Nuclear Fission and Nuclear Fusion — AQA GCSE Physics
Nuclear fission and fusion both release energy from the nucleus, but in opposite ways.
Nuclear fission
Nuclear fission is the splitting of a large, unstable nucleus (e.g. uranium-235 or plutonium-239) into two smaller nuclei.
- Fission is usually triggered when the nucleus absorbs a neutron.
- It releases energy plus two or three more neutrons and gamma rays.
- The released neutrons can cause further fission — a chain reaction.
In a nuclear reactor the chain reaction is controlled (using control rods to absorb neutrons) so energy is released steadily. In a nuclear weapon it is uncontrolled.
Nuclear fusion
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.
- It releases even more energy than fission.
- It requires extremely high temperatures and pressures to overcome the repulsion between the positively charged nuclei.
- Fusion is the process that powers the Sun and stars (hydrogen nuclei fusing into helium).
Comparing fission and fusion
| Fission | Fusion | |
|---|---|---|
| Process | splits heavy nuclei | joins light nuclei |
| Conditions | absorb a neutron | very high temp/pressure |
| Where | nuclear reactors | stars/the Sun |
Exam tips
- Fission = splitting heavy nuclei; fusion = joining light nuclei.
- Fission releases energy + neutrons → chain reaction.
- Fusion needs very high temperature and pressure and powers stars.
- Reactors control the chain reaction with control rods.