Measurement and interpretation of the thermo-physical properties of UO2 at high temperatures: The viral effect of oxygen defects

Abstract Values are reported of specific heat, thermal conductivity and thermal diffusivity of UO 2 from 1500 K to 2900 K based on laser flash measurements. Experiment is complemented by the development of solid state physics models that aid in the interpretation of the results. Specific heat is shown to exhibit a smooth maximum at 2715 K ± 100 K, consistent with a competition between two processes - oxygen defect interactions (net attraction) and saturation of oxygen interstitial sites. The specific heat model and measurements show, for the first time that a gradual pre-melting transition is consistent with high temperature literature values – enthalpy increment measurements and independently measured high temperature oxygen defect concentrations. Thermal conductivity exhibits a minimum consistent with: 1) an increase in electronic thermal conductivity via polaron production and mobilization and 2) degradation in lattice thermal conductivity due to phonon - phonon scattering and phonon - defect scattering. It is predicted that the high concentration of oxygen defects should contribute significantly to electrical conductivity and thermal expansion at high temperatures.