Neutron moderator

# Overview
In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.
Commonly used moderators include regular (light) water (75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors).
Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

# Explanation
In a thermal nuclear reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits ("fissions") into two smaller atoms ("fission products"). The fission process for uranium atoms yields two fission products, two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self sustaining  a chain reaction  under controlled conditions, thus liberating a tremendous amount of energy. However, the probability of further fission events occurring is dependent upon the speed (energy) of the incident neutrons. Faster neutrons are much less likely to cause further fission. (Note: It is not impossible for fast neutrons to cause fission, just much less likely.) The newly-released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down or "moderated", typically to speeds of a few kilometers per second, if they are to be likely to cause further fission in neighbouring uranium nuclei and hence continue the chain reaction.
A good neutron moderator is a material full of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off. In this process, some energy is transferred between the nucleus and the neutron. More energy is transferred per collision if the nucleus is lighter; see elastic collision. After sufficiently many such impacts, the velocity of the neutron will be comparable to the thermal velocities of the nuclei; this neutron is then called a thermal neutron.
A fast reactor uses no moderator, but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction.
In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully thermalised than others; For example in a CANDU reactor nearly all fission reactions are produced by thermal neutrons, while in a PWR a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled SCWR, the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor.

# Form and location
The form and location of the moderator can greatly influence the cost and safety of a reactor.  Classically, moderators were precision-machined blocks with embedded ducting to carry away heat.  Also, they were in the hottest part of the reactor, and therefore subject to corrosion and ablation.  In some materials, notably graphite, the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of Wigner energy.  At Windscale, this problem led to the infamous Windscale fire.
Some pebble-bed reactor's moderators are not only simple, but also inexpensive: the nuclear fuel is embedded in spheres of reactor-grade pyrolytic carbon, roughly of the size of tennis balls.  The spaces between the balls serve as ducting.  The reactor is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of Wigner energy.

# Moderator impurities
Good moderators are also free of neutron-absorbing impurities such as boron.  In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power.  The German World War II nuclear program suffered a substantial setback when its inexpensive graphite moderators failed to work.  At that time, most graphites were deposited on boron electrodes, and the German commercial graphite contained too much boron.  Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators.  In the U.S., Leo Szilard, a former chemical engineer, discovered the problem.

# Non graphite moderators
Some moderators are quite expensive, for example beryllium, and reactor grade heavy water.  Reactor-grade heavy water must be 99.75% pure to enable reactions with unenriched uranium.  This is difficult to prepare because heavy water and regular water form the same chemical bonds in almost the same ways, at only slightly different speeds.
The much cheaper light water moderator ( essentially very pure regular water ) absorbs too many neutrons to be used with unenriched natural uranium, and therefore uranium enrichment or nuclear reprocessing becomes necessary to operate such reactors, increasing overall costs. Both enrichment and reprocessing are expensive and technologically challenging processes, and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material, causing proliferation concerns. Reprocessing schemes that are more resistant to proliferation are currently under development.
The CANDU reactor's moderator doubles as a safety feature. A large tank of low-temperature, low-pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss-of-coolant accident conditions.  It is separated from the fuel rods that actually generate the heat.  Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high "neutron economy".

# Materials used
- Hydrogen, as an ordinary water ("light water"), in light water reactors. The reactors require enriched uranium to operate.
- Deuterium, in the form of heavy water, in heavy water reactors, eg. CANDU. Reactors moderated with heavy water can use unenriched natural uranium.
- Carbon, in the form of reactor-grade graphite or pyrolytic carbon, used in eg. RBMK and pebble-bed reactors. Lower-temperature reactors are susceptible to buildup of Wigner energy in the material. Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium.
- Beryllium, in the form of metal. Beryllium is expensive and toxic, so its use is limited.
- Lithium-7, in the form of a fluoride salt, typically in conjunction with beryllium fluoride salt (FLiBe). This is the most common type of moderator in a Molten Salt Reactor.
Other light-nuclei materials are unsuitable for various reasons. Helium is a gas and is not possible to achieve its sufficient density, lithium-6 and boron absorb neutrons.