The second most common isotope used in nuclear fission is plutonium-239, because it is able to become fissile with slow neutron interaction. This isotope is formed inside nuclear reactors by exposing U to the neutrons released during fission. As a result of neutron capture, uranium-239 is produced, which undergoes two beta decays to become plutonium-239. Plutonium once occurred as a primordial element in Earth's crust, but only trace amounts remain so it is predominantly synthetic.