Quantifying radionuclide migration in the containment providing rock zone (CRZ) is a cornerstone of safety assessments in the context of radioactive waste disposal. Process-based transport simulations require a comprehensive and well-constrained data and knowledge base. Geological evolution, hydrogeological (boundary) conditions, pore water geochemistry, diffusive transport properties and mineral-controlled sorption processes all affect the migration behaviour of radionuclides in argillaceous formations. Many of these aspects occur over geological time and spatial scales. However, they can mostly only be investigated in short-term laboratory experiments. Numerical simulations enable quantifying the processes relevant to the safe disposal of radioactive waste over the required scales based on formation-specific knowledge obtained on smaller scales. Field scale research in underground rock laboratories, such as the Mont Terri Project in Switzerland, provides direct access to the geological environment under close-to-realistic repository conditions (Bossart et al. 2017). Since its inauguration in 1996, the Mont Terri Rock Laboratory has developed into a leading international scientific platform for research on radioactive waste disposal that is shaped by its 22 partners. Systematic investigation of the Opalinus Clay under in-situ conditions has resulted in a unique body of geological, geomechanical, hydrogeological, geochemical and transport-related knowledge. Using the example of uranium migration on the host-rock scale (Hennig et al. 2020; Hennig Schöne & Hennig 2026), we retrace how three decades of experimental and conceptual research at Mont Terri have converged to a basis for process-based transport simulations. The potential migration of uranium in the Opalinus Clay at the host-rock scale over a time span of one million years is simulated using a reactive transport model. This theoretical exercise assumes a hypothetical repository in the centre of a CRZ, with characteristics and boundary conditions based on Mont Terri data to approximate close-to-reality conditions. The model accounts for diffusive transport under formation-specific hydrogeological boundary conditions and for the retention of uranium by sorption onto clay minerals, represented by surface complexation models. The model setup requires a consistent definition of the hydrogeological framework (Section 2.1), the geochemical system (Section 2.2), diffusive transport (Section 2.3) and sorption processes (Section 2.4). These are subsequently combined with uranium-specific transport properties into an integrated reactive transport framework (Section 2.5).
Schöne et al. (2026) studied this question.