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
