A nuclear power station uses uranium as a fuel. Explain how a chain reaction occurs inside the reactor core, and why the process releases large amounts of energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nuclear power stations generate electricity by harnessing the energy released during nuclear fission. Inside the reactor core, uranium nuclei undergo fission, triggering a self-sustaining chain reaction.
Model answer (5 marks)
A neutron is captured by a heavy uranium nucleus, making it unstable.
The unstable nucleus splits into two smaller nuclei (fission fragments).
Two or three neutrons and a large amount of energy (plus gamma rays) are released.
The released neutrons are absorbed by other uranium nuclei, causing further fission events – this self‑sustaining chain reaction.
The mass of the fission fragments is slightly less than the original mass; the lost mass is converted to energy (E=mc²), giving a large energy release.
The unstable nucleus splits into two smaller nuclei (fission fragments).
Two or three neutrons and a large amount of energy (plus gamma rays) are released.
The released neutrons are absorbed by other uranium nuclei, causing further fission events – this self‑sustaining chain reaction.
The mass of the fission fragments is slightly less than the original mass; the lost mass is converted to energy (E=mc²), giving a large energy release.
Examiner tips
- Use the exact sequence: neutron capture → fission → neutron release → further fission.
- Mention mass loss and E=mc² to justify the energy release.
- Keep each point brief but complete; use UK spelling.
Common mistakes
- Confusing the neutron source with the fission fragments.
- Forgetting to state that the neutrons cause further fission (the chain reaction).
- Using vague terms like “splits” without mentioning the release of neutrons and energy.
Mark scheme (5 marks)
- A neutron is absorbed by a large, unstable uranium nucleus
- The unstable nucleus splits (undergoes fission) into two smaller nuclei
- Two or three neutrons are released during the fission event (along with energy / gamma rays)
- The released neutrons are absorbed by other uranium nuclei, causing further fission events — this is the chain reaction
- Mass is converted into energy (mass-energy equivalence), so a small loss of mass produces a very large release of energy
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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