Oxidation of UO₂ and Fission Product Release at 400-1000 °C in Air

Abstract

Studies of the oxidation of unirradiated UO₂ pellets in air show two distinct types of oxidation with a change in mechanism at 600-700 °C. At temperatures ≤ 600 °C, U₃O₈ spalls a small particles, 1-15 μm, as soon as it forms. At 700 °C, the U₃O₈ starts to form a cohesive structure in which grain growth occurs in the direction of the oxygen gradient. At higher temperatures ≥ 800 °C U₃O₈ propagates by rapid grain growth, which sweeps through the grains of irradiated UO₂, producing large crystals of U₃O₈, ≥ 1mm. Put succinctly, at T ≤ 600 °C fragmentation accompanies the formation of U₃O₈ while at T ≥ 800 °C, grain growth occurs. It was observed that in the first temperature region, volatile fission product releases are small, while in the second region, 100% release can be correlated with U₃O₈ formation. This is supported by data in the literature. 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. The Ru, probably as RuO₂, reacts rapidly with Zr so that little can escape from an element or pressure tube; iodine probably as I₂, plates out rapidly on steel so that little of this can escape from a real system. Te releases are small, as it plates out rapidly on both Zr and steel.

It appears that, within the error of measurements, burnup does not greatly affect rates of fission product release and oxidation in air at 400-1000 °C, so that oxidation rate data gathered using unirradiated pellets can be applied to irradiated fuel.