Operation of a Nuclear Reactor (SL IB Physics)

• In a nuclear reactor, a chain reaction is required to keep the reactor running
• When the reactor is producing energy at the required rate, two factors must be controlled:
  • The number of free neutrons in the reactor
  • The energy of the free neutrons

• The main components of a nuclear reactor are:
  • Control rods
  • Moderators
  • Heat exchangers
  • Shielding

The overall purpose of a nuclear reactor is to collect the heat energy produced from nuclear reactions

Control Rods

Purpose of a control rod: To absorb neutrons

• Control rods are made of a material which absorbs neutrons without becoming dangerously unstable themselves

• 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

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
  • Moderators are made from materials that are poor absorbers of neutrons, such as water

• 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

Heat exchangers

The purpose of a heat exchanger: To transfer thermal energy efficiently between the water systems of a nuclear power plant

• There are multiple water systems in a nuclear power plant that need regulating
  • The coolant (usually water) used in the reactor vessel
  • The water and steam that drives the turbine
  • The condenser that cools the steam 

• The heat exchanger mediates the thermal energy exchanges between these water systems
• The coolant is a substance, such as water, that is pumped into the reactor at a cold temperature to extract the heat released by the fission reactions
• In the heat exchanger, the coolant transfers the heat to water that is pumped in externally to produce steam
• This steam then goes on to power electricity-generating turbines

Water systems in a nuclear power plant are regulated by a heat exchanger

Shielding

Purpose: To house the reactor and absorb hazardous radiation

• The entire nuclear reactor is surrounded by shielding materials
• The daughter nuclei formed during fission, and the neutrons emitted, are radioactive
• The reactor is surrounded by a steel and concrete wall that can be nearly 2 metres thick
• This absorbs the emissions from the reactions
  • It ensures that the environment around the reactor is safe

Shielding metals in a nuclear reactor