The reaction of water on polycrystalline UO2: Pathways to surface and bulk oxidation

The reaction of polycrystalline uranium dioxide with H2O is studied by in situ Raman spectroscopy, temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The difference between UO2 and U3O8 can be seen in the bulk by XRD and Raman and on the surface by both the core level and valence band regions. In the valence band region the U 5f line was far more pronounced for UO2 than for U3O8. It was possible to monitor the near surface oxidation of UO2 to U3O8 by H2O at 300 K using Raman spectroscopy while oxygen deposition was quantified using XPS. TPD of D2O on H2-reduced U3O8 (≈UO2) showed desorption of D2. D2 desorption occurred in two temperature domains (at ≈410 K and ≈570 K). Increasing the surface exposure to D2O affected the distribution of both D2 peaks. The first desorption-peak populated first while at relatively higher D2O exposure the second desorption peak increased considerably in intensity. The second desorption of D2 during D2O-TPD can be tracked down to oxidation of deeper layers.