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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.

Steels comprise the largest class of metal-based materials encountered on nuclear sites. An understanding of how process steels interact with HNO 3 in spent fuel treatment plant environments is required to enable informed decisions to be made about the design and effective application of different steel types within nuclear environments. Stainless steels readily passivate in nitric acid. However, increasing the oxidising power of the media can lead to passive film dissolution, resulting in rapid transpassive corrosion. The corrosion of steels in nitric acid is further complicated by the autocatalytic reduction of HNO 3 to aqueous HNO 2 which attacks the steel surface. This paper describes the effect of this behaviour on process steels in stagnant and/or flowing conditions using electrochemical and microgravimetric based methods. We describe linear sweep voltammetry studies performed on 316L stainless steel rotating disk electrodes in varying concentrations of nitric acid and rotation speeds and provide a qualitative interpretation of the results and what these imply about the mechanism of HNO 3 reduction. These findings will be used in follow on studies to determine the kinetic parameters of the nitric acid reduction reaction at the surface of 316L stainless steel.

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We present the first study of the effect of acetohydroxamic acid (AHA) on the corrosion behaviour of stainless steels. Particularly, studies have been performed using steels and physicochemical conditions equivalent to those proposed for use in advanced nuclear reprocessing platforms. In these, AHA has been shown to have little effect on either steel passivation or reductive dissolution of both SS304L and SS316L. However, under transpassive dissolution conditions, AHA while in part electrochemically oxidised to acetic acid and nitroxyl/hydroxylamine, also complexes with Fe 3+ , inhibiting secondary passivation and driving transpassive dissolution of both steels.

We have prepared a range of Advanced Gas-cooled Reactor (AGR) SIMFUELs at a range of simulated burn-ups and, using Raman spectroscopy, have studied the effect of the SIMFUEL dopants on the UO 2 crystal structure. We have also studied the effect of exposure to hydrogen peroxide solutions on the SIMFUEL surface. The intensity of the fundamental U-O stretch (445 cm-1) decreases as the amount of dopant increases in each SIMFUEL burn-up composition. A simultaneous increase in the lattice damage (500-700 cm-1) peak is observed as the UO 2 cubic fluorite lattice structure becomes more distressed and moves towards a tetragonal structure. Exposure to 100 µmol dm-3 H 2 O 2 further decreases the fundamental U-O stretch and increases the lattice damage peak, suggesting that additional point defects are established as the concentration of interstitial oxygen is increased in the lattice via the H 2 O 2-induced corrosion of the SIMFUEL.

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