Actinide Separations - ACS Publications - American Chemical Society

18 Photochemistry of the Actinides L. Μ. ΤΟΤΗ, J. T. BELL, and H . A. FRIEDMAN

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch018

Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830

Prior to the advent of the laser, photo-induced reactions sat, for the most part, in the chemical background. Photo -chemists of that time typically gave l i t t l e thought to actinide elements other than uranium which was known for years to be a very excellent chemical actinometer when present as the uranyl ion. The expertise and specialized equipment required in the handling of the other actinides, coupled with their very limited supply,served to discourage photochemists from fundamental inves tigations of these elements. As a result, no report of actinide photochemistry (save that of uranium) is to be found in the open literature prior to 1969. With no supporting fundamental data, reactor technologists concerned with the chemistry of reactor fuels could merely speculate on the behavior of the actinide solutions should they be exposed to electromagnetic radiation. Then as fuel reproc essing became a topic of much interest during the past decade, the concern of possible photochemical activity began to occupy the thoughts of some researchers. The f i r s t a t t e n t i o n g i v e n t o a c t i n i d e p h o t o c h e m i s t r y was f o r the p u r p o s e o f i d e n t i f y i n g any p h o t o c h e m i c a l a c t i v i t y w h i c h might a l t e r the e f f i c i e n c y of the e x t r a c t i o n or exchange p r o c e s s e s . S u b s e q u e n t l y , the i d e n t i f i c a t i o n of p h o t o c h e m i c a l l y a c t i v e s p e c i e s o f u r a n i u m a n d p l u t o n i u m g a v e some i n d i c a t i o n t h a t t h e p h o t o r e a c t i o n s c o u l d be t u r n e d t o a u s e f u l end a n d , p e r h a p s , o f f e r a c l e a n e r way t o s e p a r a t e a c t i n i d e s f r o m e a c h o t h e r a n d f r o m t h e o t h e r e l e m e n t s a c c o m p a n y i n g them i n n u c l e a r f u e l e l e m e n t s . S i n c e t h a t t i m e , l a s e r p h o t o c h e m i s t r y h a s become a p o p u l a r s u b j e c t a n d w i t h i t h a v e come t h e l a s e r p h o t o c h e m i s t s l o o k i n g f o r a photon t a r g e t . O b v i o u s l y , t h e f i r s t l a s e r p h o t o n s w o u l d be aimed toward i s o t o p e s e p a r a t i o n s w h i c h r e q u i r e d the n a r r o w b a n d w i d t h s w h i c h the l a s e r so u n i q u e l y p r o v i d e d ; b u t s p i n - o f f t a r g e t s have s i n c e i n c l u d e d t h e s e p a r a t i o n s o f r e a c t o r f u e l components i n r e p r o c e s s i n g and/or waste i s o l a t i o n systems. A l t h o u g h much h a s been promised from the a p p l i c a t i o n of l a s e r s to the r e p r o c e s s i n g of n u c l e a r f u e l s , there has been v e r y l i t t l e evidence t h a t would T

T

0-8412-0527-2/80/47-117-253$05.00/0 © 1980 A m e r i c a n C h e m i c a l Society In Actinide Separations; Navratil, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

254

ACTINIDE SEPARATIONS

suggest t h e i r usage i s e s s e n t i a l . C o n s e q u e n t l y , most o f t h e p e r t i n e n t p h o t o c h e m i c a l work has been p e r f o r m e d w i t h c o n v e n t i o n a l l i g h t sources. A r e l a t i v e l y s m a l l amount o f e n e r g y i s r e q u i r e d t o do t h e j o b in a typical reprocessing plant. We e s t i m a t e , u n d e r t h e most i d e a l circumstances,that 2000 W a t t s o f a b s o r b e d l i g h t f o r a t y p i c a l 5 t o n / d a y p l a n t w o u l d be n e c e s s a r y t o a c h i e v e t h e r e d u c t i v e s t r i p p i n g o f p l u t o n i u m . ( T h i s i s b a s e d on t h e a s s u m p t i o n t h a t t h e d o m i n a n t p h o t o - r e a c t i o n w i l l be UO^"*" r e d u c t i o n w i t h a Quantum Y i e l d . QY, = 0 . 5 . T h e t w o - f o l d e x c e s s o f U^+ i s t h e n u s e d t o r e d u c e P u ^ w h i c h i s p r e s e n t a s 1 p e r c e n t o f t h e s p e n t LWR f u e l (1). N e p t u n i u m , o c c u r r i n g as 0 . 0 5 p e r c e n t o f t h e s p e n t f u e l , h a s b e e n n e g l e c t e d f o r t h i s e s t i m a t e . ) T h i s i s i n d e e d an a t t r a c t i v e a l t e r n a t i v e to some c h e m i c a l r e a g e n t s , b u t p r e s e n t l y , f o r want o f more d a t a , no p h o t o c h e m i c a l p r o c e s s has been s e r i o u s l y c o n s i d e r e d even though s e v e r a l i n t e r e s t i n g s u g g e s t i o n s h a v e b e e n o f f e r e d and some p a t e n t s have been o b t a i n e d .


+

T h i s r e p o r t s h a l l review the b a s i c photochemistry of uranium, p l u t o n i u m , and n e p t u n i u m w h i c h h a s b e e n s t u d i e d d u r i n g t h e p a s t s e v e r a l y e a r s , s h a l l s u g g e s t where a p h o t o c h e m i c a l p r o c e s s m i g h t be most e f f i c i e n t l y a p p l i e d a n d t h e n s h a l l a s s e s s t h e g e n e r a l a p p l i c a b i l i t y o f p h o t o c h e m i s t r y to f u e l r e p r o c e s s i n g . We s h a l l assume t h a t t h e a t t r a c t i v e f e a t u r e s o f p h o t o c h e m i c a l s e p a r a t i o n s a r e a p p r e c i a t e d and t h a t the r e a d e r i s a b l e to p e r f o r m the s i m p l e c o n v e r s i o n o f W a t t s i n p u t power t o m o l e s o f r e s u l t i n g p r o d u c t p r o v i d e d t h e quantum e f f i c i e n c i e s to t h e p h o t o c h e m i c a l r e a c t i o n s a r e known. M o s t e l e m e n t a r y t e x t s on p h y s i c a l c h e m i s t r y (2) o r p h o t o c h e m i s t r y (_3) p r o v i d e a m p l e b a c k g r o u n d t h a t n e e d n o t be repeated here. The f o c u s o f t h i s r e p o r t w i l l be on t h e p h o t o c h e m i s t r y of the a c t i n i d e s t h e m s e l v e s and the p r o m i s e w h i c h t h i s g i v e s to the p o s s i b i l i t y of s e p a r a t i n g them. H i s t o r i c a l l y , the study of a c t i n i d e p h o t o c h e m i s t r y at b o t h Oak R i d g e and e l s e w h e r e h a s p r o c e e d e d a l o n g t h e s e q u e n c e shown i n T a b l e I , moving from a s p e c t r o s c o p i c s t u d y of u r a n y l s p e c i e s to p h o t o c h e m i c a l r e d u c t i o n t h e r e o f and u l t i m a t e l y t o t h e p h o t o c h e m i s t r y o f t h e more r a d i o a c t i v e a c t i n i d e s . The f u n d a m e n t a l p h o t o c h e m i s t r y o f p l u t o n i u m and n e p t u n i u m h a s b e e n p r o b e d o n l y d u r i n g t h e p a s t t e n y e a r s and b y o n l y two l a b o r a t o r i e s ( 9 - 1 4 ) , due l a r g e l y t o t h e d i f f i c u l t y o f h a n d l i n g and t h e i r l i m i t e d s u p p l y . The R u s s i a n w o r k e r s w e r e t r u l y f i r s t t o i n v e s t i g a t e p l u t o n i u m and n e p t u n i u m , b u t t h e i r r e p o r t s a r e b r i e f and do n o t m e n t i o n e s s e n t i a l f a c t o r s s u c h a s quantum e f f i c i e n c i e s and w a v e l e n g t h e f f e c t s on t h e p h o t o - i n d u c e d r e a c t i o n s . For these r e a s o n s , the r e s e a r c h which f o l l o w e d a t Oak R i d g e was p e r f o r m e d t o answer some o f t h e e s s e n t i a l questions. U l t i m a t e l y , a number o f p h o t o c h e m i s t r y g r o u p s t u r n e d t h e i r i n t e r e s t s to r e p r o c e s s i n g u s i n g the fundamental i n f o r m a t i o n thus f a r g a t h e r e d and t h e r e a l i z a t i o n o f t h e b e n e f i t s w h i c h p h o t o c h e m i c a l p r o c e s s e s had to o f f e r . A l l of these i n v e s t i g a t o r s f o l l o w e d a s i m i l a r p a t h — t h a t o f p h o t o - r e d u c i n g u r a n y l t o U ( I V ) and u s i n g t h e r e s u l t i n g u r a n i u m as t h e c h e m i c a l r e d u c t a n t f o r P u ( I V ) i n

In Actinide Separations; Navratil, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


(IV)

(III)

(II)

(I)

2

4

(A)

Pu(IV) U(IV)

REPROCESSING

3

reduction

with

Photoredox i n HC10,, HN0

2

(B)

3

i n HC10,, H S0

Photoredox HC1, H N 0 ,

(A)

NEPTUNIUM

i

Equilibrium photoshift(

\

(C)

degradation

Polymer

II

Photoredox

(B)

(A)

PLUTONIUM

(B)

Spectroscopy

Nemodruk,

Bezrogova

and Toth

[1979]

e t a l . [1972-73]

(15,16)

(11,12,13,14)

C a r r o l l , B u r n s , W a r r e n [1961] (17) W i l s o n [1971] (18) F r i e d m a n , T o t h , B e l l [1977] (19) G o l d s t e i n e t a l . [1977] (20) D e P o o r t e r , R o f e r - D e P o o r t e r [1978] (21)

Friedman

Nemodruk

(8)

(5)

[1969]

( B e l l a n d F r i e d m a n [1976] (9) \ F r i e d m a n , T o t h , B e l l [1977] ( 1 Ό )

Palei,

Nemodruk e t a l . [ 1 9 6 7 ] ( 6 ) B e l l et a l . [1974-75] Ç7)

B e l l and B i g g e r s [ 1 9 6 5 - 6 8 ] ( 4 ) B e l l , Friedman, B i l l i n g s [1974]

P h o t o c h e m i s t r y and R e l a t e d

2+ Photoreduction of U 0

Actinide

Spectroscopy

I,

(A)

URANIUM

Table


256



o r d e r to a c h i e v e the r e d u c t i v e e x t r a c t i o n of p l u t o n i u m . The f i r s t known r e p o r t o f s u c h w o r k was by C a r r o l l a n d c o w o r k e r s (17) a t H a n f o r d i n 1961 f o l l o w e d b y a p a t e n t by W i l s o n (18) i n 1971 f r o m t h e same l a b o r a t o r y s u g g e s t i n g t h e o r g a n i c p h a s e r e d u c t i o n o f U0^+ u s i n g t r i b u t y l p h o s p h i t e and p h o s p h o r i c a c i d . L a t e r , an aqueous phase p h o t o c h e m i c a l p r o c e s s w h i c h would p r o d u c e r e d u c t i v e e x t r a c t i o n o f p l u t o n i u m f r o m a TBP o r g a n i c p h a s e c o n t a i n i n g b o t h u r a n i u m and p l u t o n i u m was p a t e n t e d (19) at Oak R i d g e . G o l d s t e i n and c o w o r k e r s ( 2 0 ) , a l t h o u g h t h e y c o u l d do no e x p e r i m e n t a l w o r k , were q u i c k t o s e e t h e m e r i t s o f u r a n y l r e d u c t i o n u s i n g p h o t o c h e m i s t r y and h a v e w r i t t e n s e v e r a l r e p o r t s s u g g e s t i n g i t s u s a g e a s a means o f s e p a r a t i n g p l u t o n i u m f r o m uranium. S i n c e t h a t time the group a t Los Alamos has p e r f o r m e d some e x p e r i m e n t s o n o r g a n i c p h a s e r e d u c t i o n s o f u r a n y l i o n (21) s i m i l a r t o t h a t o f t h e H a n f o r d w o r k i n 1971; b u t , i n s t e a d o f u s i n g t r i b u t y l p h o s p h i t e and p h o s p h o r i c a c i d as t h e r e d u c t a n t s f o r u r a n i u m , t h e y p r o p o s e d t h e u s a g e o f t h e TBP i t s e l f . T h i s has a l r e a d y b e e n t r e a t e d i n a p r e v i o u s r e p o r t (22) and t h e d e t a i l s s h a l l n o t be d i s c u s s e d any f u r t h e r h e r e . I t may seem s t r a n g e a t f i r s t t h a t a l l o f t h e g r o u p s i n v o l v e d i n t h e r e p r o c e s s i n g c h e m i s t r y c h o s e t o l o o k a t t h e same b a s i c i d e a — u r a n y l p h o t o c h e m i c a l r e d u c t i o n to U ( I V ) . However, r e a l i z i n g t h a t t h e e a r l y w o r k e r s w e r e n o t aware o f t h e p l u t o n i u m a n d n e p t u n i u m p h o t o c h e m i s t r y and t h a t t h e y knew U ( I V ) was a n a c c e p t a b l e c h e m i c a l r e d u c t a n t f o r P u ( I V ) , t h e common c o u r s e o f a c t i o n i s n o t so d i f f i c u l t t o u n d e r s t a n d . The l a t e r g r o u p s h a d t h e a d v a n t a g e o f k n o w i n g t h a t t h e quantum e f f i c i e n c y f o r d i r e c t r e d u c t i o n o f P u ( I V ) to P u ( I I I ) was much l e s s t h a n t h a t f o r p h o t o r e d u c t i o n of u r a n y l to U ( I V ) . I n a d d i t i o n , t h e r e was t h e r e a l i z a t i o n t h a t u r a n y l c o u l d be p h o t o l y z e d w i t h l i g h t o f w a v e l e n g t h s (7) w e l l o u t i n t h e v i s i b l e w h i l e p l u t o n i u m r e q u i r e d t h e u s a g e o f u l t r a v i o l e t r a d i a t i o n (

+

257

Actinides

2

R

CD

+

ox

(where M c a n be e i t h e r u r a n i u m o r p l u t o n i u m ) h a s b e e n f o u n d t o o c c u r ( 8 , 9 , 1 0 ) u s i n g a v a r i e t y o f r e d u c t a n t s , R , s u c h as a l c o h o l s , a l d e h y d e s , h y d r a z i n e and h y d r o x y l a m m i n e . The p r o d u c t o f the r e a c t i o n i s t h e p e n t a v a l e n t a c t i n i d e i o n and t h e o x i d i z e d f o r m o f the r e d u c t a n t , R J . For example: X


2PuOÎ;

+

+ C H 0 H + hv 3

2PU0* + C H 0 + 2 H 2

+

(2)

The M O * s p e c i e s o f u r a n i u m a n d p l u t o n i u m i s o r d i n a r i l y u n s t a b l e and u n d e r g o e s r a p i d d i s p r o p o r t i o n a t i o n v i a : 2M0* + 4 H

+

t

M0^~ + M

4

+

+ 2H 0

(3)

2

w i t h t h e r e s u l t i n g f o r m a t i o n o f t e t r a - and h e x a - v a l e n t actinide ions. A c o m p a r i s o n o f t h e quantum e f f i c i e n c i e s f o r t h e s e r e a c t i o n s , l i s t e d i n T a b l e I I , r e v e a l s one r e a s o n why t h e p h o t o r e p r o c e s s i n g p r o p o s a l s r e l y so h e a v i l y on t h e u r a n y l p h o t o r e d u c t i o n ; i t i s simply a matter of higher e f f i c i e n c y . O t h e r r e a c t i o n s o b s e r v e d f o r p l u t o n i u m h a v e b e e n (1) the d i r e c t photochemical r e d u c t i o n of Pu(IV) u s i n g r e d u c t a n t s such a s f o r m i c a c i d , e t h a n o l and h y d r a z i n e ( 8 , 9 , 1 0 ) i n p e r c h l o r i c a c i d : Pu For

4+

+ R + hv -> Pu

^4-

+ R

+

(4)

ox

example: 4Pu

4 +

+ Ν H . + hv -> 4 P u 2 4

3 +

+ N + 4H 2 0

(5)

+

where quantum e f f i c i e n c i e s a s shown i n T a b l e I I h a v e b e e n m e a s ured; (2) P h o t o - e q u i l i b r i u m s h i f t o f t h e P u ( I V ) d i s p r o p o r t i o n a tion reaction (9,10): 3Pu

4 +

+ 2H 0 2

Pu0

2

+

+ 2Pu

3 +

+ 4H

+

(6)

w h i c h , i n r e a l i t y , i s a two-step e q u i l i b r i u m t h a t i n v o l v e s the PuO£ i n t e r m e d i a t e ; a n d (3) P h o t o - a s s i s t e d d e p o l y m e r i z a t i o n o f Pu(IV) hydrous polymers ( 9 , 1 0 ) . None o f t h e s e w i l l be f u r t h e r d i s c u s s e d because they have a l r e a d y been p r e s e n t e d i n d e t a i l and space here i s l i m i t e d . I n s t e a d , a t t e n t i o n s h a l l be f o c u s e d on p h o t o - r e p r o c e s s i n g r e l a t e d chemistry and, i n p a r t i c u l a r , neptunium w h i c h has n o t been p r e v i o u s l y c o n s i d e r e d . Neptunium

Reactions

Neptunium, l i k e i t s p l u t o n i u m n e i g h b o r , can e x i s t i n o x i d a t i o n s t a t e s t h a t v a r y f r o m +3 t o +6 ( d i s r e g a r d i n g t h e +7 s t a t e which i s not p e r t i n e n t to r e p r o c e s s i n g - r e l a t e d s o l u t i o n s ) . In



*cio-

254

-> N p ( I V )

-> N p ( I I I )

Np(V)

-> Np (VI)

Np(V)

Np(IV)

Np(IV)

Np(V)

*C107 4

254

-> Np(V)

Np(VI)

C H 0H C H 0H

300 2

2

5

5

5

254

2

C H 0H

5

300

5

C H 0H 2

*N0~

2

254

300

254

C H OH

5

300

2

C H 0H

5

254

2

2

4

HC10. 4 HC10, 4 HC10, 4 HC10. 4 HC10, 4 HC10. 4

.005 >.010

.010 ,010 ,005

,02

.011

.011

.01

.02

.004

.008

.004

.02

.006

.002

,040

0.5

.02

.03

.006

,002

3

3

.068

,001

,068

HN0

.068

0.03

0,01

0.5

.03

.07

0,5

.04

3

0.5

HC10. 4 HC10. 4 HN0

4

C H 0H,N H

-> P u ( I I I )

Pu(IV)

5

o

U ( I V )

Quantum e f f i c i e n c i e s f o r acid concentration (mole/liter) 4.0 1.0 0.5 0,1

II. Quantum e f f i c i e n c i e s f o r t h e r e d o x r e a c t i o n s o f t h e a c t i n i d e s shown a s a f u n c t i o n o f a c i d c o n c e n t r a t i o n , when known. Redox a g e n t s m a r k e d w i t h * a r e a s s o c i a t e d d i r e c t l y w i t h t h e s o l v e n t medium.

Reaction

Table


18.

τ ο τ Η ET AL.

Photochemistry

of the

259

Actinides


c o n t r a s t t o i t s u r a n i u m and p l u t o n i u m a n a l o g s , n e p t u n i u m has a v e r y s t a b l e +5 o x i d a t i o n s t a t e w h i c h t e n d s t o d o m i n a t e t h e chemistry of inorganic a c i d s o l u t i o n s . Nemodruk and c o w o r k e r s w e r e t h e f i r s t t o s t u d y t h e p h o t o - r e d o x r e a c t i o n s o f Np and t h e y looked s e p a r a t e l y at p e r c h l o r i c a c i d (11), hydrochloric acid (12), n i t r i c a c i d (13) and s u l f u r i c a c i d (14) m e d i a . However, n o t i c e a b l y a b s e n t i n t h e i r work was any c o n s i d e r a t i o n o f quantum e f f i c i e n c i e s and w a v e l e n g t h s e f f e c t s f o r t h e v a r i o u s photoredox reactions. T h e r e c e n t p h o t o c h e m i s t r y i n t e r e s t s a t Oak R i d g e h a v e b e e n t u r n e d to an e x a m i n a t i o n o f t h e s e a s p e c t s f o r neptunium i n p e r c h l o r i c (15) and n i t r i c a c i d (16) solutions. A perchloric acid medium p r o v i d e s a much s i m p l e r s t a r t i n g p o i n t b e c a u s e t h e C l O ^ i o n i s n o t s u b j e c t t o p h o t o l y s i s as i s t h e NOg i o n when e x p o s e d t o l i g h t o f 2 5 0 - 3 6 0 nm w a v e l e n g t h . Nevertheless, n i t r i c acid media were u l t i m a t e l y examined because o f t h e i r o b v i o u s p r a c t i c a l importance. Perchloric Acid Media. In p e r c h l o r i c a c i d , s o l u t i o n s of N p ( I I I ) , ( I V ) , a n d (V) a r e o x i d i z e d a f t e r t h e a b s o r p t i o n o f 254 o r 300 nm r a d i a t i o n i n a s t e p w i s e f a s h i o n a c c o r d i n g t o t h e f o l lowing general r e a c t i o n s (15): 2(4-x)Np

3 +

+ C10~ + 2 ( 4 - x ) H

2(4-x)Np

4 +

+ C10~ + 3 ( 4 - x ) H 0 ^

2(4-x)Np0

2

+

^

2

+ C10~ + 2 ( 4 - x ) H

+

^

2(4-x)Np

+ C10~ +

4 +

2(4-x)Np0 2(4-x)Np0

2

2

+

(4-x)H 0

+ C10~ + 6 ( 4 - x ) H + C10~ +

(7)

2

+

(8)

(4-x)H 0

(9)

2

where t h e b y - p r o d u c t i o n i s s u s p e c t e d t o be e i t h e r C 1 0 o r CIO . The change i n s p e c i e s c o n c e n t r a t i o n f o r Np(IV) p h o t o - o x i d a t i o n i s shown i n F i g . 1 w h i l e t h a t f o r N p ( V ) p h o t o - o x i d a t i o n i s shown i n F i g . 2. Quantum e f f i c i e n c i e s f o r t h e s e a n d o t h e r r e a c t i o n s a r e given i n Table I I . The c o m p l e x c h a n g e s w h i c h o c c u r when N p ( I V ) is photo-oxidized (cf., F i g . 1) d e m o n s t r a t e t h a t s e v e r a l phenomena take p l a c e s i m u l t a n e o u s l y . These i n c l u d e : (1) t h e r e t a r d i n g e f f e c t s f r o m t h e p r o d u c t i o n C10~ as s e e n a t 250 m i n u t e s f o r t h e N p ( V ) s p e c i e s i n 0 . 1 Ν H ; (2) t h e a c c e l e r a t i o n o f t h e N p ( V ) f o r m a t i o n r a t e i n 4 . 0 Ν a c i d as N p ( V ) a c c u m u l a t e s a n d i s , i n t u r n , p h o t o - o x i d i z e d a t a h i g h e r quantum e f f i c i e n c y t o N p ( V I ) w h i c h t h e n c o m b i n e s r a p i d l y w i t h t h e r e s i d u a l N p ( I V ) t o f o r m N p ( V ) ; and (3) t h e b u i l d - u p o f N p ( V I ) i n t h e l o w a c i d m e d i a p r i o r t o c o m p l e t e c o n v e r s i o n o f Np(IV) to N p ( V ) , w h i c h i s i n c o n t r a s t to the o b s e r v a t i o n of the p r e v i o u s workers (11). T h e p h o t o - o x i d a t i o n o f N p ( V ) i s more s t r a i g h t - f o r w a r d since t h e r e a r e o n l y two s p e c i e s p o s s i b l e . F i g u r e 2 shows t h e e f f e c t o f a c i d as e x p e c t e d f r o m t h e r e a c t i o n g i v e n i n E q . 9. The p h o t o c h e m i c a l r e d u c t i o n o f n e p t u n i u m i n aqueous p e r c h l o r i c a c i d i s c o m p l i c a t e d b e c a u s e i t must compete a t t i m e s w i t h t h e o x i d a t i o n p r o c e s s e s c a u s e d by t h e H C I O ^ . I n c o n t r a s t to the 2

+



260

ACTINIDE

SEPARATIONS

350

EXPOSURE

MINUTES

Figure 1. Photooxidation of Ν ρ (IV) with 254-nm radiation at 22° C; formation of Ν ρ (V) and Np (VI) products shown; total [Np] = 12mM for each solution photolyzed; dotted portions on curves represent dark period interruptions during the photolysis run.

2

350 EXPOSURE

MINUTES

Figure 2. Photooxidation of Np (V) with 254-nm radiation at 22°C; total [Np] = 12mM for each solution photolyzed, Ν ρ (VI) concentration as a function of time shown.


18.

τ ο τ Η ET AL.

Photochemistry

of the

261

Actinides

p r e v i o u s study (11), t h e c u r r e n t r e s u l t s show t h a t e v e n t h e N p ( I I I ) s t a t e c a n be p h o t ο c h e m i c a l l y g e n e r a t e d . The o v e r a l l p h o t o r e d u c t i o n r e a c t i o n s u s i n g e t h a n o l have been i d e n t i f i e d a s : 2ΝρΟ^ 6H

+

+ CH CH OH ^ > 3

+ 2Np0 2Np


+

4 +

2

2

2Np0* + CH CHO + 2 H

+ CH CH OH ^ > 3

2

+ CH CH 0H ^ 3

2

2Np

2Np

4 +

+ CH CH0 + 4 ^ 0 3

+ CH CH0 + 2 H

3 +

+

3

+

(10) (11) (12)

and t h e quantum e f f i c i e n c i e s a r e g i v e n i n T a b l e I I f o r e a c h o f these r e a c t i o n s . F o r more d e t a i l on t h i s c h e m i s t r y , t h e r e a d e r i s r e f e r r e d to an e a r l i e r p u b l i c a t i o n (15). N i t r i c Acid Media. The p h o t o - r e d o x r e a c t i o n s o f n e p t u n i u m i n n i t r i c a c i d a r e , as s t a t e d e a r l i e r , c o m p l i c a t e d by t h e N O 3 p h o t o - r e d u c t i o n w h i c h p r e v e n t s the complete p h o t o - o x i d a t i o n of neptunium to the (VI) s t a t e . A t f i r s t t h i s m i g h t seem t o p o s e a d i l e m m a w h i c h o u g h t t o be a v o i d e d , b u t r e c e n t r e s u l t s h a v e shown how t h i s a p p a r e n t c o m p l i c a t i o n i s e s s e n t i a l t o a p h o t o - s e p a r a t i o n process. Two p h o t o c h e m i c a l r e a c t i o n s w o r k i n g t o g e t h e r w i l l be shown t o be n e c e s s a r y i n o r d e r t o r e a c h what we h a v e a c c e p t e d a s a practical result. When p h o t o l y z e d i n t h e a b s e n c e o f any a d d e d r e d u c t a n t s , n i t r i c a c i d s o l u t i o n s o f N p ( V I ) and N p ( I V ) a r e c o n v e r t e d t o N p ( V ) . T h e p h o t o c h e m i s t r y o f Np i n n i t r i c a c i d d i f f e r s from that i n HC10, s o l u t i o n s s i n c e i n t h e f o r m e r , c o m p l e t e r e d u c t i o n o f N p ( V I ) -> N p ( V ) u l t i m a t e l y o c c u r s , w h i l e i n t h e l a t t e r , o x i d a t i o n o f N p ( V ) -+ N p ( V I ) i s o b s e r v e d ( c f . , F i g . 2 a n d E q . 9 ) . The d i f f e r e n c e i s o b v i o u s l y due t o t h e p r e s e n c e o f p h o t o c h e m i c a l l y p r o d u c e d NO2 w h i c h s e r v e s a s t h e r e d u c t a n t f o r N p ( V I ) . The quantum e f f i c i e n c i e s f o r Np i n T a b l e I I show t h a t t h e r e a c t i o n s o c c u r b e s t when i r r a d i a t e d w i t h 254 r a t h e r t h a n 300 nm l i g h t i n a g r e e m e n t w i t h t h e i n c r e a s e d QY f o r NO3 p h o t o - r e d u c t i o n a t s h o r t e r w a v e l e n g t h s (23) b u t i n o p p o s i t i o n t o t h e d i f f e r e n c e i n N O " l i g h t a b s o r p t i o n a s shown i n F i g . 3 . The N p ( V I ) r e d u c t i o n p r o b a b l y i n v o l v e s an e x c i t e d s t a t e of N p , but the e v i d e n c e i n s u p p o r t of t h i s i s not c o n c l u s i v e . A l t h o u g h t h e quantum e f f i c i e n c y f o r N p ( V I ) r e d u c t i o n i s n o t a s g r e a t as t h a t f o r t h e r e d u c t i o n o f U 0 ^ w i t h e t h a n o l o r h y d r a z i n e , i t i s n o t so p r o h i b i t i v e l y l o w t h a t i t c a n n o t be u s e f u l . +

E x p o s u r e o f N p ( V ) s o l u t i o n s t o 254 nm r a d i a t i o n p r o d u c e s a s t e a d y - s t a t e c o n v e r s i o n o f a b o u t 10 p e r c e n t N p ( V ) -* N p ( V I ) w h i c h r e t u r n s to Np(V) i n the d a r k . [ T h e r e i s t h e r e f o r e no n e t effect o f UV l i g h t on N p ( V ) ] . T h e s e o b s e r v a t i o n s c a n be e x p l a i n e d b y the s h i f t of the e q u i l i b r i u m : 2H

+

+ 2Np0

upon p h o t o - e x c i t a t i o n

2

of

+ N0~ t

2Np0

2

+

+ N0~ + 1^0

NpO"^.


(13)


262

ACTINIDE

240

260

280

300

320

SEPARATIONS

340

360

WAVELENGTH (nm)

Figure 3.

Absorption spectrum of HN0 ; [HNO ] 1.0 cm. 3

s

= 0.1N, cell pathlength


=

18.

τοτΗ E T

AL.

Photochemistry

of the

263

Actinides

Exposure o f Np(IV) n i t r i c a c i d s o l u t i o n s produces a s i m p l e p h o t o - o x i d a t i o n o f N p ( I V ) -> N p ( V ) s i m i l a r t o t h e f o r w a r d r e a c t i o n of E q . 13.


The a d d i t i o n o f h y d r a z i n e h a s l i t t l e e f f e c t o n t h e n e p t u n i u m photo-reactions i n nitric acid. T h e r e i s some e v i d e n c e t h a t i t s e r v e s a s a r e d u c t a n t , b u t i t i s n o t enough t o be s i g n i f i c a n t . Experiments are s t i l l i n progress which should f u r t h e r d e f i n e the Np r e d o x c h e m i s t r y i n t h e p r e s e n c e o f a d d e d r e d u c t a n t s ; b u t t h e c o m p e t i t i o n between t h e v a r i o u s r e d o x r e a c t i o n s i n v o l v i n g b o t h s o l v e n t and added r e a g e n t s l i m i t s t h e p o t e n t i a l f o r a p p l i c a t i o n to p r o c e s s c o n d i t i o n s . Actinide

Separation

Processes

Taking the f o r e g o i n g neptunium photochemistry i n t o account, i t w o u l d b e u s e f u l t o a p p l y i t t o t h e d e v e l o p m e n t o f a more c o m p l e x p h o t o - s e p a r a t i o n scheme t h a n h a s b e e n m e n t i o n e d p r e v i o u s l y ( 1 9 , 2 4 ) , one w h i c h i n c l u d e s N p , U a n d P u . S i n c e N p ( V I ) c a n be p h o t o c h e m i c a l l y r e d u c e d i n t h e a b s e n c e o f a n y i n t e n t i o n a l l y a d d e d r e d u c t a n t s , i t m i g h t be p o s s i b l e t o p h o t o l y z e a n i t r i c a c i d s o l u t i o n c o n t a i n i n g the t h r e e a c t i n i d e s , r e d u c i n g the Np(VI) w h i l e l e a v i n g the Pu(IV) and U ( V I ) unchanged. A f t e r the Np(V) has been removed b y a n e x t r a c t i o n p r o c e s s , h y d r a z i n e c o u l d t h e n be a d d e d and t h e s o l u t i o n p h o t o l y z e d t o a c h i e v e t h e s i m u l t a n e o u s r e d u c t i o n (as was d e s c r i b e d p r e v i o u s l y ) o f UO^" t o and P u ^ to P u ^ w i t h the accompanying a s s i s t a n c e of the r e a c t i o n : 1-

U

4

+

+ 2Pu

4 +

+ 2 H 0 -> U 0 2

2

+

+

+ 2Pu

3 +

+ 4H

+

+

(13)

The s e p a r a t i o n o f Np f r o m U a n d Pu was a c t u a l l y a c c o m p l i s h e d i n a 0 . 5 Ν HNO3 s t a r t i n g s o l u t i o n c o n t a i n i n g a p p r o x i m a t e l y 0 . 0 0 5 M , e a c h , o f N p ( V I ) , U ( V I ) and P u ( I V ) . P r e v i o u s l y , i t was demon s t r a t e d t h a t P u a n d Np c o u l d be i n d i v i d u a l l y s e p a r a t e d f r o m U (19,24). T h e r e s u l t s o f t h e s e s e p a r a t i o n p r o c e s s e s a r e shown i n T a b l e I I I , thus d e m o n s t r a t i n g p h o t o - s e p a r a t i o n s of these ions are f e a s i b l e . Summary To s u m m a r i z e t h e p r o m i s e o f a c t i n i d e p h o t o c h e m i s t r y b r i e f l y , i t has been found t h a t a l l t h r e e major a c t i n i d e s have a u s e f u l v a r i e t y o f p h o t o c h e m i c a l r e a c t i o n s w h i c h c o u l d be a r r a n g e d i n a sequence to a c h i e v e a s e p a r a t i o n s p r o c e s s t h a t r e q u i r e s fewer r e a g e n t s , a n d , u n d e r s t a n d a b l y , a reduced volume o f waste s o l u tions. Most o f t h e impact o f p h o t o c h e m i s t r y on r e p r o c e s s i n g i s a d m i t t e d l y s p e c u l a t i v e s i n c e o n l y the c h e m i c a l f e a s i b i l i t y has been d e m o n s t r a t e d . Nevertheless, the a t t r a c t i v e c h a r a c t e r i s t i c s of photochemical r e a c t i o n s w i l l c o n t i n u e t o be s t a t e d (and s o m e t i m e s o v e r s t a t e d ) a s l a s e r - r e l a t e d r e s e a r c h i n t e r e s t s grow. D u r i n g t h e p a s t y e a r s , we



264


Table I I I .

Effect

o f l i g h t on t h e r e d u c t i v e Np-Pu-U s o l u t i o n s .

extraction

of

S o l u t i o n s (approx 1 0 m l ) t y p i c a l l y c o n t a i n e d approx 0.005 M of each a c t i n i d e i n q u e s t i o n . Wavelength, exposure time, absorbed i n t e n s i t y , s t a r t i n g a c i d i t y , and f i n a l a d j u s t e d a c i d i t y were a s f o l l o w s : A , 254 nm, 20 m i n , 1W, 0 . 5 N , 2 . 0 Ν ; B , 200 nm, 30 m i n , 1W, 1 . 4 N , 2 . 6 Ν ( c o n t a i n e d a l s o 0.5 M h y d r a z i n e n i t r a t e ) ; C , 254 nm, 10 m i n , 1W, 0 . 5 N , 2 . 0 N . A c i d i f i c a t i o n was f o l l o w e d b y e x t r a c t i o n i n t o a n e q u i v a l e n t v o l u m e o f 30% TBP i n d o d e c a n e . P a r t Β taken from p r e v i o u s r e s u l t s (19,24). Dark Sample A

Separation

Light Sample

o f Neptunium from Np-U M i x t u r e

Aqueous Layer

14% Np 12.4% U

93.8% Np 12.3% U

TBP Layer

86% Np 87.6% U

6.2% Np 87.7% U

Separation

of Plutonium from Pu-U M i x t u r e

Aqueous Layer

33% 3%

Pu U

90% 3%

Pu U

TBP Layer

67% 97%

Pu U

10% 97%

Pu U

Separation

o f Neptunium from Np-U-Pu M i x t u r e 82.3% Np

Aqueous

22%

Layer

18.1% U 16% Pu

13% 20%

TBP Layer

78% Np 81.4% U 84% Pu

17.7% Np 86% U 80% Pu

Np

U Pu



18.

Photochemistry

TOTH ET AL.

of the

265

Actinides

have r e a l i z e d s e v e r a l f e a t u r e s w h i c h m e r i t enumerating i n c o n c l u sion: (1) L a s e r p h o t o c h e m i s t r y i s n o t , a t t h e moment, as u n i q u e l y i m p o r t a n t i n f u e l r e p r o c e s s i n g as i t i s i n i s o t o p i c enrichment. T h e p h o t o c h e m i s t r y p r e s e n t l y a t h a n d c a n be s u c c e s s f u l l y accomplished with conventional l i g h t sources. (2) The e a s i e s t p l a c e t o a p p l y p h o t o - r e p r o c e s s i n g i s on t h e t h r e e a c t i nides discussed here. The s o l u t i o n s a r e p o t e n t i a l l y c l e a n e r and more a m e n a b l e t o p h o t o - r e a c t i o n s . (3) O r g a n i c - p h a s e p h o t o r e a c t i o n s a r e p r o b a b l y n o t w o r t h much a t t e n t i o n b e c a u s e o f t h e troublesome s o l v e n t redox c h e m i s t r y a s s o c i a t e d w i t h the p h o t o chemical r e a c t i o n . (4) U p s t r e a m p r o c e s s t r e a t m e n t o n t h e r a f f i n a t e ( d i s s o l v e r s o l u t i o n ) may n e v e r be t o o a t t r a c t i v e s i n c e t h e r a d i a t i o n i n t e n s i t y p r e c l u d e s t h e u s a g e o f many o p t i c a l m a t e r i a l s and t h e n a t u r e o f t h e s o l u t i o n i s s u c h t h a t l i g h t t r a n s m i s s i o n i n t o i t m i g h t be t o t a l l y i m p o s s i b l e . Acknowledgment Research U.S.

sponsored by the

Office

of Basic

Energy

Sciences,

Department of Energy under c o n t r a c t W-7405-eng-26 w i t h

the

Union Carbide Corporation.

Literature Cited 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Croff, A. G.; Tedder, D. W.; Drago, J. P.; Blomeke, J. O.; Perona, J. J. "A Preliminary Assessment of Partitioning and Transmutation as a Radioactive Waste Management Concept"; ORNL/TM-5808, Sept. 1977. See for example: Moore, W. J. "Physical Chemistry"; 4th ed., Prentice Hall Inc.; New Jersey, 1972; p. 792. Calvert, J. G.; Pitts, J. N. Jr. "Photochemistry"; John Wiley & Sons Inc., New York, 1966. Bell, J. T.; Biggers, R. E. J. Mol. Spectry., 1968, 25, 312. Bell, J. T.; Friedman, Η. Α., Billings, M. R. J. Inorg. Nucl. Chem, 1974, 36, 2563. Nemodruk, Α. Α.; Bezrogova, Ε. V. Zh. Anal. Khim, 1968, 23, 388. Bell, J. T.; Buxton, S. R. J. Inorg. Nucl. Chem, 1974, 36, 1575. Palei, P. N.; Nemodruk, Α. Α.; Bezrogova, Ε. V. Radiokhimiya, 1969, 11, 300. Bell, J. T.; Friedman, H. A. J. Inorg. Nucl. Chem, 1976, 38, 831. Friedman, Η. Α.; Toth, L. M.; Bell, J. T. J. Inorg. Nucl. Chem, 1977, 39, 123. Nemodruk, Α. Α.; Bezrogova, Ε. V.; Ivanova, S. Α.; Novikov, Yu. P. Zh. Anal. Khim. 1972, 27, 73. Nemodruk, Α. Α.; Bezrogova, Ε. V.; Ivanova, S. Α.; Novikov, Yu. P. Zh. Anal. Khim. 1972, 27, 1270.



266 13. 14. 15. 16. 17.


18. 19. 20. 21.

22. 23. 24.

Nemodruk, Α. Α.; Bezrogova, Ε. V.; Ivanova, S. Α.; Novikov, Yu. P. Zh. Anal. Khim. 1972, 27, 2414. Nemodruk, Α. Α.; Bezrogova, Ε. V.; Ivanova, S. Α.; Novikov, Yu. P. Zh. Anal. Khim. 1973, 28, 379. Friedman, Η. Α.; Toth, L. M.; Osborne, M. M. J. Inorg. Nucl. Chem. 1979, in press. Toth, L. M.; Friedman, H. A. (results to be published). Carroll, J. L . ; Burns, R. E.; Warren, H. D. "The Photoactivated Reduction of Uranium (VI) to Uranium (IV) Nitrate" HW-70543, 1961. Wilson, A. S. "Organic Phase Reduction of Plutonium in a Purex-Type Process", U.S. Patent 3,620,687 (1971). Friedman, Η. Α.; Toth, L. M.; Bell, J. T. "Method for Selectively Reducing Plutonium Values by a Photochemical Process", U.S. Patent 4,131,527 (12/26/78). Goldstein, M; Barker, J. J.; Gangwer, T. "A Photochemical Technique for Reduction of Uranium and Subsequently Plutonium in the Purex Process", BNL-22443 (1976). Rofer-DePoorter, C. K.; DePoorter, G. L.; Hayter, S. W. "Photochemically Produced Uranium (IV) as a Reductant for Plutonium (IV) and Applications in LWR Fuel Reprocessing", LA-UR-78-383 (1978). DePoorter, G. L . ; Rofer-DePoorter, C. K. "Application of Photochemical Techniques to Actinide Separation Processes", Preceding paper in this symposium. Kistiakowsky, G. B. "Photochemical Processes", ACS Monograph Series, 1928, p. 225. Toth, L. M.; Friedman, Η. Α.; Bell, J. T. "Photochemical Separation of Actinides in the Purex Process" CONF-770506-1 (1977).

RECEIVED May 14, 1979.

Research sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation.


Actinide Separations - ACS Publications - American Chemical Society

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