Thermal recovery of radiation defects and microstructural change in UO2 fuels irradiated under LWR conditions (burnup : 25 and 44 GWd/t) have been studied after annealing at temperature range of 4501, 800°C by X-ray diffractometry and transmission electron microscopy (TEM). The lattice parameter of as-irradiated fuels increase with 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∼650°C with one stage and was almost completely recovered by annealing at 850°C for 5 h. Based on the recovery of broadening of X-ray reflections and TEM observations, defect clusters of dislocations and small intragranular bubbles began to recover around 1,150∼1,450°C. Complete recovery of the defect clusters, however, was not found even after annealing at 1, 800°C for 5 h. The effect of irradiation temperature on microstructural change of sub-grain structure in high burnup fuels was assessed from the experimental results.
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