Re-Evaluation of the TSL For Yttrium Hydride

Yttrium hydride (YH_x) is of interest as a high-temperature moderator material in advanced nuclear reactor systems because of its superior ability to retain hydrogen at elevated temperatures. Thermal neutron scattering laws (TSL) for hydrogen bound in yttrium hydride (H-YH₂) and yttrium bound in yttrium hydride (Y-YH₂) were previously evaluated by Naval Nuclear Laboratory using the ab initio approach and released in ENDF/B-VIII.0. In that work, density functional theory, incorporating the generalized gradient approximation (GGA) for the exchange-correlation energy, was used to simulate the face-centered cubic structure of YH₂ and calculate the interatomic Hellmann-Feynman forces for a 2×2×2 supercell containing 96 atoms. Lattice dynamics calculations using PHONON were used to determine the phonon density of states. The calculated phonon density of states for H and Y in YH₂ were then used to prepare H-YH₂ and Y-YH₂ TSL evaluations, in the incoherent approximation, using the LEAPR module of NJOY. In addition elastic scattering was assumed to be incoherent for both H and Y. While the incoherent elastic scattering approximation is appropriate for H-YH₂, it introduces an undesirable approximation for Y-YH₂. In this work, we re-evaluate the TSL for Y-YH₂ using FLASSH (Full Law Analysis Scattering System Hub). Y-YH₂ is evaluated using the FLASSH generalized coherent elastic scattering capability in order to capture the Bragg peaks associated with the YH₂ crystal structure which were neglected in the prior NJOY-based evaluation due to limitations in LEAPR. An experimental approach to validate the Y-YH₂ TSL using neutron transmission measurements is discussed.