Corrosion of Copper Canisters Through Microbially Mediated Sulphate Reduction
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Corrosion of Copper Canisters Through Microbially Mediated Sulphate Reduction Magnus Sidborn and Ivars Neretnieks Department of Chemical Engineering and Technology, Royal Institute of Technology SE10044 Stockholm, Sweden. ABSTRACT A biogeochemical model was developed with an aim to illustrate the transport and reaction processes that may be involved in copper canister corrosion by sulphide in a deep repository for nuclear waste. Sulphide concentrations in the groundwaters in Sweden are relatively low and are generally considered to be of little importance for corrosion of the copper canisters. Sulphate, however, is present in relatively large amounts (50700 mg/L) and may be reduced to sulphide by reaction with organic matter. Mediated by microbes, the reduction rate may be increased considerably compared to abiotic reduction. Microbially mediated sulphate reduction occurs in many natural environments, provided that reactive organic matter is available. Groundwater analyses indicate relatively high concentrations of dissolved methane (up to 16 mg/L) which thermodynamically is a suitable reducing agent. This processes could occur in fractures in the bedrock provided that substrates are continuously supplied and that there is a sink for the reaction products. In the repository, the copper canisters may provide a sink for sulphide that yields a favourable environment for the microbe population. The model domain includes a canister with surrounding backfill material intersected by a hydraulically conductive fracture. Transport of substrates and reaction products in the fracture and backfill as well as microbially mediated reaction at the fracture opening is included in the model. Results indicate that microbially produced sulphide could potentially contribute to canister corrosion. INTRODUCTION The long term stability of engineered barriers is of major importance for the safety assessment of nuclear waste repositories. This paper focuses on corrosion of the copper canisters containing the high level nuclear waste. In the deep granitic rocks, reducing conditions prevail during normal conditions. Redox processes are in a steady state condition. After construction of the repository, however, the steady state conditions may be altered, as a consequence of the copper present. This may cause a favourable environment for microbe populations to grow, as will be discussed in the following. Sulphide concentrations in the groundwaters in Sweden are relatively low and are generally considered to be of little importance for corrosion of the copper canisters. Sulphate, however, is present in relatively large amounts (50700 mg/L) and may be reduced to sulphide by reaction
with organic matter. Abiotic reduction of sulphate is a relatively slow process and is generally regarded not to be of major importance for canister corrosion. Mediated by microbes, however, the reaction rate may be increased considerably. Microbially mediated sulphate reduction occurs in many natural environments, provided that orga
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