Rationale
The UO2–ZrO2 solid solution at high temperatures is the key system of modern nuclear science and technology in the context of the safety operation of nuclear cycles, the consequences of severe accidents, and the incorporation of nuclear waste. Urgent needs of the continuation of experimental studies of this system at temperatures up to 3000 K are aimed at preventing severe accidents similar to Chernobyl and Fukushima when the thermodynamic approach is used for the prediction of high‐temperature behavior of materials.

Methods
This investigation was carried out using the Knudsen effusion mass spectrometric method using the MS‐1301 magnetic sector mass spectrometer. The samples in the UO2–ZrO2 system were vaporized from a tungsten effusion cell. Vapor species effusing from the cell were ionized at an electron ionization energy of 70 eV.

Results
The vaporization and thermodynamics of pure UO2 and ZrO2 as well as of the samples in the UO2–ZrO2 system were studied in the range 2000–2730 K. The temperature dependences of the partial vapor pressures of UO and UO2 over pure UO2 were obtained at 2060–2456 K, which agreed with the literature results. The partial vapor pressures of UO, UO2, ZrO, and ZrO2, the vaporization rates, and the UO2 and ZrO2 activities in the UO2–ZrO2 solid solutions were determined at 2370, 2490, 2570, and 2730 K.

Conclusions
The component activities and excess Gibbs energies of the UO2–ZrO2 system indicated a change in deviations from the ideal behavior from positive to negative with the temperature increase from 2370 to 2730 K. The thermodynamic functions of formation from the oxides of the solid solutions in the UO2–ZrO2 system such as Gibbs energies as well as the enthalpies and entropies of formation were obtained for the first time at 2550 K in the composition range 0.89–1.00 ZrO2 mole fraction.