Thermal neutron scattering kernels for uranium mono-nitride: A potential advanced tolerant fuel candidate for light water reactors

The use of uranium mono-nitride (UN) fuel in light water reactors has recently received an increasing interest. Neutronic and thermal-hydraulic performance of UN and associated composite fuels have been studied lately for several types of light water reactors. A comprehensive and detailed analysis of the performance of these reactors using nitride fuels require a complete set of thermal neutron scattering libraries of UN. In this work, the thermal neutron scattering law, the inelastic and coherent elastic scattering cross sections of UN are calculated at different temperatures starting from the phonon density of states obtained from first-principles electronic structure calculations. Excellent agreement between the calculated phonon dispersion relations and the experimental data have been obtained. Due to the huge mass difference between the constituent elements, the calculated nitrogen optic phonon modes are well-separated from those of uranium acoustic phonon modes. The nitrogen scattering law shows a set of well-defined and equally-spaced peaks corresponding to multi-phonon scattering processes, demonstrating that the nitrogen atoms behave as nearly independent three-dimensional quantum harmonic oscillators. The effects of this behavior on the computed inelastic scattering cross sections are thoroughly discussed. As ¹⁴N has a large absorption cross section in the thermal energy range and results in a production of ¹⁴C, the scattering law and the inelastic scattering cross section of U¹⁵N are also calculated. Our calculated scattering cross sections are discussed in comparison with the ENDF/B-VIII.0 thermal neutron scattering cross sections of UN and UO₂, and it has been found that U¹⁵N fuel can be a viable alternative to the UO₂ one.