The Trancom-II project addresses the migration behaviour of radionuclides, identified as important for long-term safety (Se, U, Pu, Am as analogue for trivalent Pu), in a reducing clay environment, with special emphasis on the role of the natural organic matter (NOM). The main questions are whether the NOM increases the radionuclide concentration in a solution due to complexation/colloid formation and what the possible role of mobile NOM is as radionuclide carrier. The objective is to develop and/or demonstrate a conceptual model, for the description of the migration of these radionuclides, that can be implemented in performance assessment (PA) models. The experimental programme revealed that, in general, the transport mechanism of the studied radionuclide species (except for , SeO42-) is dominated by an immobilisation process combined with a constant release of a low concentration of the radionuclide. ln the case of Se and U the constant release is controlled by a solubility limit of the precipitated phases and the transport is independent of the NOM. Colloids were in most cases in evidence but found to be either unstable in, or filtered by, the Boom Clay. Despite the strong complexation of Am with the NOM, these mobile complexes are unstable in the Boom Clay and followed by a constant release of Am at a concentration lower than its solubility. The transport of this released Am fraction is, however, correlated with the NOM. For Pu the underlying processes are not understood but resemble those of Am. A transport model, Popcorn, was developed to describe and evaluate the influence of the NOM on radionuclide transport in clay. It accounts for attachment/detachment rates of the NOM to clay surfaces and the kinetics of radionuclides complexation to, and dissociation from, the NOM. The model described well the Am-OM transport behaviour and offers potential means of extrapolating parameter values for application of the model on a larger space scale. The general conceptual model was handled by PA using a limited set of parameters: concentration limit or solubility limit, diffusion coefficient, retardation, diffusion accessible porosity. The NOM in the Boom Clay has an influence on the migration behaviour but it does not lead to an increased radionuclide flux to the biosphere.
