THERMODYNAMICS OF THE U-O AND ZR-SYSTEMS AND APPLICATION TO ANALYSIS OF FUEL LIQUEFACTION DURING SEVERE ACCIDENTS IN LIGHT-WATER REACTORS

Severe accident modeling requires knowledge of the heat effects accomp anying simultaneous dissolution of UO2 and fusion of Zircaloy to form the U-Zr-O melt. Estimation of the enthalpy changes must rely chiefly an data from the U-O and Zr-O binary systems. The partial molar enthal pies of oxygen in the pure liquid metals are determined by application of Sieverts' law. The Sieverts' law constant is obtained by deriving the complete relationship between the oxygen pressure, the O/M, ratio of the solid or Liquid phase and temperature from the pure metal M to the dioxide MO2. This assessment utilizes the integral constraint invo lving the Gibbs free energy of formation of the compound MOC and the v ariation of the oxygen pressure p, over the entire composition range. Once the p-C-T relationship has been established, the enthalpy changes on dissolving UO2 and alpha Zr(O) into the U-Zr-O melt are computed. The heat effects of fuel/cladding dissolution in a severe accident are applied in a liquefaction model based on kinetic control by the avail able heat from fission product decay. In addition to the portion of th e decay heat reaching the inner cladding surface, the rate of melting of the cladding depends on heat flow to its outer surface from steam o xidation or radiation from other parts of the core. Depending on the r elative importance of these two heat inputs to the cladding, time to m elting is from 100 to 400 s and the fraction of the fuel dissolved var ies from 5 to 20%. (C) 1997 Elsevier Science B.V.