Nuclear fission
From Tardis Wiki, the free Doctor Who reference
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)
Process
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.
This process had been explained by Peri Brown, and confirmed as correct by the Sixth Doctor; however, he interrupted her before she could finish. (AUDIO: Power Play)
