Particle Method for Regional-Scale Modeling of Transport in Fractured Rock, 20-R9566

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Principal Investigators
Scott L. Painter
James W. Mancillas
Osvaldo Pensado

Inclusive Dates:  08/03/05 – Current

Background - Low-permeability geological formations are generally considered suitable environments for high-level radioactive waste repositories. Within low-permeability formations, interconnected networks of fractures provide the primary pathways for water movement and associated slow release of radionuclides to the accessible environment. Because conventional continuum models for simulating subsurface transport of dissolved contaminants are of questionable value for simulating transport in sparsely or moderately fractured rock, some high-level waste programs are relying on discrete fracture network (DFN) simulation. However, DFN simulation is computationally intensive and generally limited to relatively small scale (approximately 100 m) models. This project developed new simulation techniques that use the results from relatively small-scale DFN simulations in regional scale (1 to 10 km) models of repository geosphere transport.

Approach - A multiscale algorithm that uses particles to represent packets of radionuclide mass was developed. The particles are moved along transport pathways according to rules that closely mimic the underlying physical processes of advection, dispersion, and retention processes such as matrix diffusion combined with sorption. The mean transport pathways are provided by regional-scale flow models. Stochastic simulation based on the results of mesoscale (approximately 100 m) DFN simulation is used to generate subgrid velocity variations along the pathways. Decay and in-growth of radionuclides are modeled as random events.

Accomplishments - Tests of the algorithm demonstrated that it is accurate, robust, and computationally efficient. Initial results of the project were presented to two foreign high-level radioactive waste repository programs, resulting in a new jointly funded project. A new computer code (TDRW) implementing the particle algorithm was developed, successfully validated, submitted for copyright registration, and licensed to two foreign repository programs.

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