First-Principles Investigation: Effects of Molybdenum Substitution on the Elastic Properties of Uranium Dioxide

Uranium dioxide (UO2) is the standard fuel in light water reactors, but improving its mechanical performance is essential for achieving higher burnups. This study employs first-principles density functional theory with the DFT + U approach to investigate the effect of molybdenum (Mo) substitution on the elastic properties of UO2. Supercell models with Mo concentrations from 3.125 to 9.375 at.% are constructed, and elastic constants are calculated using the stress–strain method, complemented by Bader charge and charge density analyses. The results reveal a non-monotonic concentration-dependent behavior: at 3.125 at.% Mo, the shear and Young’s moduli increase by ~16% and ~14%, respectively, indicating significant stiffening; at higher concentrations (6.25 and 9.375 at.%), both moduli decrease, leading to softening of UO2 lattice. Bader charge analysis shows that Mo loses only 0.13 electrons (vs. 2.56 for U) and the Mo–O bond is much shorter than the U–O bond; this is evidence of covalent bonding between Mo and O atoms that acts as local strengthening centers at low doping. The softening at higher concentrations is attributed to increased lattice distortion and enhanced bond delocalization, supported by changes in Cauchy pressure, Debye temperature, and Vickers hardness. The calculated elastic modulus and hardness of pure UO2 are in good agreement with previously reported experimental data. For Mo-doped UO2 systems, this work establishes a quantitative composition–property relationship, providing a theoretical reference for future experimental investigations.