# Explain why nuclear fusion does not occur at low temperatures and pressures, and describe the role of the moderator in a nuclear fission reactor.

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

## Mark scheme (4 marks)

1. At low temperatures and pressures, the electrostatic repulsion between protons (in the two nuclei) cannot be overcome
2. High temperatures and/or pressures give the nuclei sufficient kinetic energy to overcome this repulsion
3. The moderator slows down (fast) neutrons produced during fission
4. So that the slow (thermal) neutrons can be absorbed by uranium-235 nuclei and induce further fission

## Key terms

- [nuclear fusion](https://www.gradenine.co.uk/glossary/nuclear-fusion)
- [moderator](https://www.gradenine.co.uk/glossary/moderator)
- [nuclear fission](https://www.gradenine.co.uk/glossary/nuclear-fission)

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain and 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/explain-why-nuclear-fusion-does-not-4a8a2a82) · Published by Druglandscape Ltd.