Describe the purpose of the moderator in a nuclear fission reactor.

The moderator material is generally graphite or water. These materials do not absorb or react with the neutrons, they simply act to slow down fast moving neutrons. If neutrons are moving too quickly they are not captured by Uranium nuclei and the chain reaction stops. Once they are moving at the correct speed they will be captured by Uranium nuclei and the chain reaction can continue.It is important to note that the moderator in a nuclear fission reactor is not the same as the control rods. Both the moderator and the control rods can be used to change and maintain the rate of the chain reaction within the reactor. But the control rods are used to absorb neutrons, reducing the number of neutrons available for further chain reactions. Whilst the moderator is installed to slow down neutrons, making them more likely to be captured by Uranian nuclei, increasing the rate of chain reactions.