---
title: "Notes and summary: nuclear fission"
subject: "Communication English"
chapter: 1
key: "semester_5/communication_english:notes-summary-nuclear-fission-chain-reaction"
source: https://www.bctengineer.com/q/semester_5/communication_english/notes-summary-nuclear-fission-chain-reaction
site: BCT Engineering
---

# Notes and summary: nuclear fission

Communication English · Chapter 1: Intensive Reading

- Asked in: 2075 Chaitra, 5+5 marks
- Video: [Nuclear Fission v Nuclear Fusion: Differences and Similarities Explained](https://www.youtube.com/watch?v=3nvkHjn1ETU) — Science ABC
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/communication_english/notes-summary-nuclear-fission-chain-reaction
- Whole chapter: https://www.bctengineer.com/md/semester_5/communication_english/chapter-1

## Question

Read the text given below carefully, make notes and write summary of it:
When fission occurs, an average of 2.5 neutrons are emitted from the nucleus. [passage of about 230 words on chain reaction in uranium, thermal neutrons, moderators and critical mass] ... The minimum quantity of uranium required to initiate the chain reaction is called the critical mass.

## Short answer

Fission of a nucleus like uranium-235 releases on average 2.5 neutrons, which can split other nuclei, causing a chain reaction and huge energy release. Thermal neutrons are most effective, so a moderator slows them. The minimum uranium mass needed to sustain the reaction is the critical mass.

## Answer

#### Notes
**Title: Nuclear chain reaction**

1. **Fission**
   - A nucleus (e.g., uranium-235) splits; on average 2.5 neutrons are emitted
2. **Chain reaction**
   - Emitted neutrons strike other nuclei, causing more fission
   - Number of fissions increases rapidly; large energy released
3. **Thermal neutrons**
   - Fast neutrons must be slowed to thermal speed to cause fission efficiently in U-235
4. **Moderator**
   - Material (graphite, heavy water) slows neutrons by collisions
5. **Critical mass**
   - Minimum quantity of uranium needed to start and sustain the chain reaction

#### Summary
When a uranium nucleus undergoes fission, it emits two or three neutrons, 2.5 on average. These neutrons can split other nuclei, so that the process continues as a chain reaction and releases a great deal of energy. Because slow (thermal) neutrons are most effective, a moderator such as graphite or heavy water is used to slow them down. The minimum quantity of uranium needed to start the chain reaction is the critical mass.

*(About 70 words.)*

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. On average, how many neutrons are emitted in fission?
   A. Exactly 1
   B. 10 to 20
   C. 2 to 5
   D. None
   Answer: C. An average of 2-5 neutrons are emitted from the nucleus. (Key point: Neutrons emitted)

2. What is the role of a moderator?
   A. To slow neutrons down to thermal speed by collisions
   B. To speed up neutrons
   C. To absorb all neutrons and stop the reaction
   D. To cool the uranium
   Answer: A. Materials like graphite or heavy water slow fast neutrons so they cause fission efficiently. (Key point: Moderator)

3. What is critical mass?
   A. The maximum quantity of uranium a reactor can hold
   B. The mass of one neutron
   C. The minimum quantity of uranium required to start the chain reaction
   D. The mass lost in fission
   Answer: C. It is the smallest amount needed to initiate and sustain a chain reaction. (Key point: Critical mass)

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