Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Wren, Jungsook Clara

Abstract

The γ-radiation emitted during radioactive decay of the wastes in a nuclear fuel container can affect container corrosion. When a metal/water system is exposed to γ-radiation both the solid metal and the liquid water (or humid air) absorb energy. This energy dissipates mainly as heat in the metal but induces decomposition of water molecules to yield a range of chemically reactive species. The different chemical environments induced in the metal and liquid phases can change the driving forces for surface reactions and thereby influence the rate and pathway of metal corrosion.

This thesis presents the development of radiolysis kinetic models that predict radiolytic oxidant concentrations relevant to used fuel container (UFC) corrosion in the anticipated deep geologic repository (DGR) environments. The different DGR environments were addressed by constructing three different radiolysis kinetic models: (1) water radiolysis model (WRM), (2) humid air radiolysis model (HARM), and (3) groundwater radiolysis model (GWRM).

The HARM predicts that HNO3 will be the dominant oxidizing species formed during humid air radiolysis with its concentration increasing linearly with dose rate. The extent of corrosion damage caused by HNO3 on a copper surface was conservatively estimated from the determined transfer rate of HNO3 in the gas phase to droplets in contact with the container. A simplified rate formula for the overall radiolytic production of HNO3 in humid air was proposed.

The radiolysis kinetics of deaerated and aerated water at temperatures ranging from 25 to 80 oC was studied using the WRM. The model predicts that the key oxidants formed by radiolysis will be H2O2 and O2. In saline groundwater, GWRM predicted the formation of an intermediate, HOCl. The model calculations were verified using experimental data performed with pure water and chloride solutions.

Radiolytic corrosion of Cu in saline solution was also investigated by performing experiments with γ-irradiation and with chemically-added radiolytic oxidants to simulate the effects of radiation. The results demonstrate that the continuous radiolysis production of reactive species at low levels has different effects on corrosion kinetics from the one-time addition of these species at concentrations equivalent to their overall radiolysis yields over long times.

Summary for Lay Audience

Many countries, including Canada, are exploring long-term disposal of used nuclear fuel in a deep geologic repository (DGR). A key engineered barrier in this concept is the used fuel container (UFC). The Canadian UFC uses a copper-coated carbon steel vessel. The UFC will be exposed to a continuous flux of gamma radiation emitted from the radioactive materials trapped in the spent fuel matrix. One of the main factors to consider in the assessment of the integrity and longevity of the UFC is the effect of gamma radiation on container corrosion under anticipated DGR conditions. When exposed to gamma radiation both solid container materials and the environments inside and outside the UFC absorb energy. In a metal, the absorbed energy dissipates mainly as heat, but gamma radiation induces chemical decomposition of water and air molecules to yield a range of reactive species that can alter the driving force for corrosion reactions. This thesis investigates the role of gamma radiation in the material degradation process. This was achieved through the development of computational models that can predict with reasonable accuracy the concentrations of key oxidants in various expected DGR environmental conditions.

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