---
title: "Nuclear Fission and Fusion"
description: "Fission splits a large nucleus after it absorbs a neutron, releasing energy and more neutrons that can keep a chain reaction going, which a reactor controls with rods. Fusion joins small nuclei instea"
canonical: https://lightmysky.com/learn/science/nuclear-fission-and-fusion-mt_jkO89KodSb
source: https://lightmysky.com/learn/science/nuclear-fission-and-fusion-mt_jkO89KodSb.md
retrieved: 2026-09-12
---

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# Nuclear Fission and Fusion

Fission splits a large nucleus after it absorbs a neutron, releasing energy and more neutrons that can keep a chain reaction going, which a reactor controls with rods. Fusion joins small nuclei instead, and needs the temperature and pressure found in stars.

Subject: Science · Area: Matter & Materials · Ages 15 to 16
Page: https://lightmysky.com/learn/science/nuclear-fission-and-fusion-mt_jkO89KodSb

## Ready when they can

- Describes a fission chain reaction and the job of the control rods and moderator
- Explains why fusion needs enormous temperature and pressure to start
- Compares the two processes by what goes in, what comes out and the waste each leaves

## Lesson: Splitting heavy, joining light

Fission starts when a heavy nucleus like uranium 235 swallows a neutron and wobbles apart into two lighter nuclei. The split releases energy plus two or three fresh neutrons, and those neutrons can split further nuclei. Left alone that doubling becomes a runaway chain reaction.

Reactors tame the chain with a moderator and control rods. The moderator slows neutrons so splits stay likely, while the rods soak up spare neutrons to hold the rate steady. Push the rods in to calm the reactor, and pull them out to raise power.

**Example.** Jon compares the two star stories. Fusion joins light hydrogen into heavier nuclei and releases energy, but both nuclei are positive and fight coming close. Only star grade temperature and pressure give them the speed and squeeze to touch. That is why fusion lights the Sun but not yet our grid.

**Tip.** Compare the two by what goes in and what comes out. Fission feeds on heavy nuclei and leaves split fragments plus long lived waste stored for centuries. Fusion feeds on light hydrogen and leaves helium plus a fast neutron, with far shorter lived waste. Fission wins on practicality for now.

**Recap.** Fission splits heavy atoms in a tamed chain, fusion joins light ones in stars.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Where Elements Come From](https://lightmysky.com/learn/science/where-elements-come-from-mt_KYx0m4OyZv)
- [Irradiation, Contamination and Dose](https://lightmysky.com/learn/science/irradiation-contamination-and-dose-mt_WyxR7ZVEhi)

## Opens up

- [The Electromagnetic Spectrum: Uses and Dangers](https://lightmysky.com/learn/science/the-electromagnetic-spectrum-uses-and-dangers-mt_P_NXNSo1zQ)
- [Energy Released in Fission and Fusion](https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY)