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A Review on How to Assess the Accessory Mineral Alteration in Bentonite in the Domestic Disposal Facility for High-level Radioactive Waste
- 언어ENG
- URLhttps://db.koreascholar.com/Article/Detail/430841
p.214
For the deep geological repository, engineering barrier system (EBS) is installed to restrict a release of radionuclide, groundwater infiltration, and unintentional human intrusion. Bentonite, mainly used as buffer and backfill materials, is composed of smectite and accessory minerals (e.g. salts, silica). During the post-closure phase, accessory minerals of bentonite may be redistributed through dissolution and precipitation due to thermal-hydraulic gradient formed by decay heat of spent nuclear fuel and groundwater inflow. It should be considered important since this cause canister corrosion and bentonite cementation, which consequently affect a performance of EBS. Accordingly, in this study, we first reviewed the analyses for the phenomenon carried out as part of construction permit and/or operating license applications in Sweden and Finland, and then summarized the prerequisite necessary to apply to the domestic disposal facility in the future. In previous studies in Sweden (SKB) and Finland (POSIVA), the accessory mineral alteration for the post-closure period was evaluated using TOUGHREACT, a kind of thermal-hydro-geochemical code. As a result of both analyses, it was found that anhydrite and calcite were precipitated at the canister surface, but the amount of calcite precipitate was insignificant. In addition, it was observed that precipitate of silica was negligible in POSIVA and there was a change in bentonite porosity due to precipitation of salts in SKB. Under the deep disposal conditions, the alteration of accessory minerals may have a meaningful influence on performance of the canister and buffer. However, for the backfill and closure, this is expected to be insignificant in that the thermal-hydraulic gradient inducing the alteration is low. As a result, for the performance assessment of domestic disposal facility, it is confirmed that a study on the alteration of accessory minerals in buffer bentonite is first required. However, in the study, the following data should reflect the domestic-specific characteristics: (a) detailed geometry of canister and buffer, (b) thermal and physical properties of canister, bentonite and host-rock in the disposal site, (c) geochemical parameters of bentonite, (d) initial composition of minerals and porewater in bentonite, (e) groundwater composition, and (f) decay heat of spent nuclear fuel in canister. It is presumed that insights from case studies for the accessory mineral alteration could be directly applied to the design and performance assessment of EBS, provided that input data specific to the domestic disposal facility is prepared for the assessment required.
