Thermodynamic properties of hyperstoichiometric urania in the UO2–UO2.25 solid solution range

Experimental data by Nakamura and Fujino [J. Nucl. Mater. 149 (1987) 80] on the x–T–p(O2) relations for hyperstoichiometric urania between 773 and 1373 K have been used for the assessment of thermodynamic properties of the UO2–UO2.25 solid solution series. The relative partial thermodynamic functions of UO2 and UO2.25 in the UO2–O2 and UO2–UO2.25 systems have been calculated using the Gibbs–Duhem equation. The results enable one to determine activity–composition relations and other thermodynamic mixing properties of UO2+x and thermodynamic properties of the UO2.25 end-member. The UO2 activity and its entropic contribution have positive deviations from the mole fraction of the UO2 end-member in the entire stability range of urania except the near stoichiometric region. A peculiar property of urania are non-zero values of these functions at the second end-member composition. Beyond the near stoichiometric region, the entropy of mixing of UO2+x is approximately half of that of an ideal one-site solid solution, and the enthalpy of mixing is positive.