FURTHER OBSERVATIONS ON OCOM MOX FUEL - MICROSTRUCTURE IN THE VICINITY OF THE PELLET RIM AND FUEL-CLADDING INTERACTION

The fuel investigated was manufactured by Siemens-KWU and irradiated a t low rating in the KWO reactor in Germany, The MOX agglomerates in th e cold outer region of the fuel shared several common features with th e high burn-up structure at the rim of UO2 fuel. It is proposed that i n both cases the mechanism producing the microstructure change is recr ystallisation. Further, it is shown that surface MOX agglomerates do n ot noticeably retard cladding creepdown although they swell into the g ap. The contracting cladding appears able to push the agglomerates bac k into the fuel. The thickness of the oxide layer on the inner claddin g surface increased at points where contact with surface MOX agglomera tes had occurred. Despite this, the mean thickness of the oxide did no t differ significantly from that found in UO, fuel rods of like design . It is judged that the high burn-up structure will form in the UO2 ma trix when the local burn-up there reaches 60 to 80 GWd/tM. Limiting th e MOX scrap addition in the UO2 matrix will delay its formation.