Possibility of YH1.85 and MgO–BeO as a moderator in high temperature reactors

Abstract

This study compared the neutronic behaviors of the graphite material utilized in standard HTGR reactors to those of yttrium hydride and MgO–BeO moderator materials using the OpenMC and Serpent Monte Carlo codes. keff values for various fuel/moderator ratios in a TRISO particle-embedded moderator matrix were computed. The effect of packing fraction was then investigated, and the keff values for different packing fractions were compared for the MgO–BeO, YH1.85, and C (graphite) moderator matrices in the geometry layout containing randomly placed TRISO particles. In contrast to graphite and MgO–BeO moderators, the keff values for YH1.85 moderator indicated that a very small moderator volume is adequate for thermalization. We also investigated how temperature reactivity feedback impacts different packing fractions. In the temperature reactivity feedback analysis, the overall temperature coefficient was determined to be negative for all moderators. The temperature feedback coefficient for the fuel with the YH1.85 moderator was found to be significantly lower in magnitude than for the system with graphite and MgO–BeO. The burnup findings revealed that graphite and MgO–BeO behaved similarly; however, YH1.85 was shown to be useful only at very low moderator/fuel ratios. With typical HTGR power density, the fuel cycle time for YH1.85 was found to be around 516 days. Also, while the YH1.85 moderator system reduces the risk of proliferation by producing small amounts of 239Pu and 241Pu, it was determined that it is more suitable for small modular and microreactor designs for high-temperature reactors with low moderator/fuel ratio requirements.