Modelling of radionuclide migration through the geologic media. Application to the Meuse/Haute-Marne site in France
Titre du congrès :21ème RST (Réunion des Sciences de la Terre)
Ville du congrès :Dijon
Date du congrès :04/12/2006
IRSN has reviewed ANDRA’s study on the feasibility of a deep geological disposal of highlevel, long-lived radioactive waste in a clay formation located in the Meuse/Haute Marne area and investigated by ANDRA in the Bure underground laboratory. IRSN has paid particular attention to the modelling of activity release and transport from buried containers and has therefore developed its own modelling methodology, based on three models. A "host rock
model", including a simplified repository design made of handling drifts connecting the disposal tunnels to the access shaft, is used to simulate the activity transport throughout the argillite layer. The model aims at evaluating the influence of components of the repository design, together with the host formation, on the activity transport. Then, a more extended "hydrogeological model", set on hydrodynamic parameters measured in situ, allows the
determination of possible water pathways, outlets and associated transfer times with respect to the remaining uncertainties associated to the hydraulic role of identified or postulated tectonic structures in the vicinity of the site. Several possible combinations for the hydraulic parameters allow to fit the available hydraulic heads in terms of difference between measured and simulated values. The first proposed combination, in which faults are not
explicitly represented, gives satisfying fitting results but the transmissivities used remain too high compared to the measured values. The second combination, which explicitly represents tectonic structures, gives satisfying results both in terms of simulated hydraulic heads and of hydraulic parameter values. This second “hydrogeological model” is finally coupled with a radionuclide "transport model", devoted to the activity transport calculations to the outlets.
Considering the uncertainties related to the diffusion process in the aquifers, a sensitivity analysis is carried out in order to assess its influence on the activity transport through the different layers above the host formation. Two specific outlets are considered. They are located: at the ground surface, above the location of the repository system (possible natural outlet), and in the host rock’s overlying aquifer, in a fault zone (possible zone for
well drilling). The lesson learnt from this modelling is that both flow regimes (advection and diffusion) need to be better quantified in order to assess the respective influence of both outlets. Indeed, it is shown that the activity reaching the natural outlet is mainly governed by diffusive fluxes whereas the activity reaching the well drilling zone depends on the advective properties of the fault zones.