[en] In the Feasibility Study on Commercialized Fast Reactor Cycle Systems (FS), transmutation technology of Long-Lived Fission Product (LLFP) using commercial fast reactors has been studied as a measure to reduce the environmental burden of nuclear waste. During phase-I of the FS (1999 - 2000), neutronic studies such as transmutation properties, effect by impurities, etc. had been mainly carried out and in the succeeding phase-II (2001) design studies on the LLFP target assembly and the core had been performed. The targeted long-lived isotopes for transmutation were ¹²⁹I, ⁹⁹Tc and ¹³⁵Cs. It was assumed that those LLFPs with neutron moderator were recycled into the core of sodium cooled fast reactor after recovering from the spent fuel by element wise separation technology. Concerning Cs, it was found out that the efficient transmutation was extremely difficult to be achieved due to the existence of considerable amount of stable isotope ¹³³Cs that generates new ¹³⁵Cs through successive neutron captures, in addition to the large production rate for ¹³⁵Cs. In the design study of the core loaded with LLFP assemblies of I and Tc, two types of LLFP assembly with zirconium hydride as neutron moderator were investigated. One was intended for core-region loading and the other was for radial blanket-region loading. The former has Tc layer in the peripheral region of the assembly to avoid thermal peak in the adjacent fuel assemblies. The integrity evaluation for LLFP assemblies led a conclusion that the most significant design limiting feature was ascribed to the temperature of the moderator pins that determines the dissociated hydrogen penetration rate. To ensure the sufficient cooling for moderator pins, 6-7% of the primary system coolant flow rate had to be distributed to the LLFP assemblies. Design modifications such as subdividing of coolant flow partition and flattening of power distribution were required for the core loaded with LLFP assemblies. (author)