The Copper Sulfide Model (CSM) has been developed to predict the evolution of the corrosion behaviour of copper canisters under repository conditions. In particular, the model predicts the time dependence of the corrosion rate and of the corrosion potential, the latter being useful for assessing the possibility of localised corrosion and stress corrosion cracking. The CSM is a 1-D model that represents the various buffer, backfill, and host rock media in the near field. The effects of the evolution of the repository environment are taken into account, from the initial warm, aerobic, unsaturated conditions to the long-term cool, anaerobic, saturated phase possibly characterised by the presence of sulfide.
A number of recent developments and improvements have been made to the model. In addition to a number of minor modifications based on the results of the latest experimental studies on the effects of sulfide on copper corrosion, the treatment of microbial sulfate reduction has also been significantly improved. Monod kinetics are now used to describe the rates of both organotrophic and chemotrophic sulfate reduction, using organic carbon and hydrogen as the energy source, respectively. The extent of microbial activity can be limited by a number of factors, including: the availability of organic C or H2, the availability of sulfate from either the ground water or the dissolution of gypsum, low water activity, the diffusive transport of nutrients or electron acceptors, and elevated temperatures. Microbial metabolism results in the growth and death of cells and the effect of these processes on the time-dependent microbial population are also taken into account. Sulfide produced by microbes in the host rock or in the interface zones around the deposition holes and tunnels must diffuse through the compacted buffer and backfill to reach the canister. Precipitation of iron sulfide acts as a sink for sulfide ions and this process and the availability of ferrous ions from the dissolution of siderite or other Fe(II)-containing minerals are also included in the model.
The results of CSM simulations of the evolution of the corrosion behaviour of copper canisters in a KBS-3 type repository will be presented. The conditions under which either the availability of organic carbon or sulfate limits the extent of sulfide formation will be described. Precipitation of iron sulfide is found to be an important sink and limits the flux of sulfide to the canister surface.
Copper, lifetime prediction, modelling, KBS-3, MIC