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A continuous flow-through reactor with a thin layer of solid particles (size ranging from 100 to 300 m) was used to obtain a deeper knowledge on the mechanism of dissolution of UO 2 under oxidizing conditions. Using this methodology the dissolution rate of uranium dioxide was determined at three different oxygen partial pressures (5, 21, and 100% in nitrogen) and as a function of pH (between 3 and 12) in a noncomplexing medium.From the results of these experiments the following rate equation was derived:In addition, XPS characterizations were performed to determine the U(IV)/U(VI) ratio on the solid surface at different experimental times and conditions. These results showed that at acidic conditions (pH below 6.7) the final solid surface presents a stoichiometry close to UO 2 , while at alkaline conditions the final solid surface average composition is close to UO 2.25 . This information was integrated with the results of the leaching experiments to present a model for the mechanism of dissolution of uranium dioxide under the experimental conditions.

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