Formation and stability of actinide oxides: a valence band photoemission study

Interaction of PuO2thin films with water

The surface adsorption and reaction of water with PuO2thin films was investigated by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS, respectively). Initial motivation of the work was to further investigate the potential role of water in promoting the surface oxidation of PuO2to Pu(IV)/Pu(V) mixed oxides formerly discussed in literature which may seriously impede the stability of spent nuclear fuel. Water may act as oxidant, as catalyst for the oxidation by O2, or as reactant leading to formation of hydrous oxide being oxidized by O2instead of PuO2(cr). In order to obtain high water coverage under the experimentally required ultra-high vacuum conditions, water was adsorbed at low temperature (77 K) as thick ice film. Results were compared to thin water layers adsorbed at room temperature.

When adsorbed at 298 K, water dissociates forming a thin hydroxyl (OH−) layer with small amounts of molecularly adsorbed water but no further reaction (in the sense of oxidation or reduction) is detected. At 77 K, water condenses as ice film. Here, a mainly non-dissociative adsorption of water is observed for layers ∼1 ML while for higher dosages, only molecular water/ice is observed and no significant contribution of OH (in the water layers) is detected. When exposing the sample to UV light while warming it up, the ice layer thaws and desorbs leaving behind a Pu2O3surface. This surprising reduction of PuO2stands in sharp contrast to the radiolytically driven oxidation of spent fuel in presence of water. It is discussed in terms of a photochemically driven interface reaction.