Approach to Assessing the Potential Effects of Colloidal Radionuclide Transport on Nuclear Waste Repository Performance

The U.S. Nuclear Regulatory Commission (NRC) is independently evaluating technical issues such as colloid-facilitated radionuclide transport in preparation for reviewing an anticipated license application from the U.S. Department of Energy (DOE) for a potential high-level nuclear waste repository at Yucca Mountain, Nevada. For performance assessment computer simulations of evolving conditions many years into the future, the influence of colloids in enhancing radionuclide transport is difficult to estimate and highly uncertain. NRC staff is conducting a multipronged approach to assessing whether or not these uncertainties are sufficiently represented by performance assessment models. Preliminary simplified calculations providing a conservative estimate of calculated dose from colloidal Pu suggest that an effect on dose is plausible. A more sophisticated effort involves analytical modeling of colloidal Pu transport that uses laboratory and field data to represent more accurately processes such as kinetic controls on sorption (attachment) and desorption (detachment) of radionuclides at colloid surfaces. This modeling effort shows that slow desorption of radionuclides from colloids is a factor that could enhance radionuclide migration. Finally, an abstraction of colloidal transport is being implemented in the NRC total-system performance assessment model in order to integrate potential colloidal effects at the system level. This implementation is flexible enough that a variety of sensitivity studies can be conducted that will aid identification of the model parameters most significant to transport.