State-of-the-Art report on the understanding of radionuclide retention and transport in clay and crystalline rocks: EURAD-Future Deliverable 5.2

After isolation of radioactive waste in deep geological formations, radionuclides can return to the biosphere by slow migration through engineered barriers and host rocks. This process typically takes many thousands to hundreds of thousands of years. The rate of transport depends on the distance of the repository from the biosphere and movement of the groundwater, and is mainly governed by the interaction of the dissolved radionuclides with minerals present in the host rock and engineered barrier systems. The WP5-FUTURE of the EURAD project deals with fundamental understanding of retention and transport processes in clay and crystalline host rocks. This state-of-the-art report (SOTA) aims at providing a comprehensive overview of our current understanding of the underlying processes contributing to the radionuclide retention and migration in clay and crystalline host rocks. For each process, a brief theoretical background is provided together with current methodologies used to study these processes as well as references to key data. This SOTA report has been conceived in two steps. First, the initial SOTA (D5.1) was prepared during the first project year addressing some key questions related to radionuclide migration and retention that were listed in the strategic research agenda (SRA) that formed the basis of the EURAD project. At the end of the project, an updated version of the SOTA was prepared that takes into account the newly acquired data and knowledge. Thanks to the innovative research on retention and migration and the extensive knowledge collected for some decades, the process understanding and insights are continuously improving, prompting to adapt and refine conceptual descriptions towards safety assessments. Nevertheless there remain important research questions to be investigated in the future remain and are enlisted at the end of this report.