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 at low rating in the KWO reactor in Germany, The MOX agglomerates in the cold outer region of the fuel shared several common features with the high burn-up structure at the rim of UO2 fuel. It is proposed that in both cases the mechanism producing the microstructure change is recrystallisation. Further, it is shown that surface MOX agglomerates do not noticeably retard cladding creepdown although they swell into the gap. The contracting cladding appears able to push the agglomerates back into the fuel. The thickness of the oxide layer on the inner cladding surface increased at points where contact with surface MOX agglomerates had occurred. Despite this, the mean thickness of the oxide did not 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 matrix when the local burn-up there reaches 60 to 80 GWd/tM. Limiting the MOX scrap addition in the UO2 matrix will delay its formation.