KINETICS OF CORROSION AND DISSOLUTION OF URANIUM-DIOXIDE AS A FUNCTION OF PH

A continuous flow-through reactor with a thin layer of solid particles (size ranging from 100 to 300 mu m) was used to obtain a deeper knowl edge on the mechanism of dissolution of UO2 under oxidizing conditions . Using this methodology the dissolution rate of uranium dioxide was d etermined at three different oxygen partial pressures (5, 21, and 100% in nitrogen) and as a function of pH (between 3 and 12) in a noncompl exing medium. From the results of these experiments the following rate equation was derived: (3 < pH < 6.7) r (mol . s(-1). m(-2)) = 3.5 (+/ -0.8). 10(-8).[H+](0.37+/-0.01).[O-2](0.31+/-0.02) In addition, XPS ch aracterizations were pe;formed to determine the U(IV)/U(VI) ratio on t he solid surface at different experimental times and conditions. These results showed that at acidic conditions (pH below 6.7) the final sol id surface presents a stoichiometry close to UO2, while at alkaline co nditions the final solid surface average composition is close to UO2.2 5. This information was integrated with the results of the leaching ex periments to present a model for the mechanism of dissolution of urani um dioxide under the experimental conditions. (C) 1997 John Wiley & So ns, Inc.