The secondary fuel used in nuclear power plants is

Answer: Option A

The three most relevant fissile isotopes are Uranium-233, Uranium-235 and Plutonium-239.
When the unstable nuclei of these atoms are hit by a slow-moving neutron, they split, creating two daughter nuclei and two or three more neutrons.
These neutrons then go on to split more nuclei.
This creates a self-sustaining chain reaction that is controlled in a nuclear reactor, or uncontrolled in a nuclear weapon.