Impact of seal configuration and pore fluid type on ion transport and cation exchange in bentonite in semi-technical scale Sandwich sealing experiments

Disposal concepts for high-level radioactive waste involve underground repositories and involving an engineer barrier system (EBS) to ensure safe containment of radionuclides. Shaft and drift seals form part of the EBS and typical sealing concepts involve bulk bentonite. However, bulk bentonite is susceptible to the formation of preferential fluid flow pathways, which impact bentonite hydration and barrier performance. The Sandwich sealing system (CMM, Nüesch et al., 2002) is an alternative solution, and consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES) which allow for homogeneous saturation of the bentonite.

Semi-technical scale experiments (d = 80 cm, h = 180 cm) were required to probe the impact of different segment configurations, pore fluids, and German Ca-Mg-bentonites on the performance of the Sandwich sealing system. Columns were saturated with various artificial fluids mimicking either pore fluid of Opalinus Clay or rock salt. Experiments were terminated when fluid was detected in the final DS, typically a few hundred days duration. Experimental columns were dismantled with up to 20 sampling points on up to 20 sampling levels through the column to provide a 3D snapshot of the system via chemical and physical characterisation of samples.

Water content decreased through bentonite sampling levels from 30-40% adjacent to the fluid inflow to <15% in the final level. Saturation was relatively consistent in DS segments, despite the presence of artificial hydraulic defects. Stark differences were observed between neighbouring ES and DS. Regardless of pore fluid and segment configuration, highest ion concentrations coincided with higher water content and were higher in DS sampling levels directly adjacent to ES. Position of a sampling level in the column was indicative of Ca-Mg- to Na-bentonite transformations as longer exposure time to the pore fluid increased the level of soluble Na⁺ for interlayer Ca²⁺ and Mg²⁺. Concentrations of soluble ions with DS and ES showed little variation. Presence of artificial hydraulic defects only impacted soluble ion concentration in the immediate vicinity of the defect, otherwise segments had relatively uniform ion concentrations. Despite the long duration of the experiments, Ca²⁺ and Mg²⁺ persist in the interlayer of DS smectite after > 400 days indicating ongoing cation exchange contribution of interlayer cations to pore fluid composition. Bentonite soluble phases also contribute to pore fluid composition and indicate the systems snapshotted here are far from equilibrium.

Bentonite swelling in DS near fluid inflow caused low final fluid flow rates, indicating the Sandwich sealing system behaved as expected and reduced hydraulic conductivity. Results show a robust system suitable for installation using a variety of different Ca-Mg-bentonites, and in different locations with exposure to pore fluids with low or high salt concentrations, without affecting long-term performance. The Sandwich sealing system is also highly effective at minimising the formation and impact of preferential flow pathways. The authors thank the German Federal Ministry for Economic Affairs and Energy for funding the Sandwich-HP (FKZ 02E11799 A+B+C).