Chemical thermodynamic representation of

The entire sup(**) data base for the dependence of the nonstoichiometry, x, on temperature and chemical potential of oxygen (oxygen potential) was retrieved from the literature and represented. This data base was interpreted by least-squares analysis using equations derived from the classical thermodynamic theory for the solid solution of a solute in a solvent. For hyperstoichiometric oxide at oxygen potentials more positive than -266700+16.5T J/mol, the data were best represented by a [UO2]-[U2Osub(4.5)] solution. For O/U ratios above 2 and oxygen potentials below this boundary, a [UO2]-[U2Osub(4.5)] solution represented the data. The data were represented by a [UO2]-[Usub(1/3)] solution. The resulting equations represent the experimental In(PO2sup(*))-In(x) behavior and can be used in thermodynamic calculations to predict phase boundary compositions consistent with the literature. Collectively, the present analysis permits, for the first time, a mathematical representation of the behavior of the total data base. (orig.)