Oxidation of Uranium Dioxide

Abstract

The oxidation of UO2 was studied by thermogravimetry and X-ray diffraction. It was clarified that the thermal history covering the first stage of the oxidation from UO2 to U3O UO7 significantly influenced the rate of the oxidation of the second stage from U3O7 to U3 O8. The entire oxidation reaction proceeded in what to all appearances, was a single stage when the specimen temperature was raised rapidly, whereas at slower rates of heating up, two distinct stages of oxidation were observed, separated by an intermediate induction period. These findings suggest the existence of a close connection between the rate of formation of the U3O7 phase and the rate of the subsequent oxidation of this phase: A slow formation of U3O7 would tend to prolong the induction period preceding the second stage of the oxidation. A similar effect was observed also with annealing of the intermediate U3O7 at 200°C: The increase of annealing time prolonged the induction stage. The rate of the second stage oxidation was fairly well expressed by Johnson & Mehl's equation, log (1/(1-y/)=(1/2.303)kntn . The time exponent n in this equation varied in the range of 1.0~2.5, and the rate constant k of 1.15×10−4~2.04 ×10−1 min−1, depending on the experimental conditions.