Effect of Grain Size on Microstructural Change and Damage Recovery in UO₂Fuels Irradiated to 23GWd/t

The effect of grain size on microstructural change and damage recovery in UO₂fuels was studied by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The as-irradiated lattice parameter of the standard fuel (grain size : 16 µm) was larger than that of the large-grained fuel (43µm), indicating a larger number of fission-induced point defects in the lattice of the former fuel. This tendency was in contrast to previously reported results for low burnup fuels below 1 GWd/t. The lattice dilation in the present high burnup fuels was mainly due to the accumulation of vacancies. The lattice parameter of both fuels began to recover around an irradiation temperature of 450~650°C, and both had a complete recovery at 850°C. On annealing at high temperatures of 1, 450~1, 800°C, the bubble diameter in the standard fuel was larger than that in the large-grained fuel. This indicated that vacancy diffusion from the grain boundaries plays an important role during bubble coarsening at high temperatures.