Not all fuel used in nuclear fission chain reaction. For example, 238Pu and some other light elements are used to generate a number of nuclear power through the process of radioactive decay in radiothermal generator, and an atomic battery. Light isotopes such as 3H (tritium) are used as fuel for nuclear fussi. When looking at the binding energy at a particular isotope, there are a number of energy which can be obtained by the merge elements with atomic numbers less than iron, and memfisikan elements with atomic numbers greater than iron.
The nuclei of U-235 consists of 92 protons and 143 neutrons (92 +143 = 235). When a nucleus of U-235 atom captures a neutron, it will split into two new nuclei and release some energy in the form of heat, accompanied by the release of two or three new neutrons.
Closed nuclear fuel cycle through recycling of spent fuel without plutonium separation process has been the top choice of nuclear energy systems development in the future.
A. Mining and Milling
Uranium can be mined by open technique (open cut) and tunnel engineering (underground) depends on the depth of the rocks found uranium. For example, Ranger uranium mine is open while the Olympic Dam mine is an underground mine (mine also produces copper, gold and silver). Both the uranium mines in Australia which is the country with the largest budget category uranium reserves in the world. Mined uranium ore was then sent to the ore processing plant which is generally located near the mine. In this plant, uranium ore is mechanically crushed, and then uranium is separated from other minerals through chemical processes using sulfuric acid solution. The end result of this process in the form of uranium oxide concentrate (U3O8) is often called yellow cake, or "Yellow Cake", although in many ways colored brown.
Some of the uranium mines in Australia, the United States, and Kazakhstan using In Situ Leaching (ISL) uranium directly to mengkstrak of rocks in the soil and bring it to the surface in the form of uranium-rich solution, which is then deposited and dried into a solid uranium oxide. This technique is mainly used to extract uranium found in rocks on the ground that are not economical when delakukan with conventional techniques.
U3O8merupakan commercial products traded on world markets. Ten major countries producing uranium are Canada, Australia, Kazakhstan, Nigeria, Russia, Namibia, South Africa, Ukraine, United States, and Uzbekistan. Canada and Australia produce uranium almost 50% of total world production.
Roughly speaking, it takes about 200 tons of uranium to power a 1000 MWe reactor capable of operating for 1 year. Current world demand for uranium is relatively stable, at around 65 000 tonnes / year.
2. The next steps for the conversion of nuclear fuel-making is a process of purification and conversion into powder Yellow Cake uranium dioxide (UO2) nuclear degree. UO2 is then converted back into gaseous form of uranium hexafluoride (UF6).
For nuclear reactors which use natural uranium fuel, which is a reactor that could produce a chain fission reaction with natural uranium fuel which contains only 0.7% U-235, UO2 powder Yellow Cake conversion results can be directly sent to the plant for processing nuclear fuel a device that is ready to use nuclear fuel in the reactor.
As for the nuclear reactor is only able to produce the fission chain reaction with enriched uranium fuel, UO2 powder conversion process results in the Yellow Cake needs to be changed into the form of UF6 as feed gas enrichment process (the process of increasing levels of U-235 in uranium fuel).
UF6 conversion of UO2 to be done in a two-step process. The first is the reaction of UO2 with anhydrous HF acid to be uranium tetrafluoride (UF4). UF4 and then reacted with F2 to form UF6 gas.
The main countries operating commercial plant converting Yellow Cake - UF6adalah Canada, France, the United States, Britain, and Russia. Some countries such as China, India, Aragentina, and Romania also operates a factory conversion but only for part of its own pass unrecognized.
3. Enrichment
The majority of nuclear power plants now operating or under construction require enriched uranium as fuel. Uranium enrichment is the process of increasing levels of U-235 in uranium fuel from 0.7% (U-235 content in natural uranium) to about 3-5% or more.
Enrichment process discard about 85% U-238 through the separation process of UF6 gas into two streams, one stream is the enriched uranium will be used bait and fuel fabrication process. While the other stream is the stream of waste or "tailings" of the flow of impoverished uranium U-235 is referred to as the depletion of uranium (U-235 content less than 0.25%).
There are two methods that are commercially used for uranium enrichment process, the method of gas diffusion and gas centrifugation methods. Both of these methods basically use the same principle, namely the weight difference between the atom and U-238 U-235 atom.
In the diffusion method of enrichment, UF6dialirkan gas into a porous membrane. Therefore, the lighter U-235 atoms will diffuse or move faster than U-238 atoms, so that the UF6 gas which passes the membrane will contain more U-235. To achieve the U-235 enrichment level between 3-5%, it takes about 1400 times the repetition of the process. So the method is very wasteful of energy, will consume approximately 3-4% of the electricity it generates.
In the enrichment method of centrifugation, UF6diputar gas with high angular velocity in a tube long and slender (1-2 m long, 15-20 cm diameter). Centrifugal force will throw the isotope U-238 which is heavier away from the center of rotation, while U-235 isotopes of the lighter will be concentrated in the center of rotation.
Gas centrifuge method is more energy efficient and can be built with a smaller unit than the gaseous diffusion method, so the method is more economical and fastest growing commercially.
Uranium enrichment plant in the world was first built in the United States with a gas diffusion method. Some modern enrichment plant in Europe (France, Britain, Germany, the Netherlands) and Russian gas centrifuge method. Other countries that operate a commercial uranium enrichment plant is Japan, China, Argentina, and Brazil.
Several types of nuclear power plants, especially nuclear power plants in Canada and the Opium early generation nuclear power plants with gas cooled reactors in the UK do not require enriched uranium fuel.
4. Fuel Fabrication
Fabrication of fuel or nuclear device begins with the conversion process UF6yang been enriched (enrichment plant output) into uranium dioxide powder (UO2) is then formed into pills (pellet) cylinders through pressing and continued with roasting in an atmosphere of hydrogen gas at high temperatures (1700 ° C) to pellet UO2berderajat membetuk dense and strong ceramic.
Pellet-pellet UO2yang meet the quality requirements and then put into a shell of zirconium alloy material (zircalloy).
After both ends of the sleeve and welded closed, the fuel rod (fuel rod) arranged to form a combustion device (fuel assembly).
1000 MWe PWR core contains fuel about 160 devices. Total fuel rods are used to reach 42 000 units. Each fuel rod contains about 300-370 pellets of UO2 pellets, each weighing 6-7 grams.
PWR fuel plant the largest in the world include the Westinghouse - USA with a production capacity of 1600 ton / year, Global Nuclear Fuel - Americas with a production capacity of 1200 ton / year, Ulba - Kazakhstan with a capacity of 2000 ton / year, TVEL Elektrosal - Russia production capacity of 1020 ton / year, TVEL Novosibirsk - Russia with a production capacity of 1000 ton / year, and FBFC - France with a production capacity of 820 tons / year.
Other countries operating nuclear power plants also produce fuel perangka are Japan, South Korea, China, India, Argentina, Brazil, Britain (UK), etc.. . Nuclear Reactor
After the fabrication process, the nuclear fuel put into the reactor core. The composition of the fuel (fuel assembly) that has shaped the structure of the core or core reactor (reactor core). PWR type nuclear power plants with 1000 MW of electrical power (MWe) contains approximately 75 tonnes of slightly enriched uranium. In the reactor core, U-235 fission experience and generate heat in a continuous process called fission chain reaction. Continuity of this process depends on the moderator such as water or graphite, and fully controlled by using control rods.
In the reactor core, a number of U-238 will absorb neutrons and fission results turned into plutonium (Pu-239).
Half of the plutonium produced is also experiencing a fission reaction and produces a third of the total energy of the reactor. To maintain the performance of the reactor, about a third of the fuel used in the core should be replaced with new fuel every year or every 18 months.
6. Temporary Storage of Spent Fuel highly radioactive used fuel and spend a lot of heat. For safe handling and safe, a new spent fuel from reactors dikelurakan stored in special ponds near the reactor to reduce the heat and radioactivity. Water in the pond serves as a barrier to radiation and heat transfer spent fuel from Baban.
Spent fuel can be stored in the storage pool for a long time (up to fifty years or more), before being reprocessed or sent to final disposal as waste (sustainable storage).
Alternatively, after the level of radioactivity and heat emission decreases drastically spent fuel, spent fuel can be removed from the storage pool and then stored in a dry way. Radiation shields are reasonably priced and maintenance-free natural cooling, making this way to be an attractive option.
7. Reprocessing (Sports Birthday)
Spent fuel still contains approximately 96% (480 kg) of uranium with a fissile material content of U-235 is less than 1%. Then 3% (15 kg) of spent fuel of fission products can be categorized as high-activity waste, and 1% (5 kg) the rest of the plutonium (Pu) produced during the fuel inside the reactor and did not experience the burning.
Separation of uranium and plutonium from fission products is done by cutting the fuel element and then dissolving into the acid. Uranium is obtained from the separation process can be converted back into uranium enrichment hexaflourida for later performed. The obtained plutonium can be blended with enriched uranium to produce MOX fuel (Mixed Oxide).
Commercial MOX fuel plant in the world is Belgium, France, Germany, Britain, Russia, Japan, China, and India. United States does not do again if the spent fuel for commercial nuclear power plants in the country. Until now the United States adheres to an open cycle systems or "open cycle".
Some of the PWR nuclear power plant in the world especially in Europe have been using MOX fuel, although its nature is still partial, namely 20-30% of the existing fuel in the core. Japan in the near future it plans to ship a third of its 54 nuclear power plants with MOX fuel.
The 3% of highly radioactive waste resulting from reprocessing is a fission product which number around 750 kg per year from 1000 MWe power reactor. This waste is initially stored in liquid form and then compacted.
Reprocessing spent fuel conducted at facilities in Europe and Russia with a capacity of 5000 tons per year, and total production for almost 40 years has reached about 90 000 tons.
8. Vitrification
High radioactivity waste from reprocessing can be calcined (heated at very high temperatures) so that a dry powder which is then put into borosilicate (pyrex) for immobilizing the waste. Glass material is then poured into stainless steel tubes, each of 400 kg of waste glass. Pengoperasiaan 1000 MWe reactor for one year will result in the waste glass as much as 5 tons or about 12 tubes of stainless high as 1.3 meters and 0.4 meters in diameter. Once given appropriate radiation protection, waste is processed and then transported to the waste storage area.
Until now, the nuclear fuel cycle back end or the "back end" just get to this point.
Final disposal of high radioactivity waste or spent fuel disposal is not reprocessed (open cycle), is still not done.
9. Final Disposal of Waste
Final disposal of waste is in principle sustainable waste storage of high radioactivity that has digelasifikasi and sealed in a stainless steel tube, as well as sustainable storage of spent fuel that has been through enough and the cooling process has been sealed in a container or "canister" is made of corrosion resistant metal such as copper or stainless steel.
Has generally been accepted that the waste is planned to be buried in stable rock in the soil to a depth of not less than 500 m in bedrock (bed rock). Most countries plan to implement the sustainable storage of spent fuel after 2010.
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