Impact of auto-irradiation on the thermophysical properties of oxide nuclear reactor fuels

The effect of α-damage on the thermophysical properties of UO2 was investigated using samples doped with 238Pu. Characterisations were performed after different storage periods, including X-ray diffraction to monitor the lattice parameter evolution, Knudsen-cell helium release experiments and transmission electron microscopy examinations. The apparent heat capacity was measured by differential scanning calorimetry and the recovery stages observed were attributed to the recombination of a certain kind of point or extended defect. The thermal diffusivity, measured by the laser-flash technique during annealing cycles, displayed similar recovery stages. The measurements show that the degradation of the diffusivity with increasing α-dose is not linear, and that saturation occurs at relatively low doses. A correlation quantifying this degradation is proposed. Comparison with the thermal diffusivity of very low burn-up rector irradiated samples, where the main source of degradation is radiation damage, shows that the annealing stages are similar.