Deliverable 5.1: State-of-the-Art report on the understanding of radionuclide retention and transport in clay and crystalline rocks: FUTuRE - Work Package 5

After isolation of radioactive waste in deep geological formations, radionuclides can only enter the biosphere by slow migration. This process typically takes many 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 FUTURE project deals with fundamental understanding of retention and transport processes in clay and crystalline host rocks. This state-of-the-art report 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. Despite that research on retention and migration has been intensive for some decades and the knowledge is extensive, the process understanding and insights are continuously improving, thanks to innovative research, prompting to adapt and refine conceptual descriptions towards safety assessments. Hence, uncertainties remain and the key uncertainties that presently need to be resolved are listed.