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Radiolysis of water is a phenomenon which alters the prevailing conditions in the nearfield of a final geological repository for high-level nuclear waste (HLW), because the nominally anoxic conditions in the repository may change due to the production of oxidants close to the solidliquid interface of the fuel, caused by the alpha radiolysis of water.The influence of water chemistry and oxidant concentration in the liquid phase was tested by experiments which simulate chemical radiolysis effects. Leaching experiments with solutions of de-ionized water (DI) and natural groundwater (GW) containing the water radiolysis product H 2 O 2 in various concentrations were performed contacting pellets of depleted UO 2 as a solid phase. After the experiment, U concentration was measured and the solid surfaces was examined with scanning electron microscopy and electron-dispersive X-ray analysis (SEM-EDX). Uranium concentrations showed an unexpected behaviour; at low H 2 O 2 concentrations in solution, U concentrations were highest. U concentrations in the groundwater leachates were always lower than in pure water. The SEM-EDX examination showed the presence of different alteration phases on the sample. The sample surfaces treated in pure water showed optically a yellow discolouration and were covered with a thick layer of oxidation products. In the case of the treatment in groundwater, the samples remained black, and no dense layer was formed. Instead, crystals of octahedral shape were found on the surface, while the matrix grain structure remained intact. All alteration phases were found to contain only U (and possibly O, H, and C also). A reaction mechanism is proposed for the oxidation reactions taking place under the various conditions. Comparisons with natural analogues and previous work are made, and geochemical solubility limits of possible newly formed substances are examined. The results suggest that different polymorphs of uranium(IV) oxyhydroxides (para-, meta-schoepite, hemihydrate) and of uranium(VI) peroxide (Studtite, meta-studtite) have formed, and that in the case of the samples treated with groundwater, a scavenging mechanism, possibly controlled by an ion present in groundwater, leads to the deactivation of H 2 O 2 in the solution.*

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Iron redox cycling is supposed to be one of the major mechanisms that control the geochemical boundary conditions in the near field of a geologic repository for UO2 spent nuclear fuel. This work investigates the impact of reactions between hydrogen peroxide (H2O2) and iron (Fe2+/Fe3+) on UO2 dissolution. The reaction partners were contacted with UO2 in oxygen-free batch reactor tests. The interaction in absence of UO2 gives a stoichiometric redox reaction of Fe2+ and H2O2 when the reactants are present in equal concentration. Predomination of H202 results in its delayed catalytic decomposition. With UO2 present, its dissolution is controlled by either a slow mechanism (as typical for anoxic environments) or uranium peroxide precipitation, depending strongly on the reactant ratio. Uranium peroxide (UO4 x nH2O, m-studtite), detected on UO2 surfaces after exposure to H2O2, was not found on the surfaces exposed to solutions with stoichometric Fe(II)/ H2O2 ratios. This suggests that H2O2 was deactivated in redox reactions before a formation of UO4 took place. ESR measurements employing the spin trapping technique revealed only the DMPO-OH adduct within the first minutes after the reaction start (high initial concentrations of the OH radical); however, in the case of Fe(II) and H2O2 reacting at 10(-4) mol/L with UO2, dissolved oxygen and Fe2+ concentrations indicate the participation of further Fe intermediates and, therefore, Fenton redox activities.

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