A Modeling and Neutron Diffraction Study of the High Temperature Properties of Sub-Stoichiometric Yttrium Hydride for Novel Moderator Applications

Abstract

Low-enriched-uranium (LEU) reactor systems utilize moderators to improve neutron economy. Solid yttrium hydride is one of the primary moderator candidates for high-temperature (>700 °C) nuclear reactor applications. This is due to its ability to retain hydrogen at elevated temperatures compared to other metal hydrides. For reactor modeling purposes, both neutronic and thermos-mechanical modeling, several high-temperature properties for sub-stoichiometric yttrium hydride (YH2−x) are needed. In this paper, we present an atomistics and a neutron diffraction study of the high-temperature properties of Y and YH2−x. Specifically, we focus on the thermal lattice expansion effects in yttrium metal and yttrium hydride, which also govern bulk thermal expansion. Previously reported physical and mechanical properties for sub-stoichiometric yttrium hydride at ambient conditions are expanded using lattice dynamics to take into account high-temperature effects. Accordingly, an array of newly generated properties is presented that enables high-fidelity neutronics, and thermomechanical modeling. These properties include various elastic moduli, thermal expansion parameters for yttrium and yttrium hydride, and single-phase (YH2−x) and two-phase (Y + YH2−x) density as a function of stoichiometry and density.

The National Criticality Experiments Research Center and its role in support of advanced reactor design

“…This was the first experiment to demonstrate and measure the positive temperature coefficient of reactivity for YH x , meaning that as the temperature of the YH x increased, the reactivity also increased. This effect has been seen before in ZrH x in the Topaz reactor (Buden, 1993), and was predicted based on new thermal scattering laws for YH x in ENDF/B-VIII.0 (Zerkle and Holmes, 2017;Zerkle et al, 2021;Brown et al, 2018) and new experimental data on YH x scattering (Mehta, 2020;Mehta et al, 2022). The positive temperature coefficient of reactivity for YH x may have design implications for advanced reactors seeking to utilize this moderator (Ade et al, 2022).…”