Electrochemical Measurements during the Anaerobic Corrosion of Steel
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Electrochemical Measurements during the Anaerobic Corrosion of Steel N.R. Smart1, P.A.H. Fennell1, R. Peat1, K. Spahiu2 and L. Werme2 1 AEA Technology plc, Culham Science Centre, Abingdon, Oxfordshire, OX14 3ED, United Kingdom. Email: [email protected] 2 SKB, Box 5864, SE-10240, Stockholm, Sweden. ABSTRACT In Sweden, it is proposed that spent fuel should be encapsulated in sealed cylindrical canisters for disposal in a geologic repository. The canisters would consist of a thick ferrous inner container and a copper overpack. If mechanical failure of the copper overpack occurred, allowing water to enter, the ferrous inner container would corrode anaerobically and liberate hydrogen. The rate of hydrogen generation due to the anaerobic corrosion of steel in anoxic groundwater has been measured using barometric cells. The aim of the work presented in this paper was to measure the redox potential, Eh and pH in the presence of anaerobically corroding steel, in a barometric cell. Two specially designed barometric cells were constructed. They were equipped with a silver chloride or calomel reference electrode, a gold Eh electrode, a glass electrode, and a steel electrode. The electrodes were allowed to stabilize in anoxic artificial groundwater and then a mass of pickled steel wire was introduced into the test cell. As the wires were added, the redox potential rapidly became more negative due to the rapid consumption of the residual oxygen. The corrosion potential of the steel was stable and a slow drift in pH was observed. The results are compared with the results of geochemical modelling. Extension of the work to investigate the electrochemical parameters in the presence of dissolved uranium species is discussed. INTRODUCTION To ensure the safe encapsulation of spent nuclear fuel elements for geological disposal, SKB is considering using the Copper-Iron Canister, which consists of an outer copper canister and an inner cast iron container. After encapsulation, the fuel would be transported to a geological repository, where the containers would be deposited in granite and surrounded by a bentonite clay backfill material. The groundwater will be oxygen-free and reducing. The redox potential below this depth ranges between -200 and -300 mV on the hydrogen scale and the water has a pH ranging from neutral to mildly alkaline [1]. If, or when the copper corrosion shield fails, the iron insert will be in contact with oxygen-free water and hydrogen-producing, anaerobic corrosion will start [2]. In order to predict the solubility and speciation of radionuclides it is necessary to know the oxidizing power or redox potential, Eh, of the aqueous environment inside the HLW canisters. In a repository situation, the low redox potential caused by corrosion of the canister material would be expected to have a significant effect on the solubility of radionuclides; this needs to be taken into account when carrying out safety assessments. The aim of the work described in this paper was to demonstrate the feasibility of experimentally monit
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