The idea of recycling minor actinides (MAs) with fast breeder reactors (FBRs) is an effective way to potentially reduce environmental burdens associated with nuclear energy production. For such FBR cores, it is necessary to find one or more promising MA loading methods that can effectively transmute MAs while minimizing deterioration of the core performance and reducing the overall fuel fabrication cost. In this study, the homogeneous MA loading core with 3 wt% MAs is used as a reference design to evaluate the impact of the americium (Am) target in-core loading on reactivity characteristics and unprotected loss-of-flow (ULOF) response of sodium-cooled mixed-oxide FBR.

The Am target loading core of this study is designed by roughly preserving the MA inventory of the homogeneous MA loading core while placing Am and curium (Cm) to the ring-shaped target region between the inner and the outer core regions with 20 wt% content.

This design can flatten core radial reactivity worth distributions and effectively reduce reactivity insertion into the core during ULOF compared with the homogeneous MA loading core. It also has relatively flat and stable radial power distributions, which allow a relatively large coolant flow rate to be distributed to the target region.

During ULOF, the power increase of the Am target loading core of this study is slower than that of the homogeneous MA loading core. The maximum fuel temperature of the target region does not become particularly high compared with that of the inner core, and it is much lower than the melting point. Hence, the proposed Am target in-core loading method does not have a significant influence on ULOF response of the core. It is promising from the viewpoints of the reactivity characteristics and ULOF response.