# A chain reaction occurs when uranium-235 undergoes nuclear fission in a nuclear reactor. Describe how a chain reaction is produced and explain how control rods prevent the chain reaction from becoming uncontrolled.

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

> In a nuclear reactor, uranium-235 is used as the fissile fuel. Without careful management, the fission process could escalate rapidly.

## Mark scheme (4 marks)

1. A uranium-235 nucleus absorbs a (slow-moving / thermal) neutron and splits into two smaller (daughter) nuclei
2. The fission event releases 2 or 3 neutrons (which go on to strike other uranium-235 nuclei)
3. Control rods absorb neutrons (reducing the number available to induce further fission)
4. Control rods ensure only one neutron per fission event goes on to induce further fission, keeping the reaction steady / controlled

## Key terms

- [chain reaction](https://www.gradenine.co.uk/glossary/chain-reaction)
- [nuclear fission](https://www.gradenine.co.uk/glossary/nuclear-fission)
- [control rods](https://www.gradenine.co.uk/glossary/control-rods)
- [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 and Explain" 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/a-chain-reaction-occurs-when-uranium-235-44fbafba) · Published by Druglandscape Ltd.