Starting in the 1950’s a new candidate emerged as a candidate for cheap, clean, and limitless power: fusion. Early tests using fusion to augment the power of nuclear weapons demonstrated the fusion could do useful work (useful being used loosely here) and indicated that chain reactions were possible outside of stars. Since that time a large number of people have tried a very great number of methods to create a fusion device that generates power in excess of its own consumption. If one of these devices worked it would be great. Fusion uses primarily hydrogen isotopes, commonly available or easily produced. The byproducts are isotopes of helium, inert and non-radioactive. There is a lot of hydrogen in the universe.
Fusion has only one major problem, it doesn’t work. Well, it does, sort of. Nuclear weapons use fusion and the sun fuses tons of hydrogen every second. I suspect that there is a reason for this. Nuclear fission creates neutrons as a byproduct. Fission requires a steady supply of neutrons to continue. Thus one fission reaction creates, or is likely to create, another, the infamous chain reaction. Fusion also creates neutrons, but neutrons are not required to create fusion. There is no chain reaction. Why does the sun then work so well? Fusion in the sun is initiated by the immense heat and pressure at the core, a condition that is largely due to the mass of the sun. Existing fusion creates heat, which eventually radiates into space as sunlight. The pressure and temperature at the center of the sun was initially caused by gravitational attraction shrinking the cloud of gas out of which the entire solar system formed. Fusion has merely paused the eventual shrinking of the sun for ten billion years by providing a source of heat. Nuclear weapons, specifically thermonuclear weapons (or the H-bomb) work by using the tremendous heat and pressure caused by the explosion of a fission bomb to cause a small amount of hydrogen to fuse, creating neutrons which then cause additional uranium to fuse. The whole process is over almost immediately, in a fraction of a second, and the apparatus that created the fusion is destroyed in the process. Again, the process is not a chain reaction like fission, but a reaction based on external events. (This style of weapon is used because it allows larger weapons to be built and, per megaton, requires less plutonium or enriched uranium.)
Fission is so simple almost anyone can achieve it. Stack enough of the right stuff together and it works. Almost any pile of uranium and graphite will achieve some sort of fission chain reaction. Fusion is also pretty well understood and easy to do, the problem is that a chain reaction is not created, so the conditions necessary for fusion must be maintained by external means. A usable fission power reactor needs a pile of uranium and a moderator. This pile will get really hot, so run a coolant through or around it. Use the hot coolant to boil water and turn a turbine. Presto, useful work is done.
A usable fusion reactor is much more complicated. Create the conditions for fusion to occur using lasers, magnets, particle accelerators or some other method. Capture the energy generated by the fusion (primarily neutrons and gamma rays) and use it to power your magnets, lasers, etc. Be efficient enough to have some energy left over to use for something useful. This seems reasonable until you realize that no process is ever 100% efficient. Recovering energy from heat is typically about 30% efficient due to thermodynamic laws. Most proposed reactors would use this method of recovering energy. Other methods are even less efficient, solar panels are about 10-15% efficient, for example. Then add the inefficiencies of the magnets, lasers, whatever. The fusion reaction needs to be producing a lot of power. And this is before you factor any energy needed for safety systems.
In short, fusion doesn’t, and probably won’t for the foreseeable future, work.
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