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As an alternative method to the homogeneous minor actinide (MA) recycling in fast breeder reactors, a heterogeneous MA loading core concept using a highly concentrated americium (Am)-containing fuel (Am target) is proposed. By the use of an extraction process for Am and curium (Cm) in the reprocessing of the spent fuel, Am (and a small amount of Cm) can be recovered and then concentrated to produce the target. The Am content in the heavy metal is assumed to range from 10 to 20 wt% in accordance with the target development scope. A mixed oxide fuel that contains uranium, plutonium, and neptunium is chosen as the base material of the target, so that the targets can generate a level of power equivalent to that of the driver fuels. It was found that a ring-shaped arrangement of Am targets between the inner and outer core regions exhibits a favorable MA transmutation performance without any significant deterioration in the core neutronic characteristics, including increases of the burnup reactivity and sodium void reactivity worth, and decreases of the breeding ratio and absolute value of the Doppler coefficient, etc., in comparison with those of a reference homogeneous MA loading case. It should be noted that the Am targets in this loading arrangement can contribute to the suppression of the core power distribution change along with burnup. A series of core designs, including core neutronics, thermal hydraulics, and fuel integrity evaluations, was also carried out for a representative Am target loading case. The results indicate that it is possible to design an Am target subassembly that can cope with the issues presented by highly concentrated Am, i.e., the deterioration of thermophysical properties and the accumulation of helium gas inside the target fuel pins. Therefore, the design feasibility of the heterogeneous target loading core has been enhanced.

This paper presents validation work on MA nuclear data by PIE analyses in the framework of fast-reactor cycle system development. The PIE analyses are in progress on MA samples (237 Np, 241 Am, 243 Am, 244 Cm) irradiated at the experimental fast reactor "JOYO." The preliminary analysis results showed that the isomeric ratio for 241 Am capture reaction lies at around 0.85 (g/(g+m)) in the fast-neutron spectra, which suggested the necessity of re-evaluation of the data both in ENDF/B-VI and in JENDL-3.3. From the results on curium isotopes, overestimations could be pointed out for the capture cross section of 244 Cm in ENDF/B-VI and that of 245 Cm in JENDL-3.3.