Physics Watch: Fusion's Future?

Will tomorrow's power plants run on a few ounces of hydrogen and boron instead of several hundred tons of coal? Physicist Hendrik Monkhorst is betting on it.

By Jeffrey Winters|Friday, May 01, 1998
Fusion enthusiasts anticipate a day in the next century when reactors, by slamming together hydrogen atoms, will generate virtually limitless amounts of energy without soot-belching smokestacks or radioactive waste. The good news on the fusion front is that last fall an experimental reactor in England produced a record 16 million watts. The bad news is that over 24 million watts went into the reactor. Physicist Hendrik Monkhorst of the University of Florida in Gainesville is pessimistic about the prospects for current research. Instead he is excited by a novel fusion scheme, one that he believes will be a clean, cheap source of power.

The record-setting English reactor, the Joint European Torus in Abingdon, like many experimental fusion reactors, uses powerful magnetic fields to confine clouds, or plasmas, of charged hydrogen in a large doughnut-shaped chamber. The fields strip electrons away from the hydrogen and smash the hydrogen atoms together. Each time two atoms fuse, they release energy. The trick is to get enough atoms to fuse to make the whole process self-sustaining.

But even if someone does pull off that feat, Monkhorst says, such reactors will not deliver the clean energy their advocates promise. The main problem lies with the hydrogen-isotope fuel the reactors use. When an atom of deuterium, a form of hydrogen with a neutron and a proton in its nucleus, collides with an atom of tritium, a hydrogen with two neutrons and a proton, the two atoms fuse, creating helium, which has two protons and two neutrons. This leaves one extra neutron, which hits the reactor’s wall, transmuting metals in the wall into radioactive isotopes. The massive shielding needed for a full-scale reactor means that fusion plants would be colossal and far from the cities that need their power.

Monkhorst has a design that, on paper at least, promises to produce far less radiation than existing reactors. His scheme is based on a well-known process. The reaction of a proton and a boron atom is one of the oldest known fusion reactions, going back to the late 1930s in England, but was studied only for its astrophysical importance, says Monkhorst. When a proton fuses with a boron atom, which has five protons and six neutrons, the new nucleus splits into three helium nuclei. With no leftover protons or neutrons, the amount of radiation is greatly reduced.

Experiments show this reaction works, but the range of speeds at which colliding boron atoms and protons fuse is exceedingly narrow. In a standard fusion reactor, where the speeds and directions of atoms are random, the probability of the right atoms meeting at the right speed is next to nil. So Monkhorst stacks the deck: rather than trying to confine the boron atoms and protons in a plasma, a different type of reactor could borrow the methods of particle physics and accelerate protons to the proper speed, then slip in some slow boron atoms for the protons to hit. One in approximately 10 million close collisions will lead to fusion, Monkhorst says. That sounds terrible, but the particles zip around the reaction chamber 100 million times a second, so the reaction takes place in just a few seconds.

The high-energy helium atoms created by boron-proton fusion would be decelerated by magnets and coils that convert the particles’ energy into electricity. Although boron-proton fusion releases only about half the energy of the deuterium-tritium process, Monkhorst thinks the reduced radiation and smaller reactor size (he claims the reactor could fit in the basement of a large office building) give his proposal an edge. There’s no radioactive fuel going in, and there’s no radioactive waste coming out, he says. During one day a power plant of this sort would use only 200 grams of boron rather than 700 tons of coal.

Some physicists question whether this scheme will work. Alan Gibson, the deputy director of the experimental fusion reactor in England, says protons scattering from near misses with boron atoms would give rise to a jumbled, hot plasma that wouldn’t produce much boron-proton fusion. Even using the authors’ own arguments, he says, things look very difficult.

Monkhorst insists his group’s work is based on decades of experimental and theoretical evidence. Someone just needs to build a test reactor, something he hopes to do once he drums up funding.
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