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.

Microscopic and spectroscopic analyses were conducted on steel/bentonite interface samples removed from four in situ experiments that were carried out in three underground research laboratories at different temperatures and under different hydraulic and geochemical conditions. The results provide valuable information about the corrosion processes occurring in high‐level radioactive waste repositories. Systematic patterns can be deduced from the results, irrespective of carbon steel grade, type of bentonite and its degree of compaction, geochemical environment or experimental setup. Thus, a clear dependence of the corrosion rates on temperature and exposure period, as well as on the availability of H₂O and O₂ provided by the surrounding bentonite buffer, is observed. Furthermore, Fe(II) ions released by corrosion interact with the structural Fe in the clay. Recent developments highlight the usefulness of reactive transport modelling in understanding the coupled corrosion and Fe–clay interaction processes.

Reactive‐transport models (RTMs) have been an integral part of the safety case and design optimization for the nuclear waste repository in Finland. Such highly resolved, internally complex RTMs tend to be computationally demanding, thus their design is often a compromise between representativeness and computational efficiency. A modeling strategy is presented, demonstrating how a 2D simplified model can be constructed based upon and calibrated against a geometrically realistic 3D RTM. Key geometrical parameters of the 2D model are consistent with the 3D model, and the 2D model is verified by a series of benchmarks for various designs and conditions of the repository. Overall, the 2D model is proven suitable for fast scoping calculations, as (1) it reduces the mesh elements by 16 times and shortens the run time by about 40 times; (2) it has much more flexible remeshing possibilities; and (3) it provides highly consistent results with the 3D model.