Nuclear Fuel Cycle

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Chapter 8 : Nuclear Fuel Cycle

Nuclear Fuel Cycle arrow_upward

  • The Nuclear Fuel Cycle is a series of industrial processes.
    • It involves the production of electricity from uranium in nuclear power reactors.
  • The fuel cycle of a nuclear reactor encompasses the following three stages:
    • Front end
    • Reactor
    • Back end
  • Front end:
    • The “Front end” refers to the preparation of uranium (U) for use as fuel in a reactor.
    • Front end includes the following steps:
    • Mining and Milling
    • Conversion
    • Enrichment
    • Fuel Fabrication
  • Reactor:
    • The second stage of the fuel cycle involves the use of the fuel in the reactor.
  • Back end:
    • Storing the used fuel discharged from reactors.
    • Ultimately disposing of the waste products.
    • Back end includes the following steps:
    • Spent Fuel Storage
    • Waste Disposal
    • Reprocessing

    Figure: Nuclear Fuel Cycle

  • The Nuclear Fuel Cycle consists of sequence of steps in which uranium ore is mined, milled, enriched and fabricated into nuclear fuel and then irradiated in a reactor for several years.

  • Uranium arrow_upward

  • Uranium is a slightly radioactive metal that occurs throughout the Earth's crust.
  • It is about 500 times more abundant than gold and as common as tin.

  • Mining arrow_upward

  • Uranium ore can be extracted through conventional mining in open pit and underground methods similar to those used for mining other metals.
  • The uranium content of the ore is often between only 0.1% and 0.2%.
  • Therefore, large amounts of ore have to be mined to get at the uranium.
  • In situ leach mining method uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant.

  • Milling arrow_upward

  • Mined uranium ores normally are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching.
  • The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake", which is sold on the uranium market as U3 O8 .

  • Conversion arrow_upward

  • The uranium oxide product of a uranium mill is not directly usable as a fuel for a nuclear reactor and additional processing is required.
  • Only 0.7% of natural uranium is 'fissile', or capable of undergoing fission, the process by which energy is produced in a nuclear reactor.
  • The form or isotope, of uranium which is fissile is the uranium-235 (U-235) isotope.
    • The remainder is uranium-238 (U-238).
  • For most kinds of reactor, the concentration of the fissile uranium-235 isotope needs to be increased – typically between 3.5% and 5% U-235.
    • This is done by a process known as enrichment.

    Enrichment arrow_upward

  • The enrichment process separates gaseous uranium hexafluoride into two streams.
    • One being enriched to the required level and known as low-enriched uranium.
    • The other stream is progressively depleted in U-235 and is called 'tails', or simply depleted uranium.
  • There are two enrichment processes in large-scale commercial use, each of which uses uranium hexafluoride gas as feed:
    • Diffusion
    • Centrifuge
  • Both these processes use the physical properties of molecules, specifically the 1% mass difference between the two uranium isotopes, to separate them.
  • Natural uranium contains 0.72 % U - 235 and 99.27% U -238.

  • Fuel Fabrication arrow_upward

  • In a fuel fabrication plant, great care is taken with the size and shape of processing vessels to avoid criticality (a limited chain reaction releasing radiation).
  • UF6 is converted into UO2 clad and then grouped into fuel bundles.

  • Reactor arrow_upward

  • Several hundred fuel assemblies make up the core of a reactor.
  • For a reactor with an output of 1000 megawatts (MW), the core would contain about 75 tons of low-enriched uranium.
  • In the reactor core the U-235 isotope fissions or splits, producing heat in a continuous process called a chain reaction.
  • The process depends on the presence of a moderator such as water or graphite, and is fully controlled.
  • Some of the U-238 in the reactor core is turned into plutonium and about half of this is also fissioned, providing about one third of the reactor's energy output.
  • As in fossil-fuel burning electricity generating plants, the heat is used to produce steam to drive a turbine and an electric generator.
    • In this case producing about 7 billion kilowatt hours of electricity in one year.
  • To maintain efficient reactor performance, about one-third of the spent fuel is removed every year or 18 months, to be replaced with fresh fuel.

  • Spent Fuel Storage arrow_upward

  • All nuclear plants have storage pools for spent fuel.
  • These pools are typically 40 or more feet deep.
  • In the bottom 14 feet are storage racks designed to hold fuel assemblies removed from the reactor.
  • In many countries, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for 10 to 20 years.
  • The water serves 2 purposes:
    • It serves as a shield to reduce the radiation levels that people working above may be exposed to.
    • It cools the fuel assemblies that continue to produce heat (called decay heat) for some time after removal.

    Waste Disposal arrow_upward

  • Radioactive wastes come in many different forms including the following:
    • Protective clothing of people in contact with radioactive materials.
    • The remains of laboratory animals used in experiments with radionuclides.
    • Cooling water, used fuel rods, and old tools and parts from nuclear power plants.
    • Mill tailings from uranium-enrichment factories.
    • Old medical radiation equipment from hospitals and clinics.
    • Used smoke detectors which contain radioactive americium-241 sensors.

    Types of Nuclear Waste arrow_upward

  • High-level Waste:
    • High-level waste consists mostly of spent nuclear reactor fuel from commercial power plants and military facilities, as well as reprocessed materials which can emit large amounts of radiation for hundreds of thousands of years.
  • Mill Tailings:
    • Mill tailings left over when ore is refined and processed; it is the largest by volume of any form of radioactive waste.
    • Only 1% of uranium ore contains uranium--the rest is left on-site as sand like residue.
    • These tailings are generally left outdoors in huge piles, where they blow around, releasing radioactive materials into the surrounding air and water.
  • Low-Level Waste:
    • Low-level wastes are usually defined in terms of what they are not.
    • They are not spent fuel, milling tailings, reprocessed materials, or transuranic materials.
    • Low-level waste includes the remainder of radioactive wastes and materials generated in power plants, such as contaminated reactor water, plus those wastes created in medical laboratories, hospitals, and industry.

    Reprocessing arrow_upward

  • Nuclear reprocessing reduces the volume of high-level waste, but by itself does not reduce radioactivity or heat generation and therefore does not eliminate the need for a geological waste repository.
  • Reprocessing has been politically controversial because of the potential to contribute to nuclear proliferation.
    • The potential vulnerability to nuclear terrorism, the political challenges of repository siting (a problem that applies equally to direct disposal of spent fuel), and because of its high cost compared to the once-through fuel cycle
  • The Objectives of Reprocessing are:
    • Recover uranium, plutonium and thorium to be used as fuel.
    • Separate radioactive and neutron- absorbing fission products.
    • Convert the radioactive waste into suitable forms for safe storage.

    Thank You from Kimavi arrow_upward

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