Reactive transport model and apparent Kd of Ni in the near field of a HLW repository in granite

Current performance assessment models for radionuclide migration through the near field of high-level radioactive waste repositories often rely on the assumption of a constant K"d for sorption. The validity of such assumption is evaluated here with a reactive transport model for Ni^2^+ in the near field of a repository in granite. Model results show that Ni^2^+ sorbs mainly by surface complexation on weak sorption sites. The apparent K"d of Ni^2^+, K"d^a, depends on the concentration of dissolved Ni and pH and is constant only when the concentration of dissolved Ni is smaller than 10^-^6mol/L. The results of the sensitivity runs show that K"d^a is sensitive to the water flux at the bentonite-granite interface, the effective diffusion of the bentonite and the concentration of weak sorption sites of the bentonite. The competition of other nuclides such as Cs^+ on Ni^2^+ sorption is not important. Corrosion products, however, affect significantly the sorption of Ni^2^+ on the bentonite. The model with a constant K"d does not reproduce the release rates of Ni^2^+ from the bentonite into the granite. A model with a variable K"d which depends on the concentration of dissolved Ni^2^+ and pH may provide an acceptable surrogate of the multicomponent reactive transport model for the conditions of the repository considered in our model. Simulations using the K"d-approach were performed with GoldSim based on the interpolation in the pH and concentration table, while the reactive transport model simulations were performed with CORE^2^D which incorporates multisite surface complexation.