Answer
Uranium-235 ($^{235}$U) is the most common fissionable isotope in a commercial nuclear power reactor.
Work Step by Step
Uranium-235 ($^{235}$U) is the most common fissionable isotope in a commercial nuclear power reactor because this isotope decays naturally through alpha radiation and, in doing so, it releases two or more neutrons. Uranium-235 also will undergo fission when it absorbs a neutron, making it eligible for use in commercial nuclear reactors.
