A nuclear power station uses uranium as a fuel for nuclear fission. Explain how a chain reaction is set up in a nuclear fission reactor and why the process releases large amounts of energy.

OCR A-Level Physics A (H556) — 6.2 Electric and magnetic fields · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Nuclear power stations generate electricity by harnessing energy released during nuclear fission. Uranium nuclei are used as the fuel because they are large and unstable, making them suitable for fission reactions.

Model answer (5 marks)

A neutron is absorbed by a large, unstable uranium nucleus, making it even more unstable. The nucleus then splits into two smaller nuclei – the fission products – and releases two or three neutrons. The released neutrons are absorbed by other uranium nuclei, causing further fission events, which sets up a self‑sustaining chain reaction. Energy is released because the total mass of the fission products and neutrons is less than the original uranium nucleus; the mass difference is converted into energy (E=mc²) and appears as electromagnetic radiation and kinetic energy of the fission products.

Examiner tips

  • Use the exact sequence: neutron absorption → fission → neutron release → further absorption
  • Show the mass‑energy conversion explicitly
  • Keep the answer concise – 5 marks, no extra detail

Common mistakes

  • Confusing the neutron release with the initial neutron absorption
  • Omitting the mass‑energy equivalence explanation
  • Using vague terms like "lots of energy" instead of "mass converted to energy"

Mark scheme (5 marks)

  1. A neutron is absorbed by a large, unstable uranium nucleus
  2. The unstable nucleus splits into two smaller nuclei (fission products)
  3. Two or three neutrons are released during the fission
  4. The released neutrons are absorbed by other uranium nuclei, causing further fission events — setting up a chain reaction
  5. Energy is released because mass is converted into energy (mass-energy equivalence) / energy is released as electromagnetic radiation with each fission event

Key terms in this question

nuclear fission · chain reaction

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