Enthalpy and heat capacity of the actinide oxides

Abstract

The available enthalpy data on UO2, ThO2, PuO2, (Th, U)O2, and (Pu, U)O2 have been analyzed and equations have been derived to fit the data. Phase transitions were found in UO2, ThO2, (Th, U)O2, and (Pu, U)O2. The high temperature PuO2 data were too scattered to determine whether a phase transition exists. Above the phase transition temperature, the enthalpy data were fit with a linear equation. Enthalpy data for PuO2 and ThO2 below the phase transition temperature were fit with two-term equations whose contributions are due to phonons and thermal expansion. For UO2 below its phase transition, a term for an electronic contribution was added to this basic equation. Below the phase transitions for (Th, U)O2, enthalpy data were fit by a mole average of the equations used to fit the ThO2 and UO2 data below their phase transitions; however, the mole average equation was not valid for 90 and 92% ThO2 in the mixed oxide. Since it was found that mole averages of the PuO2 and UO2 data do not fit the (Pu, U)O2 data, these data were fit with an equation of the same form as that that used for UO2.