Concept and applicability of sorption distribution coefficient in the radionuclide transport model

In the safety assessment of the underground disposal of radioactive wastes, present models describing nuclide migration estimate the retardation effect of solid materials mainly by using sorption distribution coefficient Kd. However, since the sorption mechanisms are not well understood, some requirements should be met in using Kd in the transport models. The problem or difficulty arises from the fact that sorption is the heterogeneous reaction occurring at or near the solid-water interface and that geological media for radionuclide transport are heterogeneous in their physical and chemical properties. Based on the understanding that sorption occurs at or near the solid-water interface, this paper discusses the effectiveness of Kd in describing sorption phenomena and the validity and applicability of Kd to predict in situ radionuclide migration. (author)