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EPISODE: The Engineering Physics Department Newsletter


Fall / Winter 2005-2006
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Advances may enable on-the-spot prostate cancer treatment

GOOD HOUSEKEEPING: New method calms unruly plasmas, cleans reactors

Engineers help turn science into interactive exhibits

CAD interface boosts modeling efficiency

BIG discoveries on a small scale

Innovative recycling project could reduce U.S. inventory of spent nuclear fuel

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CAD interface boosts modeling efficiency

Assessing radiation transport through materials is easier, thanks to CAD code that enables more precise geometric modeling. Here, a 40-degree slice, in volume format, of ITER.

Assessing radiation transport through materials is easier, thanks to CAD code that enables more precise geometric modeling. Here, a 40-degree slice, in volume format, of ITER.
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Decorative initial cap For generating geometric models of such complex systems as the fusion tokamak ITER (formerly called the International Thermonuclear Experimental Reactor), a new approach enables researchers to replace combinations of elementary shapes like spheres, cylinders and bricks with more detailed and precise CAD representations.

Those old shapes are the bane of engineers using the popular Monte Carlo transport codes MCNPX or MCNP to assess radiation transport through materials. A key part of the analysis is specifying a geometric model for the physical domain—for example, a tokamak-style reactor, says Tim Tautges, an adjunct professor here and a scientist with Sandia National Laboratories.

But for a system like ITER, where even a simplified model has 930 separate volumes, using the traditional combinatorial solid geometry approach to weave spheres, cylinders and blocks into ITER’s complex shapes is not only tedious and time-consuming, it lends itself to error. “The Monte Carlo codes have their own geometric representation, but it is much less capable of representing complex models that modern CAD tools can do easily,” says Tautges.

Seeking a more efficient solution, Tautges, Professor Doug Henderson and graduate student Mengkuo Wang modified the Monte Carlo code so that it interfaces directly with a CAD modeling engine and draws on an external library of CAD-created geometries. Assistant Professor Paul Wilson, Research Professor Mohamed Sawan and several other students and scientists also are helping to benchmark this code on a fully detailed ITER model.

While other attempts at integrating Monte Carlo and CAD engine code have resulted in drastically longer computation run times, the UW-Madison group added an extra twist that removed a time-consuming step. Most CAD systems view two coincident, or adjoining, surfaces as separate surfaces, so the ray-tracing function, which determines when a particle crosses material boundaries, must be called twice—once for each adjacent material.

“Part of the reason our approach is faster than past efforts is that when two volumes have coincident surfaces, we merged them together so that they’re single surfaces that both volumes see,” says Tautges.

The group also borrowed a technique from robotics collision detection and modified it to work on ray tracing, reducing ray tracing times even more.

As a result of their success, the researchers drew the attention of the international ITER engineers and, with funding from the U.S. Department of Energy, are continuing to improve the code, not only to apply directly to ITER, but also to share with other ITER partners. In December, these partners met in Rome to share results of the full ITER neutronics benchmark calculation.


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Date last modified: Friday, 23-Dec-2005 11:49:00 CDT
Date created: 22-Dec-2005