Inverse Modeling of Experiments to Support More Realistic Simulations of Sorbing Radionuclide Transport

A series of adsorption, desorption, and column transport experiments were conducted to evaluate the transport of uranium (U) and neptunium (Np) through saturated volcanic tuffs. For potential high-level radioactive waste sites, these experiments demonstrate that slow radionuclide desorption processes, which are typically not accounted for in transport models implementing simple partition coefficients (Kd values), may dominate field-scale transport. A complimentary interpretive numerical model couples a simplified geochemical description of the system with transport calculations where heterogeneities are represented as an ensemble of sorption sites with characteristic adsorption and desorption rate constants that have widely varying values. Adsorption and desorption rate constants were estimated through inverse modeling such that reliable upscaled predictions of reactive transport in field settings could be simulated. The inverse modeling software, PEST, was also used to perform advanced uncertainty quantification. The multicomponent model/parameters matching the combined data sets suggest that over much longer time and distance scales the transport of U and Np under the experimental conditions would result in very little transport over field scales because even a small number of strong sorption sites will have an exaggerated retarding influence on the transport of a radionuclide plume. Modeling of combined sorption/desorption experiments and column transport experiments that involve both the measurement of column effluent breakthrough curves and the distribution of radionuclides remaining in the column at the conclusion of the experiments holds significant promise for supporting an improved approach to properly account for mineralogical heterogeneity over long time and distance scales in reactive radionuclide transport models. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.