Uranium Mononitride (UN) Properties from First-Principles Calculations and Ab Initio Molecular Dynamics Simulations

Uranium mononitride (UN) is a promising nuclear fuel that combines the advantageous properties of readily used UO₂ and uranium alloys. A better understanding of the UN behavior at operating temperatures can be obtained from finite temperature data. However, studies, both experimental and computational of the finite temperature elastic properties of UN are lacking. Using density functional theory (DFT) calculations we are providing finite temperature thermal conductivity and elastic properties of UN. Initially, we wanted to establish which method to use, and thus, we are comparing the performance of PBE and PBE+<i>U</i> methodologies in reproducing experimental crystallographic properties and magnetic ordering, as well as we the phonon dispersions. We performed ab initio molecular dynamic (AIMD) simulations at 5 different temperatures using constant volume (NVT) and constant pressure (NPT) ensembles. We use the NVT results to obtain the phonon dispersion at finite temperature, and the NPT results to obtain the thermal expansion of UN. We also use the NPT results to obtain the finite temperature electronic conductivity, and the electronic and lattice contributions to the thermal conductivity. In addition, we evaluate the temperature dependent elastic properties of UN using the strain-stress method in AIMD simulations.