Dissolution of uranium dioxide in simulated Boom clay water

The solubility behavior of uranium dioxide was studied under oxidizing and reducing conditions in simulated Belgian Boom clay water at 25 degreesC, on unirradiated fuel pellets, For a comparable period (1 month). the uranium concentration and the dissolution rate (10(-12) mol m(-2) s(-1)) in reduced medium were similar to 30 times smaller than in oxidizing conditions (3 x 10(-11) mol m(-2) s(-1)). X-ray photoelectron spectroscopy analyses revealed that U(VI) and U(IV) oxidation states were present in the altered layer in both cases. In the case of the experiments performed in oxidizing conditions, this technique also revealed the formation of U-OH bonds. Although steady state was not reached, geochemical calculations allowed us to suggest a class of secondary phases able to control the uranium solubility in the long term.