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Date: Around 1993

What are the limits to the size of a fusion reactor? I have read Sci-Fi stories that include the hero carrying a portable generator and heard rumors about a (discontinued) experimental fusion aircraft. I would guess that you must have a certain mass to keep the reaction going, but what is the minimum?

The basic process of a fusion reactor has no real minimum mass. But the complex equipment required to keep it going and make itefficient is enormous. (With present technology) The basic process just needs a few atoms of tritium or deuterium. It is the equipment that brings the atoms together at high velocity or heats them to high temperature that is complicated and bulky. This is quite different from a fission reaction, where a minimum mass, called the critical mass, is needed. I think the experimental nuclear airplane used a fission reactor. A typical critical mass for a fission reaction would be several kilograms. This would be very portable, if it was not for the shielding needed to keep from frying everybody in the vicinity. Plus, you need cooling and heat exchangers to use the heat that is produced. Both fusion and fission reactors put out heat as their primary method of energy production.


Actually, so far no fusion experiment has produced any more power than went into heating the stuff up in the first place. That means that not only do we not have portable generators, we do not have ANY fusion reactors that actually produce power. There is actually a proverbial "20 year" effect involved - reactors planned for 20 years in the future should finally be producing power, maybe even commercially. Unfortunately, it hss sort of been that way for the past 30 years or so.

A. Smith

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