This study investigates the characteristics of the Doppler coefficient and sodium void reactivity of a burning fast reactor core concept that was constructed in a previous study. This concept allows for multiple recyclings of plutonium and minor actinides [transuranium (TRU)]. TRU degradation due to multiple recycling deteriorates the reactivity coefficients through indirect effects, such as by hardening the neutron spectrum and steepening the energy gradient of neutron importance.

Using silicon-carbide (SiC) structural material improves the reactivity coefficient by causing an opposite indirect effect of TRU degradation. This improvement results not only from neutron spectrum softening due to the neutron moderation effect from 12C but also from the neutron leakage effect resulting from the low structural material density. The disadvantage of increased calculation uncertainty from using SiC structural material can be practically ignored.

Furthermore, the burning core has Doppler coefficient enhancement characteristics from the moderated neutron reflection effect from outside the core. This characteristic has the potential to provide a new measure for reactivity coefficient deterioration due to TRU degradation. The reactivity coefficient characteristics clarified in this study can provide valuable knowledge for future detailed designs and design improvements of a TRU burning core.