Calorimetric determination of energetics of solid solutions of UO_2+x with CaO and Y₂O₃

Quantitative study of thermodynamic properties of solid solutions of UO_2+x with divalent and trivalent oxides is important for predicting the behavior of oxide fuel. Although early literature work measured vapor pressure in some of these solid solutions, direct calorimetric measurements of enthalpies of formation have been hampered by the refractory nature of such oxides. First measurements of the enthalpies of formation in the systems UO_2+x-CaO and UO_2+x-YO_1.5, obtained by high-temperature oxide melt solution calorimetry, are reported. Both systems show significantly negative (exothermic) heats of formation from binary oxides (UO₂, plus O₂ and CaO or YO_1.5, as well as from UO₂ plus UO₃ and CaO or YO_1.5), consistent with reported free energy measurements in the urania-yttria system. The energetic contributions of oxygen content (oxidation of U⁴⁺) and of charge balanced ionic substitution as well as defect clustering are discussed. Behavior of urania-yttria is compared to that of corresponding systems in which the tetravalent ion is Ce, Zr, or Hf. The substantial additional stability in the solid solutions compared to pure UO_2+x may retard, in both thermodynamic and kinetic sense, the oxidation and leaching of spent fuel to form aqueous U⁶⁺ and solid uranyl phases.