A uranium-235 nucleus absorbs a slow-moving neutron and undergoes nuclear fission. Describe what happens during this fission event and explain how a chain reaction develops as a result.

Pearson Edexcel International GCSE Physics (4PH1) — 7.3 Fission and fusion · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A uranium‑235 nucleus captures a slow neutron and becomes an excited, unstable compound nucleus.
1. The excited nucleus splits into two smaller (daughter) nuclei.
2. Two or three neutrons and a large amount of energy are released.
3. The free neutrons are absorbed by other U‑235 nuclei, causing further fission.
4. Each fission event releases more neutrons, so the number of fission events rises rapidly, sustaining a chain reaction.

Examiner tips

  • Use the exact terms ‘daughter nuclei’, ‘slow neutron’, ‘chain reaction’.
  • Show the sequence: capture → fission → neutron release → further fission.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Mixing up the roles of neutrons (thinking they are produced by the daughter nuclei rather than released).
  • Failing to mention that the reaction is self‑sustaining because each event releases more neutrons.

Mark scheme (4 marks)

  1. The uranium-235 nucleus splits into two smaller (daughter) nuclei
  2. Two or three neutrons (and energy) are released during the fission event
  3. The released neutrons are absorbed by other uranium-235 nuclei, inducing further fission events
  4. Each fission event releases more neutrons, so the number of fission events increases rapidly, sustaining the chain reaction

Key terms in this question

nuclear fission · chain reaction · uranium-235

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