A nuclear power station uses uranium-235 as fuel. Describe what happens during a chain reaction in the reactor core, including the role of the neutrons released.

AQA GCSE Physics (8463) — 4.4.4 Nuclear fission and fusion (Phys only) · Describe · 4 marks · View as Markdown

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

Nuclear fission of uranium-235 is used in power stations to generate electricity. The fission reaction releases energy along with several neutrons.

Model answer (4 marks)

A neutron is absorbed by a U‑235 nucleus, making it unstable and causing it to split.
The split produces two smaller, more stable nuclei (fission products) and releases heat.
Two or three neutrons are emitted in the fission.
These free neutrons are then absorbed by other U‑235 nuclei, triggering further fission and allowing the reaction to continue.

Examiner tips

  • Use the exact terms ‘neutron absorption’, ‘fission’, ‘fission products’, and ‘self‑sustaining chain reaction’.
  • Show the sequence: absorption → fission → neutron release → further absorption.
  • Keep the answer concise – 4 marks, 4 points.

Common mistakes

  • Confusing the neutron release with the heat produced; they are separate. Mentioning only one neutron or not specifying that the reaction repeats. Using vague phrases like ‘more atoms split’ instead of ‘U‑235 nuclei’.

Mark scheme (4 marks)

  1. A neutron is absorbed by a uranium-235 nucleus, causing it to split (undergo fission)
  2. The fission produces two smaller (more stable) 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 reactions / the process repeats / self-sustaining reaction

Key terms in this question

chain reaction · neutron

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