Nuclear Fuel Cycle - Research Paper Example

Nuclear Fuel Cycle The use of the nuclear energy is increasing gradually in the world. IT is estimated that the 18% of the power requirement in the world is fulfilled by the nuclear energy. The nuclear energy is produced using the nuclear fuel cycle. This cycle consists of many stages starting from the mining of the ore to the waste management. As the nuclear fuel is radio active, the utmost care is required during the handling of the nuclear fissile materials and in the disposal of the fuel after the usage. The hazardous nature of the nuclear waste requires the proper management facilities to protect the future generation from the health hazards. This paper discusses about the various stages of the nuclear cycle and the safety aspects present in it. Introduction: Nuclear energy is a part of the world’s Energy mix. The nuclear fuel cycle is the series of different stages that lead to the production of Power from the Uranium in the nuclear power reactors. A secure and adequate supply of energy is required for the world and that is promised by the Nuclear power systems. The nuclear fuel cycle starts with the extraction of the raw materials from the earth. The process of conversion of the raw material into the primary fuel is the next step, followed by the production of the energy from the source. The spent fuel and the final waste are processed, conditioned and recycled to derive more energy form the fuel. Finally the spent fuel is disposed as waste into the geosphere. The spent fuel has the greater chance for emitting the radiations; hence utmost care should be taken for the proper disposal. The production of energy from nuclear fuels is increasing because of the amount of energy that uranium can produce (1 gram of nuclear fuel produces 9 x 10 13 joules of energy) and also from the reliability of the source and the recyclability of the fuel. For long term sustainable energy production, nuclear fuel is the best source. (OECD Nuclear Energy Agency, Trends on the nuclear fuel cycle). The cost for the power plants are very high, but when compared to the other power sources, the availability of the raw material land the future scope is more for this technology. Literature Review: The OECD Nuclear energy agency has given the various activities that together make up the fuel cycle as follows: 1. The mining and milling of Uranium. 2. Uranium refining and followed by the conversion to uranium hexafluoride. 3. Enriching the Uranium. 4. Fuel fabrication. 5. The reactor operation 6. Storage of the spent fuel. 7. Reprocessing of the spent fuel. 8. Decommissioning nuclear facilities 9. Radioactive waste management and the spent fuel disposal options. 1. Mining and milling of Uranium: Of the 200 known uranium minerals, a few are mined commercially. The uranium oxides, mixed oxides, Uranium silicates, Uranium phosphates, uranium vanadates, uranium carboniferous are mined at a large level due to the economic impact. The open pit and the underground mining are practiced for the extraction of the uranium deposits. The open-pit mining has a lot of advantages such as higher productivity, higher recovery, easier dewatering, lower costs and safer working conditions. The underground mining is practiced for mining operations above 200m from the earth surface using the room and pillar method. (Wilson, 1996). The milling of the uranium ore is the next step. The grading is given to the uranium ores as low grade, high grade and Acid consuming minerals. The low grade minerals are milled first with the conventional sulphuric acid leaching, followed by the solid /liquid separation and the eluex is done. After the ion exchange precipitation and calcining are done and now the product is ready for the next step. For the high grade minerals the Caros acid procedure followed by the solid –liquid separation and base metal recovery is done. Then the by-products are recovered by the solvent extraction and the final steps are precipitation of the mineral and calcining of the ore. The acid consuming minerals are processed in a separate method. (Wilson, 1996) 2. Uranium refining and conversion to uranium hexafluoride: After the milling the uranium will be in the powder form and sometimes as the yellow cake. The impurities will be present along with the uranium in this stage. Hence further refining will remove the impurities and bring the uranium to the nuclear grade purity and quality. The various steps involved in this process are dissolution in the acid, extraction, stripping, denitrification, reduction, hydrofluorification and fluorination. The final product is uranium hexafluoride. (OECD, 2005). 3. Enriching the uranium: The uranium enrichment is done for increasing the percentage of the uranium-235 isotope in the nuclear fuel. The gaseous diffusion of the Uranium is practiced for the enrichment. This enriched uranium is then converted into its oxide and it moves for the fuel fabrication step. This enrichment was first practiced in the Manhattan project in the year 1940. The other enrichment techniques include Thermal diffusion, Gas centrifuge, Zippe centrifuge, Atomic vapor laser isotope separation, molecular laser isotope separation, Separation of isotopes by laser excitation. 4. Fuel fabrication: The fuel that is used in the reactor is usually in the form of ceramic pellets. The uranium pellets are formed under high pressure and high temperature (1400 degree Celsius). 5. The reactor operation: The nuclei of the uranium -235 are split inside the reactor for the release of the energy. This energy is used to heat the water and steam is generated. This steam drives a turbine connected to it to produce electricity. Sometimes the uranium -238 isotope undergoes reaction to form the plutonium inside the reactor. (Hore-Lacy, 2006). 6. Storage of the spent fuel: After the fuel is used, the spent fuel is removed from the reactor. This spent fuel will emit radiation and heat. This used fuel is either kept for reprocessing or prepared for permanent disposal. 7. Reprocessing of the spent fuel: The reprocessing of the spent fuel is done by dissolving it in nitric acid and extracting uranium and plutonium from the spent fuel. 8. Decommissioning nuclear facilities: The decommissioning and dismantling of the nuclear fuel may take years to complete. The first process is the removal of the spent fuel from the plant followed by the removal of the buildings and sealing the area to ensure minimal external irradiation and the cost of surveillance. The establishment of the green field status is the last stage of the decommissioning facilities. (Wilson, 1996) 9. Radioactive waste management: The radioactive waste material exists in all the three forms. The gaseous waste comes from the reprocessing step and the air borne radio nucleotides such as 85 Kr, 3 H , 14C and 131I. The liquid wastes such as aqueous waste from the reprocessing plants. The solid waste comes from the milling and mining of the uranium and thorium ores. (Saling, 2001). The waste are classified as high level and low level waste and are stored at a separate place till their half life period is over. The use of the immobilization techniques to dispose the high level waste is widely practiced in many countries. The borosilicate glass is mixed with the high level waste, melted up and poured into the stainless steel canisters and is sealed by welding. The tightly sealed containers are then isolated from the biosphere. According to the OECD Nuclear energy agency, there are three types of the fuel cycle. They are 1. Once –through fuel cycle. 2. Thermal Light water Reactor (LWR) cycle. 3. Fast Breeder Reactor (FBR) cycle. 1. In the Once through fuel cycle, the fuel is used once and sent to the storage without further processing of the fuel. This method saves the additional packaging and is welcomed by a six countries: The United States, Spain, South Africa, Sweden, Finland and Canada. 2. The Thermal LWR Cycle : This cycle is now called as once-through fuel cycle as the availability of the fuel was found to be high these two technologies have been merged into one now.(Shwageraus, 2003). 3. FBR Cycle: The fast breeder reactors are designed to produce more fissile material than it consumes. This is also called as fast neutron reactor. The large scale FBRs that are cooled by the liquid sodium are called as liquid metal fast breeder reactors. Loop type and pool type reactors are the two types of FBR system. In the pool type the circulators and the heat exchangers will be kept immersed inside the reactors and in the loop type, the primary heat exchangers will circulate the primary coolant external to the reactor tank. As the demand for energy is increasing heavily, there is more need for nuclear power plants. According to the climate control Intergovernmental panel’s special report on the Emission Senarios, the nuclear power will increase by four factors by the year 2050. (Tucek , Carlsson and Wider, 2006). The further research on the fast breeder reactors are going on and Tucek etal in their paper have discussed about the use of the lead cooled fast reactors and their effort on the FBR. Tucek , Carlsson and Wider (2006) have compared the lead cooled fast reactors (LFR) and the Sodium cooled fast reactors (SFR) and stated that both the reactors can be used as burners and self reactors. The use of the tight pin activities will also lead to the increase in the neutron economy and reduce the lower burn up of the coolants. The natural circulation behavior of the Lead coolant and the higher boiling temperature of the lead, leads LFR to be the better accident initiators. Greater the fission initiation, greater is the power generation. (Tucek , Carlsson and Wider, 2006). Results The nuclear fuel power provides about 16% of the global electricity. Nuclear power is a clean, carbon –free energy and affordable energy that is available for the people all over. For the successful continuation of the cycle, safety and efficacy must be given utmost care. The front end operations and the back end operations must be preformed according to the world nuclear academy. The development of the nuclear power was first done for the non-civilian purpose and the inventions of the nuclear power cycle enable the civilians to use it for electricity generation. There are about 200 types of uranium mineral deposits in the world and of these the most commonly used type for the milling and the mining techniques are the oxides, mixed oxides and the silicates of uranium. These ores are first converted into the usable for m of uranium. The leaching of the uranium with the sulphuric acid is the most common method for the extraction of the uranium followed by the ion exchange and the calcining method. The uranium ore is now converted into the uranium powder. This powder is in yellow color. After the milling of the uranium, the refining of the uranium takes place. Here the quality of the uranium molecule is increased by dissolving it in the acids, mainly nitric acid is used. After this the stripping and the reduction of the uranium compound takes place. This is followed by the de hydrofluorification and fluorination. (Forseberg and Dole, 2003). The final compound formed after this step is uranium hexafluoride. The uranium hexa fluoride is not rich in the uranium - 235 molecules and hence enrichment of the uranium is done by diffusion techniques, the laser techniques and the centrifugation techniques. The enrichment techniques help to increase the concentration of the uranium isotope U235, the fissile isotope of the nuclear reactor. The enrichment process increases the concentration of the uranium by ten fold. The gaseous diffusion and the gas centrifuge are the most common techniques. (Tucek, Carlsson and Wider, 2006). The gaseous diffusion techniques use the semi permeable membrane that helps to filter out the heavy uranium 238 UF6 atoms. When passed under the different diffusion barriers the product stream concentration of the uranium will increase heavily. Similarly the gas centrifuge helps to separate them based on the density gradient. When the gaseous uranium is spun at a high speed in a series of cylinders, the separation occurs. This is a very costly technique when compared with the diffusion technique but the rate of concentration of the uranium 235 is achieved more in this technique. The fuel fabrication followed by the enrichment converts the uranium hexafluoride into the real fuel for the nuclear reactor. The conversion of the uranium hexafluoride into the uranium dioxide makes uranium as the right fuel for the nuclear reactor. The power generation is done by the nuclear fission process. During this process, some of the uranium is converted into plutonium in the reactor core. After the fuel is used completely, the spent fuel is removed from the reactor before the next cycle is continued. The removed spent fuel is then stored in the container for the future reprocessing or for the disposal. The nuclear waste management must be effectively carried to make sure that no external radiation occurs. (Hore-Lacy, 2003).The three types of the waste produced from a number of sources are categorized as high level waste, low level waste and intermediate level waste. The high level waste is usually the spent fuel only. The conversion of the nuclear fuel into the various compound materials can reduce the radiation effects. If the materials are converted into powders, pastes or solvents then the rate of the radiation will differ for the each compound. Thus the more radiation wastes can be dropped at the separate biosphere and the less radiation causing material can be dropped in the containers and stored for future use. Discussion: The nuclear fuel cycle is now providing about 18% of the total electricity to the world. The nuclear energy was first used for the production of the destruction weapons as missiles. Later the discovery of the nuclear fuel cycle leads to the use of the nuclear energy for the welfare of the mankind. The nuclear fuel cycle has front end and back end steps. The steps were updated with the recent techniques for the safety and efficacy of the nuclear fuel cycle. The use of the laser techniques for the enrichment of the uranium-235 is one of the notable advancement in the cycle. Similarly the use of the sodium coolants to the reactor is the next advancement. Now the researches are going on to use lead as coolants in the reactors. The natural circulation behavior and the higher boiling temperature of lead makes it the better choice for use as coolants. The uranium oxides, mixed oxides and phosphate ores are preferred for the production of the nuclear fuel than the other types because of the easier extraction and the purification steps. The Uranium oxides are available at a large concentration than the other types. The Uranium hexafluoride is then fabricated into uranium oxide and then used for the power production. The nuclear fission produces the energy. This energy is then transmitted to the heavy water and then converted into steam. This steam runs the generators and thus electricity is produced. After the fission is over, the spent fuel is tightly sealed in the stainless steel vessel and stored at an isolated place. The radioactive spent fuel is kept isolated for many years until the radiation vanishes. The half life of the uranium is 50 years. The disposal and the reprocessing of the fuel is an important issue in the nuclear waste management. The spent fuel is reprocesses for about 5 times and used as the fuel in the reactor. References: Forseberg, C.W and Dole L.R. (2003). An integrated once-through fuel cycle with depleted Uranium- dioxide SNF multifunction Casks. Advances in Nuclear Fuel Management III Conference. American Nuclear Society. Hore-Lacy, I. (2006). Nuclear energy in the 21st century: The world nuclear University Primer. World Nuclear Association. Academic Press. Hore-Lacy, I. (2003). Nuclear Electricity. 7th ed. Uranium Information Centre limited. OECD Nuclear energy agency. (2001). Trends in the nuclear fuel cycle: Economic, environmental and social aspects. OECD Publishing. OECD Nuclear energy agency. (2005). The safety of the nuclear fuel cycle: OECD Nuclear Energy Agency. Committee on the Safety of Nuclear Installations, OECD Nuclear Energy Agency. Working Group on the Safety of the Nuclear Fuel Cycle. OECD Publishing OECD Nuclear energy agency. (2005). The safety of the nuclear fuel cycle. OECD publishing. Saling JH. (2001). Radioactive waste management. CRC Press. Shwageraus, E. (2003). Rethinking the Light Water Reactor Fuel Cycle. Department of Nuclear Engineering. Massachusetts Institute of Technology. Tucek, K., Carlsson, J and Wider, H. (2006). Comparison of sodium and lead-cooled fast reactors regarding reactor physics aspects, severe safety and economical issues. 13th International Conference on Nuclear Energy. Nuclear Engineering and Design. 236 (14-16): 1589-1598 Wilson, P.D. (1996). The nuclear fuel cycle: from ores to Wastes. 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