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Underground Movement

TDRW is efficient enough to evaluate extremely detailed pathways. Shown are pathways generated by a discrete fracture network simulator. The spatial scale is a cube 100 meters on each side. Each pathway has been color-coded by radionuclide mass discharge as calculated by TDRW. Warm colors represent larger discharges.

SwRI-developed subsurface contaminant transport software received a 2008 R&D 100 Award

A software product that simulates the transport of radioactive elements beneath the Earth’s surface has been chosen to receive a 2008 R&D 100 Award. R&D Magazine presents the awards, dubbed the “Oscars of Invention” by the Chicago Tribune, in recognition of the year’s 100 most significant technical accomplishments. 

The copyrighted software product, called TDRW (Time Domain Random Walk) Version 2, was developed by scientists in the Geosciences and Engineering Division of Southwest Research Institute (SwRI) as part of the Institute’s internally funded research program. 

As more nations develop geological repositories for radioactive waste, predicting radionuclide transport underground has become increasingly important in assessing the safety of these potential repositories. Current modeling relies on highly simplified representations of the geological medium. These simplifications have been necessary because conventional simulation tools for radionuclide transport are easily overwhelmed by the fine detail needed to represent complex geologies. 

TDRW simulates subsurface transport with a level of robustness and computational efficiency that far exceeds what is available with other approaches. This makes it possible to represent geological complexity in much greater detail, resulting in a better understanding of risks and reducing the need to be overly conservative when estimating risks. For example, discrete fracture network simulations that statistically represent tens of thousands of individual fractures are easily accommodated in TDRW and its derivative software products. 

TDRW uses a novel particle tracking algorithm in which individual particles represent packets of radionuclide mass. These particles are moved along one-dimensional pathways according to rules that closely mimic the underlying physical and chemical transport processes. The TDRW particles use fixed spatial displacements and random residence times for each step. In this conceptualization, a particle crosses a constant-property segment of a pathway in a single step based on a sample from residence time distributions that are developed from theoretical analyses. This semi-analytical nature of the algorithm is one reason for the improved robustness and computational efficiency. 

TDRW accommodates an arbitrary number of radionuclides linked through multiple decay chains. Decay of one radionuclide species with resulting in-growth of the daughter species is represented as random events with appropriate adjustments to the residence time distributions. The basic simulation algorithm produces particle arrival times at the pathway outlet. These arrival times are then converted to mass discharge using an adaptive kernel method. 

Research Scientist Dr. James W. Mancillas and Staff Scientist Dr. Scott L. Painter, of the Center for Nuclear Waste Regulatory Analyses within SwRI’s Geosciences and Engineering Division, developed the TDRW Version 2 software. They will be recognized along with other R&D 100 Award winners at a ceremony in Chicago in October.

The illustration shows an example calculation of radionuclide transport from a potential repository in fractured granite. A large number of individual pathways for radionuclide transport from the potential repository to the accessible environment are represented. These pathways were generated from discrete fracture network simulations where each pathway had a high degree of spatial variability in its transport properties. A small number of pathways are responsible for most of the transport, which demonstrates why detailed representations of potential pathways are needed. 

The Monte Carlo nature of the algorithm is another reason for the improved computational performance. In many applications the effect of interest is the aggregate radionuclide discharge from a very large number of transport pathways. TDRW samples randomly from the pathway set before launching a particle. This sampling procedure makes it possible to obtain accurate estimates of aggregate discharge without accurately resolving transport on each individual pathway. In conventional, finite-difference approaches, discharge must be calculated accurately for each pathway and then aggregated. This requires vastly more expensive computation. 

The TDRW software is available as a self-contained application, or as a collection of library routines that may be linked to an existing flow or transport simulation. The TDRW software library allows organizations to integrate the TDRW functionality into their existing computational workflow. TDRW has been licensed in this mode to organizations that are developing geological repositories for radioactive waste in Sweden and Finland. The Geosciences and Engineering Division has an ongoing project to further customize the new simulation tools and to assist the Swedish and Finnish programs in their repository safety assessments. TDRW has also been licensed to the U.S. Nuclear Regulatory Commission. 

TDRW has other applications beyond the nuclear energy industry. It can be used to assess long-term transport of some types of nonradioactive contaminants that are present in dilute concentrations within aquifers. An important example is hydrocarbon contaminants, which undergo decay and transformation in the subsurface and are often modeled using decay chains when the concentrations are small. The software may be used to assess the need for remediation of sites contaminated by hydrocarbon-based organic solvents. 

In all, SwRI scientists and engineers have won 33 R&D 100 awards, dating back to 1971.

Published in the Summer 2008 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

Summer 2008 Technology Today
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