Heavy Element Separation for Thorium-Uranium-Plutonium Fuels

25 Heavy Element Separation for Thorium-UraniumPlutonium Fuels G. R. GRANT, W. W. MORGAN, Κ. Κ. MEHTA, and F. P. SARGENT Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba

A large potential exists for resource conservation through introduction of thorium fuel cycles in CANDU (CANada Deuterium Uranium) reactors (1). While a number of fuel cycles have been suggested (2), this paper deals with one using thorium dioxide fuel topped with plutonium, and a processing scheme in which all fissile and f e r t i l e materials are separated and recycled. A l though not discussed here, other cycles not involving complete separation are also being studied as part of the International Nuclear Fuel Cycle Evaluation. To test the separation of the three actinides (Th, U and Pu) from each other, a modified Thorex solvent extraction flow sheet using 30% tributylphosphate (TBP) has been developed. Consider able work has already been reported in the literature for separa tions in Th-U systems (3, 4), but inclusion of Pu in the fuel cycle adds additional complexities (5). The flow sheet adopted is that in Figure 1, which also shows the relative flows (FL) of the inlet streams and concentrations (M or mol/L) of the major components. The flow ratios and acidities for each contactor were initially derived by constructing McCabe-Thiele operating diagrams based on unpublished distribution measurements made in our laboratories (6). Contactors I and I I are used as a decontamination c y c l e t o remove most o f the f i s s i o n products from the a c t i n i d e s . A f t e r i n t e r c y c l e c o n c e n t r a t i o n and Pu valency adjustment ( t o P u ( I I I ) ) , the next three c o n t a c t o r s make up the primary s e p a r a t i o n system and are used t o recover P u ( I I I ) . Th and U . I n t h i s work, n a t u r a l U was used in place of 233y 2 3 3

e

Experimental Laboratory Scale Contactor Tests. The f e a s i b i l i t y of each p a r t o f the f l o w sheet was t e s t e d in s m a l l , commercially a v a i l a b l e m i x e r - s e t t l e r u n i t s which had 12 o r 16 stages w i t h i n d i v i d u a l mixer and s e t t l e r volumes of 15 mL and 49 mL, r e s p e c t i v e l y . T o t a l volume through-puts up t o 600 mL/h were a t t a i n a b l e w i t h 0-8412-0527-2/80/47-117-351$05.00/0 © 1980 American Chemical Society

352

ACTINIDE SEPARATIONS

good hydrodynamic performance. The general procedure was t o prepare a t y p i c a l s y n t h e t i c feed s o l u t i o n and run the c o n t a c t o r a t constant i n l e t c o n d i t i o n s u n t i l steady s t a t e was achieved, as i n d i c a t e d by mass balances. The tendency f o r the Th-TBP complex t o form a t h i r d phase a t h i g h c o n c e n t r a t i o n s when kerosene-type d i l u e n t s are used is w e l l known (_7). To counteract t h i s phenomenon, the d i l u e n t s used were e i t h e r pure d i e t h y l benzene (DEB) o r a commercial i s o p a r a f f i n i e kerosene to which s u f f i c i e n t DEB was added as m o d i f i e r . Since much of the f l o w sheet is s i m i l a r t o that o f a s t a n dard Thorex p r o c e s s i n g scheme, only those areas where changes were r e q u i r e d because of the i n c l u s i o n o f Pu w i l l be discussed in detail. Feed s o l u t i o n s f o r Contactor I t e s t s were prepared w i t h t h e approximate composition shown as the 1AF stream in F i g u r e 1. The heavy element c o n c e n t r a t i o n s are r e p r e s e n t a t i v e of those which would be obtained from d i s s o l v i n g i r r a d i a t e d Th-Pu f u e l . F l u o r i d e and aluminum were a l s o i n c l u d e d in the feed because they would be needed in an a c t u a l d i s s o l u t i o n step f o r ThC^ ( f l u o r i d e c a t a l y z e s the d i s s o l u t i o n , and aluminum counteracts the c o r r o s i v e ness o f the f l u o r i d e ) . For e f f i c i e n t Th e x t r a c t i o n , the a c i d i t y of the feed s o l u t i o n should be in the range of 2 t o 3 mol/L in HNO3. T h i s is a s i g n i f i c a n t departure from the a c i d Thorex process which uses an a c i d - d e f i c i e n t feed s o l u t i o n and is r e p o r t e d to .achieve improved decontamination from f i s s i o n products (8). However, a c i d - d e f i c i e n t feed s o l u t i o n s were considered u n d e s i r a b l e in our f l o w sheet because Pu hydrolyzes and tends t o polymerize at low a c i d i t y (9). The e f f e c t of the higher feed a c i d i t y used here on f i s s i o n product decontamination has not yet been e s t a b l i s h e d but w i l l be assessed in l a t e r experiments. Feed p r e p a r a t i o n a l s o i n c l u d e d Pu valency adjustment where NaNU2 o r NO + O2 gas was used t o convert all Pu t o the r e a d i l y e x t r a c t a b l e Pu(IV) s t a t e . Before being fed t o Contactor I I I , the aqueous feed s o l u t i o n was t r e a t e d t o reduce Pu t o the P u ( I I I ) s t a t e , t o prevent i t s ext r a c t i o n and ensure i t s recovery in the aqueous Pu product stream (3PP). The r e f e r e n c e reductant f o r the flow sheet was h y d r o x y l a mine n i t r a t e (HAN) a t 0.3 mol/L w i t h hydrazine n i t r a t e (0.1 mol/L) as a h o l d i n g agent o r HNO2 scavenger (10). Use o f HAN as Pu r e ductant was considered d e s i r a b l e because in r e a c t i o n w i t h Pu, as w e l l as in subsequent r e a c t i o n s where the reductant is destroyed, only gaseous products are formed (equations 1 and 2). 2 NH 0H + 2 P u

4 +

2

*-2 P u

3 +

+ N + 2H0 + 2H £

2

+

(1)

and 4 NH 0H + 2 HN0 2

3

*-3 N 0 + 7 H 0 2

2

(2)

HAN does not c o n t r i b u t e any s o l i d s t o the waste streams which must u l t i m a t e l y be t r e a t e d f o r d i s p o s a l . As w i l l be shown l a t e r , however, complete recovery of Pu w i t h the aqueous product stream was not achieved u s i n g HAN as reductant and s u b s t a n t i a l l o s s e s o f

1

AS

STRIP

2 SS

FL

=

1

2

Pu=lxlO' M HNO3 =2 M

U=8xIO~3M

I AW

AQUEOUS WASTE FISSfON PRODUCTS

I SE

30% TBP

FL= 6.7

EXT Ν

Figure 1.

EVAPORATOR

Pu REDUCTION

I

|PART'N|

4 TP

PRODUCT

PRODUCT 3 PP

AQUEOUS Th

4SE

30% TBP

FL=6.7

3SR

AQUEOUS Pu

3 SE

30% TBP

FL = 6.7

EXT'N

IE

SCRUB

FL = I3.4

3

4 AS

5 UP

PRODUCT

AQUEOUS U

STRIP

FL = I3.4

3

5 AS HN0 = 0.0IMi

4SP

Η Ν 0 = 0.5M

Modified Thorex flowsheet for Th-U-Pu fuel

AQUEOUS PRODUCT Th=0.086M 2AP

FL =

Th = 0.86M

SCRUB 3 AF

1

Th = 0.86M HNO3 = I.5M RED. AGENT

=

TO SOLVENT RECOVERY

FL = I

FL = 10

IAF

FL

3

HN0 = 0.5M

RED. AGENT

s

RED. AGENT

3

3 AS HN0 O.IM

PRODUCT

3

2 AS HN0 = O.OIM

ISP

SOLVENT

5 SS

TO SOLVENT RECOVERY

Η

1

ο

CO

r

>

η

354


Pu w i t h the s o l v e n t i n v a r i a b l y occurred. This l o s s was a t t r i b u t e d to o x i d a t i o n of P u ( I I I ) t o Pu(IV) in the organic phase as has a l s o been observed in P u r e x - r e l a t e d t e s t s (10, 1 1 ) . Two a d d i t i o n a l Contactor I I I t e s t s were performed as f o l l o w s : - Plutonium was reduced in the feed s o l u t i o n w i t h hydrazines t a b i l i z e d HAN and an o r g a n i c - s o l u b l e reducing agent, 2, 5 d i tert-amylhydroquinone (DAH2Q) (12), d i s s o l v e d in the s o l v e n t (0.03 mol/L) was used to reduce any Pu(IV) formed in the contactor. - The feed s o l u t i o n (3AF) was t r e a t e d w i t h f e r r o u s sulfamate (FeSA) t o reduce Pu to P u ( I I I ) and FeSA was a l s o added w i t h the scrub stream (3AS) t o reduce any Pu(IV) formed in the scrub sect i o n . The c o n c e n t r a t i o n of FeSA in both s o l u t i o n s was 0.05 mol/L and both were s t a b i l i z e d w i t h 0.05 mol/L s u l f a m i c a c i d . Solvent E x t r a c t i o n Computer Code. For modelling the e x t r a c t i o n behaviour of the a c t i n i d e s and n i t r i c a c i d in the f i v e c o n t a c t o r s of our flow sheet, a s o l v e n t e x t r a c t i o n computer code c a l l e d SECTOR has been developed. I tisa m o d i f i c a t i o n of SEPHIS (13), a code developed a t Oak Ridge N a t i o n a l L a b o r a t o r i e s (ORNL) f o r the r e p r o c e s s i n g of f u e l by the Purex process. Attempts t o develop SECTOR based on a chemical model t o d e s c r i b e the e x p e r i mental d i s t r i b u t i o n data had some l i m i t e d success, however, the c u r r e n t v e r s i o n r e l i e s on a mathematical d e s c r i p t i o n of the d i s t r i b u t i o n data. The v a r i a b l e s used in d e r i v i n g the c o r r e l a t i o n s f o r the v a r i o u s d i s t r i b u t i o n r a t i o s , D ^ D j , DHN03> e t c . , are the concentrations of Th and HNO3 in tne aqueous phase. I t was assumed that U and Pu, being present a t low concentrations o n l y , would not a f f e c t D - j ^ o r DHN03* ^ °f i e m p i r i c a l nature, any change in the parameters d e s c r i b i n g the aqueous phase cannot be accommodated by the c u r r e n t model, e.g., a d d i t i o n of a s a l t i n g agent such as NaN03, or HAN, o r a change in o p e r a t i n g temperature r e q u i r e s remeasurement of d i s t r i b u t i o n data and d e r i v a t i o n o f new c o r r e l a t i o n s . Nevertheless, t h i s v e r s i o n of SECTOR has been extremely u s e f u l in i n t e r p r e t i n g some of the experimental cont a c t o r data obtained. A d d i t i o n a l changes i n c o r p o r a t e d i n t o SECTOR to account f o r the o x i d a t i o n - r e d u c t i o n r e a c t i o n s o c c u r r i n g w i t h Pu i n c l u d e d : - Simultaneous r e d u c t i o n of Pu(IV) (when a reductant was present) in the aqueous phase of a mixer, and r e d i s t r i b u t i o n o f both P u ( I I I ) and Pu(IV) between the two phases, f o r a time equal to the average residence time in the mixer. - Further r e d u c t i o n in the aqueous phase of any remaining Pu(IV) in the s e t t l e r , f o r a time equal to the average residence time in the s e t t l e r . No f u r t h e r r e d i s t r i b u t i o n of Pu species was assumed t o occur in the s e t t l e r . - O x i d a t i o n of P u ( I I I ) t o Pu(IV) in the organic phase o f a s e t t l e r t o an extent assumed t o be p r o p o r t i o n a l to the P u ( I I I ) and HNO3 concentrations of the organic phase, i . e . , T

e c a u s e

S

T

t s

25.

GRANT E T A L .

A[Pu(IV)]

0

355

Heavy Element Separation

= A [HN0 ] 3

o

· [Pu(III)]

(3)

0

where A is an e m p i r i c a l l y defined p r o p o r t i o n a l i t y constant, and [ ] represents concentrations of the species (mol/L)int h e organic phase. 0

R e s u l t s and D i s c u s s i o n Contactor I Data. T y p i c a l experimental c o n c e n t r a t i o n p r o f i l e s f o r Th, HNO3 and Pu are shown in Figures 2, 3 and 4 r e s p e c t i v e l y , together w i t h the p r o f i l e s c a l c u l a t e d u s i n g SECTOR. A l s o shown in the f i g u r e s are schematic diagrams o f the c o n t a c t o r and the a c t u a l feed concentrations used. The thorium p r o f i l e (Figure 2) shows that Th is e x t r a c t e d e f f i c i e n t l y and t h a t l o s s t o the aqueous waste stream is reduced to < 0.1% in f i v e e x t r a c t i o n stages ( i . e . , stages 7 to 11). The p r o f i l e f o r HNO3 (Figure 3) shows that t h i s s o l u t e r e f l u x e s c o n s i d e r a b l y at the chosen o p e r a t i n g c o n d i t i o n s . This is due to a p p r e c i a b l e e x t r a c t i o n of the HNO3 over stages 10 to 16 where the Th c o n c e n t r a t i o n is low, and somewhat poorer e x t r a c t a b i l i t y over stages 1 t o 9 where the Th c o n c e n t r a t i o n is h i g h e r . This gives a peak aqueous a c i d i t y of more than 4 mol/L compared to an average a c i d i t y of ^ 2 mol/L based on the two i n l e t streams (HNO3 is 3 mol/L and 0.5 mol/L in 3AF and 3AS r e s p e c t i v e l y ) . The experimental Pu c o n c e n t r a t i o n p r o f i l e shown in F i g u r e 4 i n d i c a t e s that Pu is e x t r a c t e d e f f i c i e n t l y over only the first few e x t r a c t i o n stages, and then is e x t r a c t e d q u i t e i n e f f i c i e n t l y over the remaining stages. This poorer e x t r a c t a b i l i t y may be due in p a r t to h y d r o l y s i s of P u ( I V ) , but it is b e l i e v e d to be mainly due to the presence of a s m a l l amount of P u ( I I I ) in the feed solution. In n i t r i c a c i d s o l u t i o n s , P u ( I I I ) is o x i d i z e d according t o the f o l l o w i n g equation (14): 2 Pu

3 +

+

+ 3 H + N0~ ^

^ 2 Pu

4 +

+ HN0 + H 0 2

(4)

2

N i t r o u s a c i d is not only a product of the r e a c t i o n but it a l s o c a t a l y z e s t h i s o x i d a t i o n . However, i t s use f o r Pu valency a d j u s t ment i n e v i t a b l y s h i f t s the e q u i l i b r i u m to the l e f t and r e s u l t s in some P u ( I I I ) remaining in the feed s o l u t i o n . The e q u i l i b r i u m f o r the r e a c t i o n is described by: 1

[Pu(III)] . [ H * ] '

5

0

· [NOT] -

K-

5

(5) U

[Pu(IV)] · [ H N 0 ] -

3

2

Using a value of Κ = 0.93 (mol/L) ^'~* (14), we estimate t h a t in the t y p i c a l feed s o l u t i o n s used in these t e s t s up to 2% of the Pu may be present as P u ( I I I ) . In Contactor I , Pu(IV) is r e a d i l y e x t r a c t e d but the P u ( I I I )

356

ACTINIDE

STAGE

SEPARATIONS

NUMBER

Figure 2. Thorium concentration profile and distribution ratio for Contactor I: (Φ) aqueous phase; (O) solvent phase; (χ) distribution ratio (D n); ( ) calcu lated. T

GRANT E T A L .


Figure 3. Nitric acid concentration profile and distribution ratio for Contactor I: (Φ) aqueous phase; (O) solvent phase; (χ) distribution ratio (Ό χ ); ( j, cal culated. Η

θ3

358

ACTINIDE

Figure 4.

SEPARATIONS

Plutonium concentration profile for Contactor I: (Φ) aqueous phase; (O) solvent phase; ( ) calculated.

25.

GRANT E T A L .


359

tends t o remain w i t h the aqueous phase. N i t r o u s a c i d is a l s o e x t r a c t e d by the s o l v e n t (15), so that the e q u i l i b r i u m should s h i f t t o the r i g h t . However, the o x i d a t i o n of P u ( I I I ) by n i t r i c a c i d takes place o n l y s l o w l y when HNO2 is no longer a v a i l a b l e in the aqueous phase t o c a t a l y z e the r e a c t i o n , and the r e s u l t is a s m a l l l o s s of Pu w i t h the aqueous waste stream (^ 0.1% in the case shown). Contactor I I . R e s u l t s f o r Contactor I I were e x a c t l y as expected, i . e . , all a c t i n i d e s were e a s i l y s t r i p p e d from the organic phase in about f i v e stages. With the low a c i d i t y used in the 2AS stream, HAN reduced Pu e a s i l y and prevented any p o l y m e r i z a t i o n o f Pu(IV). The r e l a t i v e l y h i g h flow r a t i o used here (aqueous t o o r ganic, A/0 = 1.5) was necessary t o prevent r e f l u x i n g o f U. A l though not a problem in t h i s work, where n a t u r a l U was used, r e f l u x i n g could be o f concern f o r separations i n v o l v i n g f i s s i l e U isotopes. Contactor I I I Data. A t y p i c a l experimental Pu c o n c e n t r a t i o n p r o f i l e f o r Contactor I I I where h y d r a z i n e - s t a b i l i z e d HAN was used as reductant is shown in F i g u r e 5. Plutonium in the feed s o l u t i o n was i n i t i a l l y reduced t o P u ( I I I ) using HAN. At a c i d i t i e s o f l e s s than 1.5 mol/L HNO3, more than 99% of the Pu is reduced, and it should be i n e x t r a c t a b l e . The c a l c u l a t e d p r o f i l e f o r both phases f o r P u ( I I I ) is shown as dashed l i n e s on the p l o t . F o r stage numbers l e s s than 10, the Pu is p r e d i c t e d to be e a s i l y scrubbed from the s o l v e n t . E x p e r i m e n t a l l y , the behaviour was very d i f f e r e n t and s u b s t a n t i a l amounts of Pu were e x t r a c t e d and c a r r i e d w i t h the s o l v e n t , l e a d i n g t o a l o s s of ^ 12% w i t h the 3SP stream. This behaviour is a t t r i b u t e d t o the o x i d a t i o n o f some o f the P u ( I I I ) in the organic phase and an e x c e s s i v e l y low r e d u c t i o n r a t e o f Pu(IV) by HAN which operates only on the aqueous phase. To account f o r t h i s in the computer c a l c u l a t i o n s , an o x i d a t i o n step was i n c o r p o r a t e d i n t o the SECTOR code as described e a r l i e r . The p r o p o r t i o n a l i t y constant A of equation (3) was determined e m p i r i c a l l y to be that which gave the best f i t f o r a number o f e x p e r i m e n t a l l y determined p r o f i l e s obtained under v a r i o u s c o n d i t i o n s of a c i d i t y and heavy element c o n c e n t r a t i o n . When using HAN as reductant, a value o f A = 1.5 L/mol per u n i t o f s e t t i n g time appears to be most a p p r o p r i a t e . The c a l c u l a t e d p r o f i l e i n c o r p o r a t i n g the o x i d a t i o n step is a l s o shown in F i g u r e 5 as s o l i d l i n e s . Note that the shape o f the experimental p r o f i l e is reproduced extremely w e l l , u s i n g only t h i s s i n g l e a d j u s t a b l e parameter in the pseudo k i n e t i c expression f o r the Pu(III) oxidation. In c o n t r a s t t o t h i s , McCutcheon et a l (16) have used k i n e t i c expressions f o r up t o f i v e aqueous phase r e a c t i o n s i n v o l v i n g HAN, 2 4> 2 > HNO3 and Pu in t h e i r s i m u l a t i o n model and have obtained s i m i l a r l y shaped c o n c e n t r a t i o n p r o f i l e s f o r Pu. However, the. o x i d a t i o n o f P u ( I I I ) in the organic phase was not i n c l u d e d in N

H

H N 0

360

ACTINIDE

ΙΟ

SEPARATIONS

9

STAGE

NUMBER

Figure 5. Plutonium concentration profile and distribution ratioinContactor III using HAN as reductant: (%) aqueous phase; (O) solvent phase; (χ) distribution ratio (D ); ( ) calculated profile for Pu (III); ( ) calculated; A = 1.5 (see text). Pu

25.

GRANT E T A L .

361


t h e i r model. Barney has shown that the r e d u c t i o n r a t e of Pu(IV) by HAN is retarded very s e v e r e l y a t higher a c i d i t i e s , being i n v e r s e l y prop o r t i o n a l t o the f o u r t h power of [H ] (17) as shown in equation (6): +

d[Pu(IV)]

[NH3OHV - [ P u ( I V ) ]

k

[ H ] . (1 + [N0 ].)

d t

+

4

3

2

2

· [Pu(III)]

w

2

Therefore, i f the feed a c i d i t y is decreased, the r e d u c t i o n r a t e should be enhanced and lower Pu l o s s e s should be achieved. This was confirmed by experiment where the l o s s was decreased t o ^ 2% Pu a t a feed a c i d i t y o f 0.8 mol/L. The c a l c u l a t e d e f f e c t of feed a c i d i t y on Th and Pu l o s s e s from Contactor I I I is shown in F i g u r e 6. The Pu o x i d a t i o n model was used in d e r i v i n g these r e s u l t s . An a c i d i t y of l e s s than 0.6 mol/L would apparently be r e q u i r e d t o l i m i t Pu l o s s e s to ^ 0.1%. At the same time, Th l o s s e s t o the Pu product stream would be over 10%. Experimentally thorium l o s s was reduced t o acceptable l e v e l s by adding a s a l t i n g agent (NaN0 ) t o the 3AF and 3AS streams, but t h i s s o l u t i o n u n f o r t u n a t e l y e l i m i n a t e s one o f the major advantages o f using HAN as reductant, v i z , a decrease in the amount of s o l i d wastes which must be t r e a t e d . S i m i l a r c a l c u l a t i o n s f o r a U-Pu s e p a r a t i o n using HAN as reductant and the flow r a t i o s of our modified Thorex flow sheet i n d i c a t e that about the same Pu l o s s behaviour should be expected. However, f o r f l o w c o n d i t i o n s more appropriate to the Purex process, n e g l i g i b l e Pu l o s s e s are p r e d i c t e d . Uranium l o s s e s are p r e d i c t e d to be n e g l i b l e f o r both s e t s o f c o n d i t i o n s . These p r e d i c t i o n s were a l s o confirmed in contactor t e s t s . Because the Pu o x i d a t i o n appeared to be t a k i n g place in the organic phase and the reductants u s u a l l y employed operate only in the aqueous phase, it was f e l t that Pu l o s s e s could be diminished by using an o r g a n i c - s o l u b l e reductant, e.g., 2,5 d i - t e r t - a m y l h y droquinone (DAH2Q)(12) d i s s o l v e d in the e x t r a c t a n t . Plutonium in the feed s o l u t i o n was i n i t i a l l y reduced t o P u ( I I I ) w i t h hydrazines t a b l i l i z e d HAN, but, in the one run which was completed, the r e d u c t i o n d i d not go t o completion and the feed s o l u t i o n a c t u a l l y contained ^ 12% Pu(IV). However, s i n c e there was a l a r g e molar excess of reductant over Pu(IV), very l i t t l e Pu l o s s was expected. During the first s e v e r a l hours o f the run, the reductant appeared to be f u n c t i o n i n g s a t i s f a c t o r i l y , however, Pu l o s s e s w i t h the 3SP stream s l o w l y increased u n t i l they reached about 5% a f t e r 14 hours o f o p e r a t i o n . The r e d u c t i o n o f Pu(IV) w i t h DAH Q proceeds according t o the reaction: 3

2

362

ACTINIDE

SEPARATIONS

NITRIC ACID IN 3AF (M) Figure 6. Calculated Pu and Th loss from Contactor III as function of feed acidity: A = 1.5 (see text); ( ) Pu loss with 3SP stream; ( ) Th loss with 3PP stream.

25.

GRANT E T A L .


363

OH R

^

. ..4+

J U R ,

2 P u

3+

+ 2 H

+

( ? )

OH where R represents the t e r t i a r y amyl group. This i n d i c a t e s that as a c i d i t y i n c r e a s e s in t h i s contactor (e.g., due t o r e f l u x i n g , in the same manner as seen f o r Contactor I ) r e d u c t i o n is impeded. A competing r e a c t i o n w i t h DAH Q is then p o s t u l a t e d t o become more important: 2

OH R

HN0 + H 0 2

(8)

2

OH We have i d e n t i f i e d HN0 as a r e a c t i o n product in HN03~DAH Q-TBP systems by the c h a r a c t e r i s t i c spectrum of the HN0 'TBP complex in the organic phase (18). We have a l s o obtained evidence that D A H 2 Q can r e a c t w i t h HN0 and t h e r e f o r e might a l s o be considered f o r use as a h o l d i n g reductant under c e r t a i n c o n d i t i o n s . A l though the r e a c t i o n product was not i d e n t i f i e d , n i t r o s a t i o n of the r i n g seems p l a u s i b l e . However, a t high a c i d i t y , r e a c t i o n (8) appears t o become predominant and the r e s u l t i n g HN0 probably leads t o the a u t o c a t a l y t i c o x i d a t i o n of P u ( I I I ) and the l o s s of Pu(IV). Other p o s s i b l e u n d e s i r a b l e s i d e r e a c t i o n s may a l s o have taken p l a c e in t h i s contactor run i n v o l v i n g HAN or hydrazine and the quinone, e.g., oximes or hydrazones could have been produced (19). Because o f the problems encountered w i t h the other reduct a n t s , some Contactor I I I t e s t s were conducted w i t h f e r r o u s s u l famate, the most commonly used Pu reductant. These t e s t s were s u c c e s s f u l and very low Pu l o s s e s were achieved as shown by the data in F i g u r e 7. Two s e t s of SECTOR-calculated p r o f i l e s a r e a l s o shown in the f i g u r e ; the dashed l i n e s were obtained when an o x i d a t i o n step was i n c l u d e d , whereas the s o l i d l i n e s are t h e r e s u l t of assuming zero o x i d a t i o n . For the o x i d a t i o n case it was assumed that any Pu(IV) produced is reduced r a p i d l y in t h e aqueous phase by F e ( I I ) according t o equation 9: 2

2

2

2

2

P u

*

+

+ Fe

2

+

^ - Pu

3 +

+ Fe

3 +

(9)

the k i n e t i c s o f which have been shown by Rozen e t a l . (20) to be described by: -

d

[

?

^

I

V

)

]

= \

· [Pu(IV)] - k

2

· [Pu(III)]

(10)

2+ 4+ Because o f the l a r g e molar excess of Fe over any Pu in the system, we have assumed that the second term (which Rozen i n c l uded because of the back r e a c t i o n ) could be neglected. The constant k^ was evaluated from:

364

ACTINIDE

2

4

6

8

STAGE

10

12

14

SEPARATIONS

16

NUMBER

Figure 7. Plutonium concentration profile for Contactor III using ferrous sulfamate as reductant: (Φ) aqueous phase; (O) solvent phase; ( ) calculated, A = 0.25 (see text); ( ) calculated, A = 0.0 (see text).

25.

GRANT E T A L .

k

1620 [ F e ( I I ) ]

=

1

365


(

n

)

+

(1 + 2.9[N0~])[H ]

I t is apparent that i f o x i d a t i o n o f P u ( I I I ) in the organic phase does occur, the extent o f o x i d a t i o n is much l e s s than when HAN is used as reductant. For the case p l o t t e d , the o x i d a t i o n was assumed to be only ^ 1/6 of that which gave the best f i t t o the HAN data, ( i . e . , A = 0.25) and even t h i s amount o f o x i d a t i o n appears t o be excessive when compared w i t h the experimental data. In f a c t the data i n d i c a t e that o x i d a t i o n of Pu may be i n h i b i t e d completely in t h i s system. The success o f FeSA may be due in part t o the f a c t that s u l f a m i c a c i d is s o l u b l e in the organic phase t o a s m a l l extent (D ^ 10~2) d t h e r e f o r e may operate as an HNO2 scavenger in both phases. We have determined s p e c t r o p h o t o m e t r i c a l l y that HNO2 in the organic phase is consumed a t a f a s t e r r a t e when it is mixed w i t h solvent p r e v i o u s l y contacted w i t h s u l f a m i c a c i d (see F i g u r e 8 ) , although the c o n c e n t r a t i o n s a t which the e f f e c t was observed are higher than those a p p l i c a b l e t o t y p i c a l process s o l u t i o n s . The constants (k) given f o r the three curves are the apparent first order r a t e constants f o r HNO2 d e s t r u c t i o n . a

n

O x i d a t i o n o f Pu in Organic Phase. Two sets of shaker-tube experiments were designed t o o b t a i n a d d i t i o n a l i n f o r m a t i o n on the o x i d a t i o n of P u ( I I I ) in the organic phase. In the first, t y p i c a l 3AF feed s o l u t i o n c o n t a i n i n g P u ( I I I ) and h y d r a z i n e - s t a b i l i z e d HAN was e x t r a c t e d w i t h 30% TBP s o l u t i o n . The organic phase c o n t a i n i n g the e x t r a c t e d P u ( I I I ) was then r e e q u i l i b r a t e d a f t e r v a r i o u s standing times w i t h an aqueous phase c o n t a i n i n g 1.5 mol/L HNO3 and 0.1 mol/L hydrazine (the l a t t e r was added to minimize any e f f e c t due t o HNO2), and the d i s t r i b u t i o n r a t i o o f Pu was measured. The r e s u l t s are shown in Figure 9. A f t e r an i n d u c t i o n p e r i o d of 15-20 minutes, D p rose sharply i n d i c a t i n g r a p i d o x i d a t i o n of the e x t r a c t e d P u ( I I I ) to Pu(IV). The S-shaped curve a l s o suggests that the organic-phase o x i d a t i o n , l i k e that in the aqueous phase, may be a u t o c a t a l y t i c and t h e r e f o r e f a r more complicated than that assumed by the simple o x i d a t i o n step used in SECTOR. The e f f e c t of n i t r o u s a c i d on o x i d a t i o n o f P u ( I I I )int h e organic phase was assessed in the second s e r i e s of experiments. P o r t i o n s of aqueous feed s o l u t i o n c o n t a i n i n g P u ( I I I ) as above were shaken w i t h 30% TBP s o l u t i o n s which contained i n c r e a s i n g amounts of n i t r o u s a c i d . The measured Pu d i s t r i b u t i o n r a t i o s are shown as a f u n c t i o n of shaking time in Figure 10. They i n d i c a t e that q u i t e s m a l l concentrations of HNO2 can a f f e c t D p s i g n i f i c a n t l y and the t h r e s h o l d f o r enhanced o x i d a t i o n of Pu in these t e s t s was between 10 and 100 μπιοΙ/L of HNO2. These observations on o x i d a t i o n of P u ( I I I ) in the organic phase are c o n s i s t e n t w i t h those in reference (11) where U(IV) u

u

366


Ί

1

1—

1

" ΊΓ ~Ι~χ~ 10^ TrnTn) " 4

-7

3

ΙΟ"

9

θ

k * 2.6 χ Ι Ο " (min" )

7

2

_L

JL

70

8 0

4

ΙΟ"

10

2 0

3 0

4 0

50

TIME

6 0

1

9 0

1 0 0 110

(MINUTES)

Figure 8. Effect of sulfamic acid (SA) on HN0 destructioninsolvent phase: ( ) no SA present; (O) solvent preequilibrated with 0.5 mol/L SA and 0.5 mol/L HNO ; (%) solvent preequilibrated with 1.0 mol/L SA and 0.5 mol/L HNO . 2

s

s

25.

GRANT E T AL.


367

Figure 10. Effect of nitrous acid in solvent on distribution ratio of Pu; solvent preequilibrated with 0.1 mol/L HN0 and NaN0 at the following concentra tions (mol/L): (·) ΙΟ", (Π) 10\ (M) M' , (A)10~ , (A) 10 . 3

5

2

3

2

1

368


r a t h e r than HAN was used as the reductant. Evidence of P u ( I I I ) o x i d a t i o n in contactor t e s t s of the Purex p a r t i t i o n i n g step u s i n g HAN was a l s o obtained by Richardson (10). Contactors IV and V. The p a r t i t i o n i n g of Th from U in Contactor IV was achieved r e l a t i v e l y e a s i l y although the s o l v e n t product (4SP) contained somewhat more Th than d e s i r e d . The flow r a t i o s and a c i d i t i e s used here were d i f f e r e n t from those used in the standard Thorex process and were chosen to ensure that U r e f l u x i n g would not occur. O p t i m i z a t i o n of the flow sheet which has not been attempted to any extent may w e l l lead t o changes in o p e r a t i n g c o n d i t i o n s and improved s e p a r a t i o n . S t r i p p i n g of U from Contactor V was e a s i l y achieved. Summary of R e s u l t s - The flow sheet presented here has been shown, in l a b o r a t o r y t e s t s u s i n g m i x e r - s e t t l e r s , to be s u i t a b l e f o r s e p a r a t i n g the the three a c t i n i d e s , Th, U and Pu from each other. - S u c c e s s f u l p a r t i t i o n i n g of P u ( I I I ) from Th was achieved when f e r r o u s sulfamate was used as the Pu-holding r e d u c t a n t . - The flows and a c i d i t i e s r e q u i r e d f o r recovery of Th w i t h the s o l v e n t stream from Contactor I I I are such that hydroxylamine n i t r a t e cannot prevent the o x i d a t i o n of P u ( I I I ) which occurs predominantly in the o r g a n i c phase in the s e t t l e r s , and Pu l o s s e s are s u s t a i n e d . - The o x i d a t i o n of P u ( I I I ) in the o r g a n i c phase and i t s en hancement by the presence of HNO2 in the o r g a n i c phase was conf irmed. - SECTOR, a m o d i f i c a t i o n of the computer code SEPHIS, has been used s u c c e s s f u l l y to c a l c u l a t e s o l u t e behaviour in mixers e t t l e r experiments. I t a l s o accounts f o r the o x i d a t i o n of P u ( I I I ) in the organic phase although improvements to t h i s p a r t of the code could be made when the k i n e t i c s of the o x i d a t i o n are e s t a b l i s h e d . Acknowledgements We wish to thank L . J . Clegg, R.J. P o r t h , D.G. Juhnke of the Chemical Technology Branch, and R.W. Dyck, C. Murphy and t h e i r c o l l e a g u e s in the A n a l y t i c a l Science Branch f o r t h e i r c o n t r i b u t i o n s to the above work. Literature Cited 1.

2.

Critoph, E., Banerjee, S., Barclay, F.W., Hamel, D., Milgram, M.S., Veeder, J.I., Atomic Energy of Canada Limited Report, AECL-5501 (1976). Critoph, Ε., Atomic Energy of Canada Limited Report, AECL5705 (1977).

25.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

GRANT

ET

AL.


369

Morgan, W.W., Atomic Energy of Canada Limited Report, AECL508 (1958). Ryon, A.D., Oak Ridge National Laboratory Report, ORNL-3045 (1961). Schulz, W.W., Pacific Northwest Laboratory Report, BNWL-57 (1965). Smee, J.L., Clegg, L.J. Juhnke, D.G., and Porth, R.J., unpublished data. F a r r e l l , M.S. and Goldrick, J.D., AAEC/E26 (1958). Rainey, R.H. and Moore, J.G., Nucl. Sci. & Eng., 1961, 10, 367. Schuelein, V.L. ARH-SA-233 (1975). Richardson, G.L. and Swanson, J.L., Hanford Engineering Development Laboratory Report, HEDL-TME-75-31 (1975). Biddle, P., McKay, H.A.C. and Miles, J.H., in "Solvent Extraction Chemistry of Metals", MacMillan, London, 1965. Grossi, G., International Solvent Extraction Conf., Toronto, 1977. Groenier, W.S. Oak Ridge National Laboratory Report, ORNL4746 (1972). Koltunov, V.S. and Marchenko, V.I., Soviet Radiochem., 1973, 15, 754. Gourisse, D., Paper 106, "Proc. Int. Solv. Extr. Conf.", The Hague, 1971, Soc. of Chemical Industry, London, 1971. McCutcheon, E.B., Burkhart, L.E., and Felt, R.E., Advances in Instrum., 1975, 30, 716. Barney, G.S., J. Inorg. and Nuc. Chem., 1976, 38, 1677. Woodhead, J., AERE-R-3432 (1960). Fuson, R.C., "Reactions of Organic Compounds", John Wiley & Sons, 1966. Rozen, A.M., Zel'Venskii, M. Ya., Shilin, I.V., Translated from Atomnaya Energiya, 1975, 38, 367.

RECEIVED

May

11,

1979.

Heavy Element Separation for Thorium-Uranium-Plutonium Fuels

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