8.4.6 Operation 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

Components of a nuclear reactor

• During fission, neutrons are released with high energies and must be slowed down by a moderator 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

The process of moderation changes fast neutrons to slow neutrons

• 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:

Materials for Moderators Table