A large in situ demonstration test for repository sealing in an argillaceous host rock – phase II. RESEAL II. Final report on laboratory tests (WP1).

Abstract

The RESEAL project aims at demonstrating the sealing of a borehole and a shaft in plastic clay on a representative scale. The backfilling and sealing of shafts and galleries is an important part of underground repository designs. Any opening created during the repository construction is a potential preferential pathway for water, gas and radionuclides migration, and has to be effectively sealed afterwards. Bentonite is widely studied as backfill and sealing material in geological repositories for radioactive waste because of its low permeability and large swelling capacity. The main objective of the project is to demonstrate that a seal of bentonite avoids preferential migration of water, gas and radionuclides along the seal/host rock interface and through the excavation disturbed zone. The project started in 1996 in the frame of the EC specific RTD programme on “Nuclear Fission Safety”. It has included the selection and characterisation of sealing materials, the development of the seal installation technique, the monitoring of the horizontal borehole and the shaft sealing tests performed at the HADES Underground Research Facility (URF) in Mol (Belgium) and the modelling of the main processes and phenomena observed during the tests. The first phase of the project ended in October 1999 with the installation of the shaft seal. The second phase of the project (RESEAL II), mainly focussed on the shaft sealing test, is a joint project between SCK•CEN (co-ordinator), ANDRA, CEA, CIEMAT and UPC. The project is financially supported by EC and NIRAS/ONDRAF. The experimental shaft has a diameter of about 1.4 m and has been filled up with a pellets/powder mixture of FoCa bentonite (Figure 1-1). This powder/pellets mixture has the advantage that it can be easily applied to backfill irregularly shaped volumes and that it is rather cheap to produce in comparison to compacted blocks. It also requires less compaction effort to achieve a given average dry density because of the very dense pellets of the mixture. The pellets have a size of approximately 25 x 25 x 15 mm and a dry density close to 2 g/cm³. To obtain a high-density granular backfill material, the pellets are mixed with powder of the same clay. A 50/50 weight percent pellets/powder proportion was selected during the first phase of the project. This proportion has been chosen to avoid segregation during installation in the shaft and favour a homogeneous hydration and redistribution of density, although this particular proportion does not grant the highest density (Imbert et al. 1999). The first 60 cm of the seal have been compacted by using vibro-compaction techniques to increase the density of the mixture. This resulted in a dry density of 1.6 g/cm3. The compaction was stopped just below the first instrumented level. From this level the sealing material was installed without further compaction in order to avoid damage to the instruments. The dry density of the material thus installed is 1.4 g/cm3. Artificial hydration of the seal started in May 2000. Once the seal is saturated, water and gas sealing capacity tests will be carried out, as well as injection of radioactive tracers. On the other hand, a tracer injection test through the plug (pre-compacted high-density FoCa bentonite blocks) of the borehole sealing test, installed during RESEAL I, is being performed in order to check if there are preferential pathways for radionuclide migration in the borehole seal. The experimental results will allow validating the tracer diffusion models.The laboratory work to characterise the shaft pellets/powder mixture seal has been carried out by three organisations: CEA (France), SCK·CEN (Belgium) and CIEMAT (Spain). It has been divided in two groups of activities, those aimed at the study of the hydro-mechanical properties of the sealing material, and the activities related to the determination of the migration parameters. To analyse the hydration mechanisms of the pellets/powder mixtures and their coupling with the mechanical behaviour, different infiltration tests with measurement of the swelling pressure have been performed. In addition, since the migration of radionuclides in clays is controlled by diffusion, the determination of radionuclides diffusion coefficients, as well as of their accessible porosity, has been undertaken.