Plutonium is made from uranium-238. 239Pu is normally created in nuclear reactors by transmutation of individual atoms of one of the isotopes of uranium present in the fuel rods. Occasionally, when an atom of 238U is exposed to neutron radiation, its nucleus will capture a neutron, changing it to 239U. This happens more easily with lower kinetic energy (as 238U fission activation is 6.6MeV). The 239U then rapidly undergoes two β− decays — an emission of an electron and an anti-neutrino (\bar{\nu}_e), leaving a proton — the first β− decay transforming the 239U into neptunium-239, and the second β− decay transforming the 239Np into 239Pu:
