Answer
Most of the Uranium in a reactor is $U^{238}$. Though the isotope is not fissionable, it can be converted into $Pu^{239}$ by fast neutron bombardment and absorption. And $Pu^{239}$ is fissionable and can be used to make a bomb.
Work Step by Step
Most of the Uranium in a reactor is $U^{238}$. Though the isotope is not fissionable, it can be converted into $Pu^{239}$ by fast neutron bombardment and absorption. And $Pu^{239}$ is fissionable and can be used to make a bomb.
