Prediction of Corrosion Damage to High Level Nuclear Waste Canisters

Abstract The corrosion of alloy canisters in a plutonic rock repository has been explored using five deterministic models. A Mixed Potential Model (MPM) was used to calculate the corrosion potential of the canister as a function of time after resaturation, and it is predicted that the corrosion potential will shift in the negative direction as the ambient oxygen is consumed coupled with a much smaller positive shift due to the decay in temperature. Maximum general corrosion rates have been estimated using the Maximum Rate Model (MRM) and the Radiolytic Maximum Rate Model (RMRM), both of which equate the corrosion rate to the maximum possible flux of oxygen (MRM) and oxygen plus oxidizing radiolysis products (RMRM) to the canister surface. These calculations suggest that the radiolysis of water is not of major significance in establishing the general corrosion rate. Furthermore, general corrosion is predicted not to be a threat to the integrity of a thin wall (6.35mm) titanium container within the 500 year design life. Corrosion rates have also been estimated using the WAPPA code and the container is predicted to withstand crevice corrosion for up to 1200 years and to be unbreached by general corrosion for 2000-2500 years after resaturation. Finally, we outline a deterministic model for predicting damage functions due to localized corrosion, because localized attack is considered to be the most likely failure mechanism due to high penetration rates.