Modelling of cation concentrations in the outflow of NaNO3 percolation experiments through Boom Clay cores

In Belgium, the Boom Clay is considered as a reference host formation for phenomenological studies on the deep disposal of high-level and intermediate-level radioactive waste, because of its low permeability and high sorption capacity for many radionuclides. The presence of large amounts of NaNO3 in the Belgian Eurobitum bituminized intermediate-level radioactive waste will result in a NaNO3 plume. Since the clay is considered to be the main barrier against radionuclide transport, the impact of this perturbation on the performance of the host rock with respect to retarding radionuclide migration needs to be evaluated. To this purpose, experimental data was acquired on the elution of exchangeable cations from Boom Clay by injection of Boom Clay pore water containing different NaNO3 concentrations. Reactive coupled transport modelling was used to explain the observed elution curves for the cations Na, K, Ca, Mg and Sr, based on the assumption that cation exchange was the only process controlling cation transport. The principal objective was to test the applicability of a cation exchange model with selectivity coefficients independently determined on undisturbed Boom Clay, to predict the results of a percolation experiment.