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h i g h l i g h t sThe first report of the presence of both UO 2 and polymeric UO 2 2+ in the same electrodeposited U oxide sample.The action of H 2 O 2 on electrodeposited U oxides is described using corrosion based concepts.Electrodeposited U oxide freely dissolves at hydrogen peroxide concentrations <100 lmol dm À3 .At [H 2 O 2 ] > 0.1 mmol dm À3 dissolution is inhibited by formation of a studtite passivation layer. At [H 2 O 2 ] P 1 mol dm À3 studtite formation competes with uranyl-peroxide complex formation. a b s t r a c tFor the first time the effect of hydrogen peroxide on the dissolution of electrodeposited uranium oxide films on 316L stainless steel planchets (acting as simulant uranium-contaminated metal surfaces) has been studied. Analysis of the H 2 O 2 -mediated film dissolution processes via open circuit potentiometry, alpha counting and SEM/EDX imaging has shown that in near-neutral solutions of pH 6.1 and at [H 2 O 2 ] 6 100 lmol dm À3 the electrodeposited uranium oxide layer is freely dissolving, the associated rate of film dissolution being significantly increased over leaching of similar films in pH 6.1 peroxide-free water. At H 2 O 2 concentrations between 1 mmol dm À3 and 0.1 mol dm À3 , formation of an insoluble studtite product layer occurs at the surface of the uranium oxide film. In analogy to corrosion processes on common metal substrates such as steel, the studtite layer effectively passivates the underlying uranium oxide layer against subsequent dissolution. Finally, at [H 2 O 2 ] > 0.1 mol dm À3 the uranium oxide film, again in analogy to common corrosion processes, behaves as if in a transpassive state and begins to dissolve. This transition from passive to transpassive behaviour in the effect of peroxide concentration on UO 2 films has not hitherto been observed or explored, either in terms of corrosion processes or otherwise. Through consideration of thermodynamic solubility product and complex formation constant data, we attribute the transition to the formation of soluble uranyl-peroxide complexes under mildly alkaline, high [H 2 O 2 ] conditions -a conclusion that has implications for the design of both acid minimal, metal ion oxidant-free decontamination strategies with low secondary waste arisings, and single step processes for spent nuclear fuel dissolution such as the Carbonate-based Oxidative Leaching (COL) process.

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