Volatilization of urania in steam at elevated temperatures

The volatilization rates of urania in pure steam and in mixtures of steam and argon, steam and helium, and in steam/Ar/H₂ were measured in a thermogravimetric apparatus at atmospheric pressure and temperatures from 1250 °C to 1600 °C. The objective was to clarify the kinetics and mechanism of the volatilization process and to assess the validity of existing thermodynamic data that is needed to interpret the kinetic data. The mass transfer characteristics of the system were determined independently by measuring the evaporation rates of metals of well-known vapor pressures. The measurements of urania volatilization show that the rate-controlling process is gas-phase mass transfer of uranium-bearing species such as UO₃. Analysis of the volatilization data also suggests that the calculated UO₃ vapor pressures are somewhat in error at the lower temperatures in the range tested.