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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Noël, James J.

2nd Supervisor

Shoesmith, David W.

Co-Supervisor

Abstract

The current plan for disposal of used nuclear fuel in Canada involves sealing the waste in steel containers coated with 3 mm of copper and burying them in a deep geologic repository (DGR). The purpose of the copper coating is to provide corrosion resistance. To achieve long term containment, it is necessary that the copper layer corrodes slowly and predictably via active dissolution rather than passivating due to film formation. Film formation could allow pitting corrosion to occur in early phase repository conditions when groundwater ions such as Cl– , SO4 2– and HCO3 – play a dominant role in influencing copper`s corrosion behaviour. The tendency of copper to undergo active dissolution was tested by immersing a piece of copper in a variety of solutions with different combinations of Cl– , SO4 2– and HCO3 – ions at various concentrations and temperatures while observing the electrochemical behaviour. It was found that in most scenarios active dissolution was the preferred corrosion process. While active dissolution is favoured under DGR conditions, the distribution of corrosion damage in the form of surface roughening needs to be elucidated if an acceptable corrosion allowance is to be specified. Corroded copper surfaces were examined using a combination of optical microscopy and confocal laser scanning microscopy (CLSM). Multielectrode arrays (MEA’s) were designed to simulate copper surfaces. Copper coupons were tested using galvanostatic charging or immersion in Cl– -based solutions to determine the surface roughening pattern. Using this information, an oxidizing solution was designed which could buffer the potential of the system without externally controlling the potential or current. This solution also replicated the roughening damage observed in both the galvanostatic charging and immersion experiments. This created a link between accelerated and non-accelerated testing. This solution was then used to roughen the MEA electrodes. It was found that roughening of copper surfaces in Cl– -based solutions proceeds via preferential dissolution of different grains. The depth of metal dissolution was increased or limited depending on the grain orientation of the reactive surfaces present in the copper. Therefore, corrosion of used fuel containers in the DGR will be limited by the grain structure of their copper coating.

Summary for Lay Audience

Nuclear power is a prominent source of energy used in many countries across the globe. However, the permanent and safe disposal of the waste generated by nuclear power plants is a requirement for nuclear energy to be considered a green source of power. Many countries have a plan for permanent disposal of the nuclear waste that involves sealing it in metal containers which are then buried 500 m underground in a deep geologic repository (DGR). The Canadian steel used fuel container (UFC) is designed with a thin copper coating to avoid fabrication issues and reduce the cost per container. However, this copper layer needs to be properly assessed to ensure it can provide the necessary corrosion resistance within a DGR, making it important to determine how damage will accumulate as corrosion occurs in a DGR environment. To effectively study this problem, a wide variety of solutions containing groundwater species anticipated in a DGR, such as chloride, sulphate and carbonate, were tested to determine the progression of corrosion on a UFC surface. This resulted in a large database which enabled the influences of the concentration of the groundwater species, temperature and pH to be evaluated. This database indicates copper will actively corrode under DGR conditions. However, the fine details of how the corrosion damage will progress are not well understood. A unique setup, involving microelectrode arrays fabricated on premade electronic circuit boards was utilized. The copper electrodes in these arrays were designed to be micro-sized. The microelectrode arrays were used to simulate a copper surface and specialized imaging techniques were used to create accurate 3D representations of each electrode. These 3D representations were analyzed to determine the changes in the metal surface as the copper dissolved over time. Using these 3D representations specific parameters such as the height or roughness of the copper surfaces were calculated using specialized software. This provided insight into the previously undetermined progression of corrosion as the copper dissolved over time. These insights were then used to help determine if a thin copper layer could provide the necessary corrosion resistance in conditions similar to those anticipated on the surface of a UFC buried in a DGR.

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