“…The migration of uranium as main component of spent fuel in the Opalinus Clay, the chosen host rock for the disposal of nuclear wastes in Switzerland, is used as an example for the quantification of radionuclide migration close to a real case situation. Therefore, the conceptual model already applied to simulate the present-day pore water chemistry profile measured at Mont Terri (Hennig andKühn, 2021b, 2022) is now transferred to the region of the deep geothermal borehole Schlattingen, which is in vicinity to one of the three siting areas in Switzerland ("Zürich Nordost"; Nagra, 2022). At Schlattingen, geochemistry and mineralogy of the Opalinus Clay and surrounding formations have been intensively investigated and supplemented with hydrogeological knowledge from the exploration borehole Benken (Gimmi et al, 2007;Wersin et al, 2016Wersin et al, , 2018, which provide a good database for the reactive transport simulations conducted in the present study.…”