Nuclear Fission Reactor & Waste (OCR A Level Physics)

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Structure of a Fission Reactor


The purpose of a moderator: To slow down neutrons

  • The moderator is a material that surrounds the fuel rods and control rods inside the reactor core
  • The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum
  • The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’
    • This ensures neutrons can react efficiently with the uranium fuel

Control Rods

Purpose of a control rod: To absorb neutrons

  • The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods
    • Lowering the rods further decreases the rate of fission, as more neutrons are absorbed
    • Raising the rods increases the rate of fission, as fewer neutrons are absorbed

  • This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission
  • In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way so no reaction can take place


The purpose of coolant: To remove the heat released by the fission reactions

  • The coolant carries the heat to an external boiler to produce steam
  • This steam then goes on to power electricity-generating turbines

Nuclear Reactor Components, downloadable AS & A Level Physics revision notes

Components of a nuclear reactor

Environmental Impact of Nuclear Waste

The End of the Reactor Process

  • Within the fuel rods, nuclei of uranium-238 quickly decay into nuclei of plutonium-239
    • These nuclei are extremely radioactive
    • They have a long half-life of 24 000 years

  • So, plutonium-239 decays slowly
    • It will remain radioactive for a very long time
    • So, it presents a risk of contamination for a long time
    • It is classified as high-level radioactive waste

Types of Radioactive Waste

  • There are three main types of nuclear waste:
    • Low level
    • Intermediate level
    • High level

  • Low-level waste
    • This is waste such as clothing, gloves and tools which may be lightly contaminated
    • This type of waste will be radioactive for a few years, so must be encased in concrete and stored a few metres underground until it can be disposed of with regular waste

  • Intermediate-level waste
    • This is everything between daily used items and the fuel rods themselves
    • Usually, this is the waste produced when a nuclear power station is decommissioned and taken apart
    • This waste will have a longer half-life than the low-level waste, so must be encased in cement in steel drums and stored securely underground

  • High-level waste
    • This waste comprises of the unusable fission products from the fission of uranium-235 or from spent fuel rods
    • This is by far the most dangerous type of waste as it will remain radioactive for thousands of years
    • As well as being highly radioactive, the spent fuel roads are extremely hot and must be handled and stored much more carefully than the other types of waste

  • How high-level waste is treated:
    • The waste is initially placed in cooling ponds of water close to the reactor for a number of years
    • Isotopes of plutonium and uranium are harvested to be used again
    • Waste is mixed with molten glass and made solid (this is known as vitrification)
    • Then it is encased in containers made from steel, lead, or concrete
    • This type of waste must be stored very deep underground

Radioactive waste, downloadable IGCSE & GCSE Physics revision notes

Depending on the activity of radioactive waste, it is buried in different ways

Environmental Considerations

  • Isotopes with long half-lives must not enter our water and food supplies
  • Burial locations must be geologically stable, secure from attack, and designed for safety
  • Space for such locations is limited

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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