Study on Post-Irradiation Microstructure of UO₂ Pellets in Recycled Uranium Fuel Elements

In order to evaluate the irradiation stability of UO₂ pellets in the recycled uranium fuel elements, the axial and radial distribution of irradiation swelling, crack, grain size distribution and grain growth behavior of the irradiated UO₂ pellets were observed and analyzed by means of a hot cell metallographic microscope. The results show that a large amount of cracks occur in the fuel pellet and obvious circular distribution present in the cracks of recycled uranium, and radioactive divergence distribution present in the natural uranium. For both the recycled uranium and natural uranium, the grain size of fuel pellets is equiaxed, grain size grows gradually from the edge region to the center region of pellets, and grain boundary become coarser after irradiation. No obvious difference in grain size, morphology and distribution between the recycled uranium and natural uranium has been found. Additionally, the irradiation swelling of the recycled uranium pellets is not obvious, and no significant difference in the crushing degree of the pellets and grain growth process between the recycled uranium and natural uranium has been found under the same operating conditions in the reactor.