Doctor of Philosophy
Shoesmith, David W.
In Canada, the proposed disposal method for high level nuclear waste involves sealing the waste in steel containers with a 3 mm outer copper coating and burying it in a deep geologic repository. However, the thickness of the container is significantly reduced making a reassessment of the influence of γ-radiation, emitted by the waste form, on container corrosion a potential licensing requirement. Under humid aerated conditions, the formation of nitric acid is expected, potentially resulting in the formation of droplets or wetted layers on the container surface. Currently available literature on the corrosion of copper in nitric acid does not provide sufficient evidence to determine the extent of container corrosion.
The effect of high copper surface area to solution volumes (SA/V) was investigated by monitoring the pH and dissolved copper content in a small cell. The corrosion rate was shown to be dependent on the oxygen concentration with significant damage in its presence but little damage in its absence. Nitric acid had only a minor effect on the corrosion rate. Scanning electron microscopy (SEM) showed that the deposition of corrosion products prevented significant corrosion penetration.
The kinetics of corrosion were investigated in cells with a low SA/V ratio using corrosion potential and polarization resistance measurements to follow the kinetics, and SEM and Raman spectroscopy to investigate the corrosion products. The corrosion rate was found to be independent of proton and nitrate concentrations and first order with respect to oxygen concentration. X-ray photoelectron spectroscopy showed that nitrate adsorption occurred on the copper surface when oxygen was absent. Nitrite, the product of nitrate reduction was found to react rapidly with copper leading to a high corrosion rate. Chloride, the most common anion in analyzed groundwaters, was found to have a negligible effect on corrosion despite its ability to increase cuprous ion solubility by complexation.
It was demonstrated that the key role of oxygen was to produce cuprous ions which then catalyzed the reduction of nitrate to the much more aggressive nitrite with the cupric ion formed then reacting with copper in a catalytic cycle to reproduce the cuprous ion.
Summary for Lay Audience
Many countries throughout the world have adopted a plan for the permanent disposal of high level nuclear waste that includes sealing it in metallic containers and burying it in a suitable deep geologic repository (DGR). For thick-walled containers, gamma (γ) radiation fields on the outside of the container will have a negligible influence on container corrosion. However, to overcome fabrication issues and to reduce costs, steel containers with a 3 mm outer copper, Cu, coating are being designed in Canada making a reassessment of the influence of γ-radiation on container corrosion a potential licensing requirement.
A combination of radiolysis calculations and electrochemical/corrosion experiments are underway in the key environments anticipated in the early stages of disposal when radiation fields are significant and aerated vapour conditions will be evolving to anoxic saturated conditions. HNO3 is potentially one of the most influential species produced during the aerated vapour phase and could concentrate in condensed water on the container exterior surface potentially resulting in corroded locations. The goal of this project was to investigate the mechanism and determine the rate of this corrosion process with the longer-term goal of developing a model to assess the total damage expected over the period when the gamma radiation field is significant.
The ability of NO3- to induce corrosion by itself and in the presence of other species expected to be present under repository conditions (including O2, NO2-, and Cl-) was investigated. Despite its ability to react with Cu, NO3- strongly adsorbs to the Cu surface under most conditions, forming a partially passivating layer, and induces very little corrosion. Additional species including O2 and NO2- can compete with NO3- for surface adsorption sites and induce significant, general corrosion. However, due to limited solution volume in the anticipated condensed water layer, this corrosion would lead to the deposition of corrosion products which effectively prevent further corrosion. Thus, corrosion induced by HNO3 will be minimal unless an additional oxidant is present. If corrosion does occur, it will be self regulated and its effect will be negligible in terms of the overall corrosion allowance of the used nuclear fuel container.
Turnbull, Joseph P., "The Influence of Radiolytically Produced Nitric Acid on the Corrosion Resistance of Copper-Coated Used Nuclear Fuel Containers" (2020). Electronic Thesis and Dissertation Repository. 6903.