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 – 02/01/07

Background -  Low-permeability geological formations are generally considered suitable environments for high-level nuclear waste repositories. Within low-permeability formations, interconnected networks of fractures provide discrete pathways for water movement and associated slow release of radionuclides to the accessible environment. Conventional methods for simulating subsurface transport of dissolved contaminants rely on an effective medium assumption and are not well suited for a scenario with multiple discrete pathways. This project developed an efficient and robust alternative simulation approach that makes direct use of deterministic or stochastic pathway information extracted from discrete fracture network (DFN) simulations to simulate at the repository geosphere scale (approximately 1 to 10 km).

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. Decay and in-growth of decay-product radionuclides are modeled as random events. An adaptive kernel method is used to reconstruct mass discharge from the particle histories. The transport pathways may be extracted directly from DFN simulations or based on regional-scale flow models that use upscaled effective flow properties. When using upscaled flow models, stochastic simulation based on the results of mesoscale (approximately 100 m) DFN simulation may be used to generate subgrid velocity variations along the pathways.

Accomplishments - Tests of the algorithm demonstrated that it is accurate, robust, and computationally efficient. Analyses illuminating the results of the project led directly to a new externally funded project. A new computer code (TDRW) implementing the particle algorithm was developed, verified, submitted for copyright registration, and licensed to two foreign repository programs and one federal regulatory agency. Results of the project were presented at three international conferences. Two journal articles were submitted and accepted for publication.

This project developed novel, robust, and efficient algorithms for simulating radionuclide transport along discrete transport pathways through fractured rock formations. Transport along such pathways is an important process controlling the performance of high-level radioactive waste repositories and is difficult to simulate with conventional methods. Shown are transport pathways extracted from a discrete fracture network simulation of the geosphere region surrounding a potential repository. The trajectories are color coded by the cumulative radionuclide discharge at the pathway outlet as calculated with the new particle-based algorithm. Warmer colors represent larger discharge (logarithmic scale). Note that a small number of pathways are responsible for most of the radionuclide release to the environment.

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