This is the last of these expository posts in the series. Next week, I'll put up my final evaluation of the various energy sources that I've considered. However, this week, I wanted to take a look at a possible energy source that has been talked about, it seems to me, for years, yet has never really gone anywhere. I'm referring to nuclear fusion.
Fusion reactors produce energy by fusing two light atomic nuclei into a heavier one. The idea is that bringing the two nuclei together will allow the strong nuclear force in the nuclei to pull them together into a larger atom; as this new atom has slightly less mass than the sum of the original two masses, the difference is released as energy according to good ol' E = mc2. However, if the input atoms are heavier than iron, then the output atom will be heavier than the total mass of the inputs; in this case, then, the reaction will actually consume energy rather than release it.
The trick, though, is that input atoms also have an electrostatic force -- the net positive charge of the nuclei. In order to overcome it so that the atoms can combine, energy needs to be introduced into the process. The easiest way to do this, according to Wikipedia at any rate, is to heat the atoms, usually to the point at which they become plasma. The temperature that must be reached is a function of the total charge, thus hydrogen reacts at the lowest temperature; since helium has a very low mass per nucleon (i.e., nuclear particle, a proton or neutron), it tends to be the product.
Perhaps the easiest way to harness this for electricity generation is as part of a thermal power plant, of which I have discussed several types already. The process is, again, to use heat to heat/boil water (or some other substance), then drive a turbine, which drives a generator, which produces electricity.
There are a few technical challenges facing fusion as a source of commercial electricity. The first is related to the choice of fuel. One model (D-T) takes deuterium and tritium as inputs to produce helium-4 and a neutron. Since finding tritium is quite tricky (although deuterium is not), it must be bred from lithium and a neutron. So, the cycle here is obvious: D and T produce He-4 and n; n and either Li-6 or Li-7 produce T and He-4 (and, in the case of Li-7, another n). Due to the prevalence of neutrons in these reactions, though, D-T fusion results in induced radioactivity (i.e., the absorption of neutrons by the reactor structure, creating radioactive materials). (It should be noted, though, that it might be possible to convert this radiation directly into electricity, rather than trying to transport the reactor's power by some other means.) Along the same lines, the use of tritium can be a problem, as tritium is hard to contain; thus some radiation would leak into the environment. Furthermore, lithium supplies are limited, so this form of fusion would not last forever. Finally, only about 20% of the energy output is in the form of charged particles, which basically forces the reactor to be used as part of a thermal power plant. (The relative lack of charged particles means, as I understand it, that little energy can be harvested directly from the reaction.) Another (D-D) model combines two deuterium atoms to produce, with equal probability, tritium and a proton or helium-3 and a neutron. This model, then, has a similar problem with tritium as the D-T model; and, if the tritium is burned before leaving the reactor, it will produce more neutrons, resulting in the problem of induced radioactivity again. Furthermore, the energy confinement must be significantly better, and less power is produced by the reactor. The basic advantage, though, is that the reactor doesn't require tritium breeding nor the use of lithium.
The second is related to confinement. Creating uncontrolled fusion reactions isn't that hard -- they're called hydrogen bombs. Suffice to say, an H-bomb isn't really useful for commercial electricity production; there has to be some way to control the fusion reaction. One is using magnetic confinement, in the tokamak -- a transliterated Russion word, created from the (Russian words for) "toroidal chamber in magnetic coils". Frankly, the physics of them is a bit beyond me. My sense is that a magnetic field is used to rapidly heat (and maintain the heat) of the plasma in a fusion reactor. Alternatives include the Z-pinch system (again, the physics surpasses me) and laser inertial systems. Confinement has to maintain the plasma in the fusion reactor in a dense and hot enough state that it will undergo fusion and produce energy, and to keep the plasma in this state such that it can continue to undergo fusion.
The third is the choice of materials. As said, fusion reactions can induce radioactivity in the materials used in the reactor structure. Furthermore, the temperatures in a fusion reactor are extremely high. Very few materials would be able to withstand the thermal and mechanical pressures inherent in a commercial fusion power plant.
Unfortunately, at this point in research, it's by no means clear that commercial electricity generation from fusion reactors is even possible, at least in a way that would be economically viable. The promise is of fairly high power-generation without interruption, and without significant environmental effects. But whether this promise can be fulfilled is extremely unclear.