Simulating hydrogen diffusion in a zirconium hydride moderator block and its impact on steady state neutronic-thermal behavior

Zirconium hydride is a widely used moderator in compact reactor designs due to its high thermal limits and high hydrogen density, both of which being desirable feature. However, a notable characteristic of zirconium hydride is the substantial mobility of hydrogen within the metal lattice, especially at high temperatures and under large thermal gradients. Variations in hydrogen distribution can significantly affect neutron moderation and, consequently, the reactor’s power profile. This study employs a coupled OpenMC-MOOSE simulation framework to model this complex feedback between hydrogen transport, heat transfer, and neutron behavior. A hypothetical epithermal reactor configuration is analyzed, where zirconium hydride serves as a monolithic moderator with embedded fuel pins and heat pipes. The simulation results illustrate the redistribution of hydrogen and its subsequent impact on both the thermal and neutronic behavior of the system. The magnitudes of the neutronic-thermal impacts vary depending on input reactor power and heat pipe boundary condition; this work found reactivity impacts ranging from 100 to 3500 pcm and some minor spatial impacts to power distribution.