Thermodynamics of urania volatization in steam

The thermochemistry of urania volatilization as UO3 obtained from several literature was reviewed. The predicted equilibrium pressures of UO3 over pure UO2+x in a gas of known oxygen potential and temperature fall into one of two groups that differ by one order of magnitude at 2000 K and two orders of magnitude at 1500 K. The recent theoretical and experimental work from the Lawrence Livermore National Laboratory is judged to be the most comprehensive and reliable. This work supports the high-UO3 pressure estimates and gives values about 30% lower than those obtained from the earlier work of Ackermann and Chang. The effects of other factors on urania volatility were investigated: H2 in steam produces a large reduction in the UO3 pressure; the other volatile uranium species, UO2(OH)2, is important only at low temperatures and high steam pressures; fuel burnup has a small effect on the UO2 pressure. As part of the UO3 assessment, the oxygen potential models for UO2+x of Blackburn and Lindemer/Besmann were compared. (orig.)