GCSE Physics · Edexcel 1PH0 · Topic 6E
Nuclear fission and fusion
Key points
- Nuclear fission: a large nucleus (e.g. uranium-235) absorbs a slow neutron, becomes unstable and splits into two smaller daughter nuclei, releasing 2 or 3 neutrons and energy.
- The released neutrons can cause more fissions: a chain reaction.
- In a reactor, control rods absorb neutrons to control the rate; the moderator slows neutrons so they are absorbed; the coolant carries energy away to make steam.
- Fission produces radioactive waste that must be stored safely for a long time.
- Nuclear fusion joins two light nuclei (e.g. hydrogen isotopes) to make a heavier nucleus, releasing energy. It powers stars.
- Fusion needs very high temperature and density so nuclei overcome electrostatic repulsion; this is why fusion power stations are hard to build.
Topic notesPremium
The nucleus stores enormous amounts of energy. Splitting very heavy nuclei (fission) runs today's nuclear power stations, and joining very light nuclei (fusion) powers the Sun. This sub-topic explains both processes.
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