# 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.

> Pearson Edexcel International GCSE Physics (4PH1) — 7.3 Fission and fusion · Describe · 4 marks

> 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.

## Mark scheme (4 marks)

1. Control rods absorb neutrons
2. Control rods ensure only one neutron per fission event goes on to cause further fission, keeping the chain reaction controlled/sustained
3. The moderator slows down (fast-moving) neutrons
4. Slower neutrons are more easily absorbed by uranium-235 nuclei, allowing fission to continue

## Key terms

- [control rods](https://www.gradenine.co.uk/glossary/control-rods)
- [moderator](https://www.gradenine.co.uk/glossary/moderator)
- [chain reaction](https://www.gradenine.co.uk/glossary/chain-reaction)
- [uranium-235](https://www.gradenine.co.uk/glossary/uranium-235)

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/in-a-nuclear-power-station-uranium-235-6737d325) · Published by Druglandscape Ltd.