Understanding and preventing risks related to the geological disposal of long-lived high-level (HL-LL) and intermediate-level (IL-LL) nuclear waste.
Acquired by IRSN in 1992, the Tournemire experimental station, together with the Mol (Belgium), Mont-Terri (Switzerland) and Bure (Meuse, France) laboratories, is now one of the four underground laboratories in Europe carrying out research on disposal in clay formations.
The Tournemire experimental station is solely used for scientific and technical research. There is no intention of disposing of radioactive waste there at any time in the future. Further, no radioactive components are brought to this site for the purposes of the research carried out here.
Context and purpose
The French National Radioactive Waste Management Agency (Andra) is responsible for designing, constructing and operating a geological radioactive waste disposal facility. Pending approval, this facility will be opened in eastern France in 2025. With this in mind, Andra has been operating an underground laboratory in Bure (Meuse) since 1999, where it carries out studies and research. To ensure an independent assessment of Andra’s project, IRSN has been carrying out its own research at the Tournemire experimental station in southern Aveyron for the last 21 years. Located in a former railway tunnel built over 120 years ago, this station provides access to a clay formation that has similar geological characteristics to the site chosen by Andra. The research carried out there will enable IRSN to examine certain processes that play an especially important role in ensuring the long-term safety of a geological repository.
Various experimental studies are carried out by IRSN to analyse :
- The mechanisms responsible for the transfer of water and natural substances present in the clay formation;
- The effects of excavation and of using the underground engineering structures on the rock’s containment properties;
- The effects related to interaction between the rock and the materials added, such as concrete and metal components;
- The performance of components essential to the long-term safety of a geological repository, such as seals.
Geological and tectonic features
The formation in which the tunnel was excavated is made up of compacted argillaceous sequences called argillites and marls; it is the same type of rock that Andra is studying in the Meuse – Haute-Marne. The 250-metre thick argillaceous layer is surrounded by limestone rock. It is 180 million years old and formed in a marine environment.
Usually less prone to fracturing than other types of rock (limestone and granite, etc.), the argillaceous rock at Tournemire nonetheless features some faults and fractures of various sizes that formed 40-50 million years ago (formation of the Pyrenees mountains) or earlier. It is vital to know all about these faults and fractures and to characterise them since they may play a role in the migration of water through the argillaceous layer.
General geological cross-section along the Tournemire tunnel ©IRSN
Tournemire tunnel and the experimental station
Excavated at the end of the 19th century, the old Tournemire railway tunnel is 1,885 metres long. It thus affords an opportunity to observe any disturbances generated by an underground engineering structure excavated in a clay formation over 120 years ago.
The experimental station itself is located in a central part of the massif. It is made up of the former tunnel, six drifts which are 285 metres in length in all and over 200 boreholes excavated since 1990, which lead in different directions from either the tunnel or the drifts.
A range of measuring equipment and observation techniques are deployed at the station to analyse the argillite and its behaviour, detect faults using geophysical methods, the water it contains and the rate at which it moves through the formation, as well as disturbances likely to affect this type of rock if it is used for disposal and, more recently, to assess the performance of the components used for disposal facilities.
3D diagram of the Experimental station ©IRSN
Migration of water in the argillaceous rock
The argillaceous rock at Tournemire is composed of approximately 50% clay and secondary minerals (quartz, carbonates, pyrite, etc.). Compact and hard, it contains very little water (8 to 10%), which is trapped in pores around ten nanometres in diameter.
Apart from the fractures, the rock studied at Tournemire has extremely low water permeability. Analysis of hydraulic tests and natural tracer profiles obtained for the whole argillaceous layer confirm that water circulates very slowly, by about one centimetre per million years. The major method of radionuclide transfer is therefore mainly limited to the molecular diffusion process.
Through certain tectonic fractures, water circulates more rapidly and water flows intercepted at the station are the result of exchanges with the water-bearing calcareous formations located on either side of the clay layer. Analysis of the chemical and isotopic composition of the water circulating in these fractures reveals a residence time of around 15,000 years, i.e. the speed at which water migrates through the argillaceous rock is a few centimetres per thousand years.
For the purposes of comparison, water circulating through the calcareous rock on either side of the argillite layer has residence times of around 50 years.
Effects of excavating in the argillaceous rock
The mechanical properties of the argillaceous rock at Tournemire, and the fact that it is compact and hard, make it brittle. Excavation works may therefore cause mechanical disturbances extending from a few tens of centimetres (in the drifts) to around two metres (in the one hundred year old tunnel), causing irreversible plastic deformations manifested by the appearance of fractures. The extent and the intensity of this area, called the EDZ (Excavation Damaged Zone), evolve over time due to all the different thermal, hydraulic and chemical stresses.
More permeable than the undisturbed argillite layer, the fissured zone that forms the EDZ is likely to be a preferential drain for water migration.
In addition to the mechanical disturbance caused by excavation for the engineering structures, there is further fissuring caused by ventilation in the drifts and the subsequent desiccation of the rock (hydromechanical coupling phenomena).
Characterising these fissures and how they develop in space and time is therefore an essential part of assessing the safety of long-term disposal.
Disturbances to the rock associated with materials added
Some of the experiments conducted by IRSN at the Tournemire experimental station aim to identify and characterise disturbances to the rock caused by materials brought into the geological environment from the outside.
Dedicated experiments are thus conducted on the cementing materials (lime, concrete and cement) and metal materials (iron) used to support the drifts or inserted in the boreholes.
Interactions between these materials and the clayey minerals in the rock can alter the geochemical properties of the rock and compromise its ability to contain radionuclides. The chemically-disturbed zone revealed in the samples analysed shows alteration fronts that can be as large as a few centimetres in the presence of water, over a period of contact ranging between one decade and a few decades.
The Tournemire experimental station is an exceptionally useful tool for this type of test: the lime used as cementing material for the masonry facework in the tunnel, which is similar to concrete, has been in contact with the rock for over 100 years, in other words, for a period comparable to the operating lifetime of a disposal facility.
Sample of steel left in contact with the argillite for 10 years
(Photograph, microtomographic image of iron at the interface) ©IRSN
Performance of sealing structures
The safety of a long-term geological disposal facility largely depends on the effectiveness of the systems used to close up the engineering structures. In particular, these systems imply the use of seals. These are made of a natural swelling clay-based material which ensures continuity of the containment provided by the rock.
At the Tournemire experimental station, IRSN’s SEALEX research project is dedicated to assessing the effectiveness and robustness of such seals over time. This project involves examining the key factors that regulate the long-term hydraulic performance of the seals.
Tests are carried out over periods of several years.