Reactive transport model and apparent K-d 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 Ni2+ in the near field of a repository in granite. Model results show that Ni2+ sorbs mainly by surface complexation on weak sorption sites. The apparent K-d of Ni2+, 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(-6) mol/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 Ni2+ sorption is not important. Corrosion products, however, affect significantly the sorption of Ni2+ on the bentonite. The model with a constant K-d does not reproduce the release rates of Ni2+ from the bentonite into the granite. A model with a variable K-d which depends on the concentration of dissolved Ni2+ 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 CORE2D which incorporates multisite surface complexation. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.
reactive transport model apparent k-d sorption bentonite nickel core