Molten Salt Oxidation-Reduction Processes for Fuel Processing - ACS

17 Molten Salt Oxidation-Reduction Processes for Fuel Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 15, 2015 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch017

Processing L. G. MORGAN, L. L. BURGER, and R. D. S C H E E L E Pacific Northwest Laboratory, Richland, WA 99352

Pyrochemical and dry processing methods for nuclear fuel processing may meet nonproliferation criteria more easily than aqueous processing methods. One promising pyrochemical approach is the use of molten nitrate systems as oxidants for irradiated nuclear fuel components. This paper is a progress report of work initiated at the Pacific Northwest Laboratory (PNL) for the Department of Energy in FY1978 under the Pyrochemical and Dry Processing Methods Program, Consolidated Fuel Recycle Program. The specific goals of our project are to: critically evaluate the existing literature; experimentally study the basic chemistry of nitrate melts and their reactions with irradiated fuel materials; investigate volatilization phenomena and separation of fission products from actinides; and determine the compatibility of the process with other nonaqueous steps. This project is continuing, and the results of additional studies will be published at a later date. Review of Previous Studies The use of molten nitrates as oxidants for nuclear fuel components has not been studied as much as other pyrochemical or pyrometallurgical processes. Previous studies of molten n i t r a t e systems were p r i m a r i l y conducted i n European l a b o r a t o r i e s , a l though some research was done i n Japan and the United States. A large body of information e x i s t s on molten n i t r a t e s . Most of the research concerns the p h y s i c a l p r o p e r t i e s and t h e o r e t i c a l aspects of molten n i t r a t e systems. Very l i t t l e information e x i s t s on the behavior of the v a r i o u s elements and t h e i r compounds i n molten n i t r a t e systems. A c t i n i d e Oxides i n Molten A l k a l i Metal N i t r a t e s . The chemi c a l behavior of a c t i n i d e oxides i n molten a l k a l i metal n i t r a t e s i s an area with l i t t l e a v a i l a b l e experimental data. Most i n v e s t i g a t i o n s Q , 2_, 3>, 4 ) , i n c l u d i n g our own, have shown that molten

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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.

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a l k a l i metal n i t r a t e s do not d i s s o l v e uranium d i o x i d e but r a t h e r convert i t to a diuranate according to the r e a c t i o n proposed below:

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 15, 2015 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch017

2 U0

2

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3

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(1)

Avogadro and DePlano reported that the composition o f the u r a nate formed v a r i e d with the composition of the a l k a l i metal n i t r a t e melt, while Yamagishi and Kamemoto reported that only the diuranate species was produced (1, 7). U n f o r t u n a t e l y , analyses of the uranate product were not reported by some authors Ç 3 , 4 ) . Although the evidence i s sparse, some authors have assumed that plutonium dioxide would form a plutonate by a s i m i l a r r e a c t i o n Q , 2, 3, _5, 6). Wurm, however, p r e d i c t e d that plutonium dioxide would be u n a f f e c t e d by contact with molten n i t r a t e s ( 4 ) . In c o n t r a s t to the d i r e c t formation of a uranate s p e c i e s , there i s evidence that both uranium and plutonium d i o x i d e form a s o l u b l e species i n molten a l k a l i metal n i t r a t e s i n the presence of n i t r i c a c i d vapor (8^, 9_, 10) or ammonium n i t r a t e ( 7 ) . However, the s o l u b l e species was converted to an i n s o l u b l e a c t i n i d e species when the temperature was increased over that used f o r dissolution. The s o l u b l e species was not i d e n t i f i e d and only one report (7) analyzed the i n s o l u b l e m a t e r i a l produced. Cohen reported s i m i l a r r e s u l t s f o r neptunium (IV) oxide (11). Thorium oxide would not be expected to r e a c t with molten a l k a l i metal n i t r a t e s . B r a m b i l l a c l a i m s , however, that a s o l u b l e thorium species i s produced i n the molten phase when n i t r i c a c i d vapor i s combined with f l u o r i d e i o n i n molten n i t r a t e s ( 1 0 ) . F i s s i o n Products i n Molten A l k a l i Metal N i t r a t e s . The behavior of f i s s i o n products i n molten a l k a l i metal n i t r a t e s i s a l s o an area with l i t t l e a v a i l a b l e experimental data. The r e a c t i o n of the i r r a d i a t e d oxide f u e l with the molten n i t r a t e melt destroys the e x i s t i n g c r y s t a l l i n e l a t t i c e . S e v e r a l authors have claimed the complete or p a r t i a l r e l e a s e of the rare gases Q , 4, 5, 6, 9, 10, 12, 13) and t r i t i u m Q , 4, 6, 9, 10, 13) to the o f f - g a s stream as a r e s u l t of t h i s change i n c r y s t a l line lattice. In a d d i t i o n , B r a m b i l l a has claimed that both iodine and ruthenium are v o l a t i l i z e d when the molten n i t r a t e reacts with the oxide f u e l (£). In c o n t r a s t to t h i s v o l a t i l i z a t i o n , other l i t e r a t u r e claims that both i o d i n e and ruthenium w i l l be found i n the molten phase (6^, Γ 3 ) . Avogadro and Wurm s t a t e that most of the f i s s i o n products, other than the noble metals, are e i t h e r v o l a t i l e or s o l u b l e i n the n i t r a t e melt, even without a d d i t i o n of n i t r i c acid vapor (_12). In a l a t e r paper, however, Avogadro reports that i o d i n e i s s t a b l e as i o d i d e i n molten n i t r a t e s , and that ruthenium i s p a r t i a l l y s o l u b l e i n the molten phase, and p a r t i a l l y v o l a t i l i z e s , while the m a j o r i t y remains with the


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s o l i d s produced ( 1 ) . He expects to f i n d most of the other f i s s i o n products, such as the rare earths, barium, strontium, and zirconium, i n the s o l i d s ( 1 ) . Chemistry i n Molten A l k a l i Metal N i t r a t e s . The chemical behavior of a c t i n i d e oxides and f i s s i o n products reported i n the previous s e c t i o n s appear to be based mainly on p r e d i c t e d behav i o r ; l i t t l e experimental data has been provided to support the v a r i o u s c l a i m s . T h i s , i n p a r t , may be because s e v e r a l of the claims are reported i n the patent l i t e r a t u r e , r a t h e r than i n t e c h n i c a l documents or i n j o u r n a l l i t e r a t u r e . The chemical behavior of various elements and t h e i r com pounds i s best described by Plambeck and Kerridge (14, 15, 16). However, as t h e i r a r t i c l e s recognize, much remains unknown or incomplete. Two major areas, i n which s t u d i e s are both c o n t r o v e r s i a l and incomplete, are: 1) i d e n t i f i c a t i o n of the a c i d i c and b a s i c species i n n i t r a t e melts, and 2) the thermal decompo s i t i o n of molten n i t r a t e s . Both of these areas are, of course, d i r e c t l y r e l a t e d to the chemistry of molten n i t r a t e s . This lack of i n f o r m a t i o n on chemical r e a c t i o n , as w e l l as the complexity of i r r a d i a t e d - o x i d e f u e l compositions r e s u l t s i n a s i t u a t i o n r e q u i r i n g thorough experimental s t u d i e s . Such studies w i l l e x p l a i n the chemistry of v a r i o u s a c t i n i d e oxides and f i s s i o n products i n molten a l k a l i metal n i t r a t e s . Current PNL

Studies

The goals of our l a b o r a t o r y s t u d i e s were t o : 1) i n v e s t i gate the behavior of uranium d i o x i d e and plutonium d i o x i d e i n molten a l k a l i metal n i t r a t e s , determining the s i g n i f i c a n t exper imental parameters; 2) v e r i f y the claims that s o l u b l e a c t i n i d e species could be obtained i n the molten phase through a d d i t i o n of n i t r i c a c i d vapor; 3) determine the thermal s t a b i l i t y of these s p e c i e s ; 4) determine the composition of the uranates and/or plutonates and study t h e i r conversion to d i o x i d e s ; 5) determine the behavior of s i g n i f i c a n t f i s s i o n products and t h e i r d i s t r i b u t i o n f o r each stage of any proposed process; and 6) study the e f f e c t s of s e l e c t e d anion a d d i t i v e s on the chem i s t r y of a c t i n i d e and f i s s i o n - p r o d u c t s p e c i e s . Uranium Dioxide i n Molten L i t h i u m N i t r a t e / P o t a s s i u m N i t r a t e Eutectic. The behavior of uranium d i o x i d e i n a molten l i t h i u m n i t r a t e / p o t a s s i u m n i t r a t e e u t e c t i c was i n v e s t i g a t e d . Our goal was to determine whether a s o l u b l e uranium species could be pro duced u s i n g a n i t r i c - a c i d vapor sparge. The m a t e r i a l s used were a l i t h i u m n i t r a t e / p o t a s s i u m n i t r a t e e u t e c t i c mixture, pure n i t r i c a c i d (100% Η Ν Ο 3 ) , and powdered uranium d i o x i d e . The mass r a t i o of uranium d i o x i d e to the n i t r a t e melt was 1:100. The experiments were performed u s i n g a Pyrex r e a c t i o n tube



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e q u i p p e d w i t h a g a s - s p a r g e t u b e and e x i t v e n t . The r e a c t i o n t u b e and c o n t e n t s were p l a c e d i n a p o t - t y p e f u r n a c e e q u i p p e d w i t h v i e w i n g p o r t s so t h a t v i s u a l o b s e r v a t i o n o f t h e m e l t was possible. The r e s u l t s o f the t e s t showed t h a t the u r a n i u m d i o x i d e produced a v i g o r o u s r e a c t i o n w i t h the n i t r a t e m e l t , b e g i n n i n g at approximately 4 0 0 ° C . A t t h i s t i m e t h e m e l t was open t o t h e a t m o s p h e r e , and no n i t r i c a c i d was b e i n g a d d e d . Nitrogen dioxi d e was e v o l v e d as a g a s e o u s p r o d u c t o f the r e a c t i o n , b u t t h e r e was no v i s u a l i n d i c a t i o n o f any u r a n i u m s o l u b i l i t y i n t h e m o l t e n n i t r a t e eutec t i c . C o m p l e t e r e a c t i o n , i n d i c a t e d by the cessation o f gas e v o l u t i o n , r e q u i r e d a p p r o x i m a t e l y 2 . 5 h o u r s a t 5 0 0 ° C . The p r o d u c t o f t h i s r e a c t i o n a p p e a r e d to have s m a l l e r p a r t i c l e s t h a n the o r i g i n a l u r a n i u m d i o x i d e , w h i c h s e t t l e d r a p i d l y to the bottom o f the r e a c t i o n t u b e . T h e s e p a r t i c l e s were r e c o v e r e d b y a q u e o u s d i s s o l u t i o n o f t h e s o l i d i f i e d e u t e c t i c and f i l t r a t i o n o f the i n s o l u b l e u r a n i u m - c o n t a i n i n g s p e c i e s . No h y d r o l y s i s o f t h e i n s o l u b l e u r a n i u m - c o n t a i n i n g s p e c i e s i s b e l i e v e d to o c c u r w i t h aqueous d i s s o l u t i o n o f t h e s a l t m a t r i x . To p r e c l u d e h y d r o l y s i s i n e x p e r i m e n t s where i t was more l i k e l y , t h e n i t r a t e s a l t s were d i s s o l v e d i n 0 . 5 M HNO3. A s e c o n d t e s t was c o n d u c t e d u n d e r t h e above c o n d i t i o n s . R e a c t i o n o f t h e u r a n i u m d i o x i d e was assumed t o be c o m p l e t e when gas e v o l u t i o n c e a s e d . A t t h a t p o i n t , the temperature o f the m e l t was r e d u c e d t o 2 0 0 ° C , and n i t r i c a c i d v a p o r was added t o the m e l t . The n i t r i c a c i d v a p o r was c a r r i e d f r o m a h e a t e d v e s s e l c o n t a i n i n g 100% n i t r i c a c i d w i t h the i n e r t gas s p a r g e . T r a n s f e r l i n e s were h e a t e d t o m i n i m i z e c o n d e n s a t i o n . The q u a n t i t y and t r a n s f e r r a t e o f t h e n i t r i c a c i d were n o t d e t e r m i n e d . A d d i t i o n of the n i t r i c a c i d produced a r e a c t i o n w i t h the s o l i d s p r e s e n t , shown by gas e v o l u t i o n f r o m the s o l i d ' s s u r f a c e , w h i c h y i e l d e d a s o l u b l e uranium s p e c i e s i n the n i t r a t e m e l t . The t o t a l s o l i d s were d i s s o l v e d , w h i c h p r o d u c e d a c h a r a c t e r i s t i c u r a n y l c o l o r i n the m e l t . A f t e r complete d i s s o l u t i o n of the u r a n i u m s p e c i e s , t h e n i t r i c a c i d s p a r g e was r e m o v e d , and t h e m e l t was o p e n t o t h e a t m o s p h e r e . The s o l u b l e u r a n i u m s p e c i e s p r o d u c e d was s t a b l e a t 2 0 0 ° C f o r a t l e a s t t h r e e h o u r s , a t w h i c h t i m e t h e t e m p e r a t u r e was g r a d u a l l y i n c r e a s e d f r o m 2 0 0 ° C t o 3 0 0 ° C o v e r a p e r i o d o f one hour. S u s p e n d e d s o l i d s were f i r s t n o t e d a t 3 0 0 ° C . The t e m p e r a t u r e was i n c r e a s e d t o 3 5 0 ° C f o r one h o u r , t h e n t o 4 0 0 ° C f o r 0.5 h o u r . The s o l i d s p r o d u c e d t h r o u g h t h e t h e r m a l d e c o m p o s i t i o n o f t h e s o l u b l e u r a n i u m s p e c i e s were a c c o m p a n i e d by gas evolution. We d i d n o t d e t e r m i n e i f t h i s gas e v o l u t i o n was s i m p l y r e m o v a l o f e x c e s s n i t r i c a c i d from the m e l t or a d e c o m p o s i tion product. The s o l i d s were f o r m e d u n i f o r m l y t h r o u g h o u t t h e m e l t and h a d a g e l a t i n o u s a p p e a r a n c e . These s o l i d s s e t t l e d v e r y s l o w l y to the bottom o f the r e a c t i o n t u b e . The f i n a l p r o d u c t o f t h i s s e c o n d t e s t was a l s o r e c o v e r e d by a q u e o u s d i s s o l u t i o n o f


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t h e n i t r a t e e u t e c t i c and f i l t r a t i o n o f t h e i n s o l u b l e u r a n i u m species. We have n o t c o m p l e t e d i d e n t i f y i n g the f i n a l p r o d u c t s p r o d u c e d i n t h e two e x p e r i m e n t s . However, t h e s e p r e l i m i n a r y e x p e r i m e n t s v e r i f i e d c l a i m s ( £ , 9_, 10) t h a t a s o l u b l e u r a n i u m s p e c i e s can be formed i n m o l t e n a l k a l i m e t a l n i t r a t e s t h r o u g h the a d d i tion of n i t r i c acid vapor. Uranium D i o x i d e i n M o l t e n Equimolar Sodium-Potassium Nitrate. A f t e r c o m p l e t i n g t h e above e x p e r i m e n t s , we d e c i d e d t o u s e an e q u i m o l a r s o d i u m - p o t a s s i u m n i t r a t e m e l t t o p r e c l u d e the r a p i d a t t a c k o f o u r P y r e x r e a c t i o n v e s s e l s t h a t we n o t e d when l i t h i u m n i t r a t e was u s e d . U r a n i u m d i o x i d e r e a c t e d w i t h the e q u i m o l a r NaN03~KN03 m e l t a t a p p r o x i m a t e l y 3 5 0 ° C . The r e a c t i o n was f a i r l y v i g o r o u s , b u t a t no t i m e was i t e x c e s s i v e . D u r i n g t h i s p o r t i o n o f the e x p e r i m e n t , an i n e r t gas ( a r g o n ) s p a r g e was m a i n t a i n e d to a g i t a t e t h e m e l t and to sweep o u t t h e NO2 p r o d u c e d . Complete r e a c t i o n r e q u i r e d about 2.5 hours at a maximum t e m p e r a t u r e o f 4 2 0 ° C A f t e r r e a c t i o n was c o m p l e t e and the i n e r t gas s p a r g e h a d removed the r e m a i n i n g NO2, the s u p e r n a t e was c l e a r and c o l o r less. The m e l t t e m p e r a t u r e was t h e n l o w e r e d t o 2 7 5 ° C , and n i t r i c a c i d v a p o r was added t o t h e m e l t as p r e v i o u s l y d e scribed. The u r a n i u m c o m p l e t e l y d i s s o l v e d . The t e m p e r a t u r e was s l o w l y i n c r e a s e d t o 4 2 0 ° C o v e r one h o u r . The f o r m a t i o n o f s o l i d s was f i r s t a p p a r e n t a t 3 0 0 ° C t o 3 2 0 ° C Formation of the s o l i d s p r o d u c e d a l a r g e q u a n t i t y o f NO2; s p a r g i n g removed t h i s gaseous p r o d u c t . When t h e t h e r m a l d e c o m p o s i t i o n was comp l e t e , the m e l t was a g a i n c l e a r and c o l o r l e s s . Plutonium Dioxide i n Molten Equimolar Sodium-Potassium Nitrate. The b e h a v i o r o f p l u t o n i u m d i o x i d e i n m o l t e n a l k a l i m e t a l n i t r a t e s i s an a r e a o f m a j o r c o n c e r n . Claims that a l k a l i m e t a l p l u t o n a t e s a r e formed C l , 2^, 3^, .5, 6 0 a r e n o t s u b s t a n t i a t e d by d e f i n i t i v e a n a l y t i c a l r e s u l t s . I n some c a s e s (i>, 6^), s o d i u m p e r o x i d e was added as an o x i d a n t t o e i t h e r an a l k a l i m e t a l n i t r a t e m e l t (6) o r t o an a l k a l i m e t a l h y d r o x i d e m e l t (50. I f t h e t e m p e r a t u r e i s g r e a t e n o u g h , f o r example above 7 0 0 ° C , t h e r m a l d e c o m p o s i t i o n o f the n i t r a t e m e l t p r o d u c e s p e r oxide species. O t h e r s t u d i e s ( 4 , 9^, 12^, 17) do n o t c l a i m f o r m a t i o n o f a p l u t o n a t e s p e c i e s , b u t o n l y s t a t e t h a t an i n s o l u b l e p l u t o n i u m - c o n t a i n i n g compound e x i s t s . However, i n a l l the r e f e r e n c e s c i t e d , t h e r e s u l t s were g i v e n f o r m i x e d u r a n i u m p l u t o n i u m d i o x i d e ; d e f i n i t i v e a n a l y t i c a l r e s u l t s were n o t g i v e n . O u r s t u d i e s o f the b e h a v i o r o f p l u t o n i u m d i o x i d e i n m o l t e n a l k a l i m e t a l n i t r a t e s were c o n d u c t e d i n e q u i m o l a r s o d i u m potassium n i t r a t e without a d d i t i o n of peroxide. M e l t temperat u r e s were low enough so t h a t t h e r m a l d e c o m p o s i t i o n was n o t expected to produce peroxide s p e c i e s . I n a d d i t i o n , we s t u d i e d


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b o t h the b e h a v i o r of plutonium dioxide.

plutonium dioxide

and m i x e d u r a n i u m -

The b e h a v i o r o f p l u t o n i u m d i o x i d e i s h i g h l y d e p e n d e n t u p o n the h i s t o r y o f the m a t e r i a l . Thus, although i r r a d i a t e d p l u t o n i u m d i o x i d e may d i f f e r f r o m l a b o r a t o r y s a m p l e s , i t i s e s s e n t i a l t h a t t h e s a m p l e s be w e l l d o c u m e n t e d . The p l u t o n i u m d i o x i d e u s e d i n o u r s t u d i e s was p r e p a r e d i n t h e f o l l o w i n g m a n n e r : 1) A plut o n i u m ( I V ) n i t r a t e s o l u t i o n was p u r i f i e d b y i o n e x c h a n g e . 2) The Pu ( I V ) was r e d u c e d t o Pu ( I I I ) with ascorbic acid. 3) P l u tonium ( I I I ) o x a l a t e was p r e c i p i t a t e d w i t h o x a l i c a c i d and a i r dried. 4 ) The o x a l a t e was c a l c i n e d a t 7 5 0 ° C . Typical c a l c i n a t i o n temperatures f o r plutonium d i o x i d e prepared f o r f u e l s m a n u f a c t u r e are 6 5 0 ° C to 8 5 0 ° C . O u r m a t e r i a l was n o t s i n t e r e d at h i g h e r temperatures, a l t h o u g h t h i s i s n o r m a l l y done, i n t h e p r e p a r a t i o n o f m i x e d o x i d e f u e l s f o l l o w i n g b l e n d i n g and p e l let formation. F o r t h i s s t u d y we c h o s e a mass r a t i o o f 1 : 1 0 0 , p l u t o n i u m dioxide-to-nitrate melt. The e x p e r i m e n t s were p e r f o r m e d i n a glove box, designed for containment of high a l p h a - a c t i v i t y materials. P l u t o n i u m d i o x i d e and e q u i m o l a r N a N Û 3 - K N 0 3 were added to t h e r e a c t i o n v e s s e l , and t h e t e m p e r a t u r e was i n c r e a s e d to 5 2 5 ° C . The t e m p e r a t u r e e x c e e d e d 3 5 0 ° C a p p r o x i m a t e l y t h r e e h o u r s , and 5 0 0 ° C f o r a t l e a s t one h o u r . No r e a c t i o n o f p l u t o n i u m d i o x i d e was n o t e d a t any t i m e . The m e l t r e m a i n e d c l e a r and c o l o r l e s s t h r o u g h o u t t h e e n t i r e p e r i o d . It therefore a p p e a r s t h a t p l u t o n i u m d i o x i d e does n o t r e a c t w i t h t h e a l k a l i m e t a l n i t r a t e m e l t to produce a p l u t o n a t e w i t h i n the t e m p e r a t u r e range c i t e d ( < 5 2 5 ° C ) . The t e m p e r a t u r e o f t h e m e l t was t h e n r e d u c e d t o 2 7 5 ° C , and n i t r i c a c i d v a p o r was a d d e d . A t no t i m e were t h e r e any v i s u a l i n d i c a t i o n s o f s o l u b i l i t y o r r e a c t i o n o f the p l u t o n i u m d i o x i d e i n the m e l t . A n a l y t i c a l r e s u l t s o f the study o f the b e h a v i o r o f p l u t o n i u m d i o x i d e i n e q u i m o l a r s o d i u m - p o t a s s i u m n i t r a t e show t h a t , u n d e r t h e c o n d i t i o n s c i t e d , p l u t o n i u m d i o x i d e d i d n o t r e a c t and d i d not form a s o l u b l e s p e c i e s e i t h e r w i t h the o r i g i n a l m e l t or w i t h the a d d i t i o n o f 1 0 0 % n i t r i c a c i d v a p o r . The a n a l y t i c a l r e s u l t s were b a s e d on a l p h a - e n e r g y a n a l y s i s o f s a m p l e s o f t h e m o l t e n phase t a k e n t h r o u g h o u t the e x p e r i m e n t . A l l m o l t e n phase s a m p l e s were d i s s o l v e d i n 0 . 5 M H N O 3 . Mixed Uranium-Plutonium D i o x i d e i n Equimolar SodiumPotassium N i t r a t e . The b e h a v i o r o f two c o m p o s i t i o n s o f m i x e d u r a n i u m - p l u t o n i u m d i o x i d e has b e e n i n v e s t i g a t e d t h u s f a r a t PNL. The f i r s t c o m p o s i t i o n , d e s i g n a t e d m a t e r i a l A , c o n s i s t s of 5.44% P u Û 2 / 9 4 . 5 6 % U O 2 . The s e c o n d c o m p o s i t i o n , d e s i g n a t e d m a t e r i a l B , c o n s i s t s o f 27.56% P u 0 / 7 2 . 4 4 % U 0 . B o t h m a t e r i a l s were a c q u i r e d as p e l l e t s t h a t had b e e n s i n t e r e d a t 1700°C. The b e h a v i o r o f b o t h m i x e d - o x i d e m a t e r i a l s was s t u d i e d u n d e r t h e same c o n d i t i o n s u s e d i n t h e p r e v i o u s experiments. 2

2



17.

MORGAN ET AL.

Molten

Salt Redox

Processing

239

Reaction of m a t e r i a l A was s i m i l a r to that of powdered u r a nium d i o x i d e , although a higher temperature, 400°C, was required to i n i t i a t e the r e a c t i o n . Reaction of m a t e r i a l A was completed at 450°C. The temperature o f the melt was reduced to 275°C f o l l o w i n g the completion of the o x i d a t i o n r e a c t i o n , and n i t r i c acid vapor was added as i n previous experiments. Most o f the s o l i d s d i s s o l v e d i n the melt, producing the charac t e r i s t i c u r a n y l c o l o r , but not a l l of the s o l i d s could be d i s solved w i t h t h i s treatment. P r e l i m i n a r y analyses of the molten phase taken throughout the experiment i n d i c a t e d that plutonium d i o x i d e remained i n s o l u b l e at a l l times. A l l molten phase sam ples were d i s s o l v e d i n 0.5 M H N O 3 before they were analyzed. M a t e r i a l Β required an even greater melt temperature than m a t e r i a l A to i n i t i a t e and complete the o x i d a t i v e r e a c t i o n , 500°C to 550°C. The r e a c t i o n rate a l s o appeared to be l e s s than that i n previous s t u d i e s , even at t h i s higher temperature. Reduction of the melt temperature to 275°C and a d d i t i o n of n i t r i c a c i d vapor produced the same r e s u l t s as f o r m a t e r i a l A. Although these s t u d i e s are not complete, i t appears that molten a l k a l i metal n i t r a t e s w i l l react with mixed uraniumplutonium dioxide m a t e r i a l o f v a r y i n g composition. Higher melt temperatures and longer r e a c t i o n times are r e q u i r e d , however, as the plutonium enrichment i s increased. The i n s o l u b i l i t y of p l u tonium d i o x i d e must c e r t a i n l y be i n v e s t i g a t e d f u r t h e r since i t s s o l u b i l i t y i n the molten phase upon a d d i t i o n of n i t r i c a c i d vapor has been claimed i n v a r i o u s patents (8^, 10). I t should a l s o be noted that the behavior of i r r a d i a t e d uranium-plutonium d i o x i d e may d i f f e r from the m a t e r i a l used i n our experiments. A l k a l i Metal Uranate. P r e l i m i n a r y s t u d i e s to determine the composition of the a l k a l i metal uranates formed i n the v a r i o u s a l k a l i metal n i t r a t e melts have a l s o been conducted at PNL. Uranium dioxide was reacted with the f o l l o w i n g m e l t s : sodium n i t r a t e ; equimolar sodium-potassium n i t r a t e ; and l i t h i u m potassium n i t r a t e e u t e c t i c , 42.2 mole% l i t h i u m n i t r a t e . A 20:1 mole r a t i o of n i t r a t e to uranium d i o x i d e was used i n each case. The r e a c t i o n was conducted i n a m u f f l e furnace at 425°C f o r 4.5 hours. The s o l i d i f i e d and cooled s a l t cake from the r e a c t i o n was d i s s o l v e d by a d d i t i o n of d i s t i l l e d water. Hydrolysis of the i n s o l u b l e uranium r e a c t i o n product i s not expected to occur. The remaining s o l i d s were f i l t e r e d and thoroughly washed with d i s t i l l e d water on a f r i t t e d g l a s s f i l t e r . The f i l t e r plus the s o l i d s were d r i e d i n a vacuum oven at 120°C f o r two days. The a l k a l i metal species and content were determined by atomic absorption. The uranium content was determined by x-ray fluorescence; strontium was used as an i n t e r n a l standard. X-ray



240


powder d i f f r a c t i o n p a t t e r n s were a l s o o b t a i n e d f o r c o m p a r i s o n t o known compounds. Table I l i s t s t y p i c a l analyses. E m p i r i c a l f o r m u l a s f o r the p r o d u c t s f r o m e a c h m e l t s y s t e m were o b t a i n e d ; we assumed t h a t o x y g e n was t h e o t h e r constituent o f the compounds. The e l e m e n t a l r a t i o s a r e s u b j e c t t o some v a r i a t i o n because o f u n c e r t a i n t i e s i n the a n a l y t i c a l d a t a . The u r a n i u m a n a l y s e s are e s t i m a t e d to be v a l i d w i t h i n ± 2 % . Indepen d e n t a n a l y t i c a l d e t e r m i n a t i o n s have shown t h a t t h e o r i g i n a l u r a nium d i o x i d e c o n t a i n e d a p p r o x i m a t e l y 0.5% i r o n , p l u s a t r a c e o f silica. Adjustment of the a n a l y t i c a l d a t a f o r t h e s e m i n o r i m p u r i t i e s was n o t d o n e . P r o d u c t s o f b o t h the s o d i u m n i t r a t e and e q u i m o l a r s o d i u m potassium n i t r a t e melts indicated that a diuranate species, U 2 O 7 , had been f o r m e d . The p r o d u c t o b t a i n e d f r o m e q u i m o l a r s o d i u m - p o t a s s i u m n i t r a t e i n d i c a t e d t h a t s o d i u m was t h e f a v o r e d c a t i o n i n the compound; l e s s t h a n 1 0 % o f the a l k a l i m e t a l was potassium. X - r a y powder d i f f r a c t i o n d a t a s u p p o r t t h e e v i d e n c e t h a t the d i u r a n a t e s p e c i e s i s formed i n b o t h o f t h e s e m e l t systems. The p r o d u c t o b t a i n e d f r o m the l i t h i u m n i t r a t e - p o t a s s i u m n i t r a t e e u t e c t i c i s n e i t h e r i d e n t i f i a b l e as t h e d i u r a n a t e ( U 2 O 7 ) o r u r a n a t e ( U 0 4 ~ ) s p e c i e s , n o r as a m i x t u r e com posed o f each o f t h e s e s p e c i e s . To d a t e , t h e x - r a y powder d i f f r a c t i o n p a t t e r n h a s n o t b e e n i d e n t i f i e d w i t h any known com pound. I t i s i n t e r e s t i n g to n o t e t h e r a t i o s o f t h e a l k a l i m e t a l s f o r t h i s system, however: approximately a 2 : 1 atomic r a t i o o f l i t h i u m to p o t a s s i u m , i n d i c a t i n g l i t h i u m i s the preferred cation. No c h a n g e i n c o m p o s i t i o n o f the u r a n a t e p r o d u c t was f o u n d i n the equimolar sodium-potassium n i t r a t e f o l l o w i n g d i s s o l u t i o n o f the o r i g i n a l u r a n a t e w i t h n i t r i c a c i d v a p o r and s u b s e q u e n t thermal decomposition of that s o l u b l e s p e c i e s . A n a l y s e s of the u r a n a t e produced over 3 0 0 ° C to 4 5 0 ° C i n d i c a t e d t h a t o n l y the diuranate species i s formed. There was, however, a t r e n d toward a d e c r e a s i n g p o t a s s i u m c o n t e n t as the r e a c t i o n t e m p e r a t u r e increased. the

B a s e d on t h e above d a t a , u r a n a t e p r o d u c t formed i s Table I.

Constituent,

Alkali

%

i t a p p e a r s t h a t the s t a b i l i t y i n the o r d e r o f : Na»K

Molten Salt Oxidation-Reduction Processes for Fuel Processing - ACS

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