Investigating the Effect of Bentonite Compaction Density and Environmental Conditions on the Corrosion of Coupled Copper and Steel Materials

Abstract

In Canada, the proposed long-term management plan for used nuclear fuel is to isolate it within a multiple-barrier system underground in a deep geological repository (DGR). In the Canadian design, used fuel bundles will be sealed in copper-coated carbon steel used fuel containers (UFCs), encased within blocks of compacted bentonite clay, emplaced approximately 500–800 m below ground, depending on the host geology, and surrounded by a granulated bentonite gapfill material. While unlikely, a through-copper-coating defect may be present or eventually develop, exposing the underlying carbon steel to an aqueous environment while galvanically coupled to the copper coating. To explore what might happen in such a case, a simulated defect was manufactured from canister-relevant materials (i.e., a hole was drilled through a copper coating into the underlying steel substrate). Compacted bentonite clay's role on the carbon steel's galvanic corrosion when it was exposed to an oxic, and anoxic environments was investigated. The bentonite clay materials evaluated were saturated gap fill bentonite at dry compaction densities of 1250, 1400, and 1600 kg/m³. Using microcomputed tomography (micro-CT) imaging, it was observed that, for a yearlong experiment, the corrosion rate of the carbon steel material could be suppressed, even under oxic conditions, by increasing the bentonite clay compaction density. The corrosion damage decreased from 0.0197 mm³/year when the specimen was embedded in gapfill bentonite compacted to 1250 kg/m³ to 0.0181 mm³/year in gapfill compacted to 1600 kg/m³. Cross-sections of the defect samples revealed bentonite clay stained by the iron corrosion products filled the defect volume. Raman spectroscopy identified the corrosion products of carbon steel as iron oxides and oxyhydroxides. Additional defect samples were imaged using micro-CT after five- and six-years' of exposure to an anoxic 3 M NaCl solution and found that the corrosion rates of the carbon steel remained low. This further suggested that oxygen acts as the primary oxidant in the galvanically coupled system.