Sodium void reactivity comparison for advanced liquid-metal reactor fuels

The advanced liquid-metal reactor (ALMR) program in the US is based on metal as the reference fuel because of its favorable neutronic feedback characteristics for passive safety. Favorable relationships exist between core performance and safety that provide a passively safe metal fuel system with a large margin to sodium boiling. Because of this, the reduction of the positive sodium void coefficient of reactivity is not an overriding design objective. A positive sodium void effect with metal fuel is due to neutron spectral hardening that dominates capture and leakage changes during sodium voiding. This can produce as much as 5-$ positive sodium void reactivity for mixed plutonium-uranium fuel in a smaller core designed for a near-zero burnup reactivity swing. It is possible to reduce the positive void feedback and its effect on hypothetical loss-of-flow (LOF) scenarios with a commensurate increase in burnup swing. However, metal fuel's small Doppler coefficient, excellent fuel conductivity, and resultant small temperature gradients provide less reactivity feedback to handle postulated transient overpower (TOP) events for cores with significant burnup reactivity swings. The purpose of this work was to study the relationship between reduction of the sodium void and the resultant increase in the burnup reactivity swing for an ALMR modeled with metal, nitride, and oxide fuel