Thermal conductivities of irradiated UO 2 and (U, Gd)O 2 pellets

Abstract

Thermal diffusivities of UO 2 and (U, Gd)O 2 pellets irradiated in a commercial reactor (maximum burnups: 60 GWd/t for UO 2 and 50 GWd/t for (U, Gd)O 2) were measured up to about 2000 K by using a laser flash method. The thermal diffusivities of irradiated UO 2 and (U, Gd)O 2 pellets showed hysteresis phenomena: the thermal diffusivities of irradiated pellets began to recover above 750 K and almost completely recovered after annealing above 1400 K. The thermal diffusivities after recovery were close to those of simulated soluble fission products (FPs)-doped UO 2 and (U, Gd)O 2 pellets, which corresponded with the recovery behaviors of irradiation defects for UO 2 and (U, Gd)O 2 pellets. The thermal conductivities for irradiated UO 2 and (U, Gd)O 2 pellets were evaluated from measured thermal diffusivities, specific heat capacities of unirradiated UO 2 pellets and measured sample densities. The difference in relative thermal conductivities between irradiated UO 2 and (U, Gd)O 2 pellets tended to become insignificant with increasing burnups of samples.