Gas-Permeable Seal Test (GAST)

The last step of the emplacement work done in the Gas-Permeable Seal Test (GAST) was the construction of the concrete bulkhead which is designed to withold the 50 bars water pressure generated by the water injected into the sand/bentonite plug.

Closing of experiment

Concrete mixing

Concrete injection

Concrete injection

Final plug

Gas-Permeable Seal Test (GAST)

Site preparation for the GAST project : Sealing of the water conducting features, cutting of the plug seat, preparation of the tunnel floor and the tunnel rail and casting.

Sealing of water conducting features

Sealing of water conducting features

Cutting of plug seat

Cutting of plug seat

Cutting of plug seat

Cutting of plug seat

Preparation of tunnel floor

Metal framework with the cabling for the measurement instrumentation for the GAST experiment (© Comet Photoshopping)

Tunnel rail and casting

Gas-Permeable Seal Test (GAST)

Press Release :

Testing a gas-permeable seal for deep geological repositories

Guttannen (Bern) – A multi-year, large-scale demonstration experiment has started at Nagra’s Grimsel Test Site, with the aim of confirming important aspects of the functioning of tunnel seals in future geological repositories. The experiment seal consists of a clay (bentonite)/sand mixture and will be used to demonstrate the feasibility of controlled gas transport from a radioactive waste repository on a realistic scale.

Nagra began operating the Grimsel Test Site, an underground rock laboratory located at an altitude of 1730 metres in the Swiss Alps, 28 years ago. Numerous experiments have already been carried out on the construction and safe operation of future geological repositories for radioactive waste. A new milestone in the history of the Grimsel Test Site was reached in May: the completion of the construction and the start of the operation of the GAST (Gas-Permeable Seal Test) experiment, which investigates the emplacement and behaviour of the seals (barrier) for future geological repositories.

Installation of the measurement instrumentation for the GAST experiment at the Grimsel Test Site (© Comet Photoshopping)

The focus of this experiment is the transport of water during the saturation period and the transport of gas through such barriers. The experiment will bring the current state of knowledge in this area one step forward. The seal consists of a mixture of sand and clay that has been specially developed over the last 10 years. Depending on the proportion of clay in the mixture and the density of the material, the desired properties – very low water permeability and a high transport capacity for gases – can be reached in an optimum way.

“On the 10th of May, we installed the final component of the experiment, namely the concrete plug with a diameter of 4 metres and a thickness of 2 metres” explains Joerg Rueedi, Nagra’s project manager for the GAST experiment. The concrete plug should be capable of withstanding a water pressure of up to 50 bars and will allow testing of the seal under simulated hydraulic conditions that would correspond to a repository at a depth of 500 metres.

Metal framework with the cabling for the measurement instrumentation for the GAST experiment (© Comet Photoshopping)

The GAST experiment is co-financed by Nagra’s sister organisations in France (ANDRA) and South Korea (KRMC) and is planned to run till the end of 2015. A series of laboratory experiments is being performed in parallel with the field experiment. “The experiment will verify a key component of a future geological repository”, says Joerg Rueedi.

Contact person: Stratis Vomvoris, Head of Nagra’s ISP Division: telephone (+41) 56 437 13 24

Gas-Permeable Seal Test (GAST)

 Current partners

  National Radioactive Waste Management Agency (ANDRA), France

  National Cooperative for the Disposal of Radioactive Waste (Nagra), Switzerland

  Korea Radioactive Waste Agewncy (KORAD), South Korea

  Nuclear Waste Management Organization (NWMO), Canada

The degradation of organic substances and metals leads to gas production and accumulation in the emplacement caverns of radioactive-waste repositories. A concept for gas-permeable plugs and seals was developed in the framework of phase 1 of the Swiss sectoral plan process for deep geological repositories. The main aim of these plugs and seals is to increase the gas transport capacity of the backfilled underground structures without compromising the radionuclide retention capacity of the engineered barrier system. The design option is called "engineered gas transport system" (EGTS) and involves specially designed backfill and sealing materials e.g. seals made of sand/bentonite (S/B) mixtures which are part of the L/ILW repository concept.

Figure 1: Concept of the designed gas path (red arrows) for the L/ILW repository with inlay showing the detailed design drawing of repository seal mod. V4 (after Nagra, 2008).

The GAST experiment is a large-scale demonstration and validation experiment which focuses on the specific issue of S/B seal behavior during saturation, and the gas transport capacity in the later gas invasion phase. The consolidation behavior and the gas transport capacity of the EDZ and the intact host rock formation are disregarded intentionally. They are investigated in the HG-A experiment performed in the Mont Terri rock laboratory. The GAST experiment demonstrates the efficiency of the gas permeable seal and enables providing high-quality datasets at realistic scale to validate and, if necessary, improve the existing models in order to make reliable long-term predictions of the EGTS behavior.

Figure 2: Schematic layout of the GAST experiment

Figure 3: 3D cutaway model of the GAST experiment

Previous experimental studies confirmed the high gas transport capacity of the S/B mixtures. These experiments have shown as well the ability to design S/B mixtures with specific target permeabilities for water and gas flow. First in-situ experiences were gained through the Gas Migration Test (GMT) at Grimsel Test Site (GTS), but with in-silo emplacement.A comprehensive laboratory program on combined gas/water transport in S/B mixtures has been conducted as part of the EU project FORGE.

Gas-Permeable Seal Test (GAST)