Radionuclide sorption at Yucca Mountain, Nevada - A demonstration of an alternative approach for performance assessment

An approach is developed for including aspects of mechanistic models of radionuclide sorption into performance assessment (PA) calculations. Water chemistry data from the vicinity of Yucca Mountain (YM), Nevada are screened and used to calculate the ranges in key parameters that could exert control on radionuclide sorption behavior. Using a diffuse-layer surface complexation model, sorption parameters for Np(V) and U(VI) are calculated based on the chemistry of each water sample. Model results suggest that lognormal probability distribution functions (PDFs) of sorption parameters are appropriate for most of the samples; the total calculated range is almost five orders of magnitude fur Np(V) sorption and nine orders of magnitude for U(VI) sorption, but most samples fall in a narrower range. Finally, statistical correlation between the calculated Np(V) and U(VI) sorption parameters can be included as input into PA sampling routines, so that the value selected for one radionuclide sorption parameter is conditioned by its statistical relationship to the others. The approaches outlined here can be adapted readily to current PA efforts, using site-specific information to provide geochemical constraints on PDFs for radionuclide transport parameters.