Plutonium Metal Production and Purification at Los Alamos - ACS

26 Plutonium Metal Production and Purification at Los Alamos

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch026

D. C. CHRISTENSEN and L. J. MULLINS Los Alamos National Laboratory, Material Science and Technology Division, Los Alamos, NM 87545

The production of plutonium metal by both fluoride and oxide reduction is well established at Los Alamos. The subsequent purification of this metal by electrorefining is now being performed in production on a 6-kg batch scale. The objective is the production of high-purity plutonium metal. Recent process development efforts have been devoted to more expeditious and less costly pyrochemical reprocessing of residues created by the metal preparation and purification process. We intend to establish an internal recycle which yields either reusable or discardable residues and recovers a l l plutonium for feed to the electrorefining purification system. This internal recycle is to be performed in a more timely and less costly operation than in the present reprocessing mode.

The L o s Alamos N a t i o n a l L a b o r a t o r y h a s h a d a v e r y a c t i v e p r o gram f o r t h e p r o d u c t i o n o f h i g h p u r i t y p l u t o n i u m m e t a l f o r b o t h L a b o r a t o r y and n a t i o n a l needs f o r many y e a r s u s i n g p y r o c h e m i c a l techniques. P l u t o n i u m p y r o c h e m i s t r y a t L o s Alamos d a t e s b a c k t o 1 9 5 6 . D u r i n g t h e p e r i o d 1 9 5 6 - 1 9 6 2 , t h e P l u t o n i u m C h e m i s t r y and M e t a l l u r gy g r o u p was i n v o l v e d i n a p r o g r a m o n t h e p y r o c h e m i c a l p r o c e s s i n g o f P l u t o n i u m F a s t B r e e d e r R e a c t o r F u e l s f o r t h e L o s Alamos M o l t e n P l u t o n i u m R e a c t o r E x p e r i m e n t (LAMPRE). P r o c e s s e s s u c h as o x i d a t i v e s l a g g i n g , h a l i d e s l a g g i n g , p y r o r e d o x , l i q u a t i o n , and e l e c t r o r e f i n i n g were spawned i n t h i s p e r i o d . T h e s e p r o c e s s e s were t u r n e d t o w a r d n a t i o n a l d e f e n s e p r o g r a m s i n 1 9 6 4 . I n 1976 t h e d i r e c t o x i d e r e d u c t i o n (DOR) was d e v e l o p e d i n s u p p o r t o f t h e a r t i f i c i a l h e a r t , plutonium-238 program. I n 1 9 7 8 , t h e DOR p r o c e s s was a d o p t ed f o r u s e i n p l u t o n i u m - 2 3 9 p r o g r a m s . T h i s o p e r a t i o n gave us two p r i n c i p a l p r o c e s s e s f o r making p l u t o n i u m m e t a l .

0097-6156/83/0216-0409$06.75/0 © 1983 American Chemical Society

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

PLUTONIUM CHEMISTRY

410

1. The c o n v e n t i o n a l P u F r e d u c t i o n p r o c e s s . 2. D i r e c t o x i d e r e d u c t i o n p r o c e s s . (DOR). I n a d d i t i o n t h e r e a r e two p r i n c i p l e m e t a l p u r i f i c a t i o n cesses :


4

pro

1. E l e c t r o r e f i n i n g ( E R ) . 2. H a l i d e S l a g g i n g o r M o l t e n S a l t E x t r a c t i o n (MSE). B e g i n n i n g i n 1978, t h e above f o u r p r o c e s s e s were p u t i n t o p r o d u c t i o n i n t h e new TA-55 b u i l d i n g a t L o s A l a m o s . The s o u r c e o f p l u t o n i u m f o r Los Alamos needs was b o t h p l u t o n i u m d i o x i d e f r o m aqueous r e c o v e r y p r o c e s s e s and m e t a l f r o m f a b r i c a t i o n s c r a p . I n 1980, Los Alamos embarked on a p r o g r a m t o i n c r e a s e t h e o u t p u t o f pure m e t a l . P y r o c h e m i c a l p r o c e s s e s were s e l e c t e d as t h e c a n d i d a t e o p e r a t i o n s t o r a p i d l y and c o s t e f f e c t i v e l y a c h i e v e t h e i n creased throughput. This type of p r o c e s s i n g o f f e r s the p o t e n t i a l o f l a r g e c o s t r e d u c t i o n o v e r t r a d i t i o n a l aqueous c h e m i c a l p r o c e s s e s f o r a number o f r e a s o n s . • P l u t o n i u m i s p r o c e s s e d as a l i q u i d , i n h i g h l y compact e q u i p m e n t , and t h e r e f o r e e x p e n s i v e f l o o r s p a c e i s c o n s e r v e d . • V e r y few s t e p s a r e needed i n o r d e r t o a c h i e v e h i g h l y p u r i f i e d products (Figure 1). • P r i m a r y w a s t e g e n e r a t i o n i s s m a l l b e c a u s e o f t h e h i g h den s i t y nature of the fused s a l t operations. Nearly a l l p r i mary w a s t e r e s i d u e s show e x c e l l e n t p o t e n t i a l f o r r e c y c l e . (The p r e s e n t m a i n p r o c e s s i n g sequence f o r p l u t o n i u m does n o t i n c l u d e s i g n i f i c a n t r e a g e n t and r e s i d u e r e c y c l e ) . • S e c o n d a r y w a s t e f r o m t h e p r o c e s s i n g o f w a s t e r e s i d u e ma t e r i a l s i s minimal. • The t u r n a r o u n d o f p l u t o n i u m i n r e s i d u e s i s v e r y r a p i d . The expanded t h r o u g h p u t r e s u l t e d i n a commensurate i n c r e a s e i n p r i m a r y r e s i d u e g e n e r a t i o n . The i n c r e a s e d p r o d u c t i o n o f p l u t o n i u m m e t a l was a c h i e v e d v e r y e a s i l y i n e x i s t i n g f l o o r s p a c e be c a u s e o f t h e compact n a t u r e o f e q u i p m e n t , b u t t h e i n c r e a s e d needs f o r aqueous r e s i d u e h a n d l i n g c o u l d n o t be met w i t h e x i s t i n g f l o o r space. As a r e s u l t , t h e r e s i d u e s were s t o r e d f o r f u t u r e p r o c e s s ing. I n o r d e r t o s u p p o r t t h e p r o c e s s s c a l e - u p , R&D e f f o r t s have been i n v o l v e d w i t h d e v e l o p i n g c o s t - e f f e c t i v e , h i g h - t h r o u g h p u t py r o c h e m i c a l p r o c e s s e s f o r h a n d l i n g p r o c e s s m a t e r i a l s and r e s i d u e s . I n a d d i t i o n , we a r e e v a l u a t i n g p r o b l e m s i n t h e a r e a s o f p r o c e s s and e q u i p m e n t d e s i g n have b e e n e v a l u a t e d . M o s t r e c e n t e f f o r t s a r e c o n c e r n e d w i t h t h e r e c o v e r y and r e c y c l e o f p l u t o n i u m v a l u e s i n the residues. T h i s document w i l l summarize o u r p r e s e n t m a i n p r o d u c t i o n s e quence and d i s c u s s how i t was a r r i v e d a t . I t w i l l t h e n d i s c u s s the s t a t u s of our present r e c y c l e o f plutonium values i n r e s i d u e s . T h i r d , i t w i l l d i s c u s s our proposed r e c y c l e of a l l plutonium i n residues. F i n a l l y , i t w i l l d i s c u s s our goal of a f u l l y i n t e g r a t e d p r o c e s s s e q u e n c e where p l u t o n i u m and s a l t r e s i d u e s a r e r e c y c l e d through the p r o d u c t i o n sequence. 1


CHRISTENSEN


26.

AND MULLINS

IMPURE Pu METAL

j

I I I

CASTING

' ELECTROREFINING

Pu Production and Purification

OXIDATION

IMPURE Pu0

m

„

411

DISSOLUTION

2

CALCINATION

OXALATE PRECIPITATION

ION EXCHANGE

Pu0

DISSOLUTION

PEROXIDE PRECIPITATION

PuF

CALCINATION HYDROFLUORΙΝΑΤΙΟΝ

J

2

i j I FLUORIDE REDUCTION

4

P U R E Pu M E T A L F i g u r e 1. cation.

P y r o c h e m i c a l v s . aqueous

flowsheet f o r metal


purifi-

412

PLUTONIUM CHEMISTRY

M a i n P r o c e s s Sequence F o r C o n v e r s i o n Of P l u t o n i a To H i g h - P u r i t y Metal A. Process Schematic. A schematic o f the main p r o c e s s se quence f o r t h e c o n v e r s i o n o f p l u t o n i a s c r a p t o h i g h - p u r i t y m e t a l i s shown i n F i g u r e 2. P l u t o n i a scrap i s fed to both the d i r e c t o x i d e r e d u c t i o n (DOR) p r o c e s s and t h e p l u t o n i u m t e t r a f l u o r i d e p r o duction/reduction process. D i r e c t O x i d e R e d u c t i o n . I n DOR, p l u t o n i a i s r e d u c e d w i t h c a l c i u m m e t a l t o f o r m p l u t o n i u m m e t a l and c a l c i u m o x i d e . ' The r e a c t i o n takes p l a c e i n a C a C l s o l v e n t which d i s s o l v e s the c a l c i u m o x i d e and a l l o w s t h e p l u t o n i u m m e t a l t o c o a l e s c e i n t h e b o t tom o f t h e c r u c i b l e .


2

3

2

P u 0 ( s ) + 2 Ca° + 11 C a C l U ) + 2

2

2 CaO-11 C a C l U ) + Pu (£)

(1)

2

The r e a c t a n t s a r e l o a d e d i n a m a g n e s i a c r u c i b l e and h e a t e d by a r e s i s t a n c e f u r n a c e t o 800°C ( F i g u r e 3 ) . Once t h e C a C l i s m o l t e n , a t a n t a l u m s t i r r e r and a T a - N i t h e r m o c o u p l e s h e a t h a r e l o w e r ed i n t o t h e m e l t . W h i l e s t i r r i n g , t h e r e a c t i o n i s m o n i t o r e d w i t h a thermocouple. Once t h e r e a c t i o n i s c o m p l e t e , t h e s t i r r e r and t h e r m o c o u p l e w e l l a r e r e t r a c t e d and t h e m e l t i s a l l o w e d t o c o o l . F i g u r e 4 shows a t y p i c a l DOR p r o d u c t and s a l t / c r u c i b l e r e s i d u e . A t y p i c a l p r o d u c t b u t t o n w e i g h s 600 g and t h e p r o c e s s y i e l d i s >99%. E s s e n t i a l l y no p u r i f i c a t i o n t a k e s p l a c e i n t h e r e d u c t i o n s t e p , m e a n i n g t h a t t h e p r o d u c t b u t t o n i s no p u r e r t h a n t h e f e e d . P l u t o n i u m T e t r a f l u o r i d e P r o d u c t i o n and R e d u c t i o n . In PuF p r o d u c t i o n / r e d u c t i o n , p l u t o n i a i s f e d f i r s t t o an HF r e a c t i o n f u r n a c e where t h e P u 0 i s c o n v e r t e d t o P u F . The P u F i s r e a c t e d w i t h c a l c i u m metal i n the presence of i o d i n e t o form p l u t o n i u m m e t a l , C a F , and C a l . C a l c i u m and i o d i n e r e a c t f i r s t t o i n i t i a t e the plutonium r e a c t i o n . The P u F and c a l c i u m r e a c t t o f o r m p l u t o n i u m m e t a l and C a F . B o t h t h e i o d i n e and P u F r e a c t i o n s w i t h c a l c i u m a r e v e r y e x o t h e r m i c , p r o v i d i n g enough h e a t t o m e l t t h e w a s t e s l a g and a l l o w t h e p l u t o n i u m m e t a l t o c o a l e s c e i n t h e bottom of the c r u c i b l e . 2

4

2

4

2

2

,

4

,

5

,

4

6

2

4

2

Ca + I PuF

4

2

-> C a l

2

+ 2Ca -> 2 C a F

2

+ Pu

4

-ΔΗ = 128 k c a l / m o l e

(2)

-ΔΗ = 157 k c a l / m o l e

(3)

F i g u r e 5 shows a t y p i c a l P u F p r o d u c t b u t t o n w i t h t h e s l a g and c r u c i b l e r e s i d u e . A t y p i c a l b u t t o n w e i g h s 1250 g and t h e y i e l d i s 96-98%. E s s e n t i a l l y no p u r i f i c a t i o n t a k e s p l a c e i n t h e r e d u c t i o n s t e p , m e a n i n g t h a t t h e p r o d u c t b u t t o n i s no p u r e r t h a n the feed f l u o r i d e . Molten S a l t E x t r a c t i o n . The m e t a l f r o m DOR and P u F r e d u c t i o n i s i m p u r e and p r o c e e d s t o t h e n e x t s t e p i n t h e p r o c e s s s e q u e n c e 4

4


CHRiSTENSEN

AND MULLINS



26.


413

414


PLUTONIUM CHEMISTRY

F i g u r e 3.

D i r e c t o x i d e r e d u c t i o n equipment.



26.

CHRiSTENSEN AND MULLINS

F i g u r e 4.

F i g u r e 5. metal.


D i r e c t oxide reduction metal product w i t h

residues.

P l u t o n i u m t e t r a f l u o r i d e r e d u c t i o n s l a g and p r o d u c t


415

PLUTONIUM CHEMISTRY

416

A m e r i c i u m E x t r a c t i o n (more commonly r e f e r r e d t o as M o l t e n S a l t Exo r MSE). This process i s s p e c i f i c a l l y designed to reduce the americium content of the plutonium metal. (Am spontaneously grows i n t o p l u t o n i u m as a r e s u l t o f P u d e c a y . ) When t h e im p u r e m e t a l c o n t a i n s more t h a n 1000 ppm o f a m e r i c i u m , i t i s r u n t h r o u g h t h e MSE p r o c e s s . O t h e r w i s e , i t b y p a s s e s t h e MSE s t e p and proceeds d i r e c t l y to e l e c t r o r e f i n i n g . The MSE p r o c e s s was f i r s t r e p o r t e d i n R e f e r e n c e 7 as t h e h a l i d e or c h l o r i d e slagging process. I t was l a t e r o p t i m i z e d and d e v e l o p e d i n t o a m a j o r p r o d u c t i o n p r o c e s s by w o r k e r s a t t h e R o c k y Flats Plant. I n o u r p r o c e s s , t h e f e e d m e t a l i s p l a c e d i n a mag n e s i a c r u c i b l e as shown i n F i g u r e 3. The e x t r a c t i o n p r o c e d u r e i s i d e n t i c a l t o t h e DOR p r o c e d u r e e x c e p t t h a t t h e s t i r r i n g c y c l e i s 30 m i n u t e s i n s t e a d o f o n l y a few m i n u t e s . An e q u a l m o l a r K C l - N a C l i s u s e d as a b u l k m a t r i x f o r t h e a m e r i c i u m r e a c t i o n . The o x i d i z i n g a g e n t t y p i c a l l y u s e d i s M g C l , a l t h o u g h P u F has b e e n u s e d ex t e n s i v e l y a t Los A l a m o s . The r e a c t i o n s a r e as f o l l o w s : 2 4 1


2 4 1

8

2

Pu°

+ Am°

+ 3 MgCl

->

2

PuCl

4

+ AmCl

3

3

+ 3

Mg°

+3

or,

Pu° Pu

+

+ 3 PuF 3

o

+ Am°

4

+ -> 4 Pu -*

Am

+

+ 12

(4) (5)

F

3

+

Pu°

(6)

The e x t e n t o f t h e f i r s t r e a c t i o n i s a b o u t 67%, w h i c h means t h a t some o f t h e M g C l r e m a i n s i n t h e b u l k s a l t . In a t y p i c a l 4.5-kg r u n c o n t a i n i n g 3000 ppm a m e r i c i u m , 9 0 % o f t h e a m e r i c i u m i s o x i d i z e d a t t h e e x p e n s e o f a p p r o x i m a t e l y 100 g p l u t o n i u m . A t y p i c a l p r o d u c t w e i g h s 4400 g and c o n t a i n s 9 8 % o f t h e f e e d p l u tonium. Ingot C a s t i n g . A f t e r the e x t r a c t i o n of americium from the i m p u r e m e t a l , t h e p l u t o n i u m m e t a l must be p u t i n t o a shape w h i c h i s compatible w i t h the e l e c t r o r e f i n i n g c e l l . This requires a l l o y i n g and c a s t i n g t h e m e t a l i n t o a c y l i n d r i c a l i n g o t s h a p e , hence ingot casting. The shape o f t h e i n g o t i s a 2 7/8" diam. c y l i n d e r w h i c h i s up t o 4" l o n g . The q u a n t i t y o f m e t a l needed f o r e l e c t r o r e f i n i n g i s 6 kg. Due t o n u c l e a r c r i t i c a l i t y c o n c e r n s , 6 kg o f α p h a s e ( o r n e a r f u l l d e n s i t y ) m e t a l c o u l d n o t be a l l o w e d * i n t h e c y l i n d r i c a l configuration. ( F o r an e x p l a n a t i o n o f c r i t i c a l i t y c o n c e r n s w i t h r e s p e c t t o t h e 6-kg e l e c t r o r e f i n i n g p r o c e s s , see Réf. 1 ) . As a r e s u l t , the m e t a l i s a l l o y e d d u r i n g the c a s t i n g phase w i t h g a l l i u m i n o r d e r t o change t h e b u l k d e n s i t y f r o m >19 g/cm down t o

Plutonium Metal Production and Purification at Los Alamos - ACS

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