2012:11 Issues in the corrosion of copper in a Swedish high level nuclear waste repository

The objective with this research project was to increase knowledge in the area of copper corrosion in the planned repository environment and obtain information on how copper corrosion evolves during the assessment period of 100 000 years. 

The equilibrium chemical composition of groundwater close to the canister as a function of temperature has been calculated by use of a thermodynamics code called GEMS. Based on the results, the following sulphide species (S2-, HS-, H2S, HS22-, and S22-) are predicted to be present in sufficient concentrations to cause copper corrosion in the repository environment. Among the sulphide species HS- is predicted to be in highest concentration. It must be emphasized that GEMS calculation cannot consider the influence of sulphate reducing microbes which can be an important source of sulphide concentration at repository depth. The most important variables that need to be included in defining how corrosion of copper will evolve during the assessment period are found to be temperature, pH, [HS-] and [H2]. 

Within the research program a physico-electrochemical model for copper corrosion during the assessment period of 100 000 years has been developed. The model considers, transport through the saturated buffer, temperature variation and copper corrosion kinetics with HS-, O2, H2O2 present naturally or produced by radiolysis of water by gamma radiation from the spent fuel. The output from this modeling work can be used to predict how redox potential, corrosion potential and corrosion damage of copper develops during the assessment period. This output can for example be used to predict if copper could undergo general or localized corrosion during the repository evolution. In this report only preliminary modeling trails have been performed, mainly with the intention of testing the model. A lot of input data for the model is lacking but these data will be measured in the continuation of this work.