Description

Shaft sealing systems are designed to block potential pathways of radionuclides between a nuclear waste repository and the biosphere. Their role is to limit the fluid inflow from the adjacent rock or the surface in the early stage after closure of the repository and to delay the release of possibly contaminated fluids from the repository at later stage.

The Sandwich experiment in Mont Terri includes the Sandwich sealing system, developed by KIT, which consists of sealing segments (DS) of bentonite and equipotential segments (ES) that are characterized by a higher hydraulic conductivity (Figure 1). Within the ES, fluid is evenly distributed over the cross section of the seal. Water bypassing the seal via the excavation damaged zone, or penetrating the seal inhomogeneously, is contained and a more homogeneous hydration and swelling of the DS is obtained (Emmerich et al., 2019; Wieczorek et al., 2024).

The simulation of the hydraulic-mechanical interaction of shaft sealing systems is fundamental for a qualified long-term safety analysis. Therefore, a comprehensive process understanding is needed, including the bentonite saturation process, as well as the interaction between host rock and sealing system. Constitutive models are needed which can simulate these processes and predict their future development.

The Sandwich task addresses both the behaviour of the sealing system itself as well as the interaction between the sealing system and the host rock. The aim is to understand the hydraulic-mechanical (HM) coupled process interactions between the host rock and the shaft seal elements. Model improvement will lead to increased reliability of performance assessment.

Experimental Data

The experimental program comprises an upscaling approach from laboratory tests via semi-technical scale experiments to the large-scale in-situ test. The laboratory tests include bentonite characterization experiments (swelling pressure and permeability measurements) and various small-scale (MiniSandwich) experiments representing the shaft sealing system. The large-scale in-situ experiment in Mont Terri was launched in 2019 and consists of two experimental shafts of 1.18 m diameter and 10 – 12.6 m depth (Figure 2). Vertical Sandwich sealing systems have been installed in both shafts which can be saturated from pressure chambers located at the shaft bottom (Wieczorek et al., 2024). The experiment is monitored with the help of a large number of sensors in the host rock and in the sealing system. Data are collected for, amongst others, pore pressure, relative humidity, axial and radial stresses and displacement.

Approach

The task is divided into 3 steps with increasing complexity. Step 0 addresses the simulation of the laboratory and semi-technical scale tests, thus focuses on the sealing system itself. Step 1 and Step 2 focus on the simulation of the in-situ experiment. First, only one shaft will be considered to focus on the interactions between the sealing system and the host rock. Afterwards, the in-situ experiment will be simulated in its entireness. The steps are executed in the order of blind-prediction, analysis and calibration. They are divided into sub-steps:

Step 0 – Simulation of laboratory and semi-technical scale experiments

• Step 0a – Calibration of models
• Step 0b – MiniSandwich Test 8
• Step 0c – Semi-technical scale test HTV-7

Step 1 – Simulation of shaft 1

• Step 1a – Hydration of shaft 1
• Step 1b – Excavation and hydration of shaft 1

Step 2 – Simulation of the entire in-situ experiment

• Step 2 – Simulation of the entire in-situ experiment
• Step 2a – Hydration of both shafts
• Step 2b – Adding excavation, ventilation, more complex models for the host rock

Participating Groups

Germany:

• BGE (TU Bergakademie Freiberg)
• BGR (BGR, IfG)

France:

• Andra (Lamcube)

Spain:

• Enresa (International Centre for numerical methods in engineering, CIMNE)

Republic of Korea:

• KAERI
• KIGAM

USA:

• DOE (Sandia National Laboratories)

Further Information

For further information, please contact the task leader, Larissa Friedenberg.

References

• Emmerich, K., Schuhmann, R., Königer, F., Bohac, P., Delavernhe, L., Wieczorek, K., Czaikowski, O., Hesser, J., Shao, H., Jaeggi, D., Bossart, P., Hansmann, J., Gruner, M., Hofmann, M., Aurich, J., Rölke, C., Popp, T., Diedel, R., Schellhorn, M., . . . Iglesias, R. J. (2019). Joint project: Vertical hydraulic sealing system based on the sandwich principle - preproject (Sandwich-VP) : Joint final report: 01.07.2017-30.06.2019 (24 months). Joint Project: Vertical Hydraulic Sealing System Based on the Sandwich Principle - Preproject (Sandwich-VP). Advance online publication. https://doi.org/10.2314/KXP:1692488228
• Wieczorek, K., Emmerich, K., Nagel, T., Bakker, E., Diedel, R., Furche, M., García-Siñeriz, J. L., Glaubach, U., Hesser, J., Hinze, M., Hofmann, M., Jaeggi, D., Königer, F., Mayor, J. C., Räbiger, L., Rey Mazón, M., Rölke, C., Schädle, P., Schuhmann, R., . . . Yeatman, R. (February 2024). Sandwich - HP: Vertical Hydraulic Sandwich Sealing System. Final Report. Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH. Technical Report GRS-745. https://www.grs.de/de/aktuelles/publikationen/grs-745