ABSTRACT
RATIONALE: Cesium and molybdenum are fission products of uranium dioxide fuel in nuclear reactors, which interact with each other depending on the oxygen potential of the fuel. This leads to formation of various compounds of the Cs2 O-MoO3 system, which are exposed to high temperatures during operation of a reactor or a severe accident at a nuclear power plant. This is why the study of the vaporization and thermodynamics of compounds in the Cs2 O-MoO3 system is important. METHODS: Synthesis of the compounds in the Cs2 O-MoO3 system was carried out by sintering Cs2 MoO4 and MoO3 . Characterization of the samples was accomplished with the use of XRD, TGA/DSC/DTA, IR spectroscopy, and ICP emission spectroscopy. Vaporization of the samples under study was carried out from a platinum effusion cell using an MS-1301 mass spectrometer developed for high-temperature studies of low-volatility substances. RESULTS: The temperature dependences of partial pressures of vapor species were determined over pure MoO3 and Cs2 MoO4 in the ranges 870-1000 K and 1030-1198 K, respectively. MoO3 , Mo2 O6 , Mo3 O9 , Mo4 O12 , and Mo5 O15 were shown to be the main vapor species over the Cs2 O-MoO3 system in the temperature range 850-1020 K. The component activities, Gibbs energies of mixing, and excess Gibbs energies were obtained as functions of the component concentration at 900, 950, and 1000 K. CONCLUSIONS: The thermodynamic properties of the Cs2 O-MoO3 system found in the study evidenced negative deviations from ideality. The MoO3 and Cs2 MoO4 partial molar enthalpies of mixing, the Cs2 MoO4 partial vaporization enthalpy, and the total enthalpy of mixing in the Cs2 O-MoO3 system at 1000 K were obtained for the first time.
ABSTRACT
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.
