Nuclear fission is a process where an atomic nucleus splits into two (or sometimes three)
smaller nuclei. As a result this often produces free neutrons and gamma rays, and releases a
large amount of energy.
Fission produces energy for nuclear power and the explosion of nuclear weapons. This
process occurs when nuclei of certain unstable isotopes of heavier elements (e.g. uranium
and plutonium) absorb a neutron and then decay into two smaller nuclei and several free
neutrons. The free neutrons can then cause fissions in other nuclei. As a result of this a
fission chain-reaction can then occur. In such a reaction, the free neutrons which each fission
event releases can cause more events still. These then release more neutrons and cause
more fissions. Thus it is possible to create a self-sustaining fission chain-reaction which
releases energy with a controlled rate in a nuclear reactor, or with a very rapid uncontrolled
rate in a nuclear weapon.
To produce a self-sustaining chain reaction, a defined minimum amount of fissile material is
necessary: this is called the "critical mass" . A nuclear weapon produces a powerful
explosion as a result of the release of a large amount of energy in a very short space of time.
It is necessary to have a supercritical mass of fissile material so that the rate of the fission
chain reaction increases exponentially. The use of the isotopes uranium-235 or plutonium-239
are necessary to achieve this process.
Isotopes which undergo induced fission when a fast neutron strikes them are called "fissionable".
Isotopes that also undergo fission when a thermal, slow moving neutron strikes them are also
called "fissile". A fissile material is able to sustain a nuclear fission chain reaction.
Not all fissionable isotopes are able to sustain a chain reaction. Uranium-238 (the most
common form of uranium) is fissionable but not fissile. It will only undergo induced fission
when a fast neutron has an impact on it with over 1 MeV of kinetic energy. But a very little
amount of the neutrons which U-238 fission produces has sufficient energy to cause further
fissions in U-238 nuclei. Thus a self sustaining chain reaction is not possible with this isotope.