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EPISODE: The Engineering Physics Department Newsletter


Fall / Winter 2005-2006
Featured articles

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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Susan L. Reinhold receives Distinguished Achievement Award


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A cutaway view of the magnets and internal components inside the ITER vacuum vessel. Whyte’s method for soft-landing plasmas may alleviate costly damage to interior reactor walls and could “clean” residual tritium without disrupting operation.

A cutaway view of the magnets and internal components inside the ITER vacuum vessel. Whyte’s method for soft-landing plasmas may alleviate costly damage to interior reactor walls and could “clean” residual tritium without disrupting operation.
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(© 2005

New method calms unruly plasmas, cleans reactors

Decorative initial cap Anew method developed at UW-Madison could, in a tokamak reactor, alleviate the damaging effects of a plasma gone awry—while at the same time, “clean” the buildup of radioactive tritium off the reactor’s interior walls.The method could save engineers who operate large experiments like ITER (formerly, the International Thermonuclear Experimental Reactor) both time and money.

In a tokamak, which uses a toroidal, or doughnut-shaped, magnetic field to confine the super-hot ionized gas called a plasma, sometimes the plasma builds up so much pressure that it breaks the “magnetic bottle” and crashes into the reactor wall. While it’s completely safe outside the reactor, the plasma hits with such vigor that it can damage the reactor’s internal components. When that happens, engineers must stop operations, open the reactor, and repair the materials.

Dennis Whyte

Dennis Whyte
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This plasma crash, says Assistant Professor Dennis Whyte, is typical in an experimental reactor. “You’ve pushed the parameters too far, some instability has arisen, and it’s coming,” he says, of the impending crash. “So do you just let it happen, or do you something to help alleviate it?”

In Whyte’s case, you turn the culprit—the plasma itself—into the solution. Anticipating the crash, he shoots a neutral, radiation-emitting gas like neon at the plasma. Within a few thousandths of a second, the number of particles inside the reactor multiply on the order of a factor of 100. “It completely overwhelms the plasma and essentially turns the plasma into an enormous source of light,” says Whyte. “The energy of the plasma gets converted into light energy.”

Rather than colliding into one area of the reactor’s interior wall, the newly created light energy coats the whole wall uniformly. “So what you do is you spread the pain, and if you do it efficiently enough, you stop any damage to the materials that are inside,” says Whyte.

Working first at the DIII-D National Fusion Facility at General Atomics in San Diego, and more recently at the Alcator C-MOD tokamak at the Massachusetts Institute of Technology, Whyte and his colleagues showed that after this “soft” termination of the experiment, researchers could resume additional experiments immediately. Next year, they will further their solution in Oxford, England, when they have research time scheduled on JET, the world’s largest tokamak. Particularly important, he says, is that the group is learning how to design such a system for ITER.

ITER has about 1,000 square meters of interior material surface, but in a natural disruption—something like a balloon bursting—the plasma slams into an area of only 10 or 20 square meters. “We realized for a long time that disruptions are going to be a real concern for a device like ITER,” says Whyte.

Another issue in large experimental reactors like ITER, he says, is tritium retention. A man-made radioactive hydrogen isotope with only a 12 1/2-year half life, tritium is one of two fusion reactor fuels. As a reactor operates, the unused fuel must be efficiently recirculated through the system, but the tritium can become trapped in the reactor walls. “So what happens is you have this repository of tritium inside of the vacuum vessel,” says Whyte.

As a result, occasionally engineers need to stop operations, open the reactor, and clean the tritium out—a process that eats up valuable experimental time. “It turns out, though, that there’s a very well-known way to get tritium out of materials,” he says. “You heat the materials.”

Plasmas create very thin film layers on the reactor’s interior materials, and the residual tritium resides within those layers. So by exploiting the soft termination of a plasma, which uniformly distributes energy inside the reactor, Whyte hit on a tritium-removal process that’s much like a self-cleaning oven. “The plasma’s energy forces the hydrogen molecules inside of the film layers to recombine and forces them out by diffusion,” he says. “It basically cleans out all of the hydrogen, including the tritium.”

Whyte proposed the cleaning method a year ago at the Plasma Surface Interactions in Controlled Fusion Devices meeting in Portland, Maine. Since then, he and his MIT colleagues conducted experiments on the MIT reactor and showed the method cleans tritium better than other techniques they tried. “We used the disruptions in a controlled way to force the fuel out of the wall, and then we did a very careful particle accounting,” he says. “We showed that, in fact, what we were doing was forcing more hydrogen out of the wall than we were leaving behind.”

They also showed that you don’t have to wait for a plasma disruption to come along. Rather, reactor operators could use the technique as they’re ending their experiments. “The plasma’s shutting down anyway—you just shut it down a little bit earlier than you normally would, and it’s very soft, and you get all the tritium that you just lost in the last discharge,” says Whyte. “It takes no external intervention.”


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