(339a) Thermo Physical Properties of Urania-Ceria Solid Solutions: A Monte Carlo Study

The thermophysical properties of urania-ceria solid solutions are of great importance for nuclear fuel applications due to the fact that ceria is a fission product and exhibits a high solubility in urania (UO₂). Understanding the thermodynamics of this solid solution is also important because ceria-urania mixtures have been used as a surrogate for urania-plutonia mixed oxide fuels. The structure of the solid solutions is characterized as a fluorite based cubic crystal in which cations are arranged in an fcc lattice at an oxygen to metal stoichiometric ratio near 2 [J. Inorg. Nucl. Chem., 1970, 32, 59]. It has been found, however, that the structural and energetic properties of the solid mixture (U_xCe_1-xO₂) depend on both the cation arrangement and the nature of the charge state. In addition, there is evidence of charge transfer between U⁴⁺ and Ce⁴⁺ leading to oxidized state U⁵⁺ or U⁶⁺ and reduced state Ce³⁺, respectively [Inorg. Chim. Acta, 1994, 225, 305]. A recent density functional theory (DFT) study shows that charge transfer occurs between U⁴⁺ and Ce⁴⁺ and finds an increase in mixing energy in the range of 4-14 kJ/mol of the formula unit depending on the cation ordering and hence entropy of the system [Phys. Rev. B: Condens. Matter, 2011, 84, 085131]. Their subsequent experimental and lattice dynamic work involved the calculation of the mixing enthalpy at different composition of cations as well as at different charge states at room temperature [Phys. Chem. Chem. Phys., 2012, 14, 5680]. However it is quite expected that the configurational entropy may increase with temperature and hence the possibility to have mixed charge states in the system at higher temperature. On the other hand there is an energy penalty between different charge states. Therefore the key challenge is to sample all the possible configurations, which is extremely difficult to do with MD due to large energy barriers between states. In this work, we introduce efficient Monte Carlo sampling techniques, which enable a rigorous sampling of the equilibrium probability density of the system at finite temperature. This technique allows us to compute the mixing enthalpy of solid solutions (U_xCe_1-xO₂) at different charge states as well as at different temperatures.