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.