Thermochemistry of UOS; Evaporation of US–UO2 Mixtures; On the Attainment of Equilibrium in Knudsen Cells

Abstract

Evaporation of mixtures of US and UO2 and of UOS has been studied by molecular effusion, and the gaseous species have been investigated mass spectrometrically. A eutectic occurs in the system US–UO2 at 2240°K and at approximately 45% dioxide. In the vapor the gaseous species in order of decreasing partial pressures are: UO, UO2, UOS, US, S, U, and O. Because of the formation of the gaseous monoxide, the total pressure is about twice the sum of the vapor pressures for US and UO2 along. Calculated as though the vapor had the molecular weight of the monoxide, the total “effective” vapor pressure is given by logPE(atm) = − (3.496 ± 0.064)104 / T + (10.870 ± 0.298) below the eutectic temperature and logPE(atm) = − (2.450 ± 0.181)104 / T + (6.277 ± 0.755) above this temperature. The energy of atomization of gaseous UOS is 300 ± 7 kcal/mole at 0°K. In general, the mass-spectrometric measurements were much less precise and reproducible than those in previous studies of simpler systems with the same equipment. This was believed to be due to nonequilibrium conditions in the condensed phases in the Knudsen cell. It was concluded that one must study the condensed phases as well as the vapor phase before assuming that mass spectrometrically studied isomolecular reactions will yield equilibrium thermodynamic data.