XPS, XRD and SEM study of oxidation of UO2 by air in gamma radiation at 150 deg C

UO2 disks were heated at 150 deg C in air, in O2 with 60% saturated steam, and in Ar with 60% saturated steam atmospheres for ∼2 years in gamma fields equivalent to those associated with 10- to 20-year-old used CANDU fuel. Surface analysis of the disks, using X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy, shows formation of U3O8 on the UO2 disks exposed to air or O2. This is the first report of the formation of U3O8 on UO2 by air oxidation at such a low temperature. The rate of U3O8 formation by dry air oxidation of UO2 at 150 deg C and in gamma fields of dose rate ∼15 Gy·h-1 is very low. The presence of water vapour along with O2 increases the oxidation of UO2 in gamma fields, leading to the formation of U6+ phases, e.g., UO3·xH2O along with U3O8. On the other hand, UO2 disks did not suffer any oxidation by water vapour radiolysis at 150 deg C in an O2-free (60% saturated steam in Ar) atmosphere. These are important observations for the dry storage of used fuel because the oxidation of UO2 to U3O8 is accompanied by a volume expansion, which could result in splitting of the Zircaloy cladding and powdering of the fuel matrix. (author). 46 refs., 7 tabs., 16 figs