Bentonite erosion by expansion in fractures: effect of exposed surface and clay mass

Compacted bentonite is the primary engineered barrier in a deep geological repository for high-level radioactive waste. The bentonite must exhibit a sufficiently high swelling capacity and low hydraulic conductivity to seal the waste and to hinder radionuclide migration (Sellin & Leupin, 2013). The erosion of the barrier due to the flow of groundwater, which promotes clay swelling and expansion through fractures in the crystalline host rock, would entail a loss of mass in the bentonite, which could compromise the effectiveness of the barrier.

Bentonite erosion is usually experimentally studied at laboratory scale, by analyzing the role of different parameters such as the type of clay, water chemistry or the physical conditions of the fracture in a controlled manner. The extrapolation of laboratory results to a real repository requires compulsory evaluating the role of those aspects more sensitive to scale effects. This study investigated whether the amount of clay emplaced or the clay surface in contact with fractures play a role in erosion process.

For this, an experimental set-up to simulate an artificial fracture was used (Alonso et al., 2019). In this setup, a compacted bentonite sample is placed between two methacrylate plates with a known aperture. All experiments were performed at a dry density of 1.4 g/cm³ with a previously sodium equilibrated clay (NANOCOR^®). Fractures are placed in horizontal position and filled with a low saline solution (10^-3 M NaCl), to monitor the expansion distances of the clay in the fracture by periodic photographs. The clay is allowed to expand during 30 days and the amount of clay eroded is quantified post-mortem. Two sets of experiments were carried out, the first set evaluated the impact of clay area exposed to hydration in the fracture, fixing the amount of clay mass, with samples of ring geometry. For the second set, the clay exposed surface contacting the fracture was kept constant, but the amount of clay installed was varied with cylindrical compacted samples of variable height.

Results showed that clay expansion can only occur in water conductive fractures, and no expansion was observed in the inner zone of the ring geometry tests, which were not water filled. Additionally, no large differences in the expansion distance were observed in the tests with a larger exposed surface area. For the second set of experiments with cylindrical geometry and the same exposed area, it was observed that the expansion and mass loss did not vary significantly with different clay mass. The indication is that in a genuine repository featuring substantially greater volumes of clay, the impact of this factor on its expansion should not be considerable.

REFERENCES

Alonso, U., Missana, T., García-Gutiérrez, M., Morejón, J., Mingarro, M., & Fernández, A. M. (2019). CIEMAT studies within POSKBAR project Bentonite expansion, sedimentation and erosion in artificial fractures (Technical Report TR-19-08). SKB.

Sellin, P., & Leupin, O. X. (2013). The Use of Clay as an Engineered Barrier in Radioactive-Waste Management – A Review. Clays and Clay Minerals, 61(6), 477–498. https://doi.org/10.1346/CCMN.2013.0610601