Nagra’s Grimsel Test Site – 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.

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.

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.

Grimsel Rock laboratory

The Grimsel Test Site is located at an elevation of 1730 metres above sea-level in 300 million year old granite and granodiorite of the Aar Massif, at a depth of around 400 m beneath the Juchlistock. Cored boreholes with a total length of around 5000 metres have been drilled to date in the branching research tunnel system, which extends over more than 1 kilometre. Organisations from 11 countries and the EU are involved in the Grimsel experiment programme. Year-round access to the facility is assured for the scientists involved in the experiments, in winter with a cable-car. The Test Site is open to visitors by prior arrangement from the beginning of June to the end of October.