Modeling of radionuclide migration through the geologic media. Application to the Meuse/Haute-Marne site in France

In order to review ANDRA's modeling of a deep geological disposal of HLW in a clay formation located in the Meuse/Haute Marne area, IRSN has developed its own modeling methodology, based on three numerical models. A first 'host rock model', including a simplified repository design, is used to simulate the activity transport throughout the repository and the argillite host layer. The model aims at evaluating the role of components of the repository design, together with the host formation, on the activity transport. Then, a second 'hydrogeological model', set on hydrodynamic parameters measured in situ, allows the determination of possible water pathways, outlets and associated transfer times taking into account the possible hydraulic role of identified or postulated tectonic structures in the vicinity of the site. Several possible combinations for the hydraulic parameters (hydraulic conductivities and transmissivities) allow to fit the in situ measured hydraulic heads. The 'hydrogeological model' is finally coupled with a radionuclide 'transport model', devoted to simulate the transport of radionuclides out of the host rock to the outlets. Considering the uncertainties related to both advection and diffusion processes 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: at the ground surface, a 'natural' outlet above the location of the repository system, and in the host rock's overlying aquifer, in a 'well drilling' zone. 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. The lesson learnt from this modeling is that the diffusion regime needs to be better quantified in order to discriminate the respective influence of both outlets. (authors)