The MiniCan field experiment at the Äspö Hard Rock Laboratory (Sweden) exposed miniature copper canisters with cast iron inserts, surrounded by bentonite clay of varying density, to natural granitic groundwater containing sulphate-reducing bacteria (SRB). Retrieval of MiniCan Experiment 3 (low-density bentonite, water-filled headspace) revealed SRB-driven 'graphitisation' corrosion of cast iron — selective dissolution of the iron leaving a porous graphite residue — at a lower-bound rate of at least 500 µm per year, amorphous iron sulphide deposits on all surfaces, and mixed copper oxide–sulphide films whose exact phase and stoichiometry could not be determined. Later retrieval of MiniCan 4 (high-density bentonite) and MiniCan 5 (no bentonite) showed that highly compacted bentonite strongly suppresses microbiologically influenced corrosion (MIC), partially answering one of the original questions. However, three linked questions remain open: (1) the mechanism by which SRB activity accelerates cast-iron corrosion — direct electron uptake from the metal, hydrogen- or organic-matter-mediated sulphide attack, and/or galvanic coupling between the cast iron insert and the copper shell at canister defects — and the relative contributions of these pathways; (2) whether the precipitated iron sulphide and mixed copper oxide–sulphide surface films passivate the metals or sustain accelerated localised attack over repository-relevant timescales; and (3) the identity and stoichiometry of the copper sulphide phases formed on canister surfaces under preserved anoxic conditions.

Background

In the Swedish KBS-3 concept for geological disposal of spent nuclear fuel, the waste is enclosed in a canister consisting of a cast iron insert (providing mechanical strength) inside a thick copper shell (providing corrosion protection), which is surrounded by a buffer of compacted bentonite clay — a swelling montmorillonite-rich clay — and emplaced at about 400–500 m depth in crystalline rock. Microbiologically influenced corrosion (MIC) is the acceleration of metal corrosion by microbial activity; in anaerobic groundwater the key agents are sulphate-reducing bacteria (SRB), which gain energy by reducing sulphate to sulphide using hydrogen or organic compounds as electron donors. Sulphide attacks iron and copper, and some SRB can additionally extract electrons directly from Fe(0). The MiniCan experiment, installed by SKB (Swedish Nuclear Fuel and Waste Management Co.) in 2006 at a depth of about 450 m in the Äspö Hard Rock Laboratory, tested five miniature copper–cast iron canister packages differing mainly in the presence and density of surrounding bentonite. Analysis of MiniCan Experiment 3 (retrieved 2011; low-density bentonite and a water-filled headspace inside a support cage) found that the cast iron weight-loss coupon had been completely consumed by graphitisation — a corrosion form of grey cast iron in which iron is dissolved preferentially, leaving a porous residue of graphite and corrosion products — at a rate of at least 500 µm yr−1^{-1}, that dissolved iron had precipitated as predominantly amorphous iron sulphide, and that copper corroded uniformly at only about 0.15 µm yr−1^{-1} with a mixed oxide–sulphide film. Energy-dispersive X-ray (EDX) microanalysis in the scanning electron microscope is semi-quantitative, so the stoichiometry of the copper–sulphur crystals could not be pinned down among the many possible Cu–S phases (covellite CuS, digenite Cu9_9S5_5, djurleite Cu31_{31}S16_{16}, chalcocite Cu2_2S, and others). The report also noted that the severe SRB densities in the water-filled headspace might not represent a canister surrounded by highly compacted bentonite, where microbial activity is expected to be suppressed by the clay's swelling pressure and low water availability. The mechanistic question is informed by the general MIC framework developed in parallel: chemical MIC (CMIC), where biogenic sulphide attacks the metal, and electrical MIC (EMIC), where SRB take up electrons directly from Fe(0) via conductive iron-sulphide crusts, bypassing dissolved hydrogen; both mechanisms had been demonstrated in laboratory cultures, and lithotrophic SRB capable of direct iron metabolism were detected in the MiniCan groundwater. Because the cast iron insert and copper shell are in electrical contact through machined defects in some MiniCan designs, galvanic coupling between the two metals is a further candidate accelerant.

Problem Statement

For copper–cast iron spent-fuel canisters of the KBS-3 type exposed to anaerobic, SRB-containing granitic groundwater with bentonite clay, as realised in the Äspö MiniCan experiment, establish a mechanistic and predictive account of microbially influenced corrosion of the canister materials. Specifically: (i) identify the mechanism, and the relative contributions of the candidate pathways, by which sulphate-reducing bacteria accelerate corrosion of the cast iron insert — direct electron uptake from the metal by SRB, sulphide attack fuelled by hydrogen or organic-matter metabolism, and galvanic coupling between the cast iron and the copper shell at defects in the copper canister — reproducing the observed severe graphitisation (lower-bound rate at least 500 µm/yr in the unbuffered/low-density configurations, strongly reduced in high-density bentonite); (ii) determine the long-term evolution of the corrosion process, i.e. whether the precipitated iron-sulphide deposits and mixed copper oxide–sulphide surface films passivate the underlying metals or sustain accelerated localised attack over repository-relevant timescales, under both the severe unbuffered conditions of MiniCan 3 (water-filled headspace) and MiniCan 5 (no bentonite) and the defect-adjacent conditions persisting in high-density bentonite; and (iii) determine the identity and stoichiometry of the two classes of copper-bearing corrosion product formed on canister and electrode surfaces, under sampling conditions that preserve the anoxic corrosion products: the crystalline copper sulphide — identified from among the known Cu–S phases and reported as a named phase with its stoichiometry — and the mixed copper oxide–sulphide film, for which the copper oxide and copper sulphide components and their stoichiometry are to be reported; anoxic sampling and preservation is a required condition for this determination, and reporting a preservation method alone does not resolve deliverable (iii). These three deliverables jointly support one objective: a defensible mechanistic basis for the microbiological corrosion allowance in canister-material specifications and repository safety assessment. The task is resolved only by a joint account that addresses all three deliverables (i)–(iii); no single one of them, however well supported, on its own resolves the task.

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