EFFECT OF GRAIN-SIZE ON MICROSTRUCTURAL CHANGE AND DAMAGE RECOVERY INUO2 FUELS IRRADIATED TO 23-GWD T/

The effect of grain size on microstructural change and damage recovery in UO2 fuels was studied by X-ray diffractometry (XRD) and transmissi on electron microscopy (TEM). The as-irradiated lattice parameter of t he standard fuel (grain size: 16 mu m) was larger than that of the lar ge-grained fuel (43 mu m), indicating a larger number of fission-induc ed 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 mai nly due to the accumulation of vacancies. The lattice parameter of bot h fuels began to recover around an irradiation temperature of 450 simi lar to 650 degrees C, and both had a complete recovery at 850 degrees C. On annealing at high temperatures of 1,450 similar to 1,800 degrees C, the bubble diameter in the standard fuel was larger than that in t he large-grained fuel. This indicated that vacancy diffusion from the grain boundaries plays an important role during bubble coarsening at h igh temperatures.