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Reactor safety analysis requires knowledge of the thermophysical properties of molten oxide fuel and of the thermal equation-of-state of oxide fuel in thermodynamic liquid-vapor equilibrium far above 3000 exp 0 K. The thermodynamic state of saturated UO sub 2 fuel vapor, its internal energy U(T), specific heats C/sub v/(T) and C/sub p/(T), and enthalpy functions H exp 0 (H) and H exp 0 (T)-H sub 00 have been determined by means of statistical mechanics in the temperature range 3000 to 5000 exp 0 K. The discussion of the thermodynamic state includes the evaluation of the plasma state and its contribution to the caloric variables-of-state of saturated oxide fuel vapor. The resulting enthalpy functions and specific heats for saturated UO sub 2 vapor of equilibrium composition and that for pure UO sub 2 gas are compared with the enthalpy and specific heat data of gaseous UO sub 2 at lower temperatures known from literature. Tentatively also experimental enthalpy data of the condensed phases of UO sub 2 have been converted into the vapor phase using the Clausius-Clapeyron equation. However, the correlation based on the Clausius-Clapeyron equation turns out to fail because this equation is not applicable to the chemically reactive oxide fuel system. (ERA citation 03:012442)