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In a deep geological disposal system, bentonite buffer material is an important barrier used to protect the disposal canister from the inflow of groundwater and prevent the outflow of radionuclides. This study aimed to characterize the mineralogical and chemical reactions of bentonite caused by copper corrosion of the canister in a radioactive waste repository. We investigated the d-spacings of montmorillonite in Gyeongju bentonite (Ca-type, KJ-I) under groundwater-saturated conditions over 10 years and compared their characteristics with those of Wyoming bentonite (Na-type, MX-80) in the Korea Atomic Energy Research Institute Underground Research Tunnel. Mineralogical investigations using X-ray diffraction and focused ion beam energy-dispersive spectroscopy indicated that no transformation of smectite or neo-formed clay phases occurred. In the Ca-type bentonite (KJ-I), the swelling was observed when it was in contact with rolled plate (RP) and cold-spray-coated (CSC) copper, with d-spacing expansions of 2.9% and 3.8%, respectively. In contrast, the Na-type bentonite (MX-80) showed d-spacing expansions of 17.6% and 19.6% when it was in contact with the RP and CSC Cu, respectively. The Cu concentration and distribution indicated that the corrosion products dissolved and then diffused into the surrounding bentonite, with maximum penetration depths of 2.0 and 0.5 mm over 10 years, respectively.

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