Nuclear Fission Reactors (WJEC GCSE Physics): Revision Note

Nuclear Fission Reactors

• In a nuclear reactor, a chain reaction is required to keep the reactor running

• When the reactor is producing energy at the correct rate, two factors must be controlled:

  • The number of free neutrons in the reactor

  • The energy of the free neutrons

• To do this, nuclear reactors contain control rods and moderators

Structure of a Nuclear Reactor

The overall purpose of the 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, such as boron, 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 reactions can take place

Moderator

The purpose of a moderator: To slow down neutrons

• The moderator is a material, such as water or graphite, that surrounds the fuel rods and control rods inside the reactor core

• The fast-moving neutrons produced by the fission reactions must be slowed down so they can be absorbed by the uranium atoms

• As neutrons collide with the molecules of the moderator, they transfer some energy which causes them to lose some momentum

• The neutrons are slowed down until they are in thermal equilibrium with the moderator

Shielding

The purpose of shielding: To absorb hazardous radiation

• The entire nuclear reactor is surrounded by shielding materials

• It consists of many layers of steel and concrete walls that can be up to 2 metres thick

• These materials absorb energetic emissions from the reactor to ensure that the surrounding areas are safe for workers

• Shielding is required to contain the daughter nuclei formed during fission which are highly radioactive and prevent emissions from leaking, such as

  • High-energy gamma rays

  • Fast-moving neutrons

Shielding materials in a nuclear reactor

Shielding requires many layers of thick, dense materials, such as steel and concrete to contain the energetic neutrons and gamma-rays produced during fission