Moderating the speed of neutrons so that efficient fission occurs
Efficient fission occurs if the bombarding neutrons are likely to stay in the nucleus and cause the instability that results in a fission (split of the nucleus).This is only the case if they are of thermal energy speeds (slow). If they are too fast they are less likely to stay in the nucleus long enough to cause the split. This means that if they are too fast the probablility of their causing a fission is much lower than if they are of thermal energies.
The neutrons resulting from the fission reaction (produced when the split occurs) are usually of too high a kinetic energy to interact with the nucleus successfully and therefore need their speed moderating before they will have a high probability of producing a fission.
Most early British nuclear reactors such as the Magnox and Advanced Gas Cooled reactors, relied on a graphite moderator, the role of which, is to slow down neutrons freed during nuclear fission. Graphite is therefore included in the reaction vessel. It is an excellent moderator. It takes kineic energy out of the neutrons (thereby slowing them down) as they interact with the carbon atom lattice (3-d bonded structure of graphite) but is does not absorb too many of them. It is said to have a 'low cross-section for neutrons '.
Heavy water is sometimes used as a moderator in nuclear reactors. Heavy water is the same as ordinary water except that the two hydrogen atoms (H-1) of the water molecule are replaced by two deuterium atoms (H-2). As such, heavy water is sometimes symbolized D2O instead of H2O. Because it has two extra neutrons, heavy water is approximately 10% heavier than ordinary water.
The job of a moderator is to slow neutrons down to the lower energies that are more likely to initiate fission of the uranium-235 fuel. Heavy water is not quite as good at this as ordinary light water, but it is far less likely than the latter to absorb neutrons. A reactor that uses ordinary water as the moderator must compensate for this absorption of neutrons by increasing (enriching) the percent of the uranium fuel that is uranium-235. The uranium-235 is enriched to 2.5% - 3.5% to allow ordinary light water to be the moderator. This can then be used as the heat exchange as well.
You should know that graphite OR water are used as a moderator and that it is called a moderator because it moderates the speed of the neutrons.