|From uranium to actinides|
It is the successive captures of several neutrons that lead to the formation of transuranic elements, nuclei beyond uranium. One can see on this nuclei map, how multiple captures convert uranium-235 and 238 from the fuel into isotopes of plutonium, neptunium, americium and curium, also known as actinides. Neutron captures are not frequent. In a combination of fuel that stays 3 or 4 years in a reactor, only 2.5% of uranium-238 nuclei undergo at least one capture. An unstable nucleus resulting from a capture has usually the time to be transformed by beta decay well before the next capture. A capture may also lead to a fission, in which case there will be no transuranic formed.