First-Principles Study of Molybdenum Incorporation in Uranium Mononitride*

Uranium mononitride (UN) is a leading candidate for accident-tolerant nuclear fuels due to its high uranium density and superior thermal conductivity compared to UO_2. However, fuel performance under irradiation is strongly influenced by the incorporation and evolution of fission products. Among these, molybdenum (Mo) is one of the most abundant metallic fission products, and its behavior in the UN matrix remains insufficiently understood. This study employs density functional theory to investigate the incorporation of Mo in UN.

Formation energies of single and double Mo substitutions were calculated in both 2x2x2 and 3x3x3 supercells, with multiple atomic configurations tested to identify the most favorable incorporation sites. The comparison between isolated and paired substitutions allows the evaluation of Mo-Mo binding energies, providing insight into whether Mo atoms tend to cluster or remain dispersed within the lattice. The stability of Mo incorporation was also analyzed under different stoichiometry conditions, highlighting the contrasting behavior in uranium-rich versus nitrogen-rich environments. In addition, electronic structure analysis was studied, analyzing the projected densities of states and charge redistribution of each configuration.

These results provide a comprehensive picture of Mo incorporation in UN, connecting energetics, clustering tendencies, chemical potential dependence, and electronic effects. The findings contribute to a broader understanding of fission product behavior in actinide nitrides and establish a framework for modeling impurity effects relevant to next-generation nuclear fuel design.