Study on method to achieve high transmutation of LLFP using fast reactor

The purpose of this study is to clarify the method to achieve high transmutation rates of four long-lived fission products (⁷⁹Se, ⁹⁹Tc, ¹⁰⁷Pd, and ¹²⁹I) using a fast reactor. New LLFP target assemblies were invented in consideration of the suppression of thermal spikes in adjacent fuel assemblies by combining YD₂ and YH₂ moderators or using a thermal neutron filter material. It was clarified that the high transmutation rate of about 8%/year was achieved, if the new LLFP target assemblies of 4 nuclides were loaded in the blanket region of the sodium cooled, MOX fueled fast reactor. The feasibility of the LLFP transmutation target was clarified through experiments on material properties and fabrication of the LLFP target, YH₂ and YD₂ moderators.