For seven years, after it had been discovered in 1932, the neutron appeared to be essentially just another fundamental particle, like the proton and the electron, which all matter consists of. However, it is the neutron which has been responsible for the paramount importance of the atom to the world civilization since under proper conditions it can smash the atom. When a neutron of a given energy is absorbed by a uranium nucleus, there is a probability of splitting that nucleus into two or more fragments. It is this process that we call fission. The possibility of the fission of uranium under bombardment by neutrons was first revealed in 1939. In that year, the discovery was made that a particular kind of uranium, a scarce isotope known as Uranium 235, could undergo fission.

Investigations revealed that a pound of Uranium 235 could furnish as much energy as the burning up of 3,000,000 pounds of coal. It was also found that besides its producing such a large amount of energy, the fission might also result in the release of 2 or 3 neutrons. Hence, aside from the fact that fission represents a new wonderful kind of nuclear transformation, the process in question is remarkable because it is accompanied by the liberation of neutrons. The latter in their turn are capable of inducing fission of other uranium nuclei.

The possibility of a nuclear chain reaction accompanying the fission of uranium by neutrons was investigated by scientists in Europe and America.

The chain reaction implies that each uranium nucleus capturing a neutron and undergoing fission must ultimately yield at least one neutron which can also cause fission. Hence, in the fission process, neutrons are released from the fragmentation and these neutrons are capable of creating more fission in other uranium nuclei under proper conditions (see fig.).

Thus, in theory, a single neutron could start a branching chain of fissions, the number of nuclei involved increasing at a tremendous rate. Suppose, for example, that for each nucleus undergoing fission 2 neutrons is liberated. Provided each of these splits other uranium nuclei, with the release of 2 neutrons in each case, there will be 4 neutrons available. These 4 neutrons induce further fission in 4 more uranium nuclei, accompanied by the emission of 8 neutrons, and so on. Such large numbers of fissions will result in explosions unless the process is under complete control.

The neutrons produced in the fission process are known to have high energies. Inside the reactor, these neutrons may suffer scattering collisions, mainly elastic, as a result of which their energy is decreased. They may also be absorbed by the various materials in the reactor or be lost to the core by leaking out of the system. It happens sometimes that some neutrons are absorbed in core material other than uranium. Depending upon the characteristics of the moderator (i.e., the material used to slow them down), the fuel, and other substances as well as upon the dimensions of the reactor, the main portion of the neutron absorptions by uranium leading to fission will take place within a certain energy range. However, two or three neutrons that are liberated in fission may take part in several reactions, all of which are competitive.

In any event, as mentioned above, for a chain reaction to take place at least one neutron, on the average, must be produced for each uranium nucleus capturing a neutron and undergoing fission. That neutron in its turn causes the fission of another nucleus. But the significance of nuclear fission does not lie in this fact alone. The essential point is that the process under consideration releases enormous quantities of energy.