What is moderation?
The concept of moderation as it applies to nuclear technology could be just as well called slowing-down. The moderator of a nuclear reactor is the material that allows neutrons to slow down to energies where they can easily cause a nucleus to fission.
Why moderate? Many aspects of nuclear physics are measured experimentally in the form of cross-sections, which are really measurements of probability. They represent an effective cross-sectional area of a nucleus to a certain interaction. Often, there are different probabilities for the same interaction occurring at different energies (e.g. incident neutron causes fission). For example, a fast (high-energy) neutron will not cause a fission nearly as often as a slow neutron will in most nuclear fuels. The plot shown below is measured cross-section data for the fission of a Uranium-235 nucleus, the most common nuclear fuel in today’s reactors. Since fresh neutrons come out of a fission between 1 and 10 MeV, they must slow down to the thermal range before they have a significant probability of causing a fission. Hence, the reason to moderate.
[ENDF VII data for the fission of U-235. Notice how much more probable fission is for slower neutrons. Also note logarithmic scale. ]
Won’t any material do? Why have a separate moderator material? The reason for this is that some materials are better at slowing down neutrons than others. Simple conservation of energy and momentum will show you that a neutron (mass 1 AMU) cannot slow down much after a collision with a heavy nucleus, like a fuel atom (mass 235 AMU). However, in a collision with Hydrogen (mass 1 AMU), a neutron can in fact slow down very far. Think of a ping-pong ball bouncing off a bowling ball, as opposed to the same ping-pong ball colliding with an orange ping-pong ball. Clearly, the orange ping-pong ball will do a better job. So what material has a lot of hydrogen in it and will make a good moderator? Good old H2O does the trick, giving the name to light-water reactors.