text: In a reactor, when a neutron hits a fissile atom (such as certain isotopes of uranium), it either splits the nucleus or is captured and transmutes the atom. In the case of , the transmutations tend to produce useful nuclear fuels rather than transuranic waste. When  absorbs a neutron, it either fissions or becomes . The chance of fissioning on absorption of a thermal neutron is about 92%; the capture-to-fission ratio of , therefore, is about 1:12 – which is better than the corresponding capture vs. fission ratios of  (about 1:6), or  or  (both about 1:3). The result is less transuranic waste than in a reactor using the uranium-plutonium fuel cycle.
  , like most actinides with an even number of neutrons, is not fissile, but neutron capture produces fissile .  If the fissile isotope fails to fission on neutron capture, it produces , , , and eventually fissile  and heavier isotopes of plutonium. The  can be removed and stored as waste or retained and transmuted to plutonium, where more of it fissions, while the remainder becomes , then americium and curium, which in turn can be removed as waste or returned to reactors for further transmutation and fission.
  However, the  (with a half-life of ) formed via (n,2n) reactions with  (yielding  that decays to ), while not a transuranic waste, is a major contributor to the long-term radiotoxicity of spent nuclear fuel. While  can in principle be converted back to  by neutron absorption, its neutron absorption cross section is relatively low, making this rather difficult and possibly uneconomic.