Impact of flow velocity and direction on bentonite erosion and sedimentation in vertical fractures

Compacted bentonite serves as an engineered barrier in deep geological repositories designed for confining high-level radioactive waste. Its role relies on maintaining a high swelling capacity to effectively seal the host rock fractures and limit radionuclide migration (Sellin & Leupin, 2013). However, groundwater flow can favor bentonite swelling and expansion through fractures in the crystalline host rock, leading to mass loss and potentially undermining the barrier's effectiveness. To ensure the safety of the repository, it is necessary to predict the long-term erosion of the bentonite barrier.

Laboratory-scale experiments simulating an artificial fracture were developed to study bentonite erosion and sedimentation in vertical fractures, focusing on parameters like clay type, water chemistry, flow and fracture aperture. To extend these findings, this research examined the impact of the groundwater flow velocity and direction in the erosion and sedimentation of compacted bentonite simulating the clay barrier in a fracture of a granitic formation.

The experimental setup consists on compacted bentonite (SWy-3, Wyoming) pre-equilibrated with sodium and compacted to a dry density of 1.4 g/cm³ (Alonso et al., 2019) emplaced in an artificial fracture of desired aperture (0.2 mm and 0.4 mm). A low-saline solution (10⁻³ M NaCl) is injected with peristaltic pumps, simulating the flow of groundwater at desired experimental velocities (3.5·10^-7 m/s; 1.4·10^-6 m/s and 2.1·10^-6 m/s) and direction (upward, downward and lateral). Tests  in the absence of flow were used as reference.

Over a 30-day period, the clay expanded into the fractures, and its progression was tracked through periodic photographs. At the end of the experiment, the amount of extruded and sedimented clay in the bottom of the fractured was collected and weighted, alongside the mobilized colloid generation by measuring their concentration and particle size using Photon Correlation Spectroscopy.

As soon as the bentonite was hydrated, expanded in the fracture with radial geometry. The expansion of bentonite ceased after 10 days, reaching similar maximum expansion distances for the three flow velocities and flow directions analyzed. Continuous flow promotes particle mobilization, as evidenced by a reduction in the radius of the expanded ring, which is more pronounced at higher flow velocities. However, in tests conducted at lower flow velocities, the behavior was comparable to that observed in the absence of flow. The comparison of test carried out at different flow directions suggested that flow can only mobilize the fraction of the initially expanded halo  accessible to the flow, being lower the removal in the lateral direction compared to that upward or downward. These results suggest that, in clay barrier sedimentation processes, gravity plays a secondary role compared to factors like chemistry, flow velocity, duration, or fracture aperture.

REFERENCES

Alonso, U., Missana, T., Gutiérrez, M. G., 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