Fast breeder reactor core concept for heterogeneous minor actinide loading

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.