# Explain why enriched uranium, rather than natural uranium, is typically used as fuel in most nuclear fission reactors.

> IB DP Physics Standard Level (2023 syllabus) — E.4 Fission · Explain · 4 marks

> Natural uranium consists of approximately 99.3% uranium-238 and only 0.7% uranium-235. Reactor-grade enriched uranium typically contains between 3% and 5% uranium-235.

## Mark scheme (4 marks)

1. Fission in a thermal reactor is initiated primarily by slow (thermal) neutrons absorbed by uranium-235 nuclei.
2. Uranium-238 tends to absorb slow neutrons without undergoing fission, removing them from the chain reaction.
3. The low natural abundance of U-235 (0.7%) means there is insufficient fissile material to sustain a chain reaction in natural uranium in most reactor designs.
4. Increasing the proportion of U-235 through enrichment raises the probability of fission events per neutron, allowing a self-sustaining chain reaction to be maintained.

## Key terms

- [enriched uranium](https://www.gradenine.co.uk/glossary/enriched-uranium)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "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/explain-why-enriched-uranium-rather-than-14873ff9) · Published by Druglandscape Ltd.