This paper examines some previously reported experimental results on the migration behavior of selected radionuclides in compacted bentonite and demonstrates that a much better agreement between observed and theoretically predicted migration behavior can be obtained when the model considers nonlinear concentration effects. The radionuclide transport model used in this study employs a semianalytic, integral equation approach to the solution of the diffusion equation with nonlinear retardation mechanisms. The effect of radionuclide concentration on species mobility is modeled as a highly nonlinear Freundlich isotherm.
Several empirical correlations using the Freundlich isotherm form for retardation are presented and are shown to account for the deviation from linearity in the diffusion model for migration behavior. These correlations are then used in further numerical studies of long term backfill barrier performance for 3 and 4 member chain species after a hypothetical canister failure. The overall importance of nonlinear concentration effects to radionuclide migration estimation is discussed.