The primary fuel used in nuclear power plants is

SOLUTION

Concept:

• The fuel used in the Nuclear power plant is usually Uranium although Plutonium can also be used.
• Thorium is not a nuclear fuel itself, it can be turned into a suitable isotope of Uranium in a nuclear reactor.
• Thorium is naturally occurring like, Uranium, but Plutonium is only produced during nuclear reactions so its main source is nuclear reactors. 
• However for most of the world's reactors, naturally occurring Uranium is not sufficiently active and it must be enriched so that the proportion of U-235, the main uranium isotope responsible for fission, is increased. 
• Naturally, occurring Uranium only contains around 0.7% of this isotope.
• Uranium enrichment can be carried out by two methods i.e. gaseous diffusion and gas centrifuge enrichment.
• In both cases, the proportion of U-235 is increased by up to 5%.
• Due to high enrichment, the primary fuel used in nuclear power plants is U-235.