Radionuclide migration laboratory studies for validation of batch-sorption data

801To validate batch-sorption transport models of sorbing and non-sorbing radionuclides, the authors conducted advective and diffusive migration experiments in the laboratory using columns of saturated crushed, intact, and fractured tuff from Yucca Mountain, Nevada, and pure mineral separates. For crushed tuff and pure minerals separates, batch-sorption coefficients agree with those they obtained under flowing conditions for alkaline and alkaline earth elements; for actinides, they found a smaller sorption coefficient caused by elution of a small fraction of the actinides. Studies of solid-tuff columns indicate that batch-sorption coefficients will fit elution data of alkaline and alkaline earth elements provided that they have a detailed characterization of the hydrologic properties of the tuff. Studies of fractured-tuff columns indicate good agreement between fracture transport models and elution of nonsorbing radionuclides. Diffusion is one of the most important retardation mechanisms in fractured media; their studies indicate that the diffusion coefficient for nonsorbing radionuclides into saturated Yucca Mountain tuff is approximately 10-6 cm2/s. Coefficients of diffusion of the sorbing radionuclides into saturated tuff could not be fit, assuming reversible, instantaneous, and linear sorption. The authors believe that conservative transport modelling calculations will result from fitting the batch-sorption coefficient and the diffusion coefficient for tritiated water. Their validation strategy, which begins with laboratory transport experiments on columns of crushed Yucca Mountain tuff to study the effects of kinetics of speciation, mass transfer kinetics, or non-linear, non-reversible, or non-instantaneous sorption, is presented. When the differences between batch-sorption data and the results of their crushed-tuff column experiments are resolved, the next level of complexity will be tackled. 20 refs., 10 figs., 2 tabs