Reactive hot pressing of nonstoichiometric uranium dioxide
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c o m p o u n d - u r a n i u m m o n o n i t r i d e ; in this i n s t a n c e the s t a r t i n g m a t e r i a l s were u r a n i u m d i n i t r i d e and u r a n i u m m e t a l powder. MATE RIALS P r e l i m i n a r y e x p e r i m e n t s were p e r f o r m e d to obs e r v e the c o m p a c t i o n of s e v e r a l d i f f e r e n t t h e r m a l l y d e c o m p o s a b l e u r a n i u m compounds such as a m m o n i u m d i u r a n a t e (A.D.U.), u r a n o u s oxalate, u r a n y l oxalate, and also m i x t u r e s of u r a n i u m trioxide with o r g a n i c r e d u c t a n t s such a s oxalic acid, a m m o n i u m oxalate and h e x a m e t h y l e n e t e t r a m i n e . However, the m a j o r e x p e r i m e n t a l study was confined to the r e a c t i v e hotp r e s s i n g of u r a n y l oxalate and its d e c o m p o s i t i o n product UOz+x, as none of the other compounds or m i x t u r e s a p p e a r e d to yield a product of c o m m e r c i a l inte re st. U r a n y l oxalate (UO2C204 9 3H20) was p r e c i p i t a t e d f r o m hot, 90~ 0.5 M u r a n y l n i t r a t e solution by the slow addition of 10 pct oxalic acid solution. The p r e c i p i t a t e s were f i l t e r e d and washed with d i s t i l l e d water and then with acetone and d r i e d at room t e m p e r a t u r e . Also m i x t u r e s of UOa and oxalic acid were wet m i x e d in a v i b r a t o r y ball m i l l . X - r a y a n a l y s i s of the product showed c o m p l e t e c o n v e r s i o n of UO3 to UO~C204-nH~O d u r i n g mixing. EQUIPMENT AND PROCEDURE The hot p r e s s i n g was c a r r i e d out i n a h o t - p r e s s i n g unit, which was i n d u c t i v e l y heated by a P h i l l i p s 600 kc g e n e r a t o r . The e q u i p m e n t is capable of h o t - p r e s s i n g up to 2000~ u n d e r 6000 psi, u s i n g a graphite die, or up to 800~ under 20,000 psi, u s i n g a s u p e r a l l o y die. The f u r n a c e c h a m b e r and the p r e s s a s s e m b l y can be e v a c u a t e d to 10 -s t o r r with a f o r e - p u m p b a c k e d by an oil diffusion pump. Although m o s t of the e x p e r i m e n t s were c a r r i e d out in v a c u u m , a g a s e o u s e n v i r o n m e n t , p a r t i c u l a r l y Nf, was used in c e r t a i n e x p e r i m e n t s . The t e m p e r a t u r e of the s p e c i m e n was m e a s u r e d by a P t - P t + Rh t h e r m o c o u p l e i n s e r t e d into one of the p l u n g e r s . The s a m e t h e r m o c o u p l e was a l s o used for a u t o m a t i c t e m p e r a t u r e c o n t r o l of the die. However, the h e a t i n g rate of the die was c o n t r o l l e d m a n u a l l y , on VOLUME 1,JANUARY 1970 199
a s c h e d u l e which was p r e d e t e r m i n e d by t r i a l and e r r o r . A c o n s t a n t h e a t i n g r a t e could not be a c h i e v e d by this t e c h n i q u e . The d i m e n s i o n a l change of the c o m p a c t during uniaxial p r e s s i n g was d e t e r m i n e d by two m e a s u r e m e n t s a) by a dial gage a t t a c h e d to the
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