The multinational Materials Corrosion Test (MaCoTe) in-situ experiment at the Grimsel Test Site, Switzerland, measured low anaerobic corrosion rates for candidate waste-canister materials — carbon steel (~2 µm/year) and copper (0.13–0.32 µm/year) — embedded in compacted bentonite and exposed to natural granitic porewater for 394 days, but the reported rates carry caveats that bear on any longer-term extrapolation: possible oxygen ingress during handling (damaged packaging; up to ~20 days of aerobic conditions before emplacement), analytical detection limits for very thin copper corrosion films, the short exposure duration relative to longer in-situ tests elsewhere, drying-induced microfracturing that may be confused with in-situ change, and transport of iron-bearing corrosion products along an interconnected microfracture network. The open problem is to determine the time-dependent anaerobic corrosion behavior of canister materials in compacted bentonite over repository-relevant anoxic durations: the rate-law evolution (persistently low, decaying, or regime-shifting), the coupled corrosion-product and bentonite alteration, the possible late activation of native sulfate-reducing microbes under altered bentonite densities, and the transport of iron- and copper-derived species through the buffer. Solving it would replace conservative assumptions with an evidence-based extrapolation basis for canister corrosion allowances in the safety cases of deep geological repositories.

Background

Deep geological repositories are the internationally adopted solution for spent nuclear fuel and high-level radioactive waste (HLW): the waste is sealed in metallic canisters that are embedded in an engineered barrier system (EBS) — most commonly blocks or pellets of compacted bentonite, a swelling smectite clay — hundreds of metres inside a stable host rock such as granite. The canister is a principal engineered barrier, and its corrosion behavior over assessment timescales (conventionally 10410^4–10610^6 years in repository safety cases) is a core input to safety assessments. Two candidate canister materials dominate granitic-repository concepts: carbon steel, which corrodes anaerobically by reaction with water producing magnetite and hydrogen, and copper, which is thermodynamically stable in pure anoxic water but corrodes in the presence of sulphide. Bentonite protects the canister mechanically and chemically, and its high swelling pressure and low water activity are also expected to suppress microbial activity; however, native groundwater and bentonite carry microorganisms, among which sulfate-reducing bacteria (SRB, bacteria that reduce sulfate to sulphide while oxidizing hydrogen or organic electron donors) are the main potential drivers of microbially influenced corrosion (MIC) under anoxic conditions.

The Materials Corrosion Test (MaCoTe) is a long-term multinational in-situ experiment operated since 2014 in a 9-m-deep borehole (13.001) at the Grimsel Test Site, a granitic underground research laboratory in Switzerland. Its modules are stainless steel vessels containing metal coupons (carbon steel, several types of copper, and stainless steel) surrounded by Wyoming MX-80 bentonite — the reference buffer clay for several national programs — compacted to two target dry densities (mass of dry clay per bulk volume: 1250 and 1500 kg m−3^{-3}, i.e. 1.25 and 1.50 g cm−3^{-3}), and exposed to natural granitic porewater with its native microbial populations under nominally anoxic, ambient-temperature conditions. Reddy and co-workers reported the analysis of the first two retrieved modules after 394 days of exposure: weight-loss measurements gave an average carbon-steel corrosion rate of about 2 µm year−1^{-1}, much lower average rates of 0.13–0.32 µm year−1^{-1} for the copper variants, and no detectable corrosion of stainless steel, with no significant effect of bentonite dry density. X-ray photoelectron spectroscopy (XPS, a surface-chemical analysis technique) and complementary mineralogical and microbiological characterization were also performed; SRB were not enriched in the inner bentonite over this period.

The paper itself flags caveats bearing on longer-term extrapolation of these 394-day results: (1) possible oxygen ingress — some module packaging was damaged during transport, so modules may have experienced up to roughly 20 days of aerobic conditions before emplacement, and the authors note that the reported corrosion rates "may not be representative of totally anoxic conditions"; (2) methodological detection limits — weight-loss uncertainty for the low-mass copper coupons and a weak XPS copper signal (about 1 at.%, complicated by adhering bentonite) limit the ability to quantify very thin corrosion films; (3) temporal scope — 394 days is negligible against repository timescales, and the measured copper rate was higher than the 0.02 µm year−1^{-1} found after 9 years of emplacement in the MiniCan experiment at the Äspö rock laboratory, indicating possible time-dependence of the rate; (4) sample-preparation artefacts — drying-induced microfracturing may be confused with in-situ microstructural change, although fractures lined by secondary precipitates appear to have formed during the experiment; and (5) microbial evolution — SRB were not enriched in the inner bentonite over this period with no measurable MIC contribution, and whether they remain suppressed over longer emplacement times or at altered buffer density is left to the experiment's long-term periodic-retrieval design. The stated objective of the experiment is to provide measurements of the in-situ corrosion rate of stainless steel, carbon steel and different forms of copper and copper coatings, while studying the effect of the bentonite buffer density on microbial activity and microbiologically influenced corrosion, with one density (1250 kg m−3^{-3}) explicitly simulating altered bentonite or bentonite that was not emplaced at the target density. Related in-situ evidence at the time came from the companion IC-A experiment in Opalinus Clay at Mont Terri (carbon steel and copper coatings in compacted bentonite exposed to clay porewater for up to 3 years, showing low anaerobic rates and magnetite-dominated steel corrosion products), and from laboratory programmes on carbon steel and copper in bentonite.

Problem Statement

Determine the long-term anaerobic corrosion behavior of candidate spent-fuel canister materials — carbon steel and copper — in compacted bentonite saturated with natural granitic porewater containing native microbial populations, under strictly anoxic, repository-representative conditions, as needed to extrapolate the low rates measured in the 394-day MaCoTe modules to repository timescales. Concretely: (i) establish the time-dependence of the anaerobic corrosion rate for each material in compacted bentonite over multi-year to decadal exposure — whether the initial low rates persist, decay (e.g. as a power law in time), or give way to different corrosion-rate regimes or secondary-phase evolution — together with the nature of the corrosion products and the coupled mineralogical alteration of the bentonite at the metal-clay interface; (ii) establish whether native microbes, in particular sulfate-reducing bacteria, remain suppressed in the interior of the compacted buffer at longer times and under altered bentonite dry densities, or can become active and influence corrosion; and (iii) establish how corrosion-derived iron and copper species migrate through the compacted buffer, including migration along the interconnected microfracture network (fractures lined by iron-rich particles were observed up to ~15 mm from the steel coupon), and how to distinguish genuine in-situ microstructural evolution at the metal-bentonite interface from drying-induced microfracturing introduced during sample preparation. These three deliverables jointly serve one objective — a defensible, evidence-based extrapolation basis for canister corrosion allowances in repository safety cases. The resolving deliverable is an evidence-based extrapolation for carbon steel and copper, grounded in the cited multi-year in-situ and laboratory data, covering all three components; establishing any one component on its own does not resolve the task.

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