- 27 a.
Fuel Accidents
These accidents arise from abnormal q u a n t i t i e s of f u e l i n the coreu This condition could arise from f u e l accumulation i n the core; from sudden l o s s of such accumulation; from deposition i n the external system, with subsequent return of t h i s f u e l t o the core, Examples of possible d i f f i c u l t i e s of this type are:
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(1) Formation of uranium-rich l i q u i d phase by l o c a l overheating; ( 2 ) p r e c i p i t a t i o n of uranium, containing mixed sulphate brought on by build-up of dissolved nickel (most probable i n h o t t e s t region); (3) i n inclusion of uranium i n (or absorption on) the corrosion product on zirconium a l l o y core vessel, o r s i m i l a r association with external system s t a i n l e s s s t e e l s o l i d corrosion product ; (4) deposition of uranium-bearing residue by l o c a l boiling; (5) s e t t l i n g of s l u r r y i n r e a c t o r ; ( 6 ) s e t t l i n g o r caking of s l u r r y i n external system,
(1) Formation of s o l i d 3LiFeThF4 (including some uranium), a consequence of too low a temperature; ( 2 ) p r e c i p i t a t i o n of mixed trifl.uorides, when t h e sum of the concentrations of rare earths, t r i v a l e n t plutonium and uranium (produced by reaction of the f u e l with the nickel-bearing container material) exceeds the s o l u b i l i t y l i m i t ; ; ( 3 ) p r e c i p i t a t i o n o f UOz9 U03, o r U02F2 by the inleakage of oxygen o r water; (4) association of f u e l with deposited f i s s i o n products such as molybdenum and ruthenium ( t h i s seems unlikely, judging from t h e limited information available) ; ( 5 ) association o f f u e l with INOR-8corrosion products ,
m (1) P r e c i p i t a t i o n of uranium from t h e solution o r p a r t i a l freezing of the bismuth with possible g r e a t e r than normal uranium content i n the s o l i d f o r s o l u t i o n f u e l , t h e increased uranium cantent is expected; f o r s l u r r y f u e l i t can be t r u e ; (2) accumulation of m a s s of s l u r r y p a r t i c l e s (presumably by f l o t a t i o n ) i n t h e reactor; ( 3 ) accumulation of m a s s s l u r r y p a r t i c l e s i n external system; (4) possible contribution of above items by enough oxygen inleakage t o prevent wetting of the s l u r r y by bismuth; ( 5 ) association of f u e l with i r o n and chromium deposited on cold areas (mass transfer).
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In addition t o accidents a r i s i n g from f b e l inhomogeneity, there is a l s o the p o s s i b i l i t y of a f u e l accident a r i s i n g from a concentrat i o n error. Such wrong concentrations could arise from mismanagement during start-up.
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