Final answer:
The two most common nuclear fuels are uranium-235 and plutonium-239. Uranium-235 is naturally occurring and must be enriched for use in reactors, while plutonium-239 is synthetic and can be produced in breeder reactors.
Step-by-step explanation:
The two most commonly used nuclear fuels are uranium-235 (U-235) and plutonium-239 (Pu-239). U-235 is naturally occurring and is the primary fuel used in nuclear reactors; however, it must be enriched from its natural state to increase its concentration in the fuel.
Enrichment is typically achieved by processes such as gaseous diffusion of uranium hexafluoride (UF6). Although Pu-239 does not occur naturally, it can be synthesized in breeder reactors from uranium-238. Both U-235 and Pu-239 have the ability to maintain a self-sustaining chain reaction, which is crucial for the operation of nuclear power plants.