A review on the effect of iron on bentonite stability

High-Level Radioactive Waste (HLW) repositories may include iron or steel-based containers/overpack and bentonite buffers. Over the last approximately 25 years, research efforts have attempted to elucidate the nature of iron-bentonite interactions, especially the potential for the swelling clay component (smectite), to undergo alteration to iron-rich clay minerals which may lack intracrystalline swelling. This could reduce swelling pressure in the bentonite buffer which is intended to protect waste containers from shear forces and restrict water and solute transport. Most data on iron-bentonite interactions come from experimental and geochemical modelling studies, as natural analogue data are lacking. The data suggests that there is the potential for the development of an iron-rich bentonite alteration zone with primary smectite (montmorillonite) undergoing alteration to iron-rich solids, including clay minerals and typical steel corrosion products such as “green rust” or magnetite (Fe₃O₄). The evidence available is complex with many potential complex couplings. Uncertainties remain despite efforts taken over many years, but plausible scenarios for iron-bentonite interactions have been identified and implications for buffer properties have been suggested. Future efforts to further elucidate iron-bentonite interactions could focus on validating thermodynamic data for iron-rich clays, exploring the transport and engineering properties of altered bentonite, and developing more advanced reactive-transport models that include multi-phase phenomena, and a more realistic approach to bentonite porewater representation.