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Hybrid nuclear fusion-fission (hybrid nuclear power) is a proposed means of generating power by use of a combination of nuclear fusion and fission processes. The basic idea is to use high-energy fast neutrons from a fusion reactor to trigger fission in otherwise nonfissile fuels like U-238 or Th-232. Each neutron can trigger several fission events, multiplying the energy released by each fusion reaction hundreds of times. This would not only make fusion designs more economical in power terms, but also be able to burn fuels that were not suitable for use in conventional fission plants, even their nuclear waste. The concept dates to the 1950s, and was strongly advocated by Hans Bethe during the 1970s. At that time the first powerful fusion reactors were being built, but it would still be many years before they could be economically competitive. Hybrids were proposed as a way of greatly accelerating their market introduction, producing energy even before the fusion systems reached break-even. However, detailed studies of the economics of the systems suggested they could not compete with existing fission reactors. The idea was abandoned and lay dormant until the 2000s, when the continued delays in reaching break-even led to a brief revival around 2009, notably as the basis of the LIFE program. The concept has since become dormant again. ==Fission basics== Conventional fission power plants rely on the chain reaction caused when nuclear fission events release neutrons that cause further fission events. Each fission event in uranium releases two or three neutrons, so by careful arrangement and the use of various absorber materials, you can balance the system so one of those neutrons causes another fission event while the other one or two are lost. This careful balance is known as criticality. Natural uranium is a mix of several isotopes, mainly a trace amount of U-235 and over 99% U-238. When they undergo fission, both of these elements release neutrons with energies around 1 to 2 MeV. This energy is too low to cause fission in U-238, which means it cannot sustain a chain reaction. U-235 will undergo fission when struck by neutrons of this energy, but does so much easier when the neutrons have much less energy, the so-called thermal neutrons. To provide this, neutron moderator materials are used to slow the neutrons, giving off heat that is extracted for power. Common moderators like water often absorb some of the neutrons, reducing the number available for fission to a point too low to keep the chain reaction going. To keep criticality, most reactor designs concentrate, or ''enrich'', the fuel, increasing the amount of U-235 to produce enriched uranium, while the leftover, now mostly U-238, is a waste product known as depleted uranium. A fission reactor burns off its supply of U-235 over a short period, on the order of a few months. A combination of burnup of the U235 along with the creation of neutron absorbers, or ''poisons'', as part of the fission process eventually results in the fuel mass not being able to maintain criticality. This burned up fuel has to be removed and replaced with fresh fuel. The result is nuclear waste that is highly radioactive and filled with long lived radionuclides that present a safety concern. The waste contains most of the U-235 it started with, only 1% or so of the energy in the fuel is extracted by the time it reaches the point were it is no longer fissile. One solution to this problem is to reprocess the fuel, which uses chemical processes to separate the U-235 (and other non-poison elements) from the waste, and then uses that U-235 in fresh fuel loads. This reduces the amount of new fuel that needs to be mined, and also concentrates the unwanted portions of the waste into a smaller load. Reprocessing is expensive, however, and has generally been more expensive than simply buying fresh fuel from the mine. Another possibility is to ''breed'' Pu-239 from the U-238 through neutron capture, or various other means. In order to do this, higher energy neutrons are required, which means they cannot be moderated as in a conventional reactor. The simplest way to achieve this is to further enrich the original fuel well beyond what is needed for use in a moderated reactor, to the point where the U-235 maintains criticality even with the fast neutrons. The extra fast neutrons escaping the fuel load can then be used to breed fuel in a U-238 assembly surrounding the reactor core, most commonly taken from the stocks of depleted uranium. The Pu-239 is then chemically separated and mixed into fresh fuel for conventional reactors, in the same fashion as normal reprocessing, but the total volume of fuel created in this process is much greater. In spite of this, like reprocessing, the economics of breeder reactors has proven unattractive, and commercial breeder plants have ceased operation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nuclear fusion-fission hybrid」の詳細全文を読む スポンサード リンク
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