Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Noël, James J.

Abstract

Used nuclear fuel poses significant risks to human health and the environment, necessitating its safe and permanent disposal. The universally proposed plan for this is to bury the fuel-containing containers in a multi-barrier system known as a deep geological repository (DGR) at least 500 metres underground. These containers, crucial for withstanding long-term mechanical loads and a corrosive environment, vary in design across countries, with some featuring copper (Cu) shells on nodular cast iron insert (Sweden, Finland) or Cu-coated carbon steel vessels (Canada). Upon emplacement, the containers undergo evolving conditions underground, transitioning from warm, humid, and oxidizing to cool, dry, and anoxic environments over time.

During the initial oxidizing phase, oxygen entrapped upon sealing the DGR and water-radiolysis will lead to the formation of an oxide/hydroxide film on the Cu container surface. Subsequently, as anoxic conditions prevail, bisulfide (SH−) ions produced by the action of sulfate-reducing bacteria (SRB) remote from the container will become the primary oxidant.

While considerable efforts have been devoted to investigating exclusively either the oxic or anoxic periods, the current study has addressed the gap in understanding how early oxide growth impacts later stages, particularly in the presence of SH− ions. In a series of experiments, various methods were employed to create copper oxide/hydroxide layers with known compositions and structures to investigate their role in bisulfide-induced corrosion of the Cu substrate under de‑aerated conditions. The morphology of the oxide film and the concentration of bisulfide species influence potential interaction mechanisms, including chemical conversion, galvanic coupling, and direct corrosion of Cu by bisulfide species.

Our findings have shown that regardless of the composition or structure of the oxide film, it underwent partial conversion to copper sulfide via chemical and/or galvanic processes. Moreover, an unreacted remnant of the oxide layer detected on the surface was non-protective and permitted direct Cu corrosion by bisulfide species.

Electrochemically- and radiolytically-grown oxides exhibited quick conversion to copper sulfide, whereas hydrothermally-grown oxides, thicker in nature, underwent slower conversion, with regions remaining unreacted. These results highlight the importance of electrochemical pathways in facilitating rapid oxide-to-sulfide conversion, contrasting with slower chemical pathways.

Summary for Lay Audience

The safe disposal of used nuclear fuel is crucial to protect both people and the environment from chemical and radiological hazard. One widely accepted method is to bury the used fuel container (UFC) underground in a DGR, at least 500 m below the surface. In Canada, UFCs are made of a strong carbon steel vessels coated with a 3-mm-thick layer of Cu. Copper has been chosen as the main corrosion barrier for the container in Finland, Sweden, Canada, and some other countries (e.g., Switzerland, South Korea, and Japan) are considering Cu coating due to its known corrosion resistance under anoxic conditions. Over time, the conditions around the container will change from warm and humid to cool and dry. Initially, oxygen trapped in the repository causes an oxide film to form on the container surface. Eventually, as the environment becomes anoxic, bisulfide ions produced by sulfate-reducing bacteria will become the main threat for Cu corrosion.

The current research aims to understand how the early growth of oxide layers on copper containers affects their later corrosion when exposed to bisulfide ions. Through experiments, different types of oxide layers have been created via a variety of methods to study how the oxide interacts with bisulfide ions under specific conditions. This interaction can lead to chemical conversion of copper oxide to copper sulfide, galvanic processes between oxide reduction and copper oxidation by sulfide, or direct corrosion of copper by bisulfide. Understanding the conversion process is crucial for ensuring the long-term integrity of the containers storing nuclear fuel.

The mechanism and extent of conversion of different types of copper (hydr)oxides with known compositions and thicknesses have been analyzed using various electrochemical and corrosion measurements such as corrosion potential, cyclic voltammetry (CV), and cathodic stripping voltammetry. Additionally, a range of surface analytical techniques including Raman spectroscopy, scanning electron microscopy (SEM) and focused ion beam (FIB) milling were used to figure out the chemical composition, surface film morphology and thickness measurement at the film/Cu interface.

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