Application of an On-line Corrosion Probe and a Reference Electrode for Copper Corrosion Studies in Repository Condition
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$SSOLFDWLRQRIDQ2QOLQH&RUURVLRQ3UREHDQGD5HIHUHQFH(OHFWURGHIRU&RSSHU &RUURVLRQ6WXGLHVLQ5HSRVLWRU\&RQGLWLRQV Iva Betova, Jussi Heinonen1, Petri Kinnunen, Christina Lilja2, Esko Ruokola1 and Timo Saario VTT Technical Research Centre of Finland, VTT Industrial Systems, Espoo, FIN-02044 Finland 1 Radiation and Nuclear Safety Authority, Finland 2 Swedish Nuclear Power Inspectorate, Sweden $%675$&7 Two sensors, an on-line corrosion sensor based on the wire-resistance probe design and a reference electrode which are applicable in repository conditions are introduced. Both sensors have been tested in simulated repository conditions in bentonite saturated with highly saline groundwater at 80 oC temperature and 14 MPa pressure up to 150 h exposure. In addition, it has been demonstrated that after an initial transient period of ca. 5-8 h, Cu corrosion in highly saline groundwater filled with bentonite becomes undetectably low probably because of the anoxic conditions reached in the free groundwater. At the same time, the measured corrosion potential of Cu is still in the active corrosion region. Furthermore, redox potentials in the wetted bentonite remain significantly higher than in the free groundwater. This may mean that the decomposition of oxygen contained in the pores of the bentonite is a process occurring in a time scale larger than that of the present experiments. ,1752'8&7,21 The Scandinavian concepts suggested until now to dispose of high-level radioactive wastes adopt a near-field multibarrier engineering system, consisting of the form of the waste itself, a containment of the waste form (i.e. a metal canister) and a bentonite buffer which will surround and isolate the canister from the host bedrock. The canisters are designed to resist corrosion, pressure, thermal and radiation effects. Experimental studies aiming at estimating the corrosion rate of the canister materials in relevant conditions should be able to take into account the exceptionally long time period of exposure required for the canisters and also to quantify the extent of interaction between the canister and the bentonite buffer system. In Finland at the selected site of Olkiluoto and in Sweden at the candidate sites the groundwater has been found to contain appreciable amounts of chlorides. Based on thermodynamics, highly saline groundwater has been predicted to subject the canister material, pure copper, to general corrosion under quasi-anoxic conditions [1,2]. When bentonite is present it is suggested to act as a diffusion barrier and to slow down the corrosion rate of copper markedly. The present work addresses the effect of bentonite in contact with groundwater on the corrosion of copper in a closely simulated final disposal vault environment. The longer-term goal of this project is to perform corrosion monitoring tests of Cu in simulated final nuclear waste repository in the presence of wetted bentonite for extended time periods. In order to effectively verify the extent of simulation of the redox conditions and to obtain a
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