Explain how a chain reaction occurs during nuclear fission.

Eduqas GCSE Physics — 9.4 Nuclear fission and fusion · Explain · 4 marks · View as Markdown

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

Nuclear fission is used in nuclear power stations to generate electricity. During fission, uranium-235 nuclei are split apart.

Model answer (4 marks)

A neutron is absorbed by a uranium‑235 nucleus, making it unstable.
The unstable nucleus splits, or undergoes fission, and releases two or three neutrons.
The released neutrons are absorbed by other uranium‑235 nuclei, causing further fission events.
Because each fission releases more neutrons than were absorbed, the reaction is self‑sustaining and the number of fissions increases rapidly.

Examiner tips

Use the exact sequence: absorption → fission → neutron release → further absorption → self‑sustaining.
Show the cause–effect chain clearly; each step earns a mark.
Mention the number of neutrons released to justify the self‑sustaining nature.
Keep the answer concise – one sentence per point.

Common mistakes

Confusing the roles of neutrons (absorbed vs released).
Failing to state that the reaction is self‑sustaining or that the number of fissions increases.

Mark scheme (4 marks)

A neutron is absorbed by a uranium-235 nucleus
The nucleus splits / undergoes fission, releasing two or three neutrons
These neutrons go on to be absorbed by other uranium nuclei / cause further fission events
The reaction is self-sustaining / the number of fissions increases rapidly

Key terms in this question

nuclear fission · chain reaction