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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

James Jamie Noel

Abstract

The development of a safe permanent disposal plan is essential for the long-term disposal of used fuel bundles. Nuclear Waste Management Organization (NWMO) has been investigating the deep geologic disposal of nuclear waste which offers the optimum passive safety system with a negligible probability of release of radionuclides into the environment.

The proposed used fuel containers (UFC) for the permanent disposal of high-level nuclear waste in Canada is comprised of a carbon steel vessel coated with a 3 mm corrosion-resistant copper layer deposited using a combination of electrodeposition and cold spray deposition. Although copper is often considered to be thermodynamically stable in anoxic conditions, different corrosion processes might occur upon exposure to aggressive anions, such as chloride, sulfate, and sulfide. The presence of oxygen could increase the possibility of passive film formation on Cu which might result in localized corrosion (specifically pitting) and unexpected failure of UFC; so, the pitting corrosion probability of Cu in these conditions should be evaluated carefully.

This research has focused on the development of a new technique to investigate the pitting probability of copper (Cu) in Deep Geological Repository (DGR) conditions using multielectrode arrays and statistical models. Investigation of pitting probability of Cu required the combination of different types of electrochemical experiments including potentiodynamic and potentiostatic polarizations since potentiodynamic polarization test gives us information about the Eb and Erp, however, potentiosatic polarizations simulates the natural condition in which a constant potential is applied to the Cu electrode. As a result, two different applied potentials were chosen based on histograms of Ecorr and Eb which had been collected using potentiodynamic polarization experiments to simulate the natural conditions. Ecorr+ 20 mV and Eb-20 mV vs SCE were selected as the highest and lowest applied potentials, respectively. Also, the morphology and composition of various types of passive films were investigated using SEM and XPS techniques. These information gave us useful information to evaluate the pitting possibility of Cu in the presence of different types of passive films such as Cu2O, CuO, and Cu(OH)2 as well as which film was more resistant to pitting corrosion.

Summary for Lay Audience

The development of a safe permanent disposal plan is essential for the long-term disposal of used fuel bundles. Nuclear Waste Management Organization (NWMO) has been investigating the deep geologic disposal of nuclear waste which offers the optimum passive safety system with a negligible probability that radionuclides will be released into the environment. In this proposed plan, used fuel bundles will place into used fuel containers (UFC) buried around 500 m underground in suitably dense intact rock or sedimentary deposit, while employing a multi-barrier system. The proposed UFC for the permanent disposal of high-level nuclear waste in Canada is comprised of a carbon steel vessel coated with a 3 mm corrosion-resistant copper layer deposited using a combination of electrodeposition and cold spray deposition. Uniform corrosion of Cu under these conditions has been investigated extensively and the corrosion allowance of Cu in these conditions is determined to be less than 1.5 mm over a million years. However, the presence of oxygen could increase the possibility of passive film formation on Cu which might result in localized corrosion (specifically pitting) and unexpected failure of UFC. This research focused on the pitting probability of Cu in unary and binary solutions containing chloride, sulfate, and carbonate. It is important to note that corrosion parameters are deterministic, however, they are sensitive to different factors such as the composition of the solution, temperature, surface condition, etc. As a result, we are dealing with distributed data either because of the solution to which the surface is exposed or the stochastic nature of passive film rupture. We have developed two different types of methodologies to investigate the breakdown of materials.

I. Evaluating the susceptibility of Cu to pitting corrosion based on the difference between corrosion potential (Ecorr) and breakdown potential (Eb).

II. Investigating the pitting probability of Cu based on the difference between Ecorr and Repassivation potential (Erp) which we named it conservative pitting probability. In this method, pitting initiates when Ecorr rose above the Erp and repassivation occurs when the Ecorr drops below the Erp.

A new type of electrochemical experiment has been developed that enables the instrument to perform electrochemical experiments for up to 50 electrodes all in one test. This feature allowed us to have the same conditions for all electrodes as much as possible which improved the accuracy and reliability of statistical models significantly by decreasing the possibility of errors including human errors. Moreover, Python and R programming languages were used to fit the distribution curves of Ecorr, Eb, and Erp with possible distribution models. As a result, the probabilities of pitting, conservative pitting, and repassivation of Cu were determined by calculating the overlap between distribution curves of Ecorr and Eb, Ecorr and Erp (When the distribution curve of Erp was located on the left side of Ecorr), and Ecorr and Erp (When distribution curve of Erp was located on the right side of Ecorr) respectively. Various electrochemical experiments including potentiodynamic polarization, potentiostatic polarization, and statistical analysis have been conducted to investigate the pitting probability of Cu in different conditions. Furthermore, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) have been used to investigate the composition and morphology of corrosion products formed under these conditions. Moreover, FIB-SEM has been used to evaluate the morphology and thickness of the different types of passive films as well as the distribution of corrosion damage on the Cu/film interface.

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This work is licensed under a Creative Commons Attribution 4.0 License.

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