Water Plasma-Induced Redox Reactions on Thin Uranium (IV, V and VI) Oxide Films—A Surface Science Model Study of Nuclear Fuel Surface Corrosion.

Oxidation properties of uranium have a great influence on the stability of nuclear waste. U(VI) has a 1000 times higher solubility in water than U(IV), and so the dissolution of UO₂ based spent fuel and release of matrix contained radionuclides strongly increases with the pre-oxidation of the surface. Incorporation of oxygen into surface UO₂ has therefore been a subject of research for many years.<br/>A particularly important route for UO₂ oxidation is the surface reaction with oxidants formed by radiolysis of water (H₂O₂, OH●, O2, etc). Such reaction would happen, when water intrudes into a nuclear waste repository and comes into direct contact with spent nuclear fuel surface. The water would be subject to the intense radiation field and radiolytic oxidants form. Surface oxidation proceeds by oxygen incorporation transforming U⁴⁺ into U⁵⁺ and eventually U⁶⁺.<br/>We simulate the surface - oxidant reaction by exposing thin films of uranium oxides to a water plasma, produced by an ECR plasma source (electron cyclotron resonance) under UHV conditions. While these conditions are very different from real world aqueous environment, lacking complexation and dissolution reactions of the surface atoms, this has the advantage that the reaction products remain on the surface and can be analysed in-situ by spectroscopies. We were in particular interested in the evolution of the surface oxidation state upon contact with the plasma. We studied three different oxides, UO₂, U₂O₅, and UO₃, presenting uranium at the pure oxidation states +4, +5 and +6.<br/>The films were prepared in situ by reactive sputter deposition from a U metal target and exposed to water plasma. The plasma constituents were analysed using a residual gas analyser mass spectrometer. It was found that both oxidizing (O₂, O) and reducing (H₂, H) species where formed. Surface analysis was conducted using X-Ray and ultraviolet photoelectron spectroscopy before and after exposure, by measuring the U 4f, O 1s core levels and the valence band region. The evolution of the peaks was monitored as a function of temperature and time of exposure. The sample was transferred from the plasma to the analysis chamber under UHV conditions.<br/>When UO₂ is exposed to water plasma at 400 °C the following evolution of the uranium oxidation state with increasing water plasma exposure time is observed: U(IV) is rapidly (within a couple of minutes) consumed and this consumption is accompanied by rapid formation of U(V) and U(VI). The fraction of U(V) reaches a maximum after less than 10 minutes after which the fraction is rapidly decreased. The U(VI) fraction continues to increase and reaches a maximum after ca 15 minutes. The maximum U(VI) fraction is just above 80 %. After reaching this maximum, the U(VI) fraction slowly decreases and at the same time the U(V) fraction increases again. After 90 minutes exposure the fractions of U(VI) and U(V) are equal.<br/>This switch from surface oxidation to reduction under an identical plasma properties (this was checked) is highly surprising. It can be accounted for by considering the oxidizing and reducing properties of the plasma constituent. Two different models are discussed. First, a simple kinetic model reproduces the experimental data fairly well by supposing formation of two different kinds of oxides containing U(V), an initial one capable of further oxidizing into U(VI), and a final inert one, which does not re-oxidize again. A second model assumes the existence of surface adsorption sites for oxidants and reductants, U⁺⁴ and U⁶⁺. In the early phase oxidant adsorption sites prevail (UO₂) and fast oxidation takes place. In the final phase, the oxidant adsorption sites disappeared while reduction sites (U⁶⁺) are present. Under these conditions partial reduction could take place.