Due to the longevity of many radionuclides in radioactive waste, the long-term safety of a geological repository has to be demonstrated for a very long time period (in the order of one million years). This so-called safety case has to demonstrate that within the above-mentioned period, no impermissible concentrations of radionuclides will reach from the repository into the biosphere. In this context, the transport of radionuclides through geological strata is also referred to as migration. For such long periods of time, the migration behaviour of long-lived radionuclides cannot be simulated in laboratory experiments. However, the effects of similar processes that have developed over comparably long periods can be observed in naturally occurring geological systems. If the processes that have developed in such naturally occurring systems are similar to those expected in a repository system, one speaks of "Natural Analogues".
The study of such Natural Analogues offers i.a. the opportunity to obtain information about the migration behaviour of radionuclides in a natural environment over geological periods and thereby to validate the models and initial parameters used in long-term safety analyses. Within the framework of the NATAN (NATural ANalogues) project, GRS - together with the Czech "Nuclear Research Institute (NRI)" and other German and foreign research institutions - is currently studying at the Ruprechtov site (CZ) the migration behaviour of uranium in argillaceous sediments under conditions similar to those expected in the overburden of a repository in Germany. The analyses show the essential immobilisation processes and thus demonstrate the highly effective retaining properties of such sediments against uranium.
Figure Sketch view of significant processes which caused release of uranium at Ruprechtov site, followed by retention (details)