Surface Reduction of Neptunium Dioxide and Uranium Mixed Oxides with Plutonium and Thorium by Photocatalytic Reaction with Ice

The surface reductions of neptunium dioxide (NpO 2 ) and two mixed oxides of uranium (U–Pu–O 2 and U–Th–O 2 ) with adsorbed water ice were studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). The oxides were produced as thin films by reactive sputter deposition. Water was condensed as a thick ice overlayer on the surface at low temperature. Subsequent warming led to desorption of the ice. When warmed up under ultraviolet light (UV light, HeI and HeII radiation), the surface was reduced. NpO 2 was reduced to surface neptunium sesquioxide (Np 2 O 3 ). In the uranium–plutonium mixed oxide (U–Pu MOX), plutonium was reduced from plutonium dioxide (PuO 2 ) to plutonium sesquioxide (Pu 2 O 3 ). In uranium–thorium mixed oxide (U–Th MOX), the uranium was reduced from hyperstoichiometric uranium dioxide (UO 2+ x ) to stoichiometric UO 2 but not to lower oxides: the lowest thermodynamically stable oxides are formed. In the mixed oxides, uranium reduction seems to be activated for oxides with both thorium and plutonium. Surface reduction is explained as a photocatalytic reaction of the surface, triggered by the excitation of electrons from the valence (or impurity) band into the conduction band. The enhancement of reactivity of the mixed oxides compared to pure uranium is explained by the higher band gap of thorium dioxide (ThO 2 ) and plutonium dioxide (PuO 2 ) compared to uranium dioxide (UO 2 ).