Abstract
Shaft seals are geotechnical barriers in nuclear waste deposits and underground mines. The Sandwich sealing system consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES). MiniSandwich experiments were performed with blended Ca-bentonite (90 mm diameter and 125 mm height) to study hydration, swelling, solute transport and cation exchange during hydration with A3 Pearson water, which resembles pore water of Opalinus Clay Formation at sandy facies. Two experiments were run in parallel with DS installed either in one-layer hydrate state (1W) or in air-dry two-layer hydrate (2W) state. Breakthrough at 0.3 MPa injection pressure occurred after 20 days and the fluid inlet was closed after 543 days, where 4289 mL and 2984 mL, respectively, passed both cells. Final hydraulic permeability was 2.0–2.7 × 10−17 m2. Cells were kept for another 142 days before dismantling. Swelling of DS resulted in slight compaction of ES. No changes in the mineralogy of the DS and ES material despite precipitated halite and sulfates occurred. Overall cation exchange capacity of the DS does not change, maintaining an overall value of 72 ± 2 cmol(+)/kg. Exchangeable Na+ strongly increased while exchangeable Ca2+ decreased. Exchangeable Mg2+ and K+ remained nearly constant. Sodium concentration in the outflow indicated two different exchange processes while the concentration of calcium and magnesium decreased potentially. Concentration of sulfate increased in the outflow, until it reached a constant value and chloride concentration decreased to a minimum before it slightly increased to a constant value. The available data set will be used to adapt numerical models for a mechanism-based description of the observed physical and geochemical processes.
Highlights
The Sandwich sealing system, which consists of sealing segments (DS) of bentonite and equipotential segments (ES) of higher hydraulic conductivity, is a component in the German concept of shaft seals for nuclear waste deposits
An in-situ large-scale demonstration experiment is running at the Mont Terri rock laboratory (Switzerland) that addresses the interaction between the Sandwich sealing system and the Opalinus clay
Prediction and evaluation of the in-situ behavior of the Sandwich sealing system require numerical models for a mechanism-based description of the observed physical and geochemical processes. These models can be validated with MiniSandwich experiments as performed in the current study
Summary
Shaft and drift-sealing systems for a nuclear waste repository and in deep underground mines limit the fluid inflow from the adjacent rock in the early stage after closure of the repository or a mine. Partial active areas are protected from sudden fluid inflow, and in repositories and mines the release of possibly contaminated fluids at later stage are prevented, or at least delayed [1]. Current German concepts of shaft seals of nuclear waste repositories contain the hydraulic Sandwich sealing system as a component of the lower seal in the host rock [2]. The Sandwich sealing system consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES), which are characterized by a higher hydraulic conductivity. Water bypassing the seal via the excavation damaged zone (EDZ) or penetrating the seal inhomogeneously is contained and a more homogeneous hydration and swelling of the DS is obtained
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
