Impact of magnetic structure and thermal effects on vibrational excitations and neutron scattering in uranium mononitride


J.L.Wormalda, A.I.Hawarib, M.L.Zerklec

Annals of Nuclear Energy
Volume 143, August 2020, 107447

Uranium mononitride (UN) is a nuclear fuel material of interest in the design of advanced reactors. Phonon spectra and dispersion relations of UN in its anti-ferromagnetic and paramagnetic structures were calculated using ab initio lattice dynamics. Subsequently, the dynamic structure factors of uranium and nitrogen in UN were generated in the incoherent approximation using the phonon expansion method for inclusion in the US National ENDF/B-VIII.0 database of neutron thermal scattering law evaluations. Phonons from spin polarized density functional theory simulations demonstrate good agreement with inelastic neutron scattering measurements; however, phonons from non-spin polarized simulations deviate from experiment, indicating a previously unexplored impact of magnetism on the vibrational characteristics. An oscillatory behavior due to multi-phonon scattering, previously observed in neutron scattering experiments at low temperatures, was captured in the dynamic structure factor. Moreover, calculated dynamic structure factors at 296 K and 1200 K demonstrate that the oscillatory behavior is present at elevated temperatures.