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Subcritical reactor
A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of a sustaining chain reaction, a subcritical reactor uses additional neutrons from an outside source. The neutron source can be a nuclear fusion machine or a neutron source producing neutrons through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator.
Such a device with a reactor coupled to an accelerator is called an Accelerator-driven system (ADS). See also Accelerator-driven sub-critical reactor.
== 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 fissioned, 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 sections and therefore require a fast reactor for being fissioned. 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 theses 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 reactors.
Besides nuclear waste incineration, there is interest in this type reactor because it is perceived as 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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