The absorption of slow neutrons can induce fission in particular nuclei, which are referred to as the fissile nuclei. Neutrons emitted from such fission can lead to a sustainable chain reaction. If neutrons are moving too fast, fission will not occur.
Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors. Their atoms have relatively large nuclei that are easy to split, especially when hit by neutrons. The nuclear symbol for a neutron is .
When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following happens:
The equation can be written as
The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei.
This is called a chain reaction.
The chain reaction in nuclear reactors is controlled to stop it moving too quickly.
Many of the fission products are also radioactive and they decay with a large range of half-lives. Technetium-99 has a half-life of 211,100 years, but other fission products may have half-lives of about 5 years.