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S E V E R A L liquid m e t a l s a r e b e i n g c o n s i d e r e d as h i g h t e m p e r a t u r e coolants for use in a d v a n c e d e n e r g y s y s t e m s . Liquid m e t a l s , such as s o d i u m , p o t a s s i u m , and s o d i u m - p o t a s s i u m a l l o y s , a r e e i t h e r used o r s u g g e s t e d as h e a t - t r a n s f e r m e d i a in Liquid Metal F a s t B r e e d e r (LMFBR) and C o n t r o l l e d T h e r m o n u c l e a r P e a c t o r s (CTR). Liquid l i t h i u m has b e e n p r o p o s e d a s the t r i t i u m - b r e e d i n g b l a n k e t and, p o s s i b l y , as the p r i m a r y coolant in c u r r e n t CTR concepts. A n u m b e r of liquid m e t a l s a r e also c o n s i d e r e d as s e e d m a t e r i a l s for gas s t r e a m s to make an e l e c t r i c a l l y c o n d u c t i n g fluid for use in m a g n e t o h y d r o d y n a m i c (MHD) p o w e r - g e n e r a t i o n s c h e m e s . In these a p p l i c a t i o n s , the s t r u c t u r a l m a t e r i a l of the r e a c t o r s m u s t be c o m p a t i b l e with the liquid m e t a l and m u s t also m a i n t a i n s t r u c t u r a l i n t e g r i t y for long t i m e s at e l e v a t e d t e m p e r a t u r e s upon e x p o s u r e to l a r g e t e m p e r a t u r e g r a d i e n t s , t h e r m a l c y c l i n g , and, p o s s i b l y , intense irradiation. Austenitic stainless steels, f e r r i tic s t e e l s with low c h r o m i u m and m o l y b d e n u m c o n t e n t s , n i c k e l - b a s e a l l o y s , and r e f r a c t o r y m e t a l alloys a r e e i t h e r b e i n g used o r c o n s i d e r e d as s t r u c t u r a l m a terials in these systems. ADVANCED ENERGY SYSTEMS Fig. 1 shows a s c h e m a t i c d i a g r a m of a s o d i u m cooled f a s t - b r e e d e r r e a c t o r . In t h e s e r e a c t o r s , heat is g e n e r a t e d f r o m f i s s i o n r e a c t i o n s in a n u c l e a r fueled core. The heat is r e m o v e d by the liquid m e t a l flowing through the c o r e , the heat in the liquid m e t a l is t r a n s f e r r e d to a s t e a m s y s t e m , and the s t e a m is u s e d to d r i v e a t u r b i n e . B e c a u s e of e x c e l l e n t h e a t - t r a n s f e r p r o p e r t i e s , liquid s o d i u m has b e e n s e l e c t e d as the h e a t - t r a n s p o r t fluid in these r e a c t o r s . Types 304 and 316 a u s t e n i t i c s t a i n l e s s s t e e l a r e b e i n g u s e d for the fuel cladding, i n - c o r e s t r u c t u r a l c o m p o n e n t s , piping, and i n t e r m e d i a t e heat e x c h a n g e r (IHX). F e r r i t i c s t e e l s a r e , at p r e s e n t , used for the s t e a m g e n e r a t o r in the s e c o n d a r y sodium coolant s y s t e m . The s e l e c t i o n of a p a r t i c u l a r m a t e r i a l and c o m b i n a t i o n s t h e r e o f that will be used in both the p r i m a r y and s e c o n d a r y c i r c u i t s is K. NATESANis Metallurgist, Argonne National Laboratory (Materials Science Division) Argonne, IL 60439. Work s