A study has been made of self-sufficient, equilibrium thorium (SSET) cycles in 1200 MW(e) (i.e. electrical p o w e r ) , CANDU-PHW (Canada Deuterium l[rani um-PYessuri zed h[eavy W a t e r ) units. In these eyries a mixture of thorium and fissile material is used as feed fuel. The spent fuel is reprocessed and the uranium (mainly U 2 3 3 ) s e p a r a t e d , combined with thorium and fed back to the reactor. Once an equilibrium mode of operation has been reached no external supply of fissile material is required. We find that equilibrium burnups of about 10 MWd/kg of heavy element per pass are feasible in 1200 M W ( e ) , CANDU-PHW units operating on an SSET cycle provided careful attention is given to neutron e c o n o m y , and reprocessing plus fabrication losses can be kept below 1%. Using highly enriched U-235 as the externally supplied fissile material for the early operating period before e q u i l i b r i u m , mined uranium requirements as low as 1 Mg ( U ) / M W ( e ) of power capacity can be achieved for credible cases. The studies show that with economic parameters in the range foreseen for the near f u t u r e , SSET cycles incur a penalty of some 20% in electrical energy costs compared to optimum thorium c y c l e s . This result depends quite strongly on values assumed for urani u m , reprocessing and fabrication c o s t s . Future trends in these parameters are expected to be in a direction to reduce the percentage penalty. We conclude that for a country like Canada, which cannot afford the expense associated with d e v e l o p m e n t of a new reactor concept such as the fast b r e e d e r , SSET cycles can provide insurance against severe uranium s h o r t a g e s . For this reason they deserve further analytical and experimental c o n s i d e r a t i o n . * Presented at the ANS 1975 Winter M e e t i n g , San F r a n c i s c o , 16-21 N o v e m b e r , 1975. ATOMIC ENERGY OF CANADA LIMITED Chalk Rlxjzfi Hucltaft LabonatoKio.i> Chalk Rive.fi, Qntanio March 1976 AECL-5501 P R O S P I I C T S F O R S F L F S l i F F I C J r N T F n r i L U R T U " T » : O P I U e v e n s i!'! CA;:DU REACTOR? C. Ciitcph, S. B a n t ' l j C C , F.( ' . ' . Raicilatj, V. Hao M . S . M i C g t a m , J . T . V a e d e x 1. I N T R O D U C T I O N In t h i s p a p e r w e c o n s i d e r one e x t r e m e o f t h e rar.qe o f p o s s i b l e t h o r i u m c y c l e s in C A N D U r e a c t o r s t h e e x t r e m e w h i c h m i n i m i z e : o v e r a l l u r a n i u m r e q u i r ^ e n t s . 1 . 1 The. CA.VPU Ue.act.c-i Concept C a n a d a h a s d e v e l o p e d a he a v y w a t e r m o d e r a t e d r e a c t o r for p r o d u c t i o n of n u c l e a r e l e c t r i c p o w e r c a l l e d the C A N D U P H W 2 ) * . T h e m a i n f e a t u r e s of the r e a c t o r a r e s h e w n in F i g u r e 1. H e a v y w a t e r m o d e r a t o r is h e l d in the c a l a n d r i a t a n k w h i c h is p e n e t r a t e d by p r e s s u r e t u b e s c o n t a i n i n g n u c l e a r f u e l . T h e fuel is c o o l e d by s e p a r a t e h e a v y w a t e r , t y p i c a l l y a t a b o u t 10 MPa ( 1 4 0 0 p s i ) , w h i c h is c i r c u l a t e d t h r o u g h t he p r e s s u r e t u b e s . T he he a t r e m o v e d is u s e d to p r o d u c e s t e a m to d r i v e a t u r b i n e as s h o w n s c h e m a t i c a l l y i n Fi g u r e 2. I n i t i a l l y , n a t u r a l u r a n i u m w a s u s e d as the fuel and p r e s s u r i z e d h e a v y w a t e r as the c o o l a n t . T h i s e n a b l e d C a n a d a to d e v e l o p a n u c l e a r i n d u s t r y w i t h o u t the h i g h i n i t i a l c a p i t a l e x p e n s e o f u r a n i u m e n r i c h m e n t and fuel r e p r o c e s s i n g p l a n t s , a l t h o u g h D 2 0 p r o d u c t i o n p l a n t s a r e r e q u i r e d . H o w e v e r an i m p o r t a n t f e a t u r e o f the b a s i c C A N D U c o n c e p t is t h a t it c a n e v o l v e to u s e d i f f e r e n t c o o l a n t s ( l i g h t w a t e r or o r g a n i c l i q u i d s ) and d i f f e r e n t f u e l s ( u r a n i u m or t h o r i u m ) . D e v e l o p m e n t of t h e use of * C A N D U P H W s i g n i f i e s a C a n a d i an d e s i g n u s i n g d e u t e r i u m o x i d e ( h e a v y w a t e r ) as m o d e r a t o r , n a t u r a l u / a n i u m as fuel and p r e s s u r i z e d j^eavy w a t e r as c o o l a n t .