U-O AND ZR-O THERMOCHEMISTRY FROM THE PURE METAL TO THE DIOXIDE AND APPLICATION TO CORE CONCRETE INTERACTIONS IN SEVERE FUEL DAMAGE ACCIDENT ANALYSIS

The thermodynamic constraint for the oxygen potential of tetravalent m etal oxides relates the standard free energy of formation to the relat ive partial molal energy by DELTAG(f)-degrees(MO2) = 1/2integral-2/0DE LTAG(O2)dCBAR. This constraint and its variant have been utilized to c onstruct the full spectrum of oxygen potentials of the U-O and Zr-O sy stems at high temperatures. Henry's law is assumed to apply in the oxy gen-containing metal regions. The ensemble of the results are displaye d in the form of oxygen isobars superimposed on the phase diagrams of U-0 and Zr-O systems. The oxygen potentials obtained are then used in the analysis of core-concrete interactions resulted from the severe fu el-damage accident in a nuclear reactor. Allowance for dissolution of oxygen and nonideal behavior of the elements in the metal phase accord ing to regular solution theory and reduced oxidation states of uranium and zirconium ions in the oxide phase are also part of the analysis.