Many large, and even some small atoms undergo nuclear fission reactions naturally. One of the isotopes of carbon - isotopes are atoms of a single element with different numbers of neutrons - called carbon-14 behaves in this way. Carbon-14 exists at a constant concentration in natural sources of carbon. Thus living entities which constantly recycle their carbon maintain this constant concentration. However when they die, the carbon-14 is no longer renewed and it gradually decays. Measuring the residual concentration gives a good estimate of the time since the organism died. It is this property which allows archaeologists to use carbon-14 to date ancient artefacts and remains.
Other atoms can be induced to undergo fission by bombarding them with subatomic particles. One of the isotopes of uranium, the element most widely used in nuclear reactors, behaves in this manner.
Naturally occurring uranium is composed primarily of two slightly different isotopes called uranium-235 and uranium-238 (the numbers refer to the sum of protons and neutrons each atom contains). Most uranium is uranium-238, but 0.7% is uranium-235.
When an atom of uranium-235 is struck by a neutron it may be induced to undergo a nuclear fission reaction. The most frequent products of this reaction are an atom of krypton, an atom of barium, three more neutrons and a significant quantity of energy.
In theory each of the three neutrons produced during this reaction could cause three more atoms of uranium-235 to split. This would lead to a rapidly accelerating reaction, called a chain reaction, which would release an enormous amount of energy. A chain reaction of this type forms the basis for the atomic bomb.
In fact a lump of natural uranium will not explode because the uranium-235 atoms will only react when struck by slow moving neutrons; the ones created during the fission process move too fast to cause further fission reactions to take place. They need to be slowed down first. This is crucial to the development of nuclear power.
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