[en] Compacted bentonite has been proposed as a suitable backfill material in the conceptual model for underground storage of high-level radioactive waste. The backfill barrier will serve not only as a physical barrier preventing convective water flow and allowing radionuclide penetration only by diffusion but also as a chemical barrier capable of chemically interacting with the radionuclides and retarding the radionuclide migration. Until now a linear isotherm model such as a distribution coefficient concept has been applied to analyze the sorption phenomena of the radionuclide ions in bentonite backfill material. However, this model does not explicitly incorporate the physicochemical characteristics of the system. A concentration-dependent nonlinear isotherm such as a two components ion-exchange isotherm which is similar to the Langmuir isotherm form is included in this study. (author)