GEOTRAP is the OECD/NEA Project on Radionuclide Migration in Geologic, Heterogeneous Media carried out in the context of site evaluation and safety assessment of deep repository systems for long-lived radioactive waste. Retention of radionuclides within the geosphere for prolonged periods is an important safety function of deep geologic disposal concepts for radioactive waste. The extent to which retention processes can be relied upon in repository performance assessment depends upon the existence of well-established theoretical bases for the processes. It also depends on support for the opera.