uranium 23

The uranium isotope with mass number 235 and half-life 7.04 × 10⁸ years, fissionable with slow neutrons and capable in a critical mass of sustaining a chain reaction that can proceed explosively with appropriate mechanical arrangements.

The most common isotope of uranium, having mass number 238 and half-life 4.46 × 10⁹ years, nonfissionable but irradiated with neutrons to produce fissionable plutonium 239.