Results of molecular dynamics (MD) simulation of UO2 in a wide temperature range are presented and discussed. A new approach to the calibration of a partly ionic Busing-Ida-type model is proposed. A potential parameter set is obtained reproducing the experimental density of solid UO2 in a wide range of temperatures. A conventional simulation of the high-temperature stoichiometric UO2 on large MD cells, based on a novel fast method of computation of Coulomb forces, reveals characteristic features of a premelting lambda transition at a temperature near to that experimentally observed (T(lambda)=2670 K). A strong deviation from the Arrhenius behavior of the oxygen self-diffusion coefficient was found in the vicinity of the transition point. Predictions for liquid UO2, based on the same potential parameter set, are in good agreement with existing experimental data and theoretical calculations.

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The ionization and dissociation energies of the different uranium and plutonium oxides have been measured by mass spectrometry of molecular beams produced by Knudsen effusion at high temperature. The values obtained constitute a set of self-consistent quantities, which are in agreement with the existing thermodynamic data of these oxides. On the basis of the experimental molecular parameters, general formulas for the ionization and dissociation/ionization cross sections due to electron inelastic scattering have been obtained for collision energies up to about 60 eV. These formulas are sufficiently accurate to calculate the composition of equilibrium vapor mixtures over UO2 and PuO2 from conventional mass spectrometric measurements.

The thermal diffusivity and heat capacity of uranium dioxide have been measured from 500 to 2900 K with an advanced laser-flash technique. These two quantities were determined simultaneously by means of an accurate numerical fitting of the experimental thermograms. At high temperatures the precision of the method used is much better than that associated with conventional laser-flash measurements. It was found that the heat capacity continues to increase even at temperatures above the expected lambda transition (2670 K). The inverse of the thermal diffusivity increases linearly with temperature up to 2600 K, whilst at higher temperatures the slope markedly decreases. A new expression for the thermal conductivity as a function of temperature is proposed, which is corroborated by some theoretical considerations on the underlying heat transport mechanisms.