Nuclear Fission

This lesson covers nuclear fission — the splitting of heavy nuclei to release energy — as required by the Edexcel GCSE Physics specification (1PH0), Topic 6: Radioactivity. You need to understand what fission is, how a chain reaction works, how nuclear power stations use controlled fission to generate electricity, and the advantages and disadvantages of nuclear power.

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What Is Nuclear Fission?

Nuclear fission is the process in which a heavy, unstable nucleus splits into two smaller nuclei (called daughter nuclei or fission fragments), releasing:

• Two or three neutrons
• A large amount of energy (as kinetic energy of the fragments and neutrons)
• Gamma radiation

Key Points

• Fission occurs in heavy nuclei — the two fuels used in nuclear power are uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu).
• Fission is triggered (induced) when the nucleus absorbs a slow-moving neutron (called a thermal neutron).
• The neutron is absorbed by the nucleus, making it even more unstable → the nucleus splits.
• The two daughter nuclei are not always the same — many different combinations of fission products are possible.