Distribution of Fission Products in Irradiated UO2

Inhomogeneities of fission products and plutonium distribution in irradiated UO₂ were examined. UO₂ pellets were ground so as to let them fracture along the grain boundaries, and the fragments were then dissolved successively into several fractions. The specific activities of these fractions were compared, and the distribution of fission products in the grain and grain boundaries were studied. It was found that at lower temperatures of irradiation (～1,400°C), the fission products, except Cs, were distributed fairly uniformly over the grain, while Cs had accumulated at the grain boundaries. At higher temperatures (near or above the melting point of UO₂), inhomogeneity was noticed in the Zr-Nb and Cs distribution. The concentration of these elements varied along the temperature gradient of the fuel rod than within the grains. Zr-Nb was found to be concentrated in the higher temperature areas of the fuel rod, while conversely, the concentration of Cs was smaller in the same areas. Distribution of Ce, Ru and Pu was found to be fairly uniform throughout the grains. The process of fission product migration in UO₂ is also discussed in the paper.