Structure of a Fission Reactor
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
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
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
