Actinide Separations - American Chemical Society

22 Electroredox Procedures for Plutonium in Power Reactor Fuel Reprocessing

Downloaded by UNIV OF PITTSBURGH on April 11, 2017 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch022

FRANZ BAUMGÄRTNER, HUBERT GOLDACKER, and H E L M U T SCHMIEDER Kernforschungszentrum Karlsruhe GmbH, Institut für Heisse Chemie, Postfach 36 40, 7500 Karlsruhe 1, West Germany

Reduction and o x i d a t i o n (redox) steps are major process steps in the Purex process. Use is made o f redox r e a c t i o n s t o a l t e r the valency o f plutonium, uranium o r neptunium with the o b j e c t o f producing these metals with a high degree o f p u r i t y . E l e c t r o - r e d u c t i o n and - o x i d a t i o n processes are easy t o operate and c o n t r o l remotely. Unlike the use o f redox chemicals, they do not give r i s e t o waste s a l t s . Convenient remote c o n t r o l and operation and avoidance o f waste s a l t s are e s p e c i a l l y a t t r a c t i v e features f o r commercial p r o c e s s i n g o f any type o f power r e a c t o r f u e l . Accordingly, the e l e c t r o d e r e a c t i o n s were introduced q u i t e e a r l y as intermediate steps in r e p r o c e s s i n g . The e l e c t r o chemical decladding o f spent f u e l s was the first process in t h i s field to be advanced up to the technical scale in the USA (1,2,3, 4).

In the sixties hydrazine s t a b i l i z e d U(IV) s o l u t i o n s , e l e c t r o c h e m i c a l l y produced, s u c c e s s f u l l y s u b s t i t u t e d the traditional c o r r o s i v e and salt generating f e r r o u s sulfamate (5) r e ducing agent f o r the U/Pu separation (e.g. Eurochemic in Mol, Karlsruhe Reprocessing P l a n t WAK). Further development work on e l e c t r o c h e m i c a l methods s t a r t e d again both in the USA and in Germany a l s o in the sixties (6,7). The o b j e c t i v e s were t o make the U/Pu separation more e f f e c t i v e and t o improve the product q u a l i t y by e l e c t r o - r e d u c t i o n t a k i n g p l a c e in the e x t r a c t i o n apparatus itself ( i n - s i t u p r o c e s s i n g ) . Today there e x i s t t e c h n i c a l l y mature apparatuses: an e l e c t r o r e d u c t i o n column (JL7) f o r the U/Pu s e p a r a t i o n in the AGNS P l a n t , Barnwell and an in-situ e l e c t r o - r e d u c t i o n mixer s e t t l e r , ins t a l l e d in the 2nd Pu-cycle in the German WAK P l a n t , Karlsruhe. P a r a l l e l t o in-situ r e d u c t i o n , e l e c t r o - o x i d a t i o n o f products has been developed a t Karlsruhe {8). The f o l l o w i n g p a r t d e s c r i b e s the development o f e l e c t r o redox processes a t KfK, Karlsruhe.

0-8412-0527-2/80/47-117-303$05.00/0 © 1980 American Chemical Society Navratil and Schulz; Actinide Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

304

ACTINIDE

SEPARATIONS

Main E l e c t r o d e Reactions and Side Reactions The cathodic r e d u c t i o n o f the u r a n y l i o n is described by the f o l l o w i n g equation: UO^

+

2.

+ 4H

+

4 +

+ 2e~ = U

+ 2H„0

Ε

2

ο

= + 0,33 V

(1)

At n i t r i c a c i d concentrations 0,2 t o 2,0 M HNO^ u s u a l l y a p p l i e d in the separation step the primary r e d u c t i o n product is U(V) and U(IV) is formed by d i s p r o p o r t i o n a t i o n .


υθ^

+ e" — ^ u o *

+

(2)

2U0* + 2H — ^ υ θ ^ + U 0 +

+

2 +

+ H0

(disproportionation)

2

Oxidation experiments showed that U(IV) cannot be o x i d i z e d below the oxygen generation p o t e n t i a l , t h e r e f o r e the d i s p r o p o r t i o n a t i o n step can be considered as i r r e v e r s i b l e below t h i s l i m i t ( 8 , 1 9 , 2 0 ) . Oxidation o f the s t a b i l i z e r hydrazine, u s u a l l y a p p l i e d in the Purex process takes p l a c e as the p r e f e r r e d anodic r e a c t i o n , so t h a t p r a c t i c a l l y no or only l i t t l e oxygen generation occurs under the process c o n d i t i o n s . N_H* = N„ + 5H 2 5 2

+

+ 4e~

Ε

Ο

= -

0,23

V

(3)

Consequently the in-situ process needs p r i n c i p a l l y no diaphragms. Plutonium is reduced d i r e c t l y a t the cathode o r by the e l e c t r o c h e m i c a l l y produced U(IV): Pu

4 +

+ e~ = P u

3 +

Ε

= + 0,92 V

(4)

ο For the cathodic n i t r i c a c i d r e d u c t i o n the f o l l o w i n g mechanism is proposed by Vet t e r (9^) : H

+

+ N0~

HN0 N 0 2

N0 H

3

+ HN0

^ " ^

2

+ e

+

^ N 0 2

^

+ N0~

N

0

f

3 4

^ 2N0

4

2

> H N Q

+ H0 2

a

t

( 5 )

slow fast

2

s

2

CT"^ HN0

s

l

o

w

fast

2

Hydrazine r e a c t s f i n a l l y with n i t r i t e in excess as f o l l o w s : N H 2

4

+ 2HN0 —5> N 2

2

+ N 0 + 3H 0 2

(6)

2

Since hydrazine r e a c t s q u i c k l y with HN0 the r e a c t i o n preceding the e l e c t r o n t r a n s f e r step is hindered. T h i s i n t e r r u p t i o n o f 2

Navratil and Schulz; Actinide Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

22.

BAUMGÀRTNER E T A L .

Electroredox Procedures

305

n i t r a t e r e d u c t i o n (eqn. 5) has been confirmed in p r a c t i c e (8). The p r e f e r r e d hydrazine o x i d a t i o n a t the anode f a r below the oxygen generation p o t e n t i a l is the reason why Pu (III) is not or only in small amounts o x i d i z e d t o Pu(IV) under these c o n d i t i o n s . The complete conversion o f Pu(III) t o Pu(IV) is only p o s s i b l e a f t e r the hydrazine has been destroyed. Pu(VI) is formed in minor amounts a t the anode, depending on the current d e n s i t i e s and on the n i t r i c a c i d concentration, because a s i m i l a r mechanism l i k e in the U(VI)-U(IV)-couple can be assumed.


E l e c t r o - r e d u c t i o n in Mixer S e t t l e r s (£) A 16 stage mixer s e t t l e r (MILLI-EMMA) made from t i t a n i u m has been operated f o r a t o t a l o f about 2000 hours coupled t o the MILLI f a c i l i t y (Hot P i l o t P l a n t o f the KfK, throughput about 1 kg of f u e l per day). Figure 1 shows the b a s i c layout. The t i t a n i u m c a s i n g serves as cathode. In the s e t t l i n g s e c t i o n , the platinum anode has been placed in the anodic chamber l i n e d with ceramic i s o l a t i n g m a t e r i a l . The e l e c t r i c a l l y conducting connection t o the t i t a n i u m c a s i n g is provided by the opening below the base l e v e l , thus preventing macroamounts o f organic phase p e n e t r a t i n g i n t o the anode space in case o f o p e r a t i o n a l f a i l u r e . A d e t a i l e d desc r i p t i o n o f the mixer s e t t l e r with c o n s t r u c t i o n and operation data is given in KfK-report no. 2082 (8). U/Pu_split Table 1 is a compilation o f some c h a r a c t e r i s t i c t e s t r e s u l t s f o r the U/Pu separation. With about 5 t h e o r e t i c a l stages (only 30 to 50 % stage e f f i c i e n c y f o r U(VI) e x t r a c t i o n was r e a l i z e d with the miniature m i x e r - s e t t l e r ) r e s i d u a l Pu contents in the 1BU flow of 1 mg/1 were obtained. These values are i n f l u e n c e d by the HDBP concentration in the organic phase and by the excess o f U(IV) in the aqueous phase a t the 1BU discharge p o i n t . The U(IV) is necessary t o s t r i p the r e s i d u a l Pu complexed by HDBP. Figure 2 shows the concentration p r o f i l e in the b a t t e r y measured in t e s t number 33 o f t a b l e 1. The separation e f f i c i e n c y is a l s o d e c i s i v e l y i n f l u e n c e d by the aqueous a c i d , a p r o f i l e o f which can be seen in f i g u r e 2. In the BX p a r t o f the separation b a t t e r y concentrations o f 0,2 t o 1,2 Μ ΗΝ0 and in the BS p a r t o f 1,2 t o 1,5 Μ HN0 must be considered as optimum a c i d i t i e s . In s p i t e o f t h a t q u i t e low a c i d c o n d i t i o n s plutonium c o l l o i d formation have never been observed because plutonium e x i s t s mainly in the t r i v a l e n t s t a t e . The behaviour o f neptunium has been s t u d i e d in the same con t a c t o r in q u i t e a number o f experiments, r e s u l t i n g in r e s i d u a l Np concentrations in the U product o f about 1 %. Nearly 99 % o f the Np went with the Pu product stream under optimized process con d i t i o n s , e.g. 0,2 M HNO in the BXS flow and a maximum phase 3



ACTINIDE SEPARATIONS

1

I I

I

/

0 -M-Jf

/ ,T T

Figure 1. Principle of the electroreduction mixer settler MILLI-EMMA (S): A, mixing chamber; B, aqueous phase; C, organic phase; D, interface; E, anode cell; F, Ti containment; G, anode; H, insulator; K. stirrer; ( ) aqueous phase; ( ) organic phase.


Electroredox Procedures

307


22. BAUMGÀRTNER ET AL.

Figure 2. U-Pu separation (Experiment no. 33); concentration profile in the 16-stage electroreduction mixer settler, MILLI-EMMA (S): [HNO ], [N H N0 ] in mol/h; [U], [Pu] in g/L; [TBP] in vol %;flowsinmL/hr. s


2

5

3

308

r a t i o AP


ο

/BXS

a


EH J2ËE?Sfî * î: E^Ç. t

of 5

(14). —

_ iEHï i

f

ion_ cy c l e s )_

A c i d concentration and HDBP content are a l s o e s s e n t i a l parameters f o r the e l e c t r o l y t i c Pu b a c k e x t r a c t i o n in the Pu p u r i f i c a t i o n c y c l e s . As shown experimentally the maximum a c i d concent r a t i o n should be l e s s than 1,0 M HNO_ in the Pu product. The small amount of U entrained from the U/Pu separation b a t t e r y markedly reduces the Pu l o s s e s . C h a r a c t e r i s t i c r e s u l t s shown in t a b l e 2 demonstrate t h i s e f f e c t . Figure 3 gives the corresponding concentration p r o f i l e s of t e s t number 18. Again a c e r t a i n amount of U(IV) has to be kept in the discharge stage f o r the BW flow by a s u i t a b l e d i s t r i b u t i o n of anodes over the 16 stage b a t t e r y . A s p e c i a l advantage o f the e l e c t r o l y t i c technique f o r the Pu p u r i f i c a t i o n c y c l e s is in the use of high Pu product concentrat i o n , which in subsequent t e s t s was increased up to 60 g/1. The decontamination f a c t o r s achieved in the experiments surpassed the r e s u l t s obtained with the conventional b a c k e x t r a c t i o n r e l y i n g on d i l u t e d n i t r i c a c i d by a f a c t o r of at l e a s t 10. At the WAK P l a n t (Demonstration P l a n t a t Karlsruhe, throughput about 40 tons o f f u e l per year) the 2BW stream is returned to the first c y c l e on account of the high r e s i d u a l Pu content (^ 100 mg/1). The development of in-situ r e d u c t i o n in mixer s e t t l e r s s t a r ted in 1969 and was f i n a l i s e d q u i t e r e c e n t l y in a 6 week long t e s t using a 12 stage e l e c t r o - r e d u c t i o n mixer s e t t l e r on the WAK l e v e l in a Pu t e s t c y c l e i n s t a l l e d in the I n s t i t u t f u r Heisse Chemie (15). Using t h i s t e c h n i c a l t i t a n i u m contactor, the design p r i n c i p l e s o f which are s i m i l a r to t h a t shown in f i g u r e 1, r e s i d u a l Pu concentrations in the 2BW of 0,0002 to 0,005 g Pu/1 have been achieved at product concentrations up t o 45 g Pu/1 and a maximum throughput of 1,5 kg Pu per day. The mixer s e t t l e r is now ins t a l l e d in the second Pu c y c l e o f the WAK P l a n t . The c o r r o s i o n r a t e s of the t i t a n i u m c a s i n g under c a t h o d i c a l c o n d i t i o n s were smaller than that of s t a i n l e s s s t e e l in n i t r i c a c i d (~0,05 mm Ti/y) . T h i s value as w e l l as the c o r r o s i o n r a t e of the p l a t i n i z e d tantalum anode is w i t h i n the expected order of magnitude (< 0,005 mm/y). E l e c t r o - r e d u c t i o n in Pulsed Columns

(16)

P a r a l l e l with the e l e c t r o - r e d u c t i o n mixer s e t t l e r an e l e c t r o r e d u c t i o n column with a 10 cm diameter has been developed at Karlsruhe. The most simple design of such a column is shown in f i g u r e 4. The column w a l l and the s i e v e p l a t e s are made o f t i t a n i u m and work as cathodes. The c e n t r a l rod made of p l a t i n i z e d tantalum and separated e l e c t r i c a l l y from the sieve p l a t e s by means of ceramic r i n g s , serves as the anode. A d d i t i o n a l c y l i n d r i c a l metal sheets



750

750

750

375

20

30

30

11

30

33

Experiment no.

7 20

79.5

73.7

49.1

68

U

7.5

0.20

0.20

0.46

0.57

10

12.5

48

40

18

28

37

38

Pu

7.1

0.4

4.99

69

61

41

49

èlL

Pu DF P u

2000 - 5000 1700 — 3200 430 — 480 1600 - 2400

U produc t

0.0005 - 0.001 0.0013 - 0.0025 0.0008 - 0.0009 0.0002 - 0.0003

U

IB o U

1.5 1.5

3.7

0.13

21 3

0.13

U

19

Pu

Pu produet 11

1.3 0.5

2.9 2.5

0.02 0.005

0.003 0.0008

3.5 2.5

0.6

0.6

0.1

U

0.25

Pu loss %

0.33

1

0.4

0.1

Pu ai ί ο. 00015

1.5

a

0

C

2BW >

0.0015

1

0.5

0.3

2BP

2AP HNO M

a

U

2BP

3.9 3.5 10.5 16.6

20 7 5

2.08 0.54 0.49

1000 800

1900

2800

19

U 1.68

DF

22 43 0.84 400

1.2 1.2

20

0.73

^ ο

u

U 1,0 Μ HN0 ) (J_8) . Figure 6 shows s c h e m a t i c a l l y a f l a t t i t a n i u m c e l l with four p a r a l l e l channels. The anodes are made o f p l a t i n i z e d tantalum nets and are i s o l a t e d from the cathode by ceramic p l a t e s . Together with the long-running t e s t o f the e l e c t r o - r e d u c t i o n mixer s e t t l e r , an e l e c t r o - o x i d a t i o n c e l l a t a Pu throughput o f 1,5 kg per day was operated over 6 weeks without any i n t e r r u p t i o n (15). The average feed concentrations were 0,1 t o 0,15 M N H NO and 25 t o 45 g Pu(III) per l i t e r . The average energy requirement was 38 Ah/1. The design p r i n c i p l e s correspond t o t h a t o f f i g u r e 6. The c e l l is now i n s t a l l e d in the 2nd Pu c y c l e o f WAK P l a n t d i r e c t l y a f t e r the e l e c t r o - r e d u c t i o n mixer s e t t l e r .


3

2

5

Further Development Steps To prove the s u i t a b i l i t y o f the electro-redox methods f o r the i n d u s t r i a l a p p l i c a t i o n the e l e c t r o d e behaviour e.g. with respect t o c o r r o s i o n or c o a t i n g by i m p u r i t i e s has t o be i n v e s t i gated in long-running t e s t s under r e a l process c o n d i t i o n s . This is the aim o f the t e s t s in the WAK P l a n t . A Pu t e s t c y c l e (PUTE) in KfK Karlsruhe w i l l give the p o s s i b i l i t y t o t e s t e l e c t r o l y t i c a l pulsed columns in t e c h n i c a l s c a l e and design. The c o n s t r u c t i o n w i l l be f i n i s h e d in 1979. In p a r a l l e l a f u l l s c a l e e l e c t r o - r e d u c t i o n column w i l l be i n s t a l l e d in an U t e s t c y c l e f o r 4 tons o f U per day in KfK and set in operation by the German i n d u s t r y . Literature cited 1. 2. 3. 4. 5. 6. 7. 8.

Progress in Nuclear Energy, Ser. III, Process Chemistry, V o l . 4, p. 82, Pergamon Press, 1970 C a r a c c i o l o , V.P., Kishbaugh, A.A., USAEC-Report, DP-896, 1964 Kerr, W.B., Lakey, L.T., Denney, R.G., USAEC-Report, IDO14643, 1965 Bjorklund, W.J., USAEC-Report, ICP-1028, 1974 Schlea, C.S., USAEC-Report, DP-808, 1963 Newman, R.I., Nuclear Engineering I n t e r n a t i o n a l , p. 938/941, Nov. 1972 Baumgärtner, F., Schwind, Ε., Schlosser, P., Deutsches Pat. Nr. OS 1965519 (6.8.1970), US Pat. 3730851 Schmieder, H., Baumgärtner, F., Goldacker, H., Hausberger, H., G e s e l l s c h a f t für Kernforschung Karlsruhe, Report KfK 2082, 1974



316

9. 10. 11. 12. 13. 14. 15.


16.

17.

18. 19. 20.

V e t t e r , K.J., Elektrochemische K i n e t i k , Springer V e r l a g , 1961, S. 409/910 K o l t h o f f , I.M., H a r r i s , W.E., J.Am.Chem.Soc. 68, 1175 (1946) P i t z e r , E.C., USAEC-Report KAPL 653, Dec. 1951 Hartland, S., Spencer, A.J.M., Trans.Inst.Chem.Engrs. 41 (1963), p. 328/335 Chemical Engineering D i v i s i o n , ANL, Summary Report J u l y , August, Sept. 1953, USAEC-Report ANL-5169 Warnecke, E., D i s s e r t a t i o n , Universität Heidelberg, 1975 in p r e p a r a t i o n , H. Schmieder, Goldacker, Η., F i n s t e r w a l d e r , L., Hausberger, Η., Kernforschungszentrum Karlsruhe Schmieder, H., Huppert, K.C., Goldacker, Η., in "Chemie der Nuklearen Entsorgung", von F. Baumgärtner, V e r l a g K a r l Thiemig, München (1978), Part I I , p. 50-87 Cermak, A.E., Gray, J.H., Murbach, E.W., Neace, J.C., Spaunburgh, R.G., Proc. o f the American Nuclear Soc. T o p i c a l Meeting, March 1978, Savannah Georgia p. V-11 Schmieder, H., Goldacker, H., Interner B e r i c h t Kernforschungszentrum Karlsruhe, 1975 K o l t h o f f , I.M., H a r r i s , W.E., J.Am.Chem.Soc. 68, 1175 (1946) McDuffie, B., R e i l l e y , C.N., Anal.Chem. 38, 1881 (1966)

RECEIVED May

11,

1979.


Actinide Separations - American Chemical Society

Recommend Documents