Evaluation of the thermal scattering law and cross sections for α-U3O8 using ab initio lattice dynamics

Abstract

Triuranium octoxide (U₃O₈) is a starting material in the nuclear fuel cycle and a preferred form for the safe storage of nuclear fuel and for the disposal and containment of radioactive waste, due to its thermodynamically stable characteristics compared to other uranium oxides. Despite extensive experimental and computational investigations on its properties, discrepancies exist regarding its correct magnetic state. Recent inelastic neutron scattering experiments and density functional theory calculations have elucidated that U₃O₈ exhibits an antiferromagnetic (AFM) [0.5 1 1] ordering. In this study, we model the α-U₃O₈ crystal with AFM [0.5 1 1] ordering at the atomic level to evaluate its thermal scattering law (TSL). Ab initio lattice dynamics simulations based on density functional theory are coupled with phonon vibration analysis to derive the phonon density of states of α-U₃O₈, based on which the scattering cross section is generated. The optimized lattice structure of our α-U₃O₈ model closely aligns with experimental data, and the electronic band gap reasonably matches the experimental range. Phonon vibration analysis under the harmonic approximation indicates that the heat capacity, entropy, and enthalpy deviate by less than 10% from experimental observations. The calculated phonon density of states (DOS) demonstrates overall reasonable agreement with the data obtained from inelastic neutron scattering measurements. Using the DOS, the TSL and thermal neutron scattering cross sections are produced, and the anticipated systematic behavior is exhibited across various temperatures.