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The nuclear fuel cycle, also called '''nuclear fuel chain''', consists of ''front end'' steps that lead to the preparation of Uranium for use as fuel for reactor operation and ''back end'' steps that are necessary to safely manage, prepare, and dispose of Radioactive Waste . DIFFERENT FUEL CYCLES Once-through fuel cycle Technically not a cycle ''per se'' fuel is used once and then sent to storage without further processing save repackaging to provide for better isolation from the ; Canada ; Sweden ; Finland ; Spain and South Africa . {Link without Title} Some countries, notably Sweden and Canada, have designed repositories to permit future recovery of the material should the need arise, while others plan for permanent sequestration. Plutonium cycle Many countries are using the reprocessing services offered by BNFL and COGEMA , here the Fission Products , Uranium and Plutonium are separated for disposal or further use. Already BNFL have started to make MOX fuel which has been supplied to power reactors in many parts of the world. This use of fuel which was created in a reactor closes the cycle. Minor actinides recycle It has been proposed that in addition to the use of plutonium, that the is irradiated with Neutrons it will form the very heavy actinides Californium and Fermium which undergo spontaneous Fission . As a result the neutron emission from a used fuel element which had included curium will be much higher. A number of reactor designs (for example, the Integral Fast Reactor ) have been designed for this rather different fuel cycle. In principle, it should be possible to derive energy from the fission of any Actinide nucleus. With a careful reactor design, all the actinides in the fuel can be consumed, leaving only lighter elements with short half-lives. No such reactor has ever been operated on a large scale. It is vital for will be generated. These high energy neutrons and photons will then be able to cause the fission of the heavy actinides. It so happens that the neutron cross section of many actinides decreases with increasing neutron energy, but the ratio of fission to simple activation (ng reactions) changes in favour of fission as the neutron energy increases. Depending on the neutron source the energy will differ.
Hence it should be possible to destroy even Curium without the generation of the transcurium metals if the neutron energy is high, as an alternative the curium (244Cm, half life 18 years) could be left to decay into 240Pu before being used in fuel in a fast reactor. (''Reference'' V. Artisyuk, M. Saito and A. Shmelev, ''Progress in Nuclear Energy'', 2000, 37, 345-350) It is likely that the fuel will have to be able to tolerate more thermal cycles than conventional fuel, this is because if the accelerator is likely stop working on a regular basis. Each time the accelerator stops then the fuel will cool down, it is normal in many conventional power reactors to run the plant at full power for weeks or months at a time, rather than switching it on and off each day.
To date the nature of the fuel (targets) for actinide transformation has not been chosen. Depending on the matrix the process can generate more transuranics from the matrix, this could either be viewed as good (generate more fuel) or can be viewed as bad (generation of more ''radiotoxic'' transuranic elements). A series of different matrixs exist which can control this production of heavy actinides.
The actinide will be mixed with a metal which will not form more actindies, for instance a Solid Solution of an actinide in a solid such as Zirconia could be used.
The actinide oxide when mixed with Thorium oxide will on neutron bombardment form 233U (While is fissile), it is likely that the 233U on further neutron bombardment would undergo fission and it is unlikely that the transuranium elements will be generated from the matrix.
This is likely to lead to the generation of new 239Pu. The Thorium fuel cycle The Thorium fuel cycle has Thorium absorbing a slow Neutron (in a reactor) to ultimately form Uranium-233 ; which in turn is burned as fuel. Hence like Uranium-238 it is a Fertile Material . As a fuel, U-233 is superior to uranium-235 and plutonium-239 from a neutronic standpoint, because of its higher neutron yield per neutron absorbed. Another positive is that Thorium Oxide melts around 3300°C compared to 2800°C for Uranium Dioxide . U-233 also keeps its good neutronic properties with high temperatures, better than either U-235 or Pu-239. This stability means high burn-ups and higher operating temperatures, with thermal yields of 50-55%. Also, from the respective position of uranium and thorium on the Periodic Table , the long-lived minor actinides resulting from fission are in much lower quantity with the thorium cycle, especially compared with the plutonium fuel cycle. Finally all of the mineable thorium is potentially usable in a reactor, compared with the 0.7% of natural uranium, so some 40 times the amount of energy per unit mass might be available. After starting the reactor with some other Fissile Material (U-235 or Pu-239), a breeding cycle similar to but more efficient than that with U-238 and plutonium can be created. The Th-232 absorbs a neutron to become Th-233 which normally decays to Protactinium-233 and then U-233. The irradiated fuel is then discharged from the reactor, the U-233 extracted, then used in another reactor forming a closed fuel cycle. :References: Thorium Fuel Links and Perspectives of the Thorium Fuel Cycle Current industrial activity Currently the only isotopes used as nuclear fuel are Uranium U235 , Uranium U238 and Plutonium Pu239 , although the proposed thorium fuel cycle has advantages. Some modern reactors, with minor modifications, can use Thorium , which is more plentiful than uranium. Heavy-water reactors and graphite-moderated reactors can use uranium as it is mined and refined, but the vast majority of the world's reactors require that the ratio of Uranium-235 (U235) to Uranium-238 (U238) be increased. In civilian reactors the enrichment is increased to as much as 5% U235 and 95% U238, but in naval reactors there is as much as 93% U235. The term ''nuclear fuel'' is not normally used in respect to Fusion Power , which fuses isotopes of hydrogen into helium to release energy. FRONT END See Also: Uranium Market |
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