A nuclear power station uses uranium as its fuel. Explain how a chain reaction is set up inside the reactor and why engineers must be able to control the rate at which the chain reaction proceeds.
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. Uranium nuclei are used as the fuel because they are large and unstable.
Model answer (5 marks)
1. A uranium nucleus captures a neutron and becomes unstable, then splits into two smaller nuclei.
2. The fission releases further neutrons (and energy/gamma radiation).
3. The released neutrons are absorbed by other uranium nuclei, causing them to split.
4. This creates a self‑sustaining chain reaction where each split produces more splits, releasing increasing energy.
5. If the reaction is not controlled, the fission rate and energy release rise uncontrollably, risking a dangerous melt‑down.
2. The fission releases further neutrons (and energy/gamma radiation).
3. The released neutrons are absorbed by other uranium nuclei, causing them to split.
4. This creates a self‑sustaining chain reaction where each split produces more splits, releasing increasing energy.
5. If the reaction is not controlled, the fission rate and energy release rise uncontrollably, risking a dangerous melt‑down.
Examiner tips
- Use the exact sequence: capture → fission → neutron release → further fission → chain reaction. Show why control is needed: uncontrolled rate = dangerous. Keep each point short and to the point.
Common mistakes
- Mixing up the order of events (e.g. saying neutrons are released before fission). Using vague terms like "more energy" without linking to neutron release. Forgetting to mention the danger of an uncontrolled reaction.
Mark scheme (5 marks)
- A uranium nucleus absorbs a neutron and becomes unstable / splits into two smaller nuclei
- The fission of uranium releases further neutrons (and energy / gamma radiation)
- The released neutrons go on to be absorbed by other uranium nuclei, causing them to split in turn
- This creates a self-sustaining chain reaction in which each split causes further splits, releasing increasing amounts of energy
- If the chain reaction is not controlled, the rate of fission (and energy release) increases uncontrollably, which could be dangerous / cause a meltdown
Key terms in this question
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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