* Uranium-235 (²³⁵U): This is the most common isotope used in nuclear reactors and nuclear weapons.

* Plutonium-239 (²³⁹Pu): This is also used in nuclear weapons and is a byproduct of uranium fission in nuclear reactors.

* Uranium-233 (²³³U): This isotope can be produced through neutron capture by thorium-232 and is used in some nuclear reactor designs.

Important Note: While these are the most common fissionable isotopes, there are others, such as Thorium-232 (²³²Th), which can be made fissile through neutron capture.