Master of Science
Noël, James J.
Deep geological containment of used nuclear fuel will rely on multiple engineered and natural barriers, two of which are the copper-coated used fuel containers and the compacted bentonite clay buffer boxes in which the containers will rest. This work focused on possible galvanic interactions between the copper coating and the steel substrate of the container, which may occur at a hypothetical through-coating defect. In the presence of various amounts of chloride, bentonite, and oxygen, corrosion of the copper/carbon steel couple was studied using electrochemical tests complemented by surface characterization and 3D X-ray imaging. We investigated the effects of copper-to-steel area ratio, chloride, bentonite, and coating technique on corrosion of the carbon steel. This work provided insight into the conditions which may exacerbate or mitigate galvanic corrosion in the deep geological repository, contributing to the body of research which will support prediction of the long-term performance of the used fuel container.
Summary for Lay Audience
The long-term management of used nuclear fuel is an important aspect of the continued viability of nuclear energy, which is one of Canada’s largest sources of reliable, carbon-free electricity. A deep geological repository has been proposed to provide containment and isolation of the used fuel bundles for up to one million years, at which point the radioactivity of the fuel will have decayed to ambient levels. Multiple barriers will be in place to contain the harmful radionuclides, but the focus of this work is the copper-coated carbon steel used fuel container, which houses the fuel bundles, and the bentonite clay which surrounds it. Corrosion will be an ongoing threat to container integrity once the container is placed in the repository, though if intact, the copper coating is predicted to provide adequate protection. However, in the scenario of a defect penetrating through the copper coating to the steel substrate, galvanically accelerated corrosion of the steel substrate would be possible. This work seeks to investigate several aspects of such a system: the effective copper-to-steel area ratio, groundwater chloride concentration, oxygen availability (as some oxygen will be trapped upon closure), presence of bentonite, and nature of the copper/steel interface, and how these aspects affect the corrosion of carbon steel. Immersion tests, electrochemical tests, surface characterization, and X-ray imaging were used to evaluate the corrosion behaviour of steel galvanically coupled to copper. The corrosion of the steel was found to be less severe when copper-to-steel area ratio was low (which can be achieved in low conductivity environments) and when bentonite was present, due to its role as a barrier to transport oxygen. These studies provide key insights into the interactions between the copper/steel galvanic couple and the repository environment, laying the groundwork for future studies in compacted bentonite and long-term oxidants like sulphide. Due to the impracticality of performing laboratory experiments on the time scales relevant to the deep geological repository, this research contributes to the building of predictive models for how the used fuel containers will perform during long-term containment, and ultimately, the licensing basis for the repository itself.
Braithwaite, Lindsay J., "Galvanic Corrosion of Carbon Steel Coupled to Copper in Used Nuclear Fuel Containers" (2021). Electronic Thesis and Dissertation Repository. 7676.
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