Incorporation of conceptual and parametric uncertainty into radionuclide flux estimates from a fractured granite rock mass

Abstract

Detailed numerical flow and radionuclide simulations are used to predict the flux of radionuclides from three underground nuclear tests located in the Climax granite stock on the Nevada Test Site. The numerical modeling approach consists of both a regional-scale and local-scale flow model. The regional-scale model incorporates conceptual model uncertainty through the inclusion of five models of hydrostratigraphy and five models describing recharge processes for a total of 25 hydrostratigraphic-recharge combinations. Uncertainty from each of the 25 models is propagated to the local-scale model through constant head boundary conditions that transfer hydraulic gradients and flow patterns from each of the model alternatives in the vicinity of the Climax stock, a fluid flux calibration target, and model weights that describe the plausibility of each conceptual model.The local-scale model utilizes an upscaled discrete fracture network methodology where fluid flow and radionuclides are restricted to an interconnected network of fracture zones mapped onto a continuum grid. Standard Monte Carlo techniques are used to generate 200 random fracture zone networks for each of the 25 conceptual models for a total of 5,000 local-scale flow and transport realizations. Parameters of the fracture zone networks are based on statistical analysis of site-specific fracture data, with the exclusion of fracture density, which was calibrated to match the amount of fluid flux simulated through the Climax stock by the regional-scale models. Radionuclide transport is simulated according to a random walk particle method that tracks particle trajectories through the fracture continuum flow fields according to advection, dispersion and diffusional mass exchange between fractures and matrix. The breakthrough of a conservative radionuclide with a long half-life is used to evaluate the influence of conceptual and parametric uncertainty on radionuclide mass flux estimates. The fluid flux calibration target was found to correlate with fracture density, and particle breakthroughs were generally found to increase with increases in fracture density. Boundary conditions extrapolated from the regional-scale model exerted a secondary influence on radionuclide breakthrough for models with equal fracture density. The incorporation of weights into radionuclide flux estimates resulted in both noise about the original (unweighted) mass flux curves and decreases in the variance and expected value of radionuclide mass flux.

Upscaling of a dual-permeability Monte Carlo simulation model for contaminant transport in fractured networks by genetic algorithm parameter identification

“…position, orientation, length and conductivity, from the corresponding distributions; these parameters are shown to be sufficient to determine a fracture network whose geometry is geometrically similar to that observed [10] [11]; ii) each fracture network is mapped into continuum grids with constant cell size [5] In what follows, the details of the upscaling procedure are provided with reference to the migration of a single species of non-reactive contaminant, for simplicity.…”

“…The modeling of solute transport through networks of fractures is of extreme importance for the risk assessment of radioactive waste repositories [1][2] [3][4] [5].…”

“…A detailed characterization and mapping of the fractured fields is usually not feasible due to the high degree of complexity and heterogeneity of the network, and information on geometric and conducting properties can be obtained only in terms of probability distributions and correlation functions [6] [7][10][11] [5].…”

“…These methods do not rely on volume averaging techniques, but solve the flow and transport problem in individual fractures by accounting for both anisotropy and stochasticity [13] [14]. Nevertheless, an important drawback of these methods is related to the high degree of complexity of the modeling which requires an intensive computational effort and thus limits their range of applicability to near-field scales [6] [7][14] [5], and actually prevents the use in the probabilistic uncertainty and sensitivity analyses required in the PA of radioactive waste repositories.…”

“…Different realizations of fracture fields are generated on small sub-domains by random sampling from the assigned distributions and by mapping into continuum grids with constant cell size, according to a recently suggested methodology [5] [22]. Then, the MODFLOW and MT3DMS solvers [23] [24] are employed to get detailed flow and transport solutions over these sub-domains [5].…”