Low-temperature Oxidation Behaviour of Fine UO2 Powders in Moisturized Atmospheres

The sequence of oxidation behavior at low-temperature (between 40 °C and 190 °C) of a variety of fine UO2 powders under humid atmosphere has been investigated by simultaneous thermogravimetric analysis and x-ray diffraction. A systematic comparison of the oxidation of powders with different specific surface area (SSA) prepared by the ADU process in dry and humid condition was performed. Small parameter change in the UO2 powder preparation by the ADU route led to significant changes in the obtained SSA. It was found that centrifugation, used in this thesis to separate the precipitated ADU from ammonia, provided significantly lower SSA than vacuum filtering previously employed [1-4]. In humid atmosphere at low temperature the sequence of UO2 oxidation reaction was found to be the same as in dry air, UO2 → U4O9 →U3O7 At almost all the temperatures (40, 70, 100, 130 and 160 °C) tested, it was found that humid atmosphere conditions leads to more oxidation than in dry air. Only at 190 °C does dry air lead to more oxidation than moisturized air.