Copper, mainly used as a material for outer canister, generates various corrosion products under aerobic and anaerobic conditions in the operational and/or post-closure phases of the deep geological repository. These products could affect performance of engineering barrier system (EBS) through interaction with surrounding bentonite that makes up the buffer and backfill materials. Accordingly, in this study, we suggested research items to be conducted to minimize degradation of EBS due to copper corrosion products, based on the phenomenological review results for copper corrosion mechanisms and interaction between resultant product and bentonite in the deep geological disposal environment. During the post-closure phase, condition in the disposal facility changes form aerobic to anaerobic over time, and thereby, causes and products of copper corrosion vary. Under aerobic condition, copper corrosion is mainly induced by oxygen (O2) in the repository, chloride (Cl-) and carbonate (CO3 2-) ions from groundwater flowing into the facility, resulting in corrosion products such as cuprite (Cu2O), tenorite (CuO), atacamite (CuCl2·3Cu(OH)2) and malachite (Cu2CO3(OH)2). And, copper corrosion under anaerobic condition is primarily due to hydrogen sulfide (H2S) and sulfate (SO4 2-) in groundwater flowing into the facility, leading to formation of chalcocite (Cu2S) and covellite (CuS) as corrosion products. Depending on environment of the disposal facility, copper corrosion products are dissolved and ionized to Cu2+ in groundwater, and subsequently adsorbed on the nearby smectite. Then, it causes a cation exchange reaction with exchangeable cations in the interlayer of smectite. As a result of reviewing the previous experiments, it was confirmed that Cu2+-exchanged bentonite has a slightly reduced basal spacing and swelling capacity. From the results as above, there is a possibility that performance of EBS may be degraded due to copper corrosion products. To minimize its effect of degradation in the domestic facility, items to be further studied are as follows: (a) Method for reducing copper corrosion such as selection of appropriate material and structure for the canister, and (b) How to control dissolution of copper canister product into groundwater through predicting type and ionization process. The results of this study could be directly used to developing design concept of EBS for the domestic disposal facility and to establishing roadmap of future R&D programs.

In the deep geological repository, a considerable quantity of cementitious materials is generally used for structural stability of subcomponents such as grout and concrete plug of disposition tunnel. Strong alkaline leachates (pH>13) are produced after cement is dissolved by groundwater inflow from bedrock. When the leachates are transported to bentonite porewater (e.g. buffer and backfill) and thereby water exchange occurs, the physical properties of bentonite such as swelling capacity and hydraulic conductivity are changed, which eventually affects the safety function and long-term stability of engineered barrier system (EBS). Thus, in this paper, we reviewed the performance assessment methodology for cement-bentonite interaction in the operating license application for the Finnish deep geological repository, and suggested what to prepare for the analysis on the domestic disposal facility. In Finland, thermal-hydraulic-chemical analysis for dissolution of montmorillonite by alkaline leachates resulting from cement degradation during the saturation of bentonite was carried out using PRECIP code. From this analysis, it was confirmed that effect on pH was considered to be more significant than that on temperature and bentonite saturation. As a result of this analysis, it was predicted that all primary minerals (including montmorillonite, quartz, and calcite) were dissolved and some secondary minerals (e.g. chalcedony and celadonite) was precipitated by alkaline cement leachates transported to the bentonite. In addition, it was shown that silica was preferentially released while the montmorillonite was dissolved, thus cementation of the bentonite was occurred. Through this phenomenon, the swelling capacity of bentonite is reduced and the hydraulic conductivity of bentonite is increased, which have a significant impact on the performance of the buffer and backfill. Considering this, study on spreading of alkaline leachates, which is a condition for dissolution of montmorillonite, is necessary for the performance assessment of the domestic deep geological repository. However, this requires the site-specific data for the following in the disposal site: (a) distribution in fractured bedrock and pore structure (e.g. porosity, pore size distribution and pore morphology) in the bedrock, (b) hydraulic gradient and salinity concentration of groundwater, and (c) flux and velocity of groundwater. Results of this study is considered to be directly utilized to the conceptual design and performance assessment of the deep geological repository in Korea, provided that additional data on microbiological properties of groundwater are obtained for the site selected.