Models and simulations allow a prognosis of how processes in the geosphere might occur in the future, considering physical and chemical processes. They are the only way to test future scenarios and hypotheses and to evaluate the long-term evolution of a repository site, e.g. by quantifying potential radionuclide migration in the hydrogeological system of the containment providing rock unit.
An example is used to demonstrate the extent to which simulated migration lengths can vary for a million years, depending on the model concept as well as on the underlying data and parameters. In the case of uranium in the potential host rock Opalinus Clay (Switzerland), the range extends from 5 m applying experimentally determined transport parameters, over 50 m using process-based approaches and taking hydrogeology into account and up to 80 m depending on the thermodynamic data set used.
The degree of reliability of the models is derived from comparison with laboratory tests and data from boreholes and underground laboratories. This is the only way to assess the simulation results. In addition, indications can be provided where new data need to be collected. To reduce the uncertainty related to the migration length of uranium in the Opalinus Clay, the calcite-carbonate ion system as well as the hydrogeological setting at a potential disposal site need to be known, whereas the amount of clay minerals plays a subordinate role as long as it is enough, which is the case in argillaceous formations.