UNCERTAINTY ANALYSIS OF RADIONUCLIDE TRANSPORT IN THE UNSATURATED FRACTURED ROCK AT YUCCA MOUNTAIN, NEVADA

This study used Monte Carlo simulations to assessed parametric uncertainty of radionuclide transport in the unsaturated fractured rock at Yucca Mountain, the DOE proposed high-level nuclear waste repository. A three-dimensional, mountain-scale conceptual model was developed and implemented using the numerical code TOUGH2 to simulate unsaturated flow and radionuclide transport. Matrix porosity, saturated hydraulic conductivity, van Genuchten water retention parameters (α and n), and sorption coefficient are treated as statistically homogeneous random variables. Distributions of the random parameters were determined based on site measurements. For each random parameter in each model layer, the Latin Hypercube Sampling (LHS) was used to generate parameter realizations. The correlations between matrix porosity and permeability and between the retention parameters α and n were incorporated in the random field generating. Monte Carlo simulations were conducted and convergence of the Monte Carlo approach was examined. Mean, variances, 5% and 95% percentiles of saturation, pressure, and fluxes were estimated to predict radionuclide transport and associated predictive uncertainty. The 5% and 95% percentiles of saturation and pressure bracketed a large portion of site measurements, indicating the success of the uncertainty analysis. Uncertainty of the reactive and conservative tracer transport in the fractured rock was represented by the mean, 5%, and 95% percentile of the breakthrough curves. This study presented a comprehensive method of assessing radionuclide transport at the unsaturated fractured rock at Yucca Mountain by rigorous random field generation and thorough Monte Carlo simulation.