Answer:
Uranium
Step-by-step explanation:
The nuclear power plants on Earth work based on the principle of nuclear fission.
Nuclear fission occurs when a heavy unstable nucleus splits into lighter nuclei: in this process, the total mass of the final product is smaller than the mass of the initial heavy nucleus, so part of the mass is converted into energy according to Einstein's equation:
where E is the energy released when an amount of mass m is converted (c is the speed of light).
Generally, the "fuel" (the initial heavy unstable element) used in nuclear power plants is uranium. A nucleus of uranium is bombarded with slow neutrons, which cause the process of nuclear fission to start: the nucleus of uranium split into lighter nuclei, releasing also additional neutrons, which are used to start further nuclear reactions.