Effects of Gamma-Radiation on the Evolution of Copper Corrosion Dynamics in Deep Geological Repository Solution Environments
Abstract
The Canadian plan for permanent disposal of used nuclear fuel involves a used fuel container (UFC), a carbon steel vessel with a copper coating to provide the corrosion protection. The corrosion conditions for the copper layer of the UFC are expected to involve small, stagnant water volumes, pH values between 6 and 9, and a continuous flux of γ-radiation emitted from the radionuclides trapped in the fuel matrix. While γ-radiation does not affect the copper metal directly, humid air and water will radiolytically decompose into redox-active and acidic species which will be produced at constant concentrations and can alter the corrosion behaviour of the copper coating. A fundamental understanding of the corrosion behaviour in the presence of γ-radiation is needed to predict the long-term corrosion progression of the copper layer of the UFC with confidence and assure its long-term integrity.
This work investigates the combined effects of γ-radiation and solution conditions (initial pH, cover gas composition, and solution depth) on the dynamics of copper corrosion. The results show that the elementary steps involved in the overall metal oxidation process change as corrosion progresses (i.e., multiple steady-states during corrosion process), from which four distinct corrosion stages can be identified, each with characteristic corrosion product time-dependent behaviour. This work showed that the solution conditions – including the presence of radiation – do not change the stepwise behaviour of the corrosion stage; however, the solution conditions affect the rates of the elementary reactions in each stage, thereby affecting the overall metal oxidation rate. It is changes in the rates, rather than in the nature of the elementary reactions, that influence how fast the overall corrosion dynamics evolve through the different dynamic stages.
This study has shown that the relationship between the copper dissolution and pH during copper corrosion can be correlated to the Cu(OH)2 solubility equilibrium in all studied solution conditions. This key thermodynamic relationship can help simplify the long-term modeling of the corroding system. Additionally, this work has clearly shown that the system experiences feedback loops that render the linear extrapolation of corrosion rate inaccurate. The elementary steps and the thermodynamic limitations for copper concentration values identified in this study provides a simplified approach toward developing a high-fidelity corrosion model.