Suppressing the positive temperature reactivity coefficient in metal hydride moderated reactor

The Lunar and Martian nuclear reactors space applications include equipment operation, experiment execution, and powering crew habitats. Metal hydrides are being seriously considered for space nuclear reactor applications due to their solid physical state and high hydrogen density. Using hydrides for space applications poses several research and technological challenges, one of which relates to neutron up-scattering in the thermal energy regime. These hydrides, including zirconium hydride and yttrium hydride, result in a positive temperature coefficient of reactivity for several microreactor and space reactor designs. In this study, we consider one such design, which exhibits positive feedback from metal hydrides, and thoroughly investigate the neutronic aspects of the core. A parametric study of four fuel materials and two moderator materials is investigated in this work. Temperature reactivity coefficients for each case are studied for various fuels and moderator configurations, and the total temperature coefficients were found to be positive for all designs. Accordingly, a neutron energy spectrum shift is utilized alongside epi-thermal absorbers to suppress the excess positive feedback in the core. Promising results are shown for this core design, which can be extended to other hydride-moderated space reactors. (authors)