex-

I n t h e s e t e r n a r y systems t h e

r e l a t i v e measure of F- c o n c e n t r a t i o n i s more d i f f i c u l t t o d e t e r m i n e t h a n i n t h e b i n a r y system LiF-BeF2 s i n c e t h e r e a r e two "acidic"* competing f o r f l u o r i d e i o n s .

c a t i o n s i n each

I t i s c u r r e n t l y r e g a r d e d t h a t t h e MSRE s o l -

v e n t i s more F- d e f i c i e n t t h a n L2B and r e s u l t s of a p r e v i o u s e l e c t r o c h e m i 20 c a l s t u d y of UF3 s t a b i l i t y by Manning support t h i s contention. L i t t l e *By " a c i d i c 1 ' w e mean, i n t h e L e w i s a c i d c o n c e p t , t h e tendency t o c o o r d i n a t e w i t h F-.


26 a t t e n t i o n h a s been g i v e n t o t h e UF3 s t a b i l i t y i n MSBR s o l v e n t s .

We have

f i r s t a t t e m p t e d t o p r e d i c t i t and f i n a l l y w e h a v e measured i t d i r e c t l y . R e a l i z i n g t h e s o l v e n t e f f e c t s on UF

-

3

s t a b i l i t y a r i s e from changes i n

t h e a v a i l a b l e f r e e F , a n a t t e m p t was made t o estimate t h e F- c o n c e n t r a t i o n i n MSRE and MSBR s o l v e n t s b a s e d on t h e e a r l i e r o b s e r v a t i o n 1 8 t h a t t h e coo r d i n a t i o n e q u i l i b r i u m of U4+ i o n s i n LiF-BeF2 s o l v e n t s depended upon Faccording t o :

4UF8

2 UF 73-

+ F-

3-

a

We h a v e p r e v i o u s l y suggested2' t h a t t h e F- c o u l d b e measured by d e t e r m i n i n g 43t h e c o n c e n t r a t i o n of UF and t h e n e s t i m a t i n g t h e F- c o n c e n t r a and UF7 8 T h i s method w a s found t o work f o r LiF-BeF2 s o l u t i o n s w i t h t i o n by Eq. 3-8. c o n c e n t r a t i o n s of up t o 52 mole % and f o r t h e MSRE s o l v e n t which i s 2 e s s e n t i a l l y a LiF-BeF2 s o l v e n t .

BeF

The method t h e n w a s used t o estimate t h e F- c o n c e n t r a t i o n i n the MSBR 4s o l v e n t . Because t h e spectrum of UF i n t h i s s o l v e n t was l a r g e l y UF8 , a

4

F- c o n c e n t r a t i o n g r e a t e r t h a n t h a t i n L B w a s s u g g e s t e d from Eq. 3-8.

2

We

concluded t h a t t h e s t a b i l i t y of UF3 would b e v e r y much l e s s t h a n i n L2B, and i n f a c t , some e a r l i e r UF

3

s t a b i l i t y measurements2'

tended t o support

t h i s conclusion. I n c o n t r a s t t o t h i s v i e w p o i n t , were a c t i v i t y c o e f f i c i e n t d a t a by C. F. Baesl'

t h e UF

3

and BF

s o l u b i l i t y d a t a by S. C a n t o r 2 2 which s u g g e s t e d t h a t 3 s h o u l d b e s l i g h t l y more s t a b l e in t h e MSBR s o l v e n t t h a n i n L2B.

We examined t h i s d i s c r e p a n c y by e x p e r i m e n t a l l y measuring t h e s t a b i l i t y of UF3 i n t h e MSBR s o l u t i o n o v e r e x c e s s UC2 i n t h e g r a p h i t e s p e c t r o p h o t o -

metric c e l l .

The r e s u l t s are shown as loglOQ v s l / T K i n F i g . 3-5 i n t h e

same form as t h a t used f o r p r e v i o u s f i g u r e s . more s t a b l e i n MSBR t h a n i n L2B.

These d a t a show t h a t UF3 i s

From t h e s t a n d p o i n t of r e a c t o r o p e r a t i o n s ,

c o n c e n t r a t i o n r a t i o s , R , of UF3 ( c . f . Eq. 3-4)

of up t o 0.03 can b e main-

t a i n e d s a f e l y down t o t h e c a . 500째C f r e e z i n g p o i n t of t h e s o l u t i o n . The d i s c r e p a n c y i n o u r e a r l i e r 2 I p r e d i c t i o n s can o n l y b e r a t i o n a l i z e d by a l l o w i n g a more complex c o o r d i n a t i o n mechanism f o r t h e MSBR s o l v e n t t h a n T h i s p r o b a b l y i n v o l v e s U4+ which are f l u o r i d e 4+ b r i d g e d t o n e i g h b o r i n g Th o r Be2+ s o t h a t , through b r i d g i n g , t h e

i s d e s c r i b e d i n Eq. 3-8.

c


27

ORNL-DWG 72-1232t

TEMPERATURE ("C) 550 600 650

500 -4

700

1 1

-2 0.6 0.5

-3

0.4

ti* 3

-4

+

(s

0.3

s!

ti* 3

7

0,

-

0

LL*

-5

3

0.2 II Q

-6 0.1 -7

-8 1.30

1.25

1.20

1.15 'Oo0/T

1.10

1.05

1.00

(OK)

3-5 Equilibrium quotients, Q = (UF?) 4/(UFb) 3 versus temperature for UC2 + 3UFq(d 4UF3(d) + 2C in the sGlvent LiF-BeF2-ThFq (72-16-12 mole %


28

c o o r d i n a t i o n number (and a c c o r d i n g l y by Eq. 3-8,

t h e F- c o n c e n t r a t i o n ) ap-

There i s some e v i d e n c e f o r t h i s i n LiF-BeF2 s o l v e n t s 23 where t h e BeF2 c o n c e n t r a t i o n i s g r e a t e r t h a n 52 mole %. p e a r s much l a r g e r .

E f f e c t of Atmospheric Contamination on t h e E q u i l i b r i u m I n e a r l i e r a t t e m p t s t o measure t h e e q u i l i b r i u m q u o t i e n t s f o r Eq. 1-1, i t was a p p a r e n t t h a t t h e e q u i l i b r i u m c o n c e n t r a t i o n of UF

solvents was unusually

3

i n LiF-BeF2

compared w i t h t h e p r e s e n t r e s u l t s .

These

r e s u l t s a r e p r e s e n t e d i n T a b l e 3-3 and were measured by f o l l o w i n g t h e rea c t i o n of UF

i n g r a p h i t e w i t h no uranium c a r b i d e s added d i r e c t l y t o t h e 3 system. These r e a c t i o n s were always accompanied by t h e f o r m a t i o n of UO 2 and o t h e r u n i d e n t i f i e d s o l i d p h a s e s . However a f t e r v a r i o u s improvements

were made which e l i m i n a t e d obvious s i g n s of a t m o s p h e r i c c o n t a m i n a t i o n , such as U 0 2 f o r m a t i o n , t h e s t a b i l i t y of UF3 w a s g r e a t l y enhanced.

We have sub-

s e q u e n t l y concluded t h a t t h e s e e a r l i e r measurements i n v o l v e d e q u i l i b r i a of

UF

3

and UF

4

s o l u t i o n s i n g r a p h i t e and an oxy-carbide p h a s e ( a s opposed t o

a pure carbide phase).

I t w a s never p o s s i b l e t o i d e n t i f y t h e oxy-carbide

phase by X-ray a n a l y s i s d e s p i t e t h e f a c t t h a t t h e e q u i l i b r i a were v e r y e a s y t o r e p r o d u c e from Eq. 3-2. The e f f e c t of a t m o s p h e r i c c o n t a m i n a t i o n i s clear

--

i t g r e a t l y reduces

t h e s t a b i l i t y of UF3 and i s t h e r e f o r e a major f a c t o r which c a n n o t b e i g n o r ed when c o n s i d e r i n g UF

3

s t a b i l i t y i n molten f l u o r i d e s o l u t i o n s .

E f f e c t of Temperature, S o l v e n t and Contamination Compared A l l of t h e e f f e c t s of t h e v a r i a b l e s have been c o l l e c t e d t o compare -1 t h e i r r e l a t i v e importance and are shown i n F i g . 3-6 as logloQ v s TK i n

t h e same f a s h i o n as t h e p r e v i o u s f i g u r e s b u t w i t h a s u b s t a n t i a l r e d u c t i o n i n scale.

The e f f e c t of i n c r e a s i n g t e m p e r a t u r e i s s i m i l a r i n a l l c a s e s ,

There i s no r e a s o n f o r t h e 3' l i n e s t o b e p a r a l l e l t o each o t h e r b e c a u s e t h e y d i f f e r p r i n c i p a l l y ( e x c e p t

c a u s i n g an i n c r e a s e i n t h e s t a b i l i t y of UF

f o r t h e c a s e of t h e a t m o s p h e r i c c o n t a m i n a t i o n ) i n t h e a c t i v i t y c o e f f i c i e n t s and UF i n t h e d i f f e r e n t s o l u t i o n s and t h e s e need n o t change pro3 4 p o r t i o n a t e l y f o r a l l s o l u t i o n s . Neither should i t be necessary t h a t t h e f o r UF

d a t a b e r e p r e s e n t e d by s t r a i g h t l i n e s , implying t h a t AH f o r t h e r e a c t i o n i s constant.

They are used h e r e o n l y b e c a u s e t h e d a t a are i n s u f f i c i e n t t o

-


29

ORNL-DWG

72-12322

0

-1

-2 0.6 -3

0.5

-Lie 3

0.4 +m

-4

LL

3

\

0.3 Lim

-5

3

II

0.2 0

-6

0 c7

-0

0.1

-7

-8

-9

-10

-1 1

-12

-13

__

-

(C) LiF-BeF2-ThF4 (72-16-12 mole % ) (D) LiF-BeF2 (66-34 mole % ) ( E ) LtF-BeF2 ( 4 8 - 5 2 m o l e % ) OX I DE CONTAM I NATED ( F ) LiF-BeF2 ( 6 6 - 3 4 mole % ) OX IDE CONTA M INATED

I 1.6

1.5

1.4

f.2

1.3

'O0yr

1.1

1 .o

(OK)

3-6 A comparison of equilibrium quotients versus temperature for U C ~+ X J F ~ (Z~ 4uF3(d) ) + 2~ in various solvent systems.


30

justify greater detail. Decreasing t h e F- c o n c e n t r a t i o n by t h e a d d i t i o n of BeF2 i s v e r y benes t a b i l i t y whereas a t m o s p h e r i c c o n t a m i n a t i o n c a u s e s 3 t h e o p p o s i t e and most d i s a s t e r o u s e f f e c t s on UF s t a b i l i t y . 3

f i c i a l i n i n c r e a s i n g UF

S i n c e t h e MSRE r e s u l t s do n o t come from our work, t h e MSRE l i n e i s The s t a b i l i t y of UF3 i n t h e MSBR s o l v e n t i s between t h a t of t h e

broken.

I t i s obvious t h a t t h e r e g i o n of g r e a t e s t UF3 sta-

MSRE s o l v e n t and L2B.

b i l i t y i s t h a t of h i g h t e m p e r a t u r e and low F- c o n c e n t r a t i o n .

W e therefore

s u g g e s t t h a t l i t t l e UF3 c o u l d b e m a i n t a i n e d i n F- r i c h s o l v e n t s such as f

LiF-NaF-KF (46.5-11.5-42.0 mole X ) even i f t h e r e p o r t e d K r e d u c t i o n by (25) were n o t t o o c c u r . Conversely, t h e g r e a t e r s t a b i l i t y of p u r e UF (4) UF3 3 ( i . e . , n o t d i s s o l v e d i n a m o l t e n f l u o r i d e s o l v e n t ) i s e x p l a i n e d by t h e a b s e n c e of s o l v a t i n g F-. Other C o n s i d e r a t i o n s

I f t h e thermodynamic d a t a are s u f f i c i e n t l y a c c u r a t e t h e n i t s h o u l d b e p o s s i b l e t o c a l c u l a t e t h e f r e e energy change f o r Eq. 3-2 i n t h e s o l v e n t LiF-BeF2

(66-34 mole

X ) and t h e n t h e e q u i l i b r i u m c o n s t a n t by: AG = -RT I n K

(3-9)

The e x p r e s s i o n s f o r t h e f r e e energy of f o r m a t i o n a r e g i v e n i n T a b l e 3-4 f o r UC2, UF4 and UF3 (where t h e l a t t e r two are f o r t h e s t a n d a r d s t a t e of

T a b l e 3-4 F r e e E n e r g i e s of Formation f o r P u r e UC UF3 and UF4 i n LiF-BeF2 0,

(66-34 mole

X)

and t h e S o l u t e s 2 w i t h standard deviations,

i n Kcal/mole f o r r e l a t i o n s h i p : AG = A

Component

uc2 uF3

UF4

+

B (TK/lOOO>

A

B

0

Source

-15.82

-8.2

-

Storms

-338.04

40.26

2

Baes

-445.92

57.85

2

Baes

26

26

13


31

one mole percent each in L2B ) . These free energy functions, when combined in the proper stoichiometric proportions yield a change of free energy for Eq. 3-2 of AG

=

1.42-4.31 (TK/lOOO) with a significantly large combined

standard deviation of + 16 Kcal/mole. At 500째C, AG is -1.91 Kcal/mole and the equilibrium constant from Eq. 3-9 is 3.5.

Since these are standard

4 solutes, then Q is also equal to 3.5 and the ratio, R, of Eq. 3-4 is 0.89. (c.f. with Q = 1.5 x lo-' and R = 0.03 in states for the UF

Fig. 3-2).

3

and UF

Therefore, from the existing free energies of formation for the

components of the reaction, practically all of a dilute UF3 solution in contact with graphite should be stable. However, neither our results, nor those from any other investigators support this high a stability of UF

3'

It may seem obvious

A word of caution should be given at this point.

stability via Eq. 1-1 by holding UF3 solutions in graph3 ite and allowing the UF to react with graphite. Furthermore, it may be 3 most convenient to generate a UF solution by reducing a dilute UF solution 3 4 Zr, or U metal within the same graphite with a strong reductant such as Be, to demonstrate UF

vessel that will be used for the stability measurement. We have observed that this results in the formation of mixtures of UC, U2C3 and UC2 phases accompanied by the consumption of more reducing metal than is expected for the complete UF

reduction. The apparent anomally is caused by the revers4 ibility of Eq. 1-1 since as soon as UF is formed in excess of its equilib3 rium concentration within the graphite vessel, it reacts with graphite forming uranium carbide phases and UF4 in solution.

T h e UF4 is, in

reduced again by the excess reductant, forming more UF

3'

turn,

An example of the

process using Zr metal is: 4UF4

+

Zr

4UF3

+

XC % 3UF4

UF4

+

Zr

4UF3 + ZrF4

+ UCx

(3-10) (3-11)

so that the net reaction is:

+

XC t' ZrF4

+ UCx

(3-12)

This is one of the majar reasons why we found it more practical to study the equilibrium by the back-reaction mechanism of Eq. 3-2.

Although the

UC and U C phases do finally react leaving ultimately UC we found that 2 3 2'

I


32

even for our small reaction system of less than 0 . 5 cc, it took an impractical length of time.

Larger systems with smaller surface-to-volume

ratios would take even longer. The question of reaction times brings up the final point to be mentioned, that is, the kinetics involved in achieving the equilibrium of Eq. 1-1. Since UF

is reacting with graphite to form uranium carbides, the mechanism 3 is obviously heterogeneous. It is considered by these authors far too dif-

ficult a mechanism to attempt to clearly describe; but if reaction rates are sought, the initial measurements should demonstrate that the mechanism is heterogeneous by varying the surface-to-volume ratios of the reacting system. We predict that the outcome of such a measurement will substantiate the heterogeneous mechanism.

Another point of caution should be made.

Since larger surface-to-volume ratios mean slower reaction rates, apparent high stabilities of UF3 may appear whereas they actually involve metastable states of the equilibrium mechanism which include uranium carbide phases other than UC

These other carbides will ultimately be converted to UC2 2' by the mechanism of Eq. 1-1; but, until the conversion is completed, the

UF

3

ratio, R, will remain fixed at a high value. The ultimate aim of the UF

ditions under which certain UF

3

3

stability study has been to describe conratios can be maintained in graphite.

demonstrate the validity of our measurements we mixed dilute UF

3

To

and UF4 in

ratio, R = 0.17. 3 The solution was maintained for a period of a week in the graphite spectrothe LB solvent

so

that the resulting solution had a UF

photometric cell at 475째C with no l o s s of UF

3

or UF4 from solution.

Fig. 3-4 which shows the maximum R at 475째C to be 0.40-0.45).

(c.f.


33 References

1)

W. R. G r i m e s , "Chemical Research and Development f o r Molten S a l t

B r e e d e r Reactors''

2)

M. W.

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,

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34

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35

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