Investigating the Effect of Bentonite Clay Compaction Density on the Corrosion of Copper Materials
Abstract
As per international consensus on the best practice for managing used nuclear fuel (UNF), the Nuclear Waste Management Organization (NWMO) plan to isolate and contain UNF within a multiple-barrier system, underground in a deep geological repository (DGR). In the proposed design, used fuel bundles will be sealed in copper-coated carbon steel used fuel containers (UFCs), encased in blocks of highly compacted bentonite clay, and emplaced ~500-800 m (depending on host rock geology) below ground. Any gaps between the rock walls and the compacted bentonite blocks will be filled with a bentonite gap fill material (GFM).
The compacted bentonite material serves as a physical and chemical barrier within the repository, limiting the diffusion of groundwater species and corrosive species to the canister surface due to its high swelling pressures and cation exchange capabilities. In this work, we investigate the role of bentonite compaction density, oxygen availability, microbially influenced corrosion, and the evolution of DGR conditions on the corrosion of copper in contact with bentonite.
The corrosion of copper materials in compacted bentonite clay, when exposed to multiple environmental conditions, showed non-uniform corrosion across the specimen surface. It was found that the presence of oxygen trapped within the bentonite clay has the most significant impact on the initial corrosion rates of embedded copper. The increase in bentonite compaction density decreased the average corrosion rates of the embedded copper in oxic and anoxic conditions. The effect of bentonite compaction density on the corrosion rates was more pronounced in oxic conditions. The corrosion rate correlated to the system's oxygen concentration in saline and microbially active conditions. Lastly, under anoxic, crystalline groundwater with low microbial activity conditions, the effect of bentonite compaction density was less prominent but clear at longer experimental durations.
This research, coupled with microbial analysis, has provided insight into the role of bentonite compaction density on the corrosion processes of copper materials embedded in bentonite clay.