The Influence of Radiolytically Produced Nitric Acid on the Corrosion Resistance of Copper-Coated Used Nuclear Fuel Containers
Abstract
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.