Fusion: Stepping closer to reality

When two physicists gather at a restaurant with steak on the menu and fusion on the agenda, you're likely to find scribbles. Or so it must have seemed to the server who cleared Robert Goldston's table recently.

A colleague had missed a talk Dr. Goldston had given on new developments in fusion-energy research. So the two repaired to a local eatery for a recap. By the time the check arrived, "the napkins and half the table cloth were covered with equations," recalls Goldston, director of Princeton's Plasma Physics Laboratory.

Fusion, in other words, is generating renewed excitement among scientists in the field.

Given the challenges facing today's nuclear reactors, they have long dreamed of harnessing the same energy source that powers the sun. In theory, they could generate power more efficiently, more safely, and with less nuclear waste than today's reactors, and use a virtually limitless source of fuel - hydrogen. Fusion reactors represent a kind of holy grail for an energy-dependent world.

Now, researchers are poised to take the next big step in evaluating the technology's commercial potential. Scientists say they are more confident than ever that they can successfully build and operate a planned experimental fusion reactor. The bigger hurdle now looks political. The six-nation project - called the International Thermonuclear Experimental Reactor, or ITER - is caught in a big-money squabble over where to put the $5 billion reactor. Japan and France both want the privilege.

Scientists, meanwhile, are chafing to loose the bulldozers.

"There have been dramatic advancements in our scientific understanding" over the past five to 10 years, Goldston notes. The basic conclusion: The "fire" in the type of reactor planned for ITER may not be as finicky to control as many had previously believed.

Initial simulations had suggested that triggering and sustaining the fusion reactions might be too difficult. But "we've made enormous steps forward," says Anne Davies, director of the US Energy Department's Office of Fusion Energy Science. An International Atomic Energy Agency meeting last month in Portugal generated considerable excitement because experiments with test reactors around the world suggested ITER's reactor would work as designed.

The idea behind fusion is fairly straightforward. Today's nuclear reactors derive their energy by splitting atoms in a process called fission. Fusion works by combining them - actually the nuclei of two forms of hydrogen known as deuterium and tritium. Fusing nuclei requires more energy than splitting them, but the payoff is larger. A fusion reaction gives off three to four times as much energy as a fission reaction does.

The challenge: For fusion to occur, the surroundings must be torrid. Researchers anticipate their experimental reactor will run at 100 million degrees C - roughly six times as hot as the sun's core. At these temperatures, atoms and their electrons part company and form a roiling particle soup called a plasma. Such temperatures also give the nuclei of the atoms enough speed to fuse with other nuclei when they hit them. But because the plasma is filled with electrically charged particles, many researchers hold that the only way to keep the plasma bottled up is with magnetic fields.

Enter ITER, which would represent a major step toward a commercial fusion reactor. Researchers have designed it to generate at least five times the amount of power it consumes in sustaining fusion reactions. It would use a reactor roughly shaped like a hollow doughnut, surrounded by magnets. The plasma forms and the reactions occur within the doughnut. The magnetic fields are designed to keep the plasma from hitting the reactor walls. If it did, it would cool sufficiently to snuff the reactions. "No one would get hurt, but if you were trying to sell electricity, you wouldn't be very happy," Goldston quips.

For years, researchers worried that at the energy levels ITER was aiming for, the plasma would fail to remain stable or that the magnetic fields would fail to keep the plasma bottled up.

But since the mid-'90s, technological advances have yielded fresh insights into the way such reactors can operate. They include improved test equipment, new ways to tweak the reactions from outside the reactor vessel, and more-powerful computers that model the conditions in the reactors. "Now we know what we're looking at," Goldston says.

For example, when the plasma grows turbulent, it forms eddies and the plasma cools. Researchers had a difficult time figuring out what determined the size of the eddies and how to control them. With the added computational horsepower and the new instruments, they determined the factors that controlled their size. Just as important, they found that they could apply more push to the flowing plasma than the system would generate on its own, shearing off the eddies almost before they got started.

"If you play that card right, you get these regions that are very quiet" and have distinctly higher temperatures than the regions surrounding them, Goldston says.

Another troublesome question revolved around how powerful a magnetic field the ITER reactor would need to contain the plasma.

"This is a key issue," he says. "Magnetic fields cost money and plasma makes fusion. If you can hold a lot of plasma in a little magnetic field, you can make money. If you can only hold a little bit of plasma in a big magnetic field, then you ought to find a different job."

Researchers are encouraged by the results they've gotten in this area so far.

Scientists are targeting other issues as well, the DOE's Dr. Davies says. A search is under way for materials that can line the reactor chamber without succumbing to the corrosive effects of the reactions. Scientists are also seeking new materials that will lose lethal levels of radioactivity faster than is currently the case. Today's fission reactors generate large amounts of long-lasting radioactive waste. Fusion reactors are expected to generate smaller amounts of highly radioactive waste. Scientists would like to use materials, such as silicone carbide, that don't become radioactive at all.

In addition, researchers are looking at alternative approaches to designing the reactor core itself.

"The fusion energy program has risen to a new level of scientific understanding," Davies says. "We're now measuring and controlling plasmas consistent with computer simulations. This represents an enormous step forward."