A Nuclear Reactor
The energy released from nuclear fission is much greater than the chemical energy released when burning fuels. 1 kg of Uranium-235 gives the same amount of energy through nuclear fission as is released by burning about 3 million tonnes of coal. The energy from nuclear fission can be used to generate electricity in a nuclear power plant.
In order to safely harness the energy from nuclear fission the chain reaction needs to be controlled. This is achieved in a nuclear reactor. The nuclear fuel used in the reactor contains a small concentration of fissionable atoms, typically 3 to 4%. This low concentration reduces the risk of a runway reaction. However, in order to establish a chain reaction with this low concentration one neutron from each fission event must go on to cause fission in another atom. This is achieved in the reactor with the use of a moderator. A moderator is a material used in a nuclear reactor to slow down the neutrons produced from fission. By slowing the neutrons down the probability of a neutron interacting with Uranium-235 nuclei is greatly increased thereby maintaining the chain reaction. Moderators are made from materials with light nuclei which do not absorb the neutrons but rather slow them down by a series of collisions. Carbon in the form of graphite is a material used for moderators as is heavy water which is Deuterium an isotope of Hydrogen with an atomic mass of 2 bonded to Oxygen.
The moderator only slows neutrons down in order to increase the interaction with Uranium nuclei. They do not give any protection if the reaction goes out of control. If a chain reaction is heading out of control the reactors needs to be able to reduce the concentration of neutrons. For this the reactor uses control rods. Control rods are made from material with the ability to absorb neutrons; Cadmium and Boron are examples of suitable materials. By inserting control rods between the fuel rods the chain reaction can be slowed down or shut down. Withdrawing the control rods can restart or speed up the reaction.
The animation below shows how the nuclear fission process in a nuclear reactor is controlled.
A large mass of moderator encases the nuclear fuel rods slowing down the neutrons to increase their interactions with the fissionable atoms. If the chain reaction shows signs of being out of control the control rods are inserted to absorb neutrons thereby slowing down the reaction. They can then be withdrawn to allow the reaction to proceed in a controlled manner.