Study on Neutronics Properties of Zirconium Hydride Moderator in Reactor

Abstract The metal hydride ZrH1.6 has a high hydrogen atom density, equivalent to that of liquid water. Compared to other hydrides (such as H2O and organic compounds), particularly at high temperatures, ZrH1.6 exhibits good strength and stability. Using hydrogen zirconium in a reactor can effectively slow down fast neutrons, thereby increasing the utilization of thermal neutrons and enabling the use of low-enriched uranium fuel. However, as a Solid state moderator matrix, hydrogen zirconium has a relatively large positive temperature coefficient in a reactor, posing a threat to the inherent safety of the reactor. The goal of this study was to find out why the temperature coefficients of zirconium hydride is positive and to determine when the moderator and fuel temperature coefficients become positive. These are analyzed in this article by simplifing fuel and moderator calculation models and applying the SARAX program, from the most fundamental physical theory, the neutron physics parameters such as reactivity changes, nuclear cross-sections, neutron energy spectra, and reaction rates of the simplified model before and after temperature rise. The results indicate that the positive temperature coefficient of metal hydride ZrH1.6 is due to the hardening of the neutron energy spectrum below 1eV when the fuel and ZrH1.6 are heated, causing some neutrons to escape from the moderator and enter the fuel, thereby increasing the fission of the fuel. Moreover, when the reactor is designed in the under moderated zone, the temperature coefficient of fuel and moderator can be guaranteed to be negative, thus ensuring the safety of the reactor.