---
title: "Chain reaction"
subject: "Communication English"
chapter: 2
key: "semester_5/communication_english:chain-reaction-describe"
source: https://www.bctengineer.com/q/semester_5/communication_english/chain-reaction-describe
site: BCT Engineering
---

# Chain reaction

Communication English · Chapter 2: Extensive Reading

- Asked in: 2072 Chaitra, 5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/communication_english/chain-reaction-describe
- Whole chapter: https://www.bctengineer.com/md/semester_5/communication_english/chapter-2

## Question

What is chain reaction? Describe it in brief. (Chain Reaction)

## Short answer

A chain reaction is a self-sustaining series of nuclear fissions in which neutrons from one fission cause more fissions, multiplying energy release. It requires slow neutrons and a critical mass. It is uncontrolled in a bomb and controlled in a nuclear reactor.

## Answer

A chain reaction is a self-sustaining series of nuclear fissions in which the neutrons released by one fission cause further fissions, so that the process continues and multiplies.

#### Description
1. A neutron strikes the nucleus of a fissile atom such as uranium-235. The nucleus splits into two smaller nuclei, releasing a large amount of energy.
2. Each fission also emits 2 or 3 new neutrons (about 2.5 on average).
3. These new neutrons strike other uranium nuclei and cause further fissions, releasing more neutrons and energy.
4. The number of fissions grows rapidly, as shown below.

```text
   n --> U --> 3 n --> 3 U --> 9 n --> 9 U ...
        (1 fission)  (3 fissions) (9 fissions)
```

#### Conditions
- **Thermal neutrons.** Neutrons are most effective at causing fission when they move slowly, so a **moderator** (graphite, heavy water) is used to slow them down.
- **Critical mass.** This is the minimum mass of fissile material needed to keep the chain reaction going. If the mass is smaller, too many neutrons escape and the reaction stops.

#### Types
- **Uncontrolled:** the number of fissions increases extremely fast and releases enormous energy at once, as in an atomic bomb.
- **Controlled:** control rods (such as cadmium or boron) absorb extra neutrons and keep the rate steady, as in a nuclear power reactor.

Thus a chain reaction is the basis of both nuclear weapons and nuclear energy.

## Check yourself

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

1. What is a chain reaction?
   A. A single fission event
   B. A self-sustaining series of fissions where released neutrons cause further fissions
   C. Fusion of hydrogen
   D. Radioactive decay of one atom
   Answer: B. Neutrons from one fission trigger more fissions. (Key point: Definition)

2. What is critical mass?
   A. The maximum uranium allowed
   B. The mass of a neutron
   C. The mass of the moderator
   D. The minimum quantity of uranium needed to sustain the chain reaction
   Answer: D. Below it too many neutrons escape and the reaction stops. (Key point: Critical mass)

3. What does a moderator do?
   A. Absorbs neutrons to control rate
   B. Slows neutrons so they are effective (thermal)
   C. Increases neutron speed
   D. Provides fuel
   Answer: B. Graphite or heavy water slow neutrons. (Key point: Moderator)

4. How is a chain reaction controlled in a reactor?
   A. Adding more uranium
   B. Removing the moderator only
   C. Letting it grow freely
   D. Control rods of cadmium or boron absorb extra neutrons
   Answer: D. Uncontrolled reactions, as in a bomb, grow extremely fast. (Key point: Control)

## Related in Communication English

- [Notes and summary: nuclear fission](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Anotes-summary-nuclear-fission-chain-reaction) — Communication English, chapter 1

## Related in other subjects

- [Priority and daisy-chain arbitration briefly](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aarbitration-priority-daisy-brief) — Embedded System, chapter 5