USE OF THERMODYNAMIC SORPTION MODELS IN REACTIVE TRANSPORT MODELING: ADVANTAGES FOR U.S. NUCLEAR REGULATORY COMMISSION LICENSEESAND STAFF AND AREAS OF RESEARCH

Staff of the U.S. Nuclear Regulatory Commission (NRC) use environmental models to evaluate the potential release of radionuclides into the accessible environment from sites containing nuclear materials. In most models used by the NRC, reactions between radionuclides and subsurface solids are modeled with a distribution coefficient, or Kd, that describes a constant proportion between the concentration of a radionuclide in the groundwater and the concentration of the radionuclide associated with the subsurface solids. Because the chemical reactions that cause the attachment of radionuclides to solids can be sensitive to water chemistry and subsurface mineralogy, generic literature ranges of Kd often span two or more orders of magnitude. Uncertainty in Kd values has been identified as a major source of uncertainty in dose analyses. Recently developments have been made in the application of Thermodynamic Sorption Models (TSMs) to natural subsurface media. Because TSMs represent sorption by modeling the chemical reactions responsible for attachment of radionuclides to natural surfaces, they may provide a more realistic method of modeling reactive transport of radionuclides. In addition, they also may provide an economically efficient method for characterizing sorption variability at complex sites. The objective of this paper is to describe the opportunities that may be afforded to NRC staff and licensees by the recent advances in thermodynamic sorption modeling as well as projects that are being pursued or considered by the NRC to develop the use of TSMs.