The most important interaction for the nuclear power industry is the one between a neutron and a heavy nucleus, its subsequent excitation, and fission into two fragments. During this reaction, 2—3 neutrons and a certain amount of energy is released. This released energy may then be used. If the liberated neutron causes the subsequent fission of other heavy nuclei, this is known as fission chain reaction. The course of a fission chain reaction depends on many factors, such as the presence and quality of a moderator that slows down the fast neutrons, the placement and concentration of absorbers used to control the fission reaction, and the amount and configuration of the fissionable fuel in the reactor core. Heat energy released during fission must always be removed safely, using various types of gas or liquid coolants.