回收铀燃料元件UO2芯块辐照后显微组织研究

为评价回收铀燃料元件中UO₂芯块的辐照稳定性，采用热室金相显微镜对辐照后高放射性UO₂芯块沿轴向及径向的辐照肿胀、裂纹分布、晶粒尺寸及分布和晶粒长大行为进行了观察和分析。结果表明:燃料元件芯块中均存在大量的裂纹，回收铀燃料元件UO₂芯块裂纹呈明显的环形分布特征，天然铀燃料元件UO₂芯块呈放射性发散分布特征。两者的燃料芯体晶粒呈等轴状，均出现从边缘区域向芯块中心区域晶粒逐渐长大现象，辐照后晶界变粗化。两者晶粒尺寸、形貌及分布特征并无明显差别。此外，在相同的堆内运行工况条件下，回收铀燃料元件UO₂芯块辐照肿胀不明显，芯块破碎程度及晶粒长大过程与天然铀并无明显差别。 

Study on Post-Irradiation Microstructure of UO_2 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.