of some fission products is relatively low and others - such as Caesium - are present as a mixture of stable, short lived and long lived isotopes in nuclear waste, making transmutation dependent on extremely expensive isotope separation. Actinide waste 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. Uranium-232 contamination is also formed in this process, via (n,2n) reactions between fast neutrons and , , and : Unlike most even numbered heavy isotopes, is also a fissile fuel fissioning just over half the time when it absorbs a thermal neutron. has a relatively short half-life (), and some decay products emit high energy gamma radiation, such as , and particularly . The full decay chain, along with half-lives and relevant gamma energies, is: decays to where it joins the decay chain of Thorium-cycle fuels produce hard gamma emissions, which damage electronics, limiting their use in bombs. cannot be chemically separated