Editing Subcritical reactor (section)

== Motivation ==
A subcritical reactor can be used to destroy heavy isotopes contained in the used fuel from a conventional nuclear reactor, while at the same time producing electricity. The long-lived [[transuranic elements]] in [[nuclear waste]] can in principle be [[Nuclear fission|fission]]ed, releasing [[energy]] in the process and leaving behind the [[fission products]] which are shorter-lived. This would shorten considerably the time for disposal of [[radioactive waste]]. However, some isotopes have threshold fission [[Cross section (physics)|cross section]]s and therefore require a [[fast reactor]] for being fissioned. While they can be transmuted into fissile material with thermal neutrons, some nuclides need as many as three successive neutron capture reactions to reach a fissile isotope and then yet another neutron to fission. Also, they release on average too few new neutrons per fission, so that with a fuel containing a high fraction of them, criticality cannot be reached. The accelerator-driven reactor is independent of this parameter and thus can utilize these nuclides. The three most important long-term radioactive isotopes that could advantageously be handled that way are [[neptunium-237]], [[americium-241]] and [[americium-243]]. The [[nuclear weapon]] material [[plutonium-239]] is also suitable although it can be expended in a cheaper way as [[MOX fuel]] or inside existing [[fast reactor]]s.

Besides nuclear waste incineration, there is interest in this type reactor because it is perceived as [[inherent safety|inherently safe]], unlike a conventional reactor. In most types of critical reactors, there exist circumstances in which the rate of fission can increase rapidly, damaging or destroying the reactor and allowing the escape of radioactive material (see [[SL-1]] or [[Chernobyl disaster]]). With a subcritical reactor, the reaction will cease unless continually fed neutrons from an outside source. However, the problem of heat generation even after ending the chain reaction remains, so that continuous cooling of such a reactor for a considerable period after shut-down remains vital in order to avoid overheating. However, even the issue of [[decay heat]] can be minimized as a subcritical reactor needn't assemble a [[critical mass]] of fissile material and can thus be built (nearly) arbitrarily small and thus reduce the required [[thermal mass]] of an emergency coolant system capable of absorbing all heat generated in the hours to days after a [[scram]].
{{see also|Energy amplifier}}

=== Delayed neutrons===
{{Main| Delayed neutron}}
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 neutron]]s to maintain safe operating conditions. Depending on the fissioning nuclide, a bit under 1% of neutrons aren't released immediately upon fission ([[prompt neutron]]s) but rather with fractions of seconds to minutes of delay by [[fission product]]s 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 {{chem|235|U|link= Uranium-235}} the currently most used fissile material has a relatively high delayed neutron fraction, {{chem|239|Pu| link= plutonium-239}} 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 {{chem|239|Pu}} and - when "fresh" - {{chem|241|Pu}}) is usually not [[nuclear reprocessing|reprocessed]] due to the ingrowth of non-fissile {{chem|240|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 {{chem|236|U}} and other "undesirable" [[isotopes of uranium]].