Characterization of discontinuities in an argillaceous medium (Tournemire site): key questions related to safety assessment of radioactive waste disposal

The safety assessment of deep radioactive waste disposal in argillaceous medium concerns the evaluation of the possibilities and transfer processes through this geological barrier. Generally, this material is characterized by a very low permeability. However, faults and fractures may affect this medium and consequently it is important to analyse the transfer possibilities along these discontinuities. The first phase of this evaluation is the detection or survey of the fractures, and the second one is the evaluation of their hydraulic properties. A discontinuity is not a synonymous of fluid transfer, which in turn may be inhomogeneous along the same fracture. The well-compacted Tournemire argillaceous medium (argilites and marls) is affected by some faults and fractures of natural (tectonic) and artificial (excavation) origin. The natural fractures are of different scales: major regional faults that limit blocks, secondary fault and fractured zones that affect the blocks, and local fractures and micro-fractures. These discontinuities are studied from the surface analyses, transversal boreholes and drifts. Geophysical methods are tested to detect these discontinuities to different scales. The present-day fluids are shown in some fractured sectors that allow us to analyse the possibilities of fluid transfer along these discontinuities. The fracture analyses show that permeability or transmissivity properties may change along the discontinuities. A decametric fluid transfer is shown along some fractures; however, their integration in a larger scale is not yet analysed. It is necessary to understand how the transfer in the near field may be connected to the possible far field transfer, or these transfers are not possible through the argillaceous medium. (author)