In order to evaluate the safety of a potential radioactive waste repository in a geological formation, it is necessary to study the host rock and the whole sedimentary sequence in which it occurs at both a local and a regional scale.
In this respect, SCK•CEN started in 1974 investigations of the Boom clay beneath the nuclear site in Mol-Dessel. Untill now, these investigations are ongoing and include both the specific site investigation of the Boom clay at the reference site Mol-Dessel, as well as its regional geological framework. The obtained information is integrated and used as input for the hydrogeological model and performance assessment calculations.
The regional geological framework of the Campine Basin in the North-East of Belgium is investigated by integration of a variety of different types of data that provide information on the thickness of the layers, the geological structure, etc. This information is obtained from surface mapping, geophysical survey, borehole information, literature data, …
The lithological and structural homogeneity/heterogeneity of the Boom clay and its overlying and underlying formations at a regional scale has been studied primarily by means of geophysical methods, applied either in boreholes (wireline logging), or from the surface (seismic reflection), with additional testing of core material. The Boom clay has also been the subject of intense fundamental and applied research in the outcrop zone or during tunnel excavation in the underground research laboratory. This resulted in the detailed geological characterisation of the Boom clay.
Detailed investigation of the physical and geochemical properties of the Boom clay is mainly done at the Mol-Dessel reference site. Data are mainly derived from detailed clay core analyses (Mol-1 borehole), drilled at the SCK•CEN domain, several boreholes performed from our underground research facility HADES), on site measurements and in-situ experiments.
The geochemical characterisation of the Boom Clay and its pore water is our most important field of expertise. The main resources needed both from the point of view of field sampling, on-site measurements, in situ experiments, laboratory measurements, and geochemical modelling are all available within our team.
- Methods for anaerobic sampling and long-term preservation of clay samples are routinely practiced by our team in support of geochemistry and migration studies. It includes the preservation of clay samples against oxidation (to avoid, or at least to minimise, contact of pyrite and organic matter with oxygen from the air), CO2 outgassing, and contamination by microorganisms capable to induce reductive perturbation (specially for sensitive measurements that could be affected by bacterially-mediated changes in the sulfur, carbon and nitrogen cycles). SCK•CEN is equipped for mineralogical characterisation of sedimentary rock samples with classical techniques: XRD, SEM-EDS.
- Water field sampling and in situ experiments: our team is acquainted with on site and in situ measurements of parameters which are difficult to preserve in the laboratory, e.g., pCO2, pH and Eh. Beside the instrumentation we installed in the underground HADES facility in Boom clay in Mol (EURIDICE), we also participate to the development of new characterisation methods for the Opalinus Clay in the Mont Terri rock laboratory (Porewater Chemistry experiments; located in Switzerland). We are used to sample pore water under anaerobic conditions taking care to minimize oxidation, outgassing, and to avoid exchange with gases present in the air. Experimental setup made of specific materials selected to minimise contamination can be constructed to tackle special problems including operating conditions at elevated pressure and temperature as encountered in large scale in situ tests in progress at the Mol underground laboratory. Various techniques are available at SCK•CEN to extract water from low hydraulic conductivity samples, from in situ piezometers installed in clay at depth, squeezing equipment, and aqueous leaching of clay suspensions.
Besides the geochemical properties of undisturbed Boom Clay, the effect of geochemical perturbations on the clay composition are studied as well. Different types of geochemical perturbations may occur in a radioactive waste repository:
- Oxygen perturbation - due to the excavation and ventilation of galleries;
- Perturbation of alkaline fluids – due to the presence of concrete (used in the supercontainer design, as well as the lining of the galleries)
- NaNO3 perturbation – in case of disposal of bituminised waste
- Increased temperatures – in case of high-level radioactive waste disposal
For the interpretation, the geochemical data obtained from the experimental work in surface laboratories and in the HADES Underground Research Facility is combined with geochemical modelling.
Our expertise is regularly shared and updated in the frame of international cooperation programs and European projects. We are also involved in collaborative studies of other deep clay formations mainly in Switzerland (Opalinus Clay, Mont Terri rock laboratory in Jura) and in France (Callovo-Oxfordian Clay, Laboratoire de Meuse Haute Marne in Bure).
The cooperation work of SCK•CEN with eastern countries for the characterisation of geological formations also concerns several countries: Bulgaria, Romania, Slovenia, Slovak republic, Czech republic, Hungary, Lithuania, Latvia, and Russia.
General collaboration agreements dealing amongst others with site characterisation activities, performance assessment studies, and training of scientists have also been recently signed with Japan Atomic Energy Agency (JAEA) and several Chinese Institutes.
Finally, SCK•CEN is also member of the so-called “Clay Club” founded by several countries from the Nuclear Energy Agency (NEA, OECD) where general information is exchanged in relation with clay formations and pore water characterisation methods.
Important publications: Geochemistry of Boom Clay pore - Status 2004 (2 MB)
Contact: De Craen Mieke