1. Uranium-235 (²³⁵U): This is the only naturally occurring fissile isotope, meaning it can sustain a nuclear chain reaction. While it makes up only about 0.7% of natural uranium, it is the primary fuel for most nuclear power plants.

2. Plutonium-239 (²³⁹Pu): This isotope is not naturally occurring but is produced in nuclear reactors through neutron capture by uranium-238. It is also fissile and is used as fuel in some nuclear reactors and in nuclear weapons.