Characterisation of crystalline UO2 oxidised in 1 Torr of oxygen at 25, 225 and 300 °C. Part 1.—X-ray photoelectron spectroscopy

Abstract

The oxidation behaviour of individual grains in bulk UO₂ has been simulated using single-crystal surfaces. To this end the oxidation of (111), (110) and (100) planar slices of crystalline UO₂ together with a polycrystalline sample have been oxidised in 1 Torr of oxygen at 25, 225 and 300 °C. The oxidation process was monitored using X-ray photoelectron spectroscopy. At 25 °C surface oxidation only occurred (to a depth of ca. 10 Å) to form the hyperstoichiometric oxide UO_2+x, with x in the range (0.03–0.06)±0.02. The surface reactivity decreased in the order (110) > (111) > (polycrystalline) > (100), and was governed by the two-dimensional surface crystal structure. At 225 °C the rate and extent of reaction was much greater, resulting in the formation and growth of a sub-stoichiometric layer of U₃O₇ on all the samples. The order of surface reactivity changed to (111) > (110) > (100) > (polycrystalline). Further oxidation at 300 °C enhanced these differences in reactivity, resulting in the growth of a layer of U₃O₈ on the (111) surface.