THERMAL RECOVERY OF RADIATION DEFECTS AND MICROSTRUCTURAL CHANGE IN IRRADIATED UO2 FUELS

Thermal recovery of radiation defects and microstructural change in UO 2 fuels irradiated under LWR conditions (burnup: 25 and 44GWd/t) have been studied after annealing at temperature range of 450 approximately 1,800-degrees-C by X-ray diffractometry and transmission electron mic roscopy (TEM). The lattice parameter of as-irradiated fuels increase w ith higher burnup, which was mainly due to the accumulation of fission induced point defects. The lattice parameter for both fuels began to recover around 450 approximately 650-degrees-C with one stage and was almost completely recovered by annealing at 850-degrees-C for 5h. Base d on the recovery of broadening of X-ray reflections and TEM observati ons, defect clusters of dislocations and small intragranular bubbles b egan to recover around 1,150 approximately 1,450-degrees-C. Complete r ecovery of the defect clusters, however, was not found even after anne aling at 1,800-degrees-C for 5h. The effect of irradiation temperature on microstructural change of sub-grain structure in high burnup fuels was assessed from the experimental results.