Nuclear Reactors

• Thermal nuclear reactor: generates energy through the fission of Uranium-235
• Uranium-235 is most likely to undergo fission when hit by slow, thermal neutrons
• Control rods: material that absorbs excess neutrons
• Coolant: a liquid that absorbs heat energy to transfer into power
• Radiation Shield: thick, concrete wall that prevents neutrons from escaping
• Difficulties with Nuclear Reactors

  • the neutrons released travel at high speeds, but must be slowed to be absorbed into another U-235 nucleus to continue the reaction
  • each U-235 releases on average 2.47 neutrons which can react with too many nuclei causing not only a chain reaction but an explosion, therefor some neutrons must be absorbed
  • the heat produced in the reactor must be collected and repurposed to create steam for turbines and generate the reactor
• Nuclear Fuel Rods: long, thin rods containing Uranium pellets (fuel)

  • Uranium samples do not contain enough U-235 to be fissile and therefore must be enriched
  • Once enriched the pellets are packed into thin aluminium tubes (the fuel rod)
  • Over time fuel rods deplete of U-235, eventually becoming unable for chain reaction and endure a 4 yr replacement strategy
• U-238 is refered to as fertile as it is not fissile but is able to capture fast moving electrons and form plutonium-239 which can is fissile
• Fast Breeder Reactor

  : when the initial neutrons emitted from U-235 sustain a chain reaction while also 'breeding' Pu-239 that can too be used as fuel
• Moderators: a material that slows (moderates) fast moving neutrons

  • substances with small nuclei can slow fast moving neutrons to speeds that they can be captured by
  • after many collisions, neutrons can be slowed from 2km/s to less than 1 eV
  • materials include: graphite, water, heavy water, carbon dioxide
• Control Rods: purpose is to achieve a steady energy release by controlling the number of neutrons involved in the fission chain

  • they must contain a material capable of absorbing neutrons i.e Cadmium and Boron steel
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• The core consists of the moderating material with fuel rods and control rods placed into it

  • the rate of chain reaction is controlled by raising/lowering the control rods
  • If wanting to shut down or slow the reactor the control rods will be lowered further into the core where the neutrons are absorbed and the chain reaction is stopped
• • fission reactions in the core creates temperatures of 500-1500c, which is removed into the coolant (coolant must have a high Specific heat capacity and is flushed around the reactor through pipes)
  • A heat exchanger transfer the energy into water pipes where it is converted into high pressure steam thats used to rotate turbines, fueling the generator
• • The reactor core is encased in a radiation shield of concrete, steel, graphite and lead that is 2 metres thick. Prevents radiation from escaping, ensuring worker safety
• Difference between Thermal Reactors and others

  how the heat is produced

  • Nuclear - through the fission process
  • Gas/coal - through the combustion of fossil fuels
• The management of wastes major problem is to dispose of unstable radioactive waste with long half-lives ethically
• To dispose nuclear waste it goes through a classification process to determine the best procedure for disposure
• Low-level Waste

  • Generated from: hospitals, industry and laboratories
  • consists of: tools, clothing, used wrapping materials and other items contaminated with radionuclides of short half-lives
  • Method of disposure: usually compacted or incinerated then buried in shallow pits on land or at sea
• Intermediate-level Waste

  • consists of: reactor components, chemical sludges, contaminated materials from reactors that have been decommissioned
  • method of disposure: solidified in bitumen/concrete then buried/stored in deep trenches
• High-level Waste

  • generated from: contaminated reactor parts, liquid waste from repurposing fuel rods containing highly radioactive fission fragments, therefore require special shielding during handling and transport
  • Method of disposure: CAN & USA: spent fuel rods are permanently stored in cooling ponds. JAP, RUS, EURO: spent fuel rods are repurposed where the uranium is extracted and repurposed as fuel