Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Wren, Jungsook C.

Abstract

The current Canadian used nuclear fuel container (UFC) design uses a pressure‑grade carbon steel (CS) vessel with its outer surface coated with a thin layer of copper. One concern regarding the structural integrity of the UFC design is the potential internal corrosion of the CS vessel. Moisture trapped inside a UFC could condense within the gap between the hemispherical head and the cylindrical body of the vessel. The internal UFC environment will be exposed to a continuous flux of ionizing radiation arising from the decay of radionuclides trapped in the used UO2 fuel matrix.

This thesis research project investigates the effects of physical and chemical solution parameters on CS corrosion, with the aim of developing a corrosion dynamics model that can be used to assess the integrity of the current Canadian UFC design with confidence. The parameters studied in this thesis project were the ratio of solution volume to surface area, pH, dissolved O2, and the presence or absence of γ-radiation. Corrosion dynamics were followed using electrochemical techniques, both conventional and non‑standard techniques developed as part of this project. The electrochemical tests were augmented with post-test surface and solution analyses to study oxides formed on corroded surfaces and to determine the dissolved metal content in the solution phase.

The results of this study clearly demonstrated that CS corrosion involves many oxidation steps that lead to the formation and growth of different oxides as well as the dissolution of metal ions. The transfer of Fe atoms between metal, oxide and solution phases provides routes for developing strong systemic feedback that can induce autocatalytic reaction cycles, resulting in oscillatory behaviours that are observable under certain solution conditions. The dynamics of CS corrosion may not approach and reach only one steady state, but continue to evolve and reach different steady states, depending on solution parameters. A mechanism that can explain the CS corrosion dynamics over long time periods under a range of solution conditions has been proposed. The mathematical formulation of a model for the long-term corrosion of CS based on this mechanism has just begun. This study has shown that the corrosion dynamics in the early stages of corrosion can be easily modeled by applying classical electrochemical reaction rate equations coupled with mass transport flux equations. However, for CS in aerated solutions or other oxidizing environments, these classical equations must be formulated for the metal oxidation process rather than the reduction of solution species (oxidant) because the former process is rate determining.

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