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.
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.
In a fusion reactor, the amount of radioactive matter that would exist in a volatile form is only a small fraction of that typically found in a fission reactor. A lithium fire, however, can be hot enough to melt the metal in the reactor structure. This could release any radioactive material trapped there. Consequently, the "worst case" of a release of radioactive material from a stainless-steel fusion reactor is estimated to be a sizable fraction (about three-fourths) of that estimated for a fission reactor, in terms of radioactivity released per megawatt of capacity of the reactor.
On the other hand, second-or third-generation fusion plants, using "advanced" materials and with reduced tritium inventories, could hav "worst case" release one-tenth to one-hundreth that of first-generation plants, Dr. Holdren believes.
In the case of radioactive waste, fusion plants begin with a considerable advantage over the breeder reactor, the fission reactor that "breeds" more nuclear fuel than it consumes, the scientist has calculated. Per unit of electricity generated, the first-generation fusion plant would produce only one-fiftieth of the waste produced by the breeder. And advanced fusion plants could be even more waste-free, generating one ten-thousandth the amount of radioactive byproducts as the breeder did.
Achieving fusion's full environmental advantages depends on realizing the benefits of these advanced designs. This may prove to be considerably more difficult than the present challenging problem of making fusion work in the laboratory. For, perversely, the approaches to controlling fusion that now seem closest to technical success are among the least attractive possibilities from the environmental standpoint, Dr. Holdren cautions. to forgo the earliest possible commercialization of fusion power in favor of realizing its full potential will be expensive in time and dollars, he says. But he believes it to be the proper course.
Another group that sees environmental and social problems with current fusion technologies is the utilities. "Public acceptability is of primary importance," says Clinton P. Ashworth of Pacific Gas & Electric Company. He is deeply disturbed by the direction taken by the Us Department of Energy's fusion program. In particular, he and a number of colleagues in the utility business are concerned about the concentration on a few, large-scale technologies such as that of the tokamak in the drive to show that controlled fusion is possible.
"We are likely to end up with fusion reactors that are well tested but totally unacceptable," Mr. Ashworth warns.
The large scale of current designs is the utility expert's major objection. Driven by design and cost factors, federal engineering studies have envisioned giant fusion plants with 3,000-megawatt capacity and each costing almost $5 billion. "These are far too large to be of use," Mr. Ashworth says.
There are alternative fusion concepts that have been given low priority in the federal effort that could result in smaller and more acceptable power plants , he says. Pacific Gas & electric, with the electric Power Research Institute, has chosen one such concept to support. And the nuclear industry is attempting to organize itself to influence the course of the federal program.
Meanwhile, some fusion researchers and influential congressmen are convinced that a demonstration fusion reactor could come on line as early as the 1990s. Last summer, this group attempted to pare the $350 million-a- year federal program down to mainstream projects while substantially reducing academic research on alternative concepts.
The resultant furor led to a compromise that will restore funding for alternative research. And Edwin Kintner, director of the Department of Energy's fusion program, says a number of concepts must be developed in parallel. Nevertheless, many experts are concerned that development of fusion technology could repeat the pattern of the fission reactor, where the pressures to come up with a commercial power source led to lower priorities for overall safety and social acceptability than, in retrospect, were justifiable.