A nuclear power station uses uranium as a fuel. Explain how a chain reaction occurs in a nuclear fission reactor and why it is able to sustain itself.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nuclear power stations generate electricity using the energy released from nuclear fission reactions. Engineers must carefully control the chain reaction taking place inside the reactor to prevent it from becoming uncontrolled.
Model answer (5 marks)
1. A large, unstable uranium nucleus absorbs a neutron.
2. The nucleus becomes even more unstable and splits (undergoes nuclear fission) into two smaller, more stable nuclei.
3. The fission event releases energy as gamma‑ray radiation and kinetic energy of the fission fragments.
4. Two or three neutrons are also released during the split.
5. These released neutrons are absorbed by other uranium nuclei, causing further fission and the process repeats, sustaining a self‑sustaining chain reaction.
2. The nucleus becomes even more unstable and splits (undergoes nuclear fission) into two smaller, more stable nuclei.
3. The fission event releases energy as gamma‑ray radiation and kinetic energy of the fission fragments.
4. Two or three neutrons are also released during the split.
5. These released neutrons are absorbed by other uranium nuclei, causing further fission and the process repeats, sustaining a self‑sustaining chain reaction.
Examiner tips
- Use the exact sequence: neutron absorption → fission → energy release + neutrons → further fission.
- Show the chain‑reaction loop clearly; each step must be linked to the next.
Common mistakes
- Confusing the energy release with only heat; neglecting gamma radiation.
- Omitting the fact that the released neutrons are what sustain the reaction.
- Using vague terms like "breaks apart" instead of "undergoes fission."
Mark scheme (5 marks)
- A large, unstable uranium nucleus absorbs a neutron / an unstable nucleus splits (undergoes nuclear fission)
- The nucleus splits into two smaller, more stable nuclei and releases neutrons (two or three neutrons)
- Energy is released in the form of electromagnetic radiation (gamma rays) / kinetic energy of products
- The released neutrons are absorbed by other uranium nuclei, causing them to become unstable and undergo further fission
- This process repeats / is self-sustaining, forming a chain reaction where each fission event triggers further fission events
Key terms in this question
nuclear fission · chain reaction
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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