Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Dr. James J. Noël

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.

Summary for Lay Audience

Canada's plan for safely managing nuclear waste relies on the long-term storage of spent nuclear fuel in an engineered multi-barrier system contained within a deep geological repository (DGR). The used fuel is stored in copper-coated steel containers, emplaced in bentonite clay, and stored deep underground.

Within the clay, the metabolic activity of organisms native to the clay may become active and produce corrosive species. These corrosion species could diffuse through the bentonite clay and corrosion the copper-coated container; this is known as microbially influenced corrosion. It has been previously observed that higher bentonite compaction densities can suppress the activity of microorganisms due to low water activity, high swelling pressures and small pore spaces within the bentonite clay material. This work aims to mitigate microbially influenced corrosion by controlling the compaction density of the bentonite clay surrounding the copper material.

Copper materials were emplaced in bentonite clay and exposed to various conditions and durations. The specimens were extracted and analyzed using various surface analysis techniques to evaluate the morphology, composition, and structure of the corrosion products on the copper. We investigated the copper corrosion embedded in various bentonite compaction densities when exposed to various oxygen concentrations and salinities over various durations.

It was determined that bentonite compaction density influenced the corrosion rate of embedded copper materials, with higher compaction densities suppressing corrosion rates. Oxygen was found to play a significant role in the corrosion of copper, with higher oxygen concentrations having higher corrosion rates at all bentonite compaction densities. The effect of salinity increased corrosion rates compared to pure water experiments, likely due to the increased concentration of corrosive species present. Through working with microbiologists, we associate the decreased corrosion rate with the decreased water activity within the clay, the rapid depletion of oxygen, and the suppression of microbe activity. Overall, we observe low corrosion rates that are suitable with respect to the DGR design.

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