Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Chemistry

Supervisor

Noel, Jamie J

Abstract

In Canada, the current design of the containers for used nuclear fuel includes a cold spray copper coating as part of a multi-barrier system for long-term disposal in a deep geological repository. The oxygen trapped during emplacement will potentially cause initial copper coating oxidation, while bisulfide ions produced by sulfate-reducing bacteria will act as a long-term oxidant by forming copper sulfides. Bisulfide ions will first meet oxide-covered used fuel containers, and conversion of oxide to sulfide will take place. Unlike wrought copper, cold spray copper contains microstructural features such as entrapped cuprous oxide inclusions at particle-particle interfaces. This study examined the impact of these entrapped oxide inclusions on the surface oxide formation and corrosion behaviour of cold spray copper in an anoxic bisulfide-containing environment. Electrochemical tests and surface characterization showed that oxide inclusions significantly influence the initial oxidation process and accelerate copper corrosion and oxide-to-sulfide conversion. These findings provided insight into the role of copper’s microstructure in the long-term performance of used fuel containers.

Summary for Lay Audience

Copper exhibits notable versatility and has also been selected as a suitable material for cold spray coating, a solid-state metal deposition technique. The cold spray copper coating will be one of the critical components that act as a corrosion barrier for used fuel containers, for long-term disposal of used nuclear fuel in a deep geological repository. Due to the fabrication process, entrapped metal oxide inclusions in the primary form of cuprous oxide are present at the cold spray copper coating’s particle-particle interfaces.

The environment in the deep geological repository will transition from warm and oxic to cool and anoxic. The oxygen trapped during emplacement will act as an oxidant and will be partially consumed to oxidize the copper coating, forming oxide layers on the used fuel container surfaces. As the conditions become anoxic, the diffusing bisulfide ions, produced by bacteria located remote from the used fuel containers, will first meet the oxide layers formed on the container’s surface during the oxic period. The conversion of copper oxides into copper sulfide will take place.

Cold spray copper exhibits different microstructural features compared to that of wrought copper due to its coating procedures. The oxide inclusions in cold spray copper particle-particle interfaces may affect the material properties and corrosion behaviour. Their presence may make the formation of copper oxide on cold spray copper and the electrochemical response of oxide-to-sulfide conversion in aqueous bisulfide solution different from those on wrought copper, which is critical for the lifetime assessment of Canadian used fuel containers.

Using cold spray copper samples with various amounts of oxide inclusions, with wrought copper specimens as controls, this project will investigate the roles of the entrapped oxide inclusions in the oxidation process of cold spray copper in sodium hydroxide solutions, how these oxide inclusions influence the initiation and development of cold spray copper corrosion in bisulfide solutions, and how the surface oxides formed during the oxic period will influence the long-term corrosion of cold spray copper under anoxic bisulfide conditions, using multiple electrochemical and surface analysis techniques.

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