Another issue in which a subcritical reactor is different from a "normal" nuclear reactor (no matter whether it operates with fast or thermal neutrons) is that all "normal" nuclear power plants rely on delayed neutrons to maintain safe operating conditions. Depending on the fissioning nuclide, a bit under 1% of neutrons aren't released immediately upon fission (prompt neutrons) but rather with fractions of seconds to minutes of delay by fission products which beta decay followed by neutron emission. Those delayed neutrons are essential for reactor control as the time between fission "generations" is on such a short order of magnitude that macroscopic physical processes or human intervention cannot keep a power excursion under control. However, as only the delayed neutrons provide enough neutrons to maintain criticality, the reaction times become several orders of magnitude larger and reactor control becomes feasible. By contrast this means that too low a fraction of delayed neutrons makes an otherwise fissile material unsuitable for operating a "conventional" nuclear power plant. Conversely, a subcritical reactor actually has slightly improved properties with a fuel with low delayed neutron fractions. (See below). It just so happens that while U the currently most used fissile material has a relatively high delayed neutron fraction, Pu has a much lower one, which - in addition to other physical and chemical properties - limits the possible plutonium content in "normal" reactor fuel. For this reason spent MOX-fuel, which still contains significant amounts of plutonium (including fissile Pu and - when "fresh" - Pu) is usually not reprocessed due to the ingrowth of non-fissile Pu which would require a higher plutonium content in fuel manufactured from this plutonium to maintain criticality. The other main component of spent fuel - reprocessed uranium - is usually only recovered as a byproduct and fetches worse prices on the uranium market than natural uranium due to ingrowth of U and other "undesirable" isotopes of uranium.