The argillaceous indurated media as a potential geological barrier for deep radioactive waste disposal: studies at the experimental IRSN Tournemire site.

Cabrera Nunez, J.; DeWindt, L.; Bruno, G.; Rejeb, A.; Genty, A.; Volant, P. actes du congrès "Safewaste 2000 -2. International Conference, Nuclear Waste: From Research to Industrial Maturity" - Montpellier (France) 2-4 Oct 2000. Société Française d'Energie Nucléaire 2000 (v.1-2) 888 p. p. 607-623.

Deep argillaceous formations are potential host media for radioactive waste disposal. In order to train its experts, the French Institute for Protection and Nuclear Safety (irsn) develops an experimental research program in the Tournemire site (Aveyron, France). These research program aims at evaluating the possibilities and processes of fluid migrations through the argillaceous formations, as well as the characterisation and long-term behaviour of the excavation-disturbed zone (EDZ). These programs cover geological, structural and tectonics, geophysical, hydrogeological, geochemical and geomechanical domains. The experimental programs necessitate the realisation of boreholes and drift excavations. The geological setting with discontinuities of different scales in time and space allows us to analyse the confining properties of the non-perturbed argillaceous formation. Concerning the perturbed or damaged argillaceous media related to drift excavations, the Tournemire site allows us to compare an old railway tunnel (one century old) with recent drifts excavations (3 years old). The results of these investigations suggest very slow fluid migrations in the non perturbed matrix (far field), mainly controlled by diffusion processes. However, the fluid flows along the discontinuities that affect these formations are poorly known. Consequently, the present investigations focus on these discontinuities: the hydraulic role of fractures and their possible perturbations that modify the barrier characteristics. Structural (geometry, scale, connection), geochemical (isotopic analyses) and hydraulic (permeability, transmissivity) approaches are developed. The investigations related to the drift excavations show mechanical, hydraulic, chemical and thermal perturbations. A mechanical fracturing is related to the tunnel and drift excavations. The hydraulic perturbation is associated to a desaturation of argilites with fracture development, a water discharge from some pre-existing fractures, and a permeable zone located in the fractures of mechanical origin. Chemical perturbations induced by the excavation can be related to oxidation phenomena as supposed through the observations of gypsum, iron oxides, iron hydroxides, jarosite and celestine. The present paper summarises the main results of the research programs of the Tournemire project, and the current research orientations.