Radionuclide migration in groundwaters: Review of the behaviour of actinides (Technical Report)
The most likely process that can lead to the release of radionuclides from a radioactive waste repository in a geological formation is transport by groundwaters. Because of this, the migration of radionuclides in groundwaters is extensively studied ; consequently works and studies to be performed in the evaluation of this phenomenon must deal with aqueous chemistry, geochemistry, hydrology, physics and modeling. The current state of the art concerning actinide migration is given in this report in order to highlight the various aspects of the subject and to point out the research required to bridge the gaps in our current knowledge. INTRODUCTION The safety of a radioactive waste or spent nuclear fuel repository is related to its capacity to confine radioactivity and isolate it from the biosphere. The most likely process that can lead to the release of radionuclides from a repository to the geosphere is transport by groundwater. Consequently, waste disposal-related safety analyses must assess the importance of the migration of radionuclides (i.e., their mobility) in the conservative assumption of leaching by groundwater (i.e., after the destruction of the engineered barriers). The evaluation of the mobility of radionuclides in groundwaters requires : Knowledge of their mobile species, i.e., knowledge of radionuclide aqueous chemistry and knowledge of the environmental waters, which condition the nature and concentration of their soluble species. This study can be carried out with a thermodynamic approach, which considers the system at steady state (reasonable approximation, taking into account the very low water velocity expected in the vicinity of a geological repository). The chemical species are calculated (with the assistance of computer based methods) by solving the equilibrium reactions which can occur between the water constituents and the nuclides. This procedure assumes that reliable thermodynamic data are available for every major reaction, including data for conditions that differ greatly from standard conditions (such as temperature up to 200°C in the near field of vitrified waste disposal and ionic strengths different from zero). Unfortunately, these data are not always available., . Knowledge of transport processes and their kinetic, convection and diffusion characteristics within the water, of sorption and of desorption from rocks and minerals in contact with the aqueous phase. The effect of these processes on radionuclide mobility and changes in sorption behaviour, as a function of time due to rock aging phenomena, are also required. Research in this field has led to both