Thermodynamic Properties of MOX Fuel: Impact of the Cationic Valence from MD Simulations

Molecular dynamics (MD) simulations were carried out to determine the impact of the cationic valence on the thermodynamic properties of the MOX (Mixed Oxide) samples. It has been found recently that U and Pu cations can coexist with various valences in near-stoichiometric and hypostoichiometric samples. Moreover, the CALPHAD method predicts a variation of valence concentrations with increasing temperature. As a consequence, this work investigates the effect of pentavalent and trivalent cations on the thermodynamic properties, i.e., lattice parameter, thermal expansion, and heat capacity. Stoichiometric conditions in MOX systems were studied by considering pentavalent and trivalent cation concentrations between 0 and 0.08. In addition, two charge compensation mechanisms arising under hypostoichiometric conditions─particularly those involving U5+ and Pu3+, together with the resulting variation in O/M─were examined. This work highlights for the first time the influence of these elements on MOX fuel properties, in particular, on the Bredig transition. The lattice parameters, linear thermal expansion coefficients, and heat capacities were calculated.