Anaerobic Corrosion of Steel in Bentonite
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Nicholas R. Smart , Andrew P. Rance1 and Lars O. Werme2 1 Serco Assurance, Culham Science Centre, Abingdon, Oxfordshire, OX14 3ED, United Kingdom. 2 SKB, Box 5864, SE-10240, Stockholm, Sweden. $%675$&7 In Sweden, spent nuclear fuel will be encapsulated in sealed cylindrical canisters, consisting of a ferrous insert and a copper outer container, for disposal in a geologic repository. Ferrous support structures will also be used in the repository, which will be backfilled with bentonite clay. Once any residual oxygen has been consumed, any ferrous material exposed to anoxic groundwater will undergo anaerobic corrosion, liberating hydrogen, forming a magnetite film, and releasing iron ions into the surrounding matrix. In order to characterise these processes the rate of hydrogen generation of steel in bentonite was measured using a barometric gas cell technique. The initial corrosion rates were found to be higher than measured previously in comparable aqueous solutions, but the long-term corrosion rates were similar. Analysis of the bentonite matrix showed that iron produced by corrosion had penetrated into the bentonite matrix, suggesting that ferrous ion exchange had occurred. ,1752'8&7,21 In Sweden, spent nuclear fuel will be encapsulated in sealed cylindrical canisters for disposal in a geologic repository. The canisters will consist of a thick ferrous inner container and a copper overpack. After emplacement in the repository they will be surrounded by bentonite clay to limit the release of radioactivity, if for any reason there is a leakage from the canister. If failure of the copper overpack occurred, allowing water to enter, the ferrous inner container would corrode anaerobically and liberate hydrogen. Although bentonite will not initially be in contact with the inner vessel, it is possible that bentonite may enter the inter-vessel annulus by passage through a breach of the outer vessel and come into contact with the ferrous insert. Ferrous metal support structures in the repository will also be in contact with bentonite clay buffer. Bentonite is most commonly formed by alteration of volcanic ash and consists predominantly of particles of montmorillonite (a magnesium aluminium silicate, with a range of possible stoichiometries), which have a flat, thin sheet morphology (typically 1 µm in the longest dimension by 0.92 nm thick) and a very large specific surface area (typically 800 m2 gm-1) [1]. Water can easily enter between the unit layers [2] causing the lattice to expand, giving bentonite its swelling properties. Exchangeable cations can permeate the interlayer sites or attach themselves to the surfaces of the crystallites; in bentonite deposits, the exchangeable cations are normally sodium or calcium. Montmorillonite has a high cation exchange capacity (CEC, measured in milli-equivalents of cations per unit mass of clay). If iron ions produced by corrosion in the repository were to come into contact with bentonite there is a possibility that ion exchange would
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