Corrosion of steel canisters containing buried high-level radioactive waste is a relevant issue for the long-term integrity of repositories. The purpose of the present study was to evaluate this issue by examining two differently corroded blocks originating from a full-scale in situ test of the FEBEX bentonite site in Switzerland. The FEBEX experiment was designed initially as a feasibility test of an engineered clay barrier system and was recently dismantled after 18 years of activity. Samples were studied by Fspatially resolved`and Fbulk`experimental methods, including Scanning Electron Microscopy, Elemental Energy Dispersive Spectroscopy (SEM-EDX), μ-Raman spectroscopy, X-ray Fluorescence (XRF), X-ray Diffraction (XRD), and 57 Fe Mössbauer spectrometry, with a focus on Fe-bearing phases. In one of the blocks, corrosion of the steel liner led to diffusion of Fe into the bentonite, resulting in the formation of large (width > 140 mm) red, orange, and blue colored halos. Goethite was identified as the main corrosion product in the red and orange zones while no excess Fe 2+ (compared to the unaffected bentonite) was observed there. Excess Fe 2+ was found to have diffused further into the clay (in the blue zones) but its speciation could not be unambiguously clarified. The results indicate the occurrence of newly formed octahedral Fe 2+ either as Fe 2+ sorbed on the clay or as structural Fe 2+ inside the clay (following electron transfer from sorbed Fe 2+). No other indications of clay transformation or newly formed clay phases were found. The overall pattern indicates that diffusion of Fe was initiated when oxidizing conditions were still prevailing inside the bentonite block, resulting in the accumulation of Fe 3+ close to the interface (up to three times the original Fe content), and continued when reducing conditions were reached, allowing deeper diffusion of Fe 2+ into the clay (inducing an increase of 10-12% of the Fe content).

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Bentonite, a common smectite-rich buffer material, is in direct contact with corroding steel in many high-level radioactive waste repository designs. The interaction of iron with the smectite-rich clay may affect its swelling and sealing properties by processes such as alteration, redox reactions and cementation. The chemical interactions were investigated by analysing the Fe/clay interfaces of eight bentonite blocks which had been exposed to temperatures up to 130 °C for five years in the ABM2 borehole at the Äspö Hard Rock Laboratory managed by the Swedish Nuclear Fuel and Waste Management Co (SKB). Eleven interface samples were characterised by high spatial resolution methods, including scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and μ-Raman spectroscopy as well as by “bulk” methods X-ray diffraction, X-ray fluorescence and 57Fe Mössbauer spectrometry. Corrosion induced an iron front of 5–20 mm into the bentonite, except for the high-Fe bentonite where no Fe increase was detected. This Fe front consisted mainly of ferric (oxyhydr)oxides in addition to the structural Fe in the smectite fraction which had been partially reduced by the interaction process. Fe(II) was also found to extend further into the clay, but its nature could not be identified. The consistent behaviour is explained by the redox evolution, which shifts from oxidising to reducing conditions during the experiment. No indication of smectite alteration was found.

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Rates and mechanisms of important reactions in the cyclingof electrons via the geochemical transformations of iron have been identified using Mössbauer spectrometry. The cycling of iron through various reservoirs (aquifer, soils, sediments, claystone) depends on high surface-area-to-volume ratios of Fe-bearing solids. The ability of Fe-bearing solids surfaces to interact chemically, through surface complexation, and ligand exchange mechanisms, with reductants such as Fe II , and oxidants such as Se, U, Tc, Co, Eu, and O 2 facilitates electron transfer as well as dissolution and precipitation. Various pathways have been assessed on the basis of laboratory experiments for application to natural and engineered systems. Fe II in the structure of layered silicates, oxides (e.g., Fe 3 O 4 ) and hydrous oxides, and sulfides, as well as Fe II surface complexes, such as on clay mineral edges, are very efficient reductants from a thermodynamic as well as from a kinetic point of view.