ABSTRACT
This chapter concerns nuclear fission and its applications. It examines the nuclear fuel and various modes of energy recovery. The fission neutrons play a major role in the functioning of a nuclear reactor. In effect, the neutron does not exist in a free state and the envisageable sources have intensities which are much too weak. The fission mechanism permits the creation on-site of the neutrons necessary for new fission. Because is of the order of 2.4 to 2.9 depending on the fissile matter under consideration, a chain reaction is possible. Nuclear energy was able to develop initially only on the basis of the first two cycles, since the only natural fissionable isotope is uranium U235. The second cycle is that envisaged for high temperature reactors moderated by graphite. This too can start off on the basis of what nature has to offer, but it requires more or less pure uranium U235.
