AQA A Level Physics

Revision Notes

8.4.6 Operation of a Nuclear Reactor

Components of a Nuclear 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

Moderation of Fission Reactors

  • During fission, neutrons are released with high energies and must be slowed down by water moderation to maintain the chain reaction
  • The first few collisions of a neutron with the moderator transfer sufficient energy to excite nuclei in the moderator with the neutrons being absorbed
  • The subsequent collisions of a neutron with the moderator are elastic
  • In these subsequent collisions, momentum is transferred to the moderator atoms
    • With each collision, the neutron slows down until the average kinetic energy of the neutrons corresponds to that of the moderator nuclei
  • Eventually (after about 50 collisions), the neutrons reach speeds associated with thermal random motion (hence the name thermal neutron)
    • At these speeds, neutrons can cause fission rather than rebound off of the uranium nuclei

Materials Used for Nuclear Reactor Components

  • Moderators must be made from light nuclei which are not fissionable and will not absorb neutrons but will absorb a large amount of energy from them
    • Graphite and water are commonly used for moderators

 

  • Control rods must be made with non-fissionable materials
    • This is so that they can absorb excess neutrons without decaying themselves
    • Boron and cadmium are commonly used for control rods

 

  • Often water is used as both the coolant and moderator
    • This is because it has a high specific heat capacity meaning it can transfer large amounts of thermal energy
  • Other materials such as molten salt or inert gas (e.g helium) are sometimes used as a coolant

 

  • Another important component of a nuclear reactor is shielding
  • Alpha and beta radiation can be stopped by a few cm of material, however, gamma radiation is much more penetrating
    • Therefore, lead or concrete is needed to ensure there are no radiation leakages

 

  • A summary of materials used are shown in the table below:
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