OPERA-PU-NRG6131 Model representation of radionuclide diffusion in Boom Clay: Model representation of radionuclide diffusion in Boom Clay

The migration of radionuclides through the host rock plays an important role for the long-term safety of disposal concepts in clay. Due to the slow transport of radionuclides in the host rock, most radionuclides will be decayed before they can enter the surrounding aquifers. For the generic OPERA disposal concept in Boom Clay, the delayed transport of radionuclides through the host rock can be assumed as the most important safety function on the long-term. It is therefore important to understand the basic processes behind the migration of radionuclides in the host rock sufficiently well to be able to make a credible quantitative assessment of the effects of long-term underground disposal of radioactive waste. Overall nuclide migration rates in Boom Clay are determined by a combination of chemical and physical processes. This report focuses on the physical aspects of nuclide migration, on a model approach and required model parameters. An important source of information for these parameters are the results of the Belgian research programme as part of which radionuclide behaviour in Boom Clay has been studied for over 30 years. Since the Boom Clay conditions in the Netherlands are expected to be slightly different from the Belgian situation, an assessment was carried out of the transferability of the data obtained on Belgian samples for predicting/describing radionuclide transport in the Netherlands. The results show that although expected higher salinity, higher pressures, and higher temperatures in the Dutch situation are likely to have some effect on diffusion coefficients, the order of magnitude of these effects limited. The resulting estimated set of transport parameters for the Dutch situation was summarized into a database that together with the set of retardation factors resulting from Task 6.1.2 forms the complete set input data for modelling migration of radionuclides through the Boom Clay.