Fusion: making it practical
Like Prometheus, the legendary fire- bringer, a dedicated group of researchers is trying to harness the celestial fire -- hydrogen fusion -- that powers the Sun and stars. But while they are increasingly optimistic that contorl of this formidable process is within their grasp, they are not at all certain they can transform successful laboratory experiments into an economically attractive source of electric power.Skip to next paragraph
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These modern heirs of Prometheus are beginning to appreciate the tremendous engineering challenges that will be involved.
For the past year, the International Atomic Energy Agency has been conducting a major assessment of these engineering requirements. According to one of the participants, Weston M. Stacey Jr. of the Georgia Institute of Technology, the conclusion is that "there is an awful lot of development and engineering to be done, but it is doable."
The IAEA review concentrated on the mainstream of experimental fusion technology, which is embodied in a class of devices called the tokamak. This is a machine in which magnetic forces confine the hot hydrogen gas in what amounts to a doughnut-shaped magnetic bottle. When heated to 10,000 times the temperature of the surface of the sun, the hydrogen atoms fuse together to form helium, releasing tremendous amounts of energy.
To create this magnetic bottle without consuming more energy than is produced by the fusion reaction itself, a special type of magnet is required. This is a "superconducting" magnet, which carries electrical current with virtually no resistance when cooled to within a few tens of degrees of absolute zero (minus 273 degrees C.).
For fusion power plants, superconducting magnets must be developed that can generate magnetic fields twice the intensity of those acheivable with present technology. "Researchers at Lawrence Livermore Laboratories are working hard on this, and the early results are positive," Dr. Stacey reports.
While the outlook for superconducting coils that generate a constant magnetic field seems good, however, the same can't be said for the coils that would generate the pulsating magnetic fields needed to control instabilities by which the hot fusion medium tends to escape its confinement. "The technology for pulsed superconducting magnets is not in place, and US efforts are too small," Dr. Stacey says.
Radiation damage to the walls of the reactor vessel is also an important engineering consideration. High-energy radiation from the fusion reaction weakens metals and causes them to become radioactive. To be economical, the reactor lining must be designed to last four to five years and then be reremoved and replaced relatively easily by remote control.
Today, about the only available material that is suitable would be stainless steel, according to Dr. Stacey. Yet it suffers more from radiation damage than other, more exotic, materials.
Niobium and vanadium, for instance, withstand radiation better and also become less radioactive than stainless steel. Unfortunately, materials such as these are also rare and expensive. Yet the material used for the reactor vessel has a major bearing on environmental and social questions.
One of the main advantages of fusion over fission power is that it involves less radioactivity. Yet this is a potential advantage rather than an automatic benefit, explains John P. holdren of the University of California, Bekerley, who has analyzed the environmental implications of present design concepts for the fusion power plant.
Current plans for tokamak plants use lithium, either as a coolant or to generate tritium, the radioactive form of hydrogen that would be used as fuel. This liquid lithium would be the largest source of stored energy in such a reactor, Dr. Holdren says, and a lithium fire may well represent the "maximum hypothetical accident." The fusion specialist has made a preliminary study of possible "worst case" accidents that would release radioactive material from tokamaks of several possible designs.