AQA GCSE Physics

Revision Notes

4.4.2 Chain Reactions

Chain Reactions

  • Only one extra neutron is required to induce a Uranium-235 nucleus to split by fission
  • During the fission, it produces 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, downloadable IGCSE & GCSE Physics revision notes

The neutrons released by each fission reaction can go on to create further fissions, like a chain that is linked several times – from each chain comes two more

Controlled Chain Reactions

  • In a nuclear reactor, a chain reaction is required to keep the reactor running
  • When the reactor is producing energy at the correct rate, the number of free neutrons in the reactor needs to be kept constant
    • This means some must be removed from the reactor
  • To do this, nuclear reactors contain control rods
  • These absorb neutrons without becoming dangerously unstable themselves

Uncontrolled Chain Reactions

  • Because each new fission reaction releases 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

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