Dealing with Geological Discontinuities in Safety Assessment: the Approach of the French Technical Safety Organisation

The present paper focuses on IRSN structural studies aiming at assessing the existence of tectonic structures in the Callovo-Oxfordian clay formation and their potential role on radionuclide transport. IRSN has first determined the type of fracturing that could be present in the Callovo-Oxfordian, by carrying out surface fieldwork in Meuse/Haute-Marne and investigations in clay and limestone layers in the Tournemire experimental station. IRSN has also examined the results of the Andra 3-D seismic survey associated with inclined boreholes around the Bure URL, as well as the limits of these survey methods, based on its own 3-D High-Resolution seismic survey campaign carried out in Tournemire on fault zones already recognised by drifts and boreholes. At the scale of the Paris sedimentary basin, on the basis of a geological model and of measured hydrodynamic parameters, IRSN has developed underground water flow schemes, which allow the determination of possible water pathways, outlets and associated transfer times. This modelling shows that several combinations in the representation of tectonic structures are possible to fit measured data, and that the remaining uncertainties regarding their hydrogeological role should be reduced. In addition, the influence of a hypothetic fault in the Callovo-Oxfordian clay close to disposal cells on radionuclide molar flows is tested at the repository scale. At last, perspectives are given regarding in particular methods to detect hydrogeologically active faults, differential fracturing patterns in clays and limestones as well as improvement of IRSN hydrogeological model.