Abstract
Exposure of microcrystalline UO2 to CO2 in a reactor flux between 40 and 500°C leads to the formation of CO and UO2+x, where x does not exceed 0.25. The UO2+x, a disordered cubic phase which is thermodynamically unstable at low temperatures, is stable to annealing at temperatures below 650°C, but when heated to 750°C recrystallizes into the thermodynamically stable forms U4O9 and UO2. The initial rate of oxidation is proportional to surface area of the UO2 and independent of CO2 pressure, the geometry of the system and of inert gas diluents. UO2 is oxidized by products of radiolysis of chemisorbed CO2. Active species produced in the gas phase remote from the solid do not take part in the oxidation. A radiation-assisted reduction of UO2+x by CO also occurs, the rate being dependent on temperature. The steady-state composition of UO2+x at a given temperature is controlled by the relative rates of the oxidation by CO2 and the reduction by CO. High G values for γ-radiolysis of CO2 in this system arise from direct energy transfer to the gas from the solid surface.
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