Fission Reactors
Current nuclear reactors are fuelled with uranium. The energy is produced by the splitting (fission) of uranium (235U) nuclei. When a 235U nucleus splits, several neutrons are released. These neutrons can then hit other 235U nuclei, causing them to split, producing further neutrons, and so on, in a chain reaction.
Some of the neutrons are absorbed by 238U nuclei, which converts them into 239U, which decay rapidly, in two steps, to plutonium (239Pu). This is also fissionable, just as 235U is. Some of it then gets split in the chain reaction, and some remains unsplit, just as the 235U does.
In an atom bomb, the chain reaction is allowed to expand rapidly, with as many neutrons as possible going on to split more uranium (or plutonium) nuclei, until all the uranium is used up – or until the energy produced has blown the whole thing apart with some uranium (and plutonium) nuclei still unsplit. In a reactor, most of the neutrons are mopped up by neutron absorbers, leaving, on average, only one from each fission to go on and cause another fission – slightly more if you’re turning the power level up, or slightly less when you’re turning it down. These small variations are achieved by moving control rods (made of strongly neutron-absorbing material) in and out of the reactor.
When uranium (or plutonium) nuclei split, apart from the neutrons, two smaller nuclei are produced in each fission. These fragments are called fission products. They’re not always the same nuclei – there’s a huge range of possibilities, and all the possible fission products are produced in different quantities in a reactor. The exact proportions of each depend on the conditions in the reactor.
Most of these fission products are radioactive – almost all of them vastly more radioactive than the original uranium. This is where the bulk of the radioactive waste produced by the nuclear industry comes from. Initially it’s billions of times more radioactive than the original uranium, but the most radioactive atoms decay away very quickly, and after a few years it’s only millions of times more radioactive than the original uranium. Nonetheless, it’ll be a couple of thousand years before the level of radioactivity gets down to that of the original uranium.
In the long term, the unsplit plutonium is also a big problem: with a half-life of 24,000 years, it’s 200,000 times more radioactive than the uranium it came from, but from a human perspective it goes on being highly radioactive forever. In principle, it might be separated from the rest of the waste, and converted into (relatively) short-lived fission products by being used as fuel in a reactor, but the separation process (reprocessing the used fuel) is extrememly difficult and hazardous and most used fuel is not reprocessed. (There are also other isotopes of plutonium and other transuranic elements produced in fission reactors by neutron absorption, but 239Pu is the main one.)
In addition to the fission products and plutonium, some radioactive waste is produced by activation of other parts of the reactor, such as structural components, the cooling system, control rod mechanisms etc. Activation occurs when atoms of these other parts absorb neutrons, changing them into different kinds of atoms, some of which are radioactive. In a fission reactor, activation products are heavily outweighed in significance by fission products and plutonium.
Radioactive waste is nuclear power’s Achilles heel – or one of them, much the biggest.
Nuclear fusion
An alternative kind of nuclear reactor would be a fusion reactor, in which small nuclei get stuck together (fused) into bigger nuclei, rather than big nuclei being smashed into smaller ones as in a fission reactor. These aren’t really practical at all – and wouldn’t be much better than fission reactors even if they were. See Fusion.