# Explain why a controlled chain reaction in a nuclear fission reactor requires the use of a moderator.

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

> In a nuclear fission reactor, uranium-235 nuclei undergo fission when struck by neutrons, releasing energy and further neutrons. Without careful control, the reaction rate can increase uncontrollably.

## Mark scheme (4 marks)

1. Neutrons released by fission are fast (high kinetic energy) neutrons
2. Fast neutrons have a low probability of causing further fission in uranium-235 / U-235 undergoes fission most readily with slow (thermal) neutrons
3. The moderator slows neutrons down by (elastic) collisions with moderator nuclei / neutrons lose kinetic energy through repeated collisions
4. This produces slow (thermal) neutrons that sustain a controlled chain reaction at a steady rate / maintaining a criticality of approximately one fission per neutron on average

## Key terms

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

## 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-a-controlled-chain-reaction-be1db951) · Published by Druglandscape Ltd.