Vaporization and thermodynamics of the Cs2O-MoO3 system studied using high-temperature mass spectrometry

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 Cs2O-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 Cs2O-MoO3 system is important. Methods Synthesis of the compounds in the Cs2O-MoO3 system was carried out by sintering Cs2MoO4 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 Cs2MoO4 in the ranges 870-1000 K and 1030-1198 K, respectively. MoO3, Mo2O6, Mo3O9, Mo4O12, and Mo5O15 were shown to be the main vapor species over the Cs2O-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 Cs2O-MoO3 system found in the study evidenced negative deviations from ideality. The MoO3 and Cs2MoO4 partial molar enthalpies of mixing, the Cs2MoO4 partial vaporization enthalpy, and the total enthalpy of mixing in the Cs2O-MoO3 system at 1000 K were obtained for the first time.