Characterisation of crystalline UO2 oxidised in 1 Torr of oxygen at 25, 225 and 300 °C. Part 2.—X-ray diffraction and scanning electron microscopy

Abstract

The surface oxidation of (111), (110) and (100) planar slices of crystalline UO2, together with a polycrystalline sample, has been studied using X-ray diffraction and scanning electron microscopy. Oxidation was carried out at 225 and 300 °C in 1 Torr O2. The rate and extent of oxidation varied with the choice of face, decreasing along the series: (111) > (110) > (100) > (polycrystalline). At 225 °C a surface layer of U3O7 several µm thick was formed on all the samples. The (111) surface showed the first signs of oxide cracking due to lattice contraction experienced by the cubic UO2 lattice when U3O7 was formed. At 300 °C the differences in reactivity between the individual crystal faces became more marked, resulting in the growth of a layer of U3O8 on the (111) surface, cracking of the (110) surface combined with extensive U3O8 nucleation at crack sites, and cracking of the (100) and polycrystalline surfaces associated with the continued growth of the U3O7 layer. Factors governing the oxidation behaviour of different crystalline UO2 slices are discussed, and it is proposed that the observed reactivity reflects the ease with which oxygen interstitial atoms are incorporated into the new oxide forming at the U3O7/UO2 interface.