Oxidation of uranium dioxide by carbon dioxide in a reactor flux

Abstract

Exposure of microcrystalline UO₂ to CO₂ in a reactor flux between 40 and 500°C leads to the formation of CO and UO_2+x, where x does not exceed 0.25. The UO_2+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 UO_2+x by CO also occurs, the rate being dependent on temperature. The steady-state composition of UO_2+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.