Study of the reduction of UO 2 by magnesium or calcium dissolved in molten chlorides
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The r e d u c t i o n of solid UO2 to u r a n i u m by m a g n e s i u m or c a l c i u m d i s s o l v e d in t h e i r m o l t e n chlor i d e s has been studied. The rate of r e d u c t i o n p e r unit a r e a of UO2 s u r f a c e , at constant t e m p e r a t u r e and c o n c e n t r a t i o n of r e d u c t a n t in the m o l t e n c h l o r i d e , was found to i n c r e a s e with time to a constant value. The rate of r e d u c t i o n per unit a r e a was o b s e r v e d to be p r o p o r t i o n a l to the c o n c e n t r a t i o n of r e d u c t a n t in the m o l t e n s a l t . The s m a l l i n c r e a s e o b s e r v e d in the r e action rate over the t e m p e r a t u r e range 750 ~ to 850~ s u g g e s t s that the r e d u c t i o n is c o n t r o l l e d by t r a n s p o r t of the r e d u c t a n t to the r e a c t i o n site. Solidified salt, c o n t a i n i n g UO2 p e l l e t s which had been p a r t i a l l y reduced, was s e c t i o n e d , polished, and e x a m i n e d m i c r o s c o p i c a l l y . The p r o d u c t s of the reduction r e a c t i o n f o r m c o n c e n t r i c l a y e r s a r o u n d the UO2 p e l l e t s . L a y e r s of m e t a l l i c u r a n i u m and oxide c o n t a i n i n g s m a l l a m o u n t s of d i s p e r s e d salt a l t e r n a t e d with l a y e r s of salt containing s m a l l a m o u n t s of m e t a l l i c u r a n i u m and oxide. The l a y e r s r u p t u r e d , p r e s u m a b l y b e c a u s e the volume of the p r o d u c t s , u r a n i u m and oxide, is g r e a t e r than the volume of the UO2. T h e r e f o r e , an i m p e r v i o u s l a y e r did not f o r m on the oxide s u r f a c e to inhibit the r e d u c t i o n r e a c tion.
I H I S i n v e s t i g a t i o n was u n d e r t a k e n as a p a r t of a study of the m e c h a n i s m of the p r o c e s s ~ in which u r a n i u m is produced by the reduction of UO2 p e l l e t s e m p l o y i n g a m o l t e n flux of CaClz-CaF~ and a liquid C u - M g - C a alloy. In this p r o c e s s , there a r e t h r e e p h a s e s , solid UO2, m o l t e n flux, and liquid alloy i n i t i a l l y , and m e t a l l i c u r a n i u m and, u n d e r c e r t a i n c o n d i t i o n s , e i t h e r CaO o r MgO appear in the product. T h e r e a r e four p o s s i b l e r e d u c t i o n r e a c t i o n s . Two r e d u c t i o n r e a c t i o n s a r e i n volved when c a l c i u m is d i s s o l v e d in the alloy in one case and in the salt in a n o t h e r case and the other two when m a g n e s i u m is d i s s o l v e d in e i t h e r the alloy or the salt. The f i r s t two r e d u c t i o n r e a c t i o n s with c a l c i u m may be r e p r e s e n t e d by the e q u a t i o n , UO2(s) + 2Ca(alloy or salt) = 2CaO(salt) + U(s)
[1]
and the other two UO2(s) + 2Mg(alloy or salt) = 2MgO(s) + U(s)
[2]
In addition to these c h e m i c a l r e a c t i o n s , t h e r e a r e four physical p r o c e s s e s , i.e., t r a n s p o r t of c a l c i u m or m a g n e s i u m through the alloy o r the flu
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