Hydrogeological issues associated with nuclear wastes geological repository in clay rocks. Example of Cigéo project - France

Nuclear wastes geological repository research activities were conducted, during the last decades, in different types of potential host rock in many countries. Crystalline or granitic rock were the focus of nuclear industry in Sweden, Finland, China while clay as host formation was mainly studied in Belgium, France and Switzerland. An overview of hydrogeology and hydrogeological modelling implication on geological repository development in sedimentary formations environment gives an insight of advances in site characterization and hydrogeological performance assessment made by the French National Agency for Radioactive Wastes Management. Several factors make, particularly challenging, the study of the flow and the transport for the evaluation of the site performance, which involves long-term predictions of hundreds of thousands or millions of years. Development of subsurface flow and transport prediction models includes present time hydrogeological model and geodynamic evolution through the next millions of years covering safety time scale. Performance and safety assessment of a deep geological repository for intermediate high level long lived (IHLL) radioactive wastes requires identification of potential flow paths and the associated travel times for radionuclides originating at repository depth. The planned French repository (Cigéo Project) will be located at great depth of 500 m in the Callovo-Oxfordian claystone formation (160 Ma) of the multi-layered system of Paris Basin. Numerical simulations coupling internal and external geodynamic evolutions with groundwater flow describe how the tectonic uplift, erosion/sedimentation and permafrost genesis, expansion and retreat processes affect (i) long-term transient flow and transport behaviour and (ii) hydrogeological performance measures. Performance is then analysed by the use of Lagrangian transport approach using a 3D transient flow fields induced by: (i) deformation of the multi-layered aquifer system resulting from differential tectonic uplift, (ii) evolution of the outcrop zones governed by erosion and incision of the geological layers and (iii) climate changes.