Evaluating Hydrogen-Based Moderators in High-Temperature Gas-Cooled Reactors With 5 wt.% Enriched Uranium Annular Fuel Rods

Abstract

Small modular reactors (SMRs) based on high-temperature gas-cooled reactor (HTGR) technology are being developed for providing high-temperature process heat and high-efficiency (>40%) electrical power generation. However, most of the HTGR-SMR concepts require high assay low enriched uranium (HALEU) fuel, with enrichments typically above 10 wt.% 235U/U, to get sufficiently high burnup levels and fuel lifetime. The goal of this study is to identify alternative fuel concepts for HTGRs that can achieve sufficiently high burnup and low temperature reactivity coefficients while using uranium with 5 wt.% 235U/U. A previous study has shown that a modified prismatic HTGR fuel assembly with hydrogen-based moderator (7LiH) and cylindrical fuel elements of 5 wt.% 235U/U enriched uranium can greatly reduce fuel consumption of an HTGR. However, such a design concept could lead to positive temperature reactivity coefficients (TRCs), making reactor control more challenging, with reduced passive safety. In this study, variations of the hydrogen-based moderator in this alternative fuel assembly concept are evaluated to identify configurations that achieve negative TRCs, thus improving passive safety characteristics. Calculation results demonstrate that negative TRCs can be achieved with reduced hydrogen mass such that natural uranium consumption is substantially less than that of the tristructural isotropic (TRISO) fuel concept, with comparable or longer core life.