Pertinent to the behaviour of carbon steel based nuclear waste packages in saline geological environments, sorption and reduction of U(VI) on real container corrosion products has been studied in an Ar/8%H 2 flushed glove box. A carbon steel was corroded in MgCl 2 -brine at elevated temperatures. U(VI) was added and the redox states both of uranium and iron and their distribution among solid and liquid phases were investigated. The corrosion products initially consisted to more than 97% of hydrous Fe(II) oxides containing CI and Mg 2+ , but always some magnetite was present. In the course of the sorption step, the amount of magnetite increased. Reaction products buffer pH values of the system in a very narrow neutral range. Less than 1% of iron or uranium were found in colloidal state and already after one day, more than 98% of initially dissolved uranium was found associated with the immobile phases. A reciprocal relationship of solution concentrations of uranium with the nominal mass of magnetite was found. Behaviour of uranium species was rationalized in terms of Eh/pH diagrams. Reduction of hexavalent to tetravalent uranium was observed but to a much less pronounced extent than expected from thermodynamical considerations.

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In previous corrosion studies, carbon steels, especially the fine-grained steel TStE355, were identified as promising materials for heat-generating nuclear waste containers acting as a barrier in a rock-salt repository. In the present study detailed investigations have been performed on fine-grained steel to determine the influence of important parameters on its corrosion behaviour in disposal-relevant salt brines. These parameters are: brine composition (Mg Cl2-rich and NaCI-rich brines), temperature (90°C, 170°C), and salt impurities, such as H2S concentrations of 25 mg/I-200 mg/I salt brine.Under the conditions of the tests used here, carbon steel was subjected to general corrosion. Pitting and crevice corrosion or stress-corrosion cracking were not observed. The increase in temperature from 90°C to 170°C strongly enhanced the corrosion rate of the steel. In the MgCl2-rich brines, considerably higher rates (37-70 μm/a at 90°C, 200-300 °m/a at 170°C) were observed than in the NaCI-rich brine (5 μm/a at 90°C, 46 μm/a at 170°C). H2S concentrations in the MgCl2-rich Qbrine of up to 200 mg/l did not influence significantly the corrosion rate of the steel. The corrosion rates determined imply corrosion allowances that are technically acceptable for thick-walled containers. In view of these results, fine-grained steel continues to be considered as a promising material for long-lived HLW containers.

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