Role of bentonite colloids on europium and plutonium migration in a granite fracture

To establish when the presence of colloids can favour contaminant migration is a critical task for the performance assessment of a high level radioactive waste repository. In this study, the effects of the presence of bentonite colloids on the migration of highly sorbing elements, such as Eu and Pu, in a granite environment were investigated. Special efforts were made to quantify the filtration of bentonite colloids in this medium and to determine the experimental uncertainties that could bias the interpretation of the results. The migration of bentonite colloids in a granite fracture was studied in the laboratory by dynamic column experiments under low water flow rates (4–11 mL/h). The breakthrough curves of bentonite colloids always showed a peak in a similar position to conservative tracers, but the colloids recovery critically depended on their initial concentration and on the water flow rate. In the presence of colloids, the breakthrough curves of Eu and Pu always showed a peak in the same position as conservative tracers, thus indicating the migration of these radionuclides to be mostly colloid-driven. The recovery of Pu adsorbed onto the colloids was approximately that expected when the experimentally determined bentonite colloid filtration in the column was accounting for but the Eu recovery was always significantly lower.