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Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium (LEU) fuel used in the light water reactors that predominate nuclear power generation. For example, a mixture of 7% plutonium and 93% natural uranium reacts similarly, although not identically, to LEU fuel. MOX usually consists of two phases, UO2 and PuO2, and/or a single phase solid solution (U,Pu)O2. The content of PuO2 may vary from 1.5 wt.% to 25–30 wt.% depending on the type of nuclear reactor. Although MOX fuel can be used in thermal reactors to provide energy, efficient fission of plutonium in MOX can only be achieved in fast reactors. One attraction of MOX fuel is that it is a way of utilizing surplus weapons-grade plutonium, an alternative to storage of surplus plutonium, which would need to be secured against the risk of theft for use in nuclear weapons.〔(Military Warheads as a Source of Nuclear Fuel )〕〔http://fissilematerials.org/blog/2011/04/us_mox_program_wanted_rel.html〕 On the other hand, some studies warned that normalising the global commercial use of MOX fuel and the associated expansion of nuclear reprocessing will increase, rather than reduce, the risk of nuclear proliferation, by encouraging increased separation of plutonium from spent fuel in the civil nuclear fuel cycle.〔(Is U.S. Reprocessing Worth The Risk? )〕〔(Plutonium proliferation and MOX fuel )〕 ==Overview== In every uranium-based nuclear reactor core there is both fission of uranium isotopes such as uranium-235 (), and the formation of new, heavier isotopes due to neutron capture, primarily by uranium-238 (). Most of the fuel mass in a reactor is . By neutron capture and two successive beta decays, becomes plutonium-239 (), which, by successive neutron capture, becomes plutonium-240 (), plutonium-241 (), plutonium-242 () and (after further beta decays) other transuranic or actinide nuclides. and are fissile, like . Small quantities of uranium-236 (), neptunium-237 () and plutonium-238 () are formed similarly from . Normally, with the fuel being changed every three years or so, most of the is "burned" in the reactor. It behaves like , with a slightly higher cross section for fission, and its fission releases a similar amount of energy. Typically about one percent of the spent fuel discharged from a reactor is plutonium, and some two thirds of the plutonium is . Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year. A single recycling of plutonium increases the energy derived from the original uranium by some 12%, and if the is also recycled by re-enrichment, this becomes about 20%. With additional recycling the percentage of fissile (usually meaning odd-neutron number) nuclides in the mix decreases and even-neutron number, neutron-absorbing nuclides increase, requiring the total plutonium and/or enriched uranium percentage to be increased. Today in thermal reactors plutonium is only recycled once as MOX fuel; spent MOX fuel, with a high proportion of minor actinides and even plutonium isotopes, is stored as waste. Existing nuclear reactors must be re-licensed before MOX fuel can be introduced because using it changes the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it; for example, more control rods are needed. Often only a third to half of the fuel load is switched to MOX, but for more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The Palo Verde Nuclear Generating Station near Phoenix, Arizona was designed for 100% MOX core compatibility but so far have always operated on fresh low enriched uranium. In theory, the three Palo Verde reactors could use the MOX arising from seven conventionally fueled reactors each year and would no longer require fresh uranium fuel. According to Atomic Energy of Canada Limited (AECL), CANDU reactors could use 100% MOX cores without physical modification.〔("Swords into Ploughshares: Canada Could Play Key Role in Transforming Nuclear Arms Material into Electricity," ) in The Ottawa Citizen (22 August 1994): "CANDU ... reactor design inherently allows for the handling of full-MOX cores"〕 AECL reported to the United States National Academy of Sciences committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3% plutonium. The content of un-burnt plutonium in spent MOX fuel from thermal reactors is significant – greater than 50% of the initial plutonium loading. However, during the burning of MOX the ratio of fissile (odd numbered) isotopes to non-fissile (even) drops from around 65% to 20%, depending on burn up. This makes any attempt to recover the fissile isotopes difficult and any bulk Pu recovered would require such a high fraction of Pu in any second generation MOX that it would be impractical. This means that such a spent fuel would be difficult to reprocess for further reuse (burning) of plutonium. Regular reprocessing of biphasic spent MOX is difficult because of the low solubility of PuO2 in nitric acid.〔 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「MOX fuel」の詳細全文を読む スポンサード リンク
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