The continuing process whereby neutrons emitted by fissioning nuclei induce fissions in other fissile or fissionable nuclei is called a fission chain reaction.
These neutrons can induce fission in a nearby nucleus of fissionable material and release more neutrons that can repeat the sequence, causing a chain reaction in which a large number of nuclei undergo fission and an enormous amount of energy is released.
Uranium-238 and thorium-232 (and some other fissionable materials) cannot maintain a self-sustaining fission explosion, but these isotopes can be made to fission by an externally maintained supply of fast neutrons from fission or fusion reactions.
Fissionable plutonium isotopes are formed as by-products of fission in reactors using uranium; when neutrons are added to uranium-238, which is not itself fissionable, it is converted to the fissionable isotope plutonium-239.
Many heavy atomic nuclei are capable of fissioning, but only a fraction of these are fissile that is, fissionable not only by fast (highly energetic) neutrons but also by slow neutrons.
This series of rapidly multiplying fissions culminates in a chain reaction in which nearly all the fissionable material is consumed, in the process generating the explosion of what is known as an atomic bomb.
As a result, the chain reaction will peter out unless the neutrons can be slowed down sufficiently.There also need to be enough fissionable atoms about for the neutrons to bump into in other words, a critical mass.
Only 0.7% of natural uranium is the fissionable isotope 235U.
An uncontrolled chain reaction within a large amount of fissionable material can lead to an explosive release of energy, as in nuclear weapons.
But in nuclear reactors, which contain far less fissionable material, the chain reaction and the release of energy are carefully controlled.War and peaceIn 1942 the physicist Enrico Fermi led a group of scientists who built the first nuclear reactor as part of the Manhattan Project America's effort to build the first atomic bomb.
Two other fissionable actinoid isotopes are important.
