THE INFLUENCE OF MOISTURE ON AIR OXIDATION OF UO2 - CALCULATIONS AND OBSERVATIONS

Phase relationships among solids in the UO2-O2-H2O system at 25, 100, and 200-degrees-C and pressures to 2 MPa have been calculated from cri tically evaluated thermodynamic data. Stability limits of the solids a re expressed in terms of oxygen and water partial pressures at each te mperature. The results are then discussed in terms of known UO2 oxidat ion reactions and uranium mineralogy. Particular attention is paid to ''UO3 hydrates, '' some of which are shown to be stable phases in air at very low relative humidities (down to approximately 0.1% at 25-degr ees-C). This is relevant to fuel storage because of the very high mola r volumes of these phases, relative to UO2, and consequent potential f or damage to defected fuel assemblies. Comparison of the calculated ph ase relationships with observed UO2 oxidation behavior helps to identi fy those phase interconversions that are kinetically constrained.