Finite temperature properties of uranium mononitride

Uranium mononitride (UN) is a promising nuclear fuel that combines the advantageous properties of readily used UO2 and uranium alloys, such as high melting temperature and high uranium density, and thermal conductivity, respectively. A better understanding of UN behavior at operating temperatures can be obtained from finite temperature data, such as elastic properties. To get this information, ab initio molecular dynamics (AIMD) simulations were performed at five different temperatures using constant volume (NVT) and constant pressure (NPT) ensembles. Initially, the performance of PBE functional in reproducing experimental crystallographic properties and magnetic ordering is assessed. The finite tem-perature phonon dispersions are calculated using NVT simulation results, which show a softening of the phonon modes with increasing temperature. The NPT results are used to obtain the thermal expansion of UN and finite temperature electronic properties. The calculated thermal expansion is compared with our measurements using neutron diffraction. Additionally, the temperature dependent elastic properties of UN are evaluated using the strain-stress method in AIMD simulations, indicating that UN becomes softer and more compressible with increasing temperature. Also, the calculated Young's modulus slope is in very good agreement with the experiment. The finite temperature heat capacity and electronic thermal conductivity are calculated from AIMD simulations, which are in better agreement with the experiment than the heat capacity and thermal conductivity calculated using the structures relaxed at 0 K. Lastly, the thermal diffusivity from AIMD has opposite temperature dependence compared to experimental results, which we argued comes from the underestimated electronic thermal conductivity.(c) 2023 Elsevier B.V. All rights reserved.

Automated summary

Uranium mononitride (UN) is a promising nuclear fuel with advantageous properties for high temperature applications. This study uses ab initio molecular dynamics (AIMD) simulations to investigate UN behavior at different temperatures. The simulations reveal softening of phonon modes and increased compressibility of UN with temperature. The calculated thermal expansion and elastic properties are in good agreement with experimental measurements. Furthermore, the electronic properties and thermal conductivity are better predicted using AIMD simulations compared to calculations at 0 K. However, the thermal diffusivity shows an opposite temperature dependence due to underestimated electronic thermal conductivity.