Joint project iCross: Integrity of radioactive waste disposal systems - cross-scale system understanding and system analysis. Subproject A. Final report

A key objective of the project was to gain a cross-scale understanding of coupled thermal-hydraulic-mechanical-chemical-biological processes (THM/CB processes) that determine the long-term evolution of the barriers in a deep-geological repository for radioactive waste as well as the release and migration of radionuclides as basis for "close to reality" predictions of the evolution of a repository system for specific scenarios. In this context, the influence of the heterogeneity of clay host rocks on radionuclide migration in the repository far field was investigated on the laboratory scale, using the sandy Opalinus Clay facies as an example. Moreover, the Representative Elementary Volumes (REV) for the diffusivity and porosity of the sandy Opalinus Clay, which are key parameters to describe material heterogeneity for reactive transport modelling on the continuum scale, were determined by a combination of pore- scale transport modelling and diffusion experiments. Contributions to the planning and implementation of corresponding diffusion experiments in the Opalinus Clay in the underground rock laboratory Mont Terri (Switzerland) were made to allow for upscaling of the results and knowledge gained from laboratory scale experiments. Other more fundamental aspects of reactive transport phenomena considered in the project were (i) radionuclide migration due to thermodiffusion (Soret effect), (ii) solute transport processes in partially water-saturated clay rocks, (iii) the effects of coupled dissolution and precipitation processes on porosity and permeability changes in porous media as well as (iv) questions of scalability (upscaling) with respect to the modelling of reactive transport processes in heterogeneous rocks. The project results contribute to the safe disposal of heat-generating and long-lived radioactive wastes and to the scientific basis of the safety case for deep geological disposal, in particular with respect to a comparative assessment of repository systems in different host rock formations and different siting regions required in the context of site selection.