Expectations, open questions to be addressed in the workshop within the context of a deep geological repository in clay formations

Precise knowledge of the clay properties in the various domains concerned by the construction feasibility, the exploitation phase of repository facilities, as well as the long term evolution of the waste and of its environment is of crucial importance in assessing the performance and the safety of the various radioactive waste disposal concepts. The knowledge to be acquired on clays as such goes well beyond solely the field of disposal of radioactive waste. For both, clay formations or bentonites in engineered barriers, the characterization in a continuous way from the nanometer to the micrometer, of their internal structure and the study of the associated physico-chemical phenomena is a fundamental issue. It aims for explaining: The 'Initial state' of the clays, in particular for the clay formations: the nature of the mechanical, hydraulic and geochemical processes, in a broad sense, and the way these processes were involved during the geological history of these formations, The fundamental processes involved by physico-chemical or hydraulic stresses, related to the evolution of the repository at the macroscopic scale. The choice of the characterization scale and relevant modeling is of first importance in the approaches leading to the establishment of the models of representation. Various research works pointed to experimental difficulties in quantifying the microstructure of the clay rocks at scales smaller than a micrometer, because of technical/instrumental limitations. This lack of knowledge at small scales does not allow to fully connect all the Thermal-hydrological-mechanical-chemical (THMC) mechanisms and to integrate them into an up-scaling approach. There already exist conceptual models and experimental approaches to describe the microstructure of argillaceous formations in terms of porosity and texture. Examples on the undisturbed Callovo-Oxfordian (COx) argillite are given in this paper. Questions and objectives to be addressed during the workshop on undisturbed argillaceous rocks concerning the characterization of the pore and mineral 3D network geometry at the infra-millimeter scales are highlighted. Also, radioactive waste disposal offer a wide range of questions connected to various research areas: - Clay minerals and clay rocks still need to be studied in detail in order to provide robust models for the evaluation of the processes occurring during the evolution of an underground disposal; - Issues related to thermo-hydro-mechanical-chemical (THMC) processes are central when evaluating the safety of an underground repository; - Nano or microscopic resolution techniques provide new insights for the understanding of clay minerals reactivity; - Some phenomena, the description and understanding of which require micro-nano approaches, still constitutes major issues for the demonstration of the safety and the dimensioning of repository components