Characterisation of crystalline UO2 oxidised in 1 Torr of oxygen at 25, 225 and 3000C: Pt. 1:

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 3000C. The oxidation process was monitored using X-ray photoelectron spectroscopy. At 250C surface oxidation only occurred (to a depth of ca. 10 A) to form the hyperstoichiometric oxide UO2+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 2250C 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 3000C enhanced these differences in reactivity, resulting in the growth of a layer of U3O8 on the (111) surface. (author)