Nuclear fission, also known as atomic fission, (AUDIO: Power Play) was a method of producing power. The Master planned to guide the citizens of Destination towards the discovery of the method in order to power his TARDIS. (AUDIO: The Destination Wars)
The process began with a large atom, such as Uranium-235, absorbing a neutron, leading to fission of the atom. This meant the atom split in two, releasing gamma radiation, kinetic energy, and free neutrons.