GCSE Physics · AQA 8463 · Topic 4.4.4
Nuclear fission and fusion
Key points
- Nuclear fission is the splitting of a large, unstable nucleus such as uranium-235 or plutonium-239. It usually starts when the nucleus absorbs a neutron.
- Fission produces two smaller nuclei, two or three neutrons and gamma rays, and releases a lot of energy as kinetic energy of the products.
- The released neutrons can cause more fissions: a chain reaction. In a reactor, control rods absorb neutrons to keep it steady; a moderator slows the neutrons.
- An uncontrolled chain reaction is used in a nuclear weapon.
- Nuclear fusion is the joining of two light nuclei, such as hydrogen isotopes, to form a heavier nucleus. Some mass becomes energy.
- Fusion powers the Sun but needs extremely high temperatures and pressures, so it is hard to use in power stations.
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The nucleus holds enormous amounts of energy. Splitting very large nuclei (fission) powers nuclear reactors, while joining very small ones (fusion) powers the Sun. This sub-topic explains both processes and how a chain reaction is kept…
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