Oxidation of UO2 at 400 to 1000 degrees C in air and its relevance to fission product release

Currently there is great interest in the behaviour of UO2 under oxidizing conditions because irradiated uranium dioxide fuel can conceivably be exposed to a hot oxidizing atmosphere as a result of accidents. The temperature range covered in this paper is 400 to 1000 degrees C. At these high temperatures, UO2 in air can oxidize rapidly to U3O8 via U3O7 and/or U4O9. The accompanying volume increase and corresponding stresses lead to fragmentation of the fuel pellets. The purpose of this work was to investigate the dependence of UO2 oxidation on temperature, rate of air supply and residence time at temperature; to determine the rate controlling steps and rate of oxygen penetration; and to characterize the oxidation products and size of fragments. In addition, detailed metallography was related to X-ray diffraction studies of the oxidized UO2 to facilitate future study of irradiated fuel, which is easier to do by metallography in hot-cells than by X-ray diffraction. Samples were heated in argon, then once at temperature they were exposed to air at a controlled flow-rate. Studies of the oxidation of unirradiated UO2 pellets in air show two distinct types of oxidation with a change in mechanism at 600-700 degrees C. At temperatures ≤ 600 degrees C fragmentation accompanies the formation of U3O8 while at T ≥ 800 degrees C, rapid grain growth occurs. In the first temperature region, volatile fission product releases are small, while in the second region, 100% release can be correlated with U3O8 formation. In the first region, only the grain boundary inventory is released while in the other, 100% of the Xe, Kr, Ru, Sb, Cs and I are released. It appears that, within the error of present measurements, burnup does not affect rates of fission product release and oxidation in air at 400 to 1000 degrees C, so that oxidation rate data gathered using unirradiated pellets can be applied to irradiated fuel. 33 refs