Nuclear Fission Reactor & Waste (OCR A Level Physics)

Moderator

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

Coolant

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

Components of a nuclear reactor

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

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