A comparative investigation on the fundamental physical properties of UX2 (X = O, N, C, Si and S) solid nuclear fuel materials: A DFT + U study

Abstract

This study investigates and compares the physical properties of various UX2 (X = O, N, C, Si and S) nuclear fuels using a combination of Density Functional Theory (DFT) and Hubbard U correction parameter (U) to accurately model the electronic structure and account for strong correlation effects among 5f orbital electrons. Spin-polarized DFT + U method is employed to optimize the structures of the considered compounds. The computational analysis reveals that UO2 and UN2 exhibit Mott-insulating properties, while the remaining UX2 compounds demonstrate metallic nature. To assess the mechanical aspects of these materials, the study evaluates their mechanical stability, Vickers hardness and machinability index through calculations of elastic constants. Notably, all the UX2 fuels are found to be mechanically stable and possess ductility. Both the two-dimensional (2D) and three-dimensional (3D) depictions of the elastic moduli for the analyzed solid fuels demonstrate the presence of elastic anisotropy. The famous Slack's equation is used to determine the lattice thermal conductivity of the UX2 fuels, providing valuable insights into their thermal properties. Among the investigated materials, UC2 exhibits the highest values of Debye temperature and lattice thermal conductivity, which amount to 412 K and 15.73 Wm−1 K−1 at 300 K, respectively. Furthermore, UN2 demonstrates the highest melting temperature. Among the compounds under investigation, UC2 and UN2 fuels exhibit the most minimal thermal expansion and the greatest heat capacity. Based on these findings, UC2 and UN2 emerge as promising alternative fuel materials to UO2 for potential use in nuclear power reactors. This research contributes to the ongoing efforts in identifying alternative fuel materials.