Neutronic analysis of Yttrium Hydride temperature feedback coefficient in HALEU UMo fuelled microreactors

The emerging interest in terrestrial and space microreactors has highlighted the need for innovative core design solutions. Selecting an adequate moderator is challenging for solid-core microreactors operating at high temperatures, where few weight-efficient options are available. Yttrium hydride (YH) is an appealing candidate due to its high hydrogen content and superior high-temperature retention compared to other hydrides. However, YH exhibits a positive temperature feedback coefficient (TFC), posing a significant safety risk that could offset its advantages. This work studies the YH TFC across various configurations, from simple unit-cells (fuel pins, moderator pins, and layers) to a realistic space heat-pipe reactor (PoliMi-TASI design) modified with moderator layers. The TFC is broken down into contributions defined by the 4-factor and 6-factor formulas. Neutron transport is simulated employing a Monte Carlo approach via OpenMC. The thermal utilisation factor provides the predominant contribution to the TFC, leading to positive values for most analysed configurations. Furthermore, analysis of the moderated PoliMi-TASI design reveals the MTC depends not only on the moderator-to-fuel ratio but also on layer thickness: for a fixed ratio, thinner layers yield a smaller MTC. This analysis clarifies the physics behind the YH TFC, guiding future design improvements for the PoliMi-TASI reactor.