Thermochemistry of UO₂ – ThO₂ and UO₂ – ZrO₂ fluorite solid solutions

The enthalpies of formation of cubic urania – thoria (c-Th_xU_1−xO_2+y) and urania – zirconia (c-Zr_xU_1−xO₂, x < 0.3) solid solutions at 25 °C from end-member binary oxides (c-UO₂, and c-ThO₂ or m-ZrO₂) have been measured by high temperature oxide melt solution calorimetry. The enthalpies of mixing for both systems are zero within experimental error. The interaction parameters for binary solid solutions MO₂ – M′O₂ (M, M′ = U, Th, Ce, Zr, and Hf), fitted by regular and subregular thermodynamic models using both calorimetric and computational data, increase linearly with the corresponding volume mismatch. Cubic UO₂ – ZrO₂ appears to be an exception to this correlation and shows a zero heat of mixing despite large size mismatch, suggestive of some short-range ordering and/or incipient phase separation to mitigate the strain. The incorporation of ZrO₂ into UO₂ stabilizes the system and makes it a potential candidate for immobilization and disposal of nuclear waste.