Chain Reactions (Oxford AQA IGCSE Physics)

Chain Reactions

• For fission to occur, a nucleus must first absorb one neutron

• The nucleus splits into two smaller nuclei (called daughter nuclei) of roughly equal size as well as two or three neutrons which move away at high speed

• Each of these new neutrons can start another fission reaction, which again creates further excess neutrons

• This process is called a chain reaction

Chain reaction analogy

Controlled chain reactions

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

• The kinetic energy of the fission products is transferred to water, causing it to boil

• The steam from this is used to generate electricity

• For the reactor to work safely in a stable state, the number of free neutrons in the reactor needs to be kept constant

  • This means some neutrons must be removed from the reactor

• To do this, nuclear reactors contain control rods

Structure of a nuclear reactor

• Control rods are made of a material, such as boron, which can absorb neutrons without becoming dangerously unstable

• 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, on average, only one neutron produced by each fission event goes on to cause another fission event

• In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way to absorb all of the neutrons so no further reactions can take place

Uncontrolled chain reactions

• Because each new fission reaction transfers energy, uncontrolled chain reactions can be dangerous

• The number of neutrons available increases quickly, so the number of reactions does too

• A nuclear weapon uses an uncontrolled chain reaction to release a huge amount of energy in a short period of time as an explosion