Chain Reactions (AQA GCSE Physics): Revision Note

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

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