The thermal neutron scattering law for hydrogen bound in plutonium dihydride and predicted critical mass for several configurations

The thermal neutron scattering law (TSL) for hydrogen bound in plutonium dihydride (H-PuH₂) is developed using first-principles density functional theory and lattice dynamics calculations. The GGA+U exchange correlation with spin-polarized magnetism is used to simulate the electronic structure of the material with a Hubbard U parameter that is selected to reproduce experimental lattice constant measurements. The partial phonon density of states from these calculations is used to develop a thermal neutron scattering law evaluation for H-PuH₂ in the incoherent approximation. This thermal scattering law is used to predict the critical mass of ²³⁹PuH₂ in various bare, reflected, and moderated spherical configurations with MC21 using ENDF/B-VII.1 cross sections. The H-PuH₂ TSL is designed to be suitable for the analysis of bulk PuH_x compositions with 0.0 < x ≤ 2.0. Water volume fractions from 0.0 to 0.95 are analyzed for the homogenized slurry. A small positive difference in k_eff is observed for the water reflected configuration when comparing calculations using the ¹H free-gas treatment instead of the H-PuH₂ thermal scattering law