If a uranium atom absorbs a neutron it will be unstable, and will generally split into two fragments. This process, the splitting of a large nucleus into two smaller ones, is known as nuclear fission.
Many nuclear reactors use uranium as fuel to generate electricity. Although radioactive by-products are produced in the reactor, generating electricity in a nuclear reactor is much more efficient than using a chemical process, such as burning oil, gas, or coal. The chemical processes that occur during burning produce a few eV of energy per molecule. Splitting a uranium nucleus into two pieces produces an average of 200 MeV per nucleus, a factor of about 108 more energy per nucleus than you get from burning something.
Uranium-238 is by far the most naturally-abundant isotope of uranium; uranium-235, however, is much more likely to absorb a neutron and break apart. For this reason many reactors use enriched uranium, uranium with about 3% of U-235 (about 4 times as much as the natural abundance).
U-235 is most easily split by a slow-moving neutron. Such neutrons, with kinetic energy of 0.04 eV or less, are known as thermal neutrons. When a thermal neutron hits a U-235 nucleus and is absorbed, the nucleus generally splits into two big pieces and a few neutrons. Two of the possible reactions are:
As many as five neutrons can be released in a reaction, but the average number is 2.5. These neutrons are used to sustain the chain reaction in the reactor: neutrons don't have to be sent in continually because they are produced in the reactions. These neutrons have kinetic energies of several MeV, however, and this energy must be removed so the neutron becomes a thermal neutron and can be used to break apart another uranium nucleus. The neutrons are slowed to thermal energies by a moderator, which is often water.
A nuclear reactor is designed to safely sustain the chain reaction of fissioning nuclei in its core. To keep a reactor operating safely it must be ensured that, on average, each reaction produces one thermal neutron that is used to split apart another nucleus. If less than one, on average, carried on the chain, the chain would soon die out; this is known as subcritical. If exactly one neutron per reaction goes on take part in the chain reaction, the reactor is critical, meaning its operating at exactly the right level. The danger comes if more than one neutron per reaction goes on to sustain the chain; in this case the reactor would be supercritical, the rate of reaction would spiral out of control, and a meltdown could occur.
The system used to control the reaction rate is a set of rods that can be moved into or out of the reactor core. These rods absorb excess neutrons. If the reaction rate is too high, the rods are moved further into the core so more neutrons are absorbed, slowing the reaction rate to a safe level; a rate too low and the rods are moved out of the core so that more neutrons are available.
To sum up, then, a nuclear reactor requires these components: