Reactor Physics Considerations for Use of Yttrium Hydride Moderator

Recent developments in manufacturing large metal hydrides are enabling their use as a moderator for advanced reactor designs. Yttrium hydride (YH_x) is particularly attractive for small reactor designs because of its ability to retain a high hydrogen density at elevated temperatures. Design iteration for the Transformational Challenge Reactor (TCR), which uses a YH_1.85 moderator, revealed positive moderator temperature coefficients. A positive temperature coefficient for YH_x is expected regardless of the core design, however, the positive moderator coefficient exceeded that of the negative fuel temperature coefficient in some early TCR design iterations. The cause of the positive moderator coefficient is analyzed, and conditions for which positive temperature coefficients would be expected are identified for a number of fuel and moderator geometry layouts for dense tristructural isotropic/silicon carbide fuel and UO₂ fuel.