# A uranium-235 nucleus absorbs a slow-moving neutron and undergoes nuclear fission. Describe what happens during this fission event and explain how a chain reaction develops as a result.

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

## Mark scheme (4 marks)

1. The uranium-235 nucleus splits into two smaller (daughter) nuclei
2. Two or three neutrons (and energy) are released during the fission event
3. The released neutrons are absorbed by other uranium-235 nuclei, inducing further fission events
4. Each fission event releases more neutrons, so the number of fission events increases rapidly, sustaining the chain reaction

## Key terms

- [nuclear fission](https://www.gradenine.co.uk/glossary/nuclear-fission)
- [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 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-uranium-235-nucleus-absorbs-a-slow-moving-4a3d0dd4) · Published by Druglandscape Ltd.