In a nuclear power station, uranium-235 is used as fuel. Describe the roles of the control rods and the moderator in maintaining a controlled chain reaction inside the nuclear reactor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nuclear power stations use the process of nuclear fission to generate electricity. Uranium-235 nuclei are split by absorbing neutrons, releasing energy and further neutrons that can go on to cause additional fission events.
Model answer (4 marks)
Control rods are inserted into the reactor core to absorb excess neutrons. By removing neutrons from the system they limit the number of neutrons that can go on to cause further fission, so that only about one neutron per fission event is available to sustain the chain reaction. The moderator, usually light or heavy water, is placed around the fuel. It slows down the fast neutrons produced in fission. Slower neutrons have a higher probability of being captured by uranium‑235 nuclei, which is necessary for the reaction to continue.
Examiner tips
- State the function of control rods and moderator separately
- Mention neutron absorption and slowing down
- Show how each keeps the chain reaction controlled
- Use terms: absorb, slow, sustain
Common mistakes
- Confusing moderator with coolant
- Saying control rods increase the reaction instead of limiting it
- Omitting the role of slowing neutrons in increasing absorption probability
Mark scheme (4 marks)
- Control rods absorb neutrons
- Control rods ensure only one neutron per fission event goes on to cause further fission, keeping the chain reaction controlled/sustained
- The moderator slows down (fast-moving) neutrons
- Slower neutrons are more easily absorbed by uranium-235 nuclei, allowing fission to continue
Key terms in this question
control rods · moderator · chain reaction · uranium-235
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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