High temperature mass spectrometric study of thermodynamic properties in the UO2 -ZrO2 system.

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 continuation of experimental studies of this system at the temperatures up to 3000 K are dictated by the aim of preventing severe accidents similar to Chernobyl and Fukushima when the thermodynamic approach is used for prediction of high temperature behavior of materials. METHODS This investigation was carried out by 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 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 UO and UO2 partial vapor pressures over pure UO2 were obtained at 2060-2456 K and agreed with the literature results. The UO, UO2 , ZrO, and ZrO2 partial vapor pressures, the vaporization rates, and the UO2 and ZrO2 activities in the UO2 -ZrO2 solid solutions were determined at 2370 K, 2490 K, 2570 K, and 2730 K. CONCLUSIONS The component activities and excess Gibbs energies of the UO2 -ZrO2 system indicated a change of deviations from the ideal behavior from positive to negative with temperature increase from 2370 K 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 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.