Effects of Rim Structure Formation on the Thermal Conductivity of UO2 Pellets

Abstract

Thermal diffusivities of UO2 pellets irradiated in a test reactor were measured by using a laser flash method. The maximum burnups of the samples were about 85 GWd/t and some of the samples contained rim structures. Thermal diffusivities of irradiated pellets decreased compared with those of unirradiated and simulated soluble fission products-doped UO2 pellets. Hysteresis phenomena in the thermal diffusivity of irradiated fuel, which had been reported before, were not clearly observed in UO2 pellets in which the rim structures had formed. The thermal conductivities for irradiated UO2 pellets were evaluated from measured thermal diffusivities, specific heat capacities of unirradiated UO2 pellets and measured sample densities. The thermal conductivities of irradiated UO2 were compared to those of unirradiated UO2 pellets. The relative thermal conductivities were normalized to those of 96.5%TD (Theoretical Density). These normalized thermal conductivities of irradiated UO2 pellets in which significant rim structures had formed tended to be slightly higher than those of irradiated UO2 pellets in which no rim structure had formed. This may be caused by an effect of recrystallization in rim structures.