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.