Evaluation of Yttrium Hydride (δ-YH_2-x) Thermal Neutron Scattering Laws and Thermophysical Properties

Yttrium hydride is being considered as a moderator material for microreactor concepts because of its excellent hydrogen retainment capacity at high temperatures. These types of reactors, operating at thermal to epithermal neutron energies, require accurate thermal scattering laws (TSLs) for yttrium hydride to predict and optimize moderator performance. Currently, TSL evaluations exist only for stoichiometric YH₂. To perform high-certainty neutronics calculations and to improve the criticality safety of yttrium hydride–moderated reactors, evaluations of substoichiometric yttrium dihydride TSLs are necessary. Ab initio density functional theory (DFT) was used to generate the phonon density of states for yttrium and hydrogen under harmonic approximation in yttrium hydride (). To obtain substoichiometric yttrium dihydride, vacancies in the YH₂ crystal were created using special quasi-random structures (SQS). Using NJOY2016, the TSLs for yttrium hydride were constructed from the DFT results as a function of stoichiometry and temperature. Our TSLs for the stoichiometric composition YH₂ were in excellent agreement with the ENDF/B-VIII.0 evaluations. As such, this study extends the yttrium hydride TSLs for compositions between YH_1.31 to YH_1.91 with the interval of H/Y ≈ 0.1 for use in the MCNP code. The substoichiometric yttrium hydride scattering cross sections deviated by as much as 30% (elastic) and 60% (inelastic) when compared to the YH₂ TSLs, underlining the necessity to have the TSLs presented here available, e.g., for safety-related reactor calculations. For the validation of the underlying DFT results of our model, quasi-harmonic approximation was used to compute the thermal lattice strain and constant pressure heat capacity for YH₂. Neutron diffraction experiments were also carried out to characterize thermophysical properties that were adopted for stoichiometric and substoichiometric model validation. Additional properties such as heat capacity c_v, and thermal displacement parameters were also computed for yttrium hydride () and compared to experimental results. Neutron diffraction validation of the YH_2-x material properties and ENDF/B-VIII.0 verification of YH₂ TSLs provide a very strong basis on the accuracy of the extended yttrium hydride TSL evaluations at thermal energies.