Nuclear Fission and Chain Reactions — AQA GCSE Physics
Nuclear fission releases energy by splitting large nuclei, and can sustain a chain reaction.
Nuclear fission
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei. Common fuels are uranium-235 and plutonium-239.
The process:
- The unstable nucleus usually absorbs a neutron first.
- It splits into two smaller nuclei, releasing energy (as kinetic energy of the fragments) plus two or three neutrons and gamma rays.
Spontaneous fission is rare; it is usually triggered by neutron absorption.
Chain reactions
The neutrons released by one fission can be absorbed by other nuclei, causing further fission — a chain reaction.
- In a nuclear reactor, the chain reaction is controlled: control rods (e.g. boron) absorb excess neutrons to keep the reaction steady, and a moderator slows the neutrons. The energy heats water to make steam that drives turbines.
- In a nuclear weapon, the chain reaction is uncontrolled, releasing a huge amount of energy very quickly.
Exam tips
- Fission = splitting a large nucleus, usually after absorbing a neutron.
- It releases energy + 2–3 neutrons, which can cause a chain reaction.
- Reactors control the reaction with control rods (absorb neutrons).
- Uncontrolled chain reaction → nuclear weapon.