Status of Americium-241 Recovery at Rocky Flats Plant - American

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Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: July 20, 1981 | doi: 10.1021/bk-1981-0161.ch004

Status of Americium-241 Recovery at Rocky Flats Plant JAMES B. KNIGHTON, P. G. HAGAN, J. D. NAVRATIL, and G. H. THOMPSON Rockwell International, Box 464, Golden, CO 80401

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Am grows i n t o plutonium by the beta decay o f Pu. Americium is p e r i o d i c a l l y removed from plutonium by a molten s a l t e x t r a c t i o n process to lower the impurity content and to lower the gamma r a d i a t i o n a s s o c i a t e d with alpha decay of Am to Np. The e x t r a c t i o n s a l t is an a t t r a c t i v e source of Am. At the Department o f Energy's Rocky F l a t s P l a n t (RFP), the p r o d u c t i o n s c a l e recovery and purification of Am from the e x t r a c t i o n s a l t s has i n v o l v e d aqueous i o n exchange and p r e c i p i t a t i o n p r o c e s s e s . P r e s e n t l y , about a kilogram per year o f >95% AmO ( c o n t a i n ing the v a l u e o f Kd' is equal t o Kd. By using the apparent d i s t r i b u t i o n c o e f f i c i e n t (Kd ) r a t h e r than the true d i s t r i b u t i o n c o e f f i c i e n t (Kd), Equation 6 reduces t o α = Kd . (s/m) (7) f

1

From Equation 7, it f o l l o w s that an i n f i n i t e number o f combinations e x i s t f o r values o f K d and s/m, the product o f which equals any s p e c i f i c d e s i r e d v a l u e o f a . Three modes o f e x t r a c t i o n ( s i n g l e - c o n t a c t , two stage c r o s s c u r r e n t , and two stage countercurrent) have been used at Rocky Flats. In s i n g l e - c o n t a c t , one e x t r a c t i o n stage is used. I n two stage c r o s s c u r r e n t e x t r a c t i o n , a batch o f plutonium metal is contacted s e q u e n t i a l l y by two separate batches o f new s a l t . In two stage countercurrent e x t r a c t i o n , the s a l t and metal s o l v e n t s move in opposite d i r e c t i o n s through the e x t r a c t i o n stages. From an o p e r a t i o n a l viewpoint, a s i n g l e - c o n t a c t removal o f americium is most d e s i r a b l e . To avoid secondary problems caused by the production o f magnesium (by Equations 2 and 3) in excess of i t s s o l u b i l i t y in plutonium, americium removals are l i m i t e d t o about 85% per e x t r a c t i o n stage. M u l t i p l e - s t a g e e x t r a c t i o n s consequently are used when 85% o r g r e a t e r removal o f americium is required. To lower americium content in the plutonium t o acceptable l e v e l s , about 90% removal o f americium is r e q u i r e d . To o b t a i n 90% removal with a two stage e x t r a c t i o n , a s e p a r a t i o n o f 68.4% is r e q u i r e d in each stage. Magnesium generation is not a problem. Crosscurrent e x t r a c t i o n was used i n i t i a l l y at Rocky F l a t s . T h i s mode o f e x t r a c t i o n has the f o l l o w i n g disadvantages: (1) the s o l v e n t c a p a c i t y o f the s a l t is not used e f f e c t i v e l y , (2) plutonium l o s s t o the s a l t is high because two s a l t s must be 1

Navratil and Schulz; Transplutonium Elements—Production and Recovery ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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e q u i l i b r a t e d with the plutonium metal, and (3) a l a r g e amount of s a l t must be processed f o r recovery o f americium and plutonium. Countercurrent e x t r a c t i o n is favored over c r o s s c u r r e n t e x t r a c t i o n at a f i x e d s a l t composition f o r the f o l l o w i n g reasons: (1) l e s s s a l t is r e q u i r e d to o b t a i n the same s e p a r a t i o n , (2) plutonium l o s s to the s a l t is lower, (3) l e s s magnesium metal by-product is generated, and (4) s a l t feed to the s a l t recovery l i n e is cut in h a l f . Table I g i v e s the values o f the e x t r a c t i o n f a c t o r (a) r e q u i r e d f o r americium removals ranging from 80 to 99%. These r e q u i r e d values o f α are given f o r the three modes of e x t r a c t i o n ( s i n g l e - c o n t a c t , two stage c r o s s c u r r e n t , and two stage countercurrent). TABLE I . Values o f the E x t r a c t i o n Factor (a) Percent Removal 80 85 90 91 92 93 94 95 96 97 98 99

Single-Contact

Countercurrent 2 Stages

4.000 5.667 9.000 10.111 11.500 13.286 15.667 19.000 24.000 32.333 49.000 99.000

Crosscurrent 2 Stages 1.236 1.582 2.162 2.333 2.536 2.779 3.083 3.472 4.000 4.773 6.071 9.000

1.562 1.933 2.540 2.719 2.928 3.179 3.489 3.887 4.425 5.208 6.519 9.460

The values o f α r e q u i r e d f o r a s p e c i f i c americium removal are p e c u l i a r to the e x t r a c t i o n mode and the number o f e x t r a c t i o n stages. The remaining terms in the e x t r a c t i o n f a c t o r r e l a t i o n s h i p (Kd, s/m, F, and β) and the i n t e r r e l a t i o n s h i p o f the K d and s/m terms are d i s c u s s e d in the f o l l o w i n g s e c t i o n s o f this paper. 1

D i s t r i b u t i o n C o e f f i c i e n t Term (Kd) In the molten s a l t e x t r a c t i o n process, the v a r i a b l e s that c o n t r o l the v a l u e s o f the americium and plutonium d i s t r i b u t i o n c o e f f i c i e n t s are temperature, metal composition, s a l t composition, and t o t a l americium. To minimize the v a r i a b l e s , the e x t r a c t i o n s are conducted at a f i x e d temperature o f about 750°C. S l i g h t changes o f magnesium content in the metal have a n e g l i g i b l e e f f e c t upon the value o f the americium and plutonium d i s t r i b u t i o n c o e f f i c i e n t s . The e f f e c t o f americium c o n c e n t r a t i o n



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in the metal upon the value o f the americium d i s t r i b u t i o n c o e f f i c i e n t is b e l i e v e d n e g l i g i b l e over the c o n c e n t r a t i o n range of 200 t o 2000 ppm. S a l t composition is t h e r e f o r e the v a r i a b l e with the major e f f e c t upon values o f the americium and plutonium distribution coefficients. Since M g C l 2 is the o x i d i z i n g agent f o r americium and plutonium, values o f the americium and plutonium d i s t r i b u t i o n c o e f f i c i e n t s a r e expressed as f u n c t i o n s of M g C l 2 content in the s a l t . I . Johnson, at Argonne N a t i o n a l Laboratory, d e r i v e d a r e l a t i o n s h i p f o r e s t i m a t i n g the value o f d i s t r i b u t i o n c o e f f i c i e n t s f o r a s o l u t e p a r t i t i o n i n g between a s a l t o f v a r y i n g M g C l 2 content and a molten metal o f f i x e d composition (13). T h i s r e l a t i o n s h i p is d e f i n e d a s : D = C (X M g C l )

3 / 2

(8)

2

where D = distribution coefficient =

mole % s o l u t e in s a l t atom % s o l u t e in metal

C = constant X M g C l 2 = mole f r a c t i o n M g C l 2 in s a l t For convenience, of Kd r a t h e r than D. obviously d i f f e r e n t .

Equation 8 may a l s o be expressed in terms The values o f the constant, however, a r e

Kd = C

f

(X M g C l )

3 / 2

(9)

2

The d i s t r i b u t i o n c o e f f i c i e n t s f o r americium and plutonium are estimated by u s i n g the a p p r o p r i a t e constants in Equation 9 as shown in Equations 10 and 11. KdAm = 273.7 (X M g C l )

3 / 2

(10)

KdPu = 0.692 (X M g C l )

3 / 2

(11)

2

2

From equations 10 and 11, values o f the americium and plutonium d i s t r i b u t i o n c o e f f i c i e n t can be estimated f o r M g C l 2 contents in the s a l t ranging from 0.02 to 1.0 mole f r a c t i o n (Table I I ) .


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TABLE I I . Estimated Values of Americium and Plutonium D i s t r i b u t i o n C o e f f i c i e n t s f o r NaCl-KCl-MgCl2 S a l t and Plutonium Metal System at 750°C


MgCl

2

Mole F r a c t i o n 0.02 0.05 0.10 0.15 0.20 0.30 0.40 0.60 0.70 1.00

S a l t - t o - M e t a l R a t i o Term

Estimated KdAm 0.77 3.1 8.7 15.9 24.5 45.0 69.2 127.3 160.4 273.4

Estimated KdPu 0.002 0.0085 0.022 0.040 0.063 0.115 0.175 0.325 0.405 0.693

(s/m)

The s a l t - t o - m e t a l term (s/m) is the weight r a t i o of the l i q u i d s a l t and metal phases present at e q u i l i b r i u m . The a c t u a l weights of the l i q u i d s a l t and metal present at e q u i l i b r i u m may be estimated from the feed weights by f a c t o r i n g in the weight changes caused by (1) t r a n s f e r of plutonium and americium from the metal to the s a l t , (2) t r a n s f e r of magnesium from the s a l t to the metal, (3) s a l t take-up of plutonium i n s o l u b l e i m p u r i t i e s a s s o c i a t e d with the plutonium metal feed, (4) b u i l d - u p and r e l e a s e of s a l t and metal on the c r u c i b l e and s t i r r e r , and (5) evaporation of v o l a t i l e s , such as Mg, from the metal. Because o f the above u n c e r t a i n t i e s in e s t i m a t i n g a c t u a l weights of l i q u i d s a l t and molten plutonium at e q u i l i b r i u m , it is more p r a c t i c a l (although not r i g o r o u s ) to base the s a l t - t o - m e t a l r a t i o on the weights of the s a l t and metal f e d to the e x t r a c t i o n r a t h e r than on the estimated weights of the s a l t and metal at e q u i l i b r i u m . T h i s puts a low b i a s on the value of the s a l t - t o - m e t a l r a t i o and a h i g h b i a s on the v a l u e of the apparent distribution coefficient. Mixing Term (F) Two u n i t o p e r a t i o n s are used in the e q u i l i b r a t i o n of the s a l t and metal phases: (1) i n t e r m i x i n g of s a l t and metal, and (2) disengagement of s a l t and metal. Because this is a batch e x t r a c t i o n , both operations ( i n t e r m i x i n g and disengagement of phases) occur s e q u e n t i a l l y in the same v e s s e l . For p r a c t i c a l o p e r a t i o n of the molten s a l t e x t r a c t i o n p r o c e s s , attainment of e q u i l i b r i u m or n e a r - e q u i l i b r i u m c o n d i t i o n s (when the v a l u e of F approaches 1) in a r e l a t i v e l y short p e r i o d of time is e s s e n t i a l .



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Complete disengagement o f phases in a r e l a t i v e l y short p e r i o d o f time a l s o is e s s e n t i a l . To provide continuous i n t e r m i x i n g o f the l i g h t and heavy phases in an u n b a f f l e d c r u c i b l e , a reverse-motion mode o f mixing was developed. T h i s reverse-motion mixing was obtained by r e p e a t i n g the f o l l o w i n g sequence o f events: 2.5 sec clockwise s t i r r e r r o t a t i o n , 0.5 sec stop, 2.5 sec counterclockwise s t i r r e r r o t a t i o n , and 0.5 sec s t o p . The above times f o r clockwise and counterclockwise mixing and f o r the stop p e r i o d s have not been optimized; however, they a r e adequate f o r o p e r a t i o n s at Rocky F l a t s . By f r e q u e n t l y r e v e r s i n g the d i r e c t i o n o f mixing, the s t i r r e r blade a l s o serves as a b a f f l e to i n t e r m i x the s w i r l i n g l i g h t and heavy phases. I t is b e l i e v e d that e q u i l i b r i u m c o n d i t i o n s a r e c l o s e l y approached with the reverse-motion mode o f mixing (F - 1 ) . Side Reaction Term (β) S a l t and metal i n s o l u b l e i m p u r i t i e s , such as P u Û 2 , a s s o c i a t e d with plutonium metal are taken up by the s a l t in Stage 1. Stage 2 is e s s e n t i a l l y f r e e of these i m p u r i t i e s . S t r i c k l a n d , e t a l . (14), r e p o r t e d that plutonium oxide e x t r a c t s americium from molten plutonium metal in a molten s a l t media. Because these s a l t and metal i n s o l u b l e i m p u r i t i e s a r e present in s i z a b l e amounts only in Stage 1, the s i d e r e a c t i o n between americium and these i m p u r i t i e s occurs o n l y in Stage 1. The s i d e r e a c t i o n term (β) is introduced to q u a n t i f y the s i d e r e a c t i o n caused by the presence o f i m p u r i t i e s such as in Stage 1. The r a t i o o f Equation 6 f o r Stages 1 and 2 g i v e s Stage 1 Stage 2

=

Kd'i Kdi « F i * βι Kd' Kd · F · β

(12)

=

2

2

2

2

Because o f the absence o f s a l t and metal i n s o l u b l e i m p u r i t i e s in Stage 2, the value o f the s i d e r e a c t i o n term f o r Stage 2 is $ 2 = 1. The value o f the true d i s t r i b u t i o n c o e f f i c i e n t (Kd) is assumed t o be the same f o r both stages. The e f f e c t o f americium c o n c e n t r a t i o n (200 to 2000 ppm) in the metal upon the value o f Kd is assumed to be n e g l i g i b l e over the above c o n c e n t r a t i o n range. Because the same mode and time o f mixing are used in Stages 1 and 2, the value o f F ( f r a c t i o n o f e q u i l i b r i u m ) is assumed to be the same f o r both stages. From the above assumptions (Kdi = K d 2 , F i = F 2 , and β = 1 ) , Equation 12 reduces to Equation 13 and p r o v i d e s a method f o r e s t i m a t i n g the value of the s i d e r e a c t i o n term (βι) f o r Stage 1. 2

1

Kd χ Kd f

2


(13)

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Process O p t i m i z a t i o n An optimum molten s a l t e x t r a c t i o n process at Rocky F l a t s would use the minimum amount o f s a l t r e q u i r e d to o b t a i n (1) a d e s i r e d removal o f americium, (2) a minimum t r a n s f e r o f plutonium to the s a l t , and (3) a minimum take-up o f magnesium by the plutonium metal product. The product s a l t must be compatible with subsequent chemical processes f o r the recovery o f americium and plutonium contained in the s a l t . To minimize the number o f glove-box o p e r a t i o n s , time in the gloves, and operator r a d i a t i o n exposure, the operations must be simple and easy to conduct. By using the minimum amount of s a l t feed, a minimum amount o f waste w i l l be generated that u l t i m a t e l y must be sent to long-term storage. To optimize the molten s a l t e x t r a c t i o n process, the values of the terms in the e x t r a c t i o n f a c t o r r e l a t i o n s h i p f

α = K d · (s/m)

(7)

and the i n t e r r e l a t i o n s h i p o f these terms must be known. The numerical values o f the e x t r a c t i o n f a c t o r (a) a r e s e t by s e l e c t i n g the d e s i r e d s e p a r a t i o n , the mode o f e x t r a c t i o n , and the number o f e x t r a c t i o n stages. F o r example, the value o f α r e q u i r e d f o r 90% americium removal by two stage countercurrent e x t r a c t i o n is α = 2.54 (see Table I ) . As shown p r e v i o u s l y , the v a l u e o f the americium d i s t r i b u t i o n c o e f f i c i e n t is a f u n c t i o n o f the s a l t composition; i . e . , the M g C l 2 content o f the s a l t and the composition o f the d i l u e n t s a l t system. The value o f the s a l t - t o - m e t a l r a t i o is s e t by the weight o f s a l t and metal f e d to the e x t r a c t i o n . When the values o f any two o f the three terms in Equation 11 are known, the value o f the t h i r d term may be c a l c u l a t e d . From Equation 11, it is noted that there a r e i n f i n i t e combinations o f values f o r K d and s/m, the product o f which equals any s p e c i f i e d value o f a . As the value o f K d becomes l a r g e , the corresponding value o f s/m becomes small when the value o f α is h e l d constant. F i g u r e 2 gives the kg s a l t per kg Pu (or s/m) and the corresponding M g C l 2 content in the NaCl-KCl-MgCl2 s a l t system. These data are f o r americium removals ranging between 80 and 99% using the two stage countercurrent mode o f e x t r a c t i o n . F i g u r e 2 provides the b a s i s f o r o p t i m i z i n g the process. The minimum amount of s a l t that can be p h y s i c a l l y handled in the e x t r a c t i o n is determined and is represented by a h o r i z o n t a l l i n e . The optimum M g C l 2 content in the s a l t occurs where this h o r i z o n t a l l i n e i n t e r s e c t s the l i n e r e p r e s e n t i n g the d e s i r e d removal o f americium. In the a c t u a l o p e r a t i o n o f the e x t r a c t i o n process, the minimum amount o f s a l t is about 0.05 kg s a l t p e r kg Pu. T h i s amount o f s a l t is b a r e l y s u f f i c i e n t to cover the molten plutonium. Even with this small amount o f s a l t , molten plutonium is exposed to the c e l l atmosphere during mixing. I m p u r i t i e s in 1

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Figure 2. Relationship between salt-to-metal ratio and MgCl content in salt for selected removals of americium from plutonium NaCl-KCl-MgClg salt t



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the c e l l atmosphere may r e a c t with the plutonium metal to form compounds that are taken up by the s a l t . High plutonium l o s s e s to the s a l t are u n d e s i r a b l e . The i n t e r s e c t i o n of the 0.05 kg s a l t per kg Pu l i n e with the curve f o r 90% americium occurs at 32 mole % M g C l 2 - For convenience purposes only, a 30 mole % M g C l 2 s a l t combined with a s a l t - t o - m e t a l r a t i o of 0.056 is near optimum. Table I I I compares the previous p r o d u c t i o n process w i t h an optimum p r o c e s s . In the previous p r o d u c t i o n process, an americium removal of 90% was obtained u s i n g a s a l t c o n t a i n i n g 5.74 mole % M g C l at a s a l t - t o - m e t a l r a t i o (s/m) of 0.557. By comparison, the optimum process gives the same americium removal (90%) with a 30 mole % M g C l 2 s a l t at a s a l t - t o - m e t a l r a t i o of 0.0564. The amount of s a l t is decreased by an order o f magnitude with the optimum process, and the americium c o n c e n t r a t i o n in the e x t r a c t i o n s a l t is increased by an order of magnitude. Optimum e x t r a c t i o n c o n d i t i o n s (minimum amount of h i g h M g C l 2 s a l t ) were implemented in the production o p e r a t i o n of the molten s a l t e x t r a c t i o n p r o c e s s . 2

PART I I AQUEOUS RECOVERY OF AMERICIUM FROM EXTRACTION SALTS C a t i o n Exchange In the c a t i o n exchange process, plutonium and americium are cosorbed on the r e s i n with mono- and d i v a l e n t c a t i o n s from the molten s a l t e x t r a c t i o n (MSE) process (15). A f t e r a c t i n i d e breakthrough, the column is washed with 0 . 3 5 M HNO3 to remove nonadsorbed ions remaining, then e l u t e d with 7M H N O 3 . Plutonium is o x i d i z e d to Pu(IV) and forms the h e x a n i t r a t o complex Pu(N03^ Although this a n i o n i c complex is not adsorbed on the r e s i n , it s t i l l does not e l u t e very r a p i d l y . T h i s is a t t r i b u t e d to both the d i f f i c u l t y the bulky complex has in d i f f u s i n g through the r e s i n matrix, and the h i g h s e l e c t i v i t y of the r e s i n f o r Pu(IV). The process was made e a s i e r by u s i n g g e l - t y p e r e s i n of lower c r o s s l i n k a g e and macroporous r e s i n , as reported e a r l i e r (16). A c o n t i n u i n g problem with the c a t i o n exchange process as used in p r o d u c t i o n operations is that it has not been s u f f i c i e n t l y s e l e c t i v e and t h e r e f o r e allows c o n s i d e r a b l e carryover of the MSE s a l t c o n s t i t u e n t s and i m p u r i t i e s with the plutonium and americium. T h i s i s n ' t s e r i o u s with plutonium s i n c e plutonium can be subsequently p u r i f i e d by anion exchange. For americium, however, the subsequent recovery process is o x a l a t e p r e c i p i t a t i o n which is l e s s s e l e c t i v e and c a r r i e s some o f the i m p u r i t i e s i n t o the f i n a l product. O r i g i n a l l y , s e l e c t i v e s e p a r a t i o n by c a t i o n exchange was expected to r e s u l t because of l a r g e d i f f e r e n c e s in the



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TABLE I I I Comparison o f Previous Production and Optimum MSE Processes Previous Production Process E x t r a c t i o n mode countercurrent E x t r a c t i o n stages 2 Pu recovery 97% Am removal 90% 2.54 E x t r a c t i o n f a c t o r (a) ~ 2.2 kg Weight Pu metal feed ~ 1.225 kg Weight s a l t feed 0.557 S a l t - t o - m e t a l r a t i o (s/m) 5.74 mole % MgCl2 in s a l t 4.56b) a

Element Removed Pu Am Ca Mg Pb Κ

0.35

Status of Americium-241 Recovery at Rocky Flats Plant - American

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