Report on model representation of radionuclide sorption in Boom Clay

The migration of radionuclides through the host rock plays an important role for the long-term safety of disposal concepts in Boom 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 many scenarios that will be considered in OPERA, 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. This report focuses on the processes that determine the fraction of radionuclides that will be bound by adsorption to the solid matter of the Boom Clay and can therefore be considered as immobile. Adsorption of inorganic compounds on different surfaces has been studied for a long time, resulting in an extensive amount of available scientific literature on this topic, from which the most relevant approaches are discussed in this report. Based on the available information, a modelling approach is presented on how to address adsorption in the OPERA safety assessment, and how to include existing uncertainties. The approach foresees a two-step procedure, during which for all radionuclides of interest so-called ‘Kd-values’ are derived, characteristic for radionuclide adsorption to Boom Clay as present in the Netherlands. The approach will be applied in the next phase of the RANMIG project, and the outcomes will be documented and discussed in a subsequent report (OPERA-PU-NRG6123).