Chemical States of Fission Products and Actinides in Irradiated Oxide Fuels Analyzed by Thermodynamic Calculation and Post-Irradiation Examination

In irradiated oxide fuels, a number of fission products (FPs) and actinides are produced. They take various chemical states depending on the conditions of the fuel, e.g. temperature and oxygen potential. The compounds composed of FPs and actinides would affect significantly the fuel characteristics. Therefore, the behavior of FPs and actinides under irradiation and the properties of their compounds are very important for evaluation of the fuel performance. It has been recognized that some FPs such as lanthanides (Ln) are soluble in the fuel matrix and form solid solutions of (U,Pu,Ln)O2. On the other hand, some FPs such as alkaline and alkaline-earth elements, molybdenum, and platinum-family metals form oxides or metallic inclusions such as Mo-Tc-Ru-Rh-Pd alloys, (Ba,Sr)(U,Zr,Mo)O3, and Cs2(Mo,U)O4, etc. However, there exist still many points that are doubtful and unclear with regard to the behavior of FPs and actinides under irradiation. In the present study, we tried to evaluate the chemical states of FPs and actinides in irradiated oxide fuels by both thermodynamic equilibrium calculation and post-irradiation examination (PIE). The calculated results were compared with the PIE results.