---
title: "Fission Reactors and Fusion in the Laboratory"
description: "A chain reaction is controlled by moderating neutrons and absorbing the surplus, while fusion needs temperatures high enough to beat the Coulomb barrier and confinement long enough to pay for itself. "
canonical: https://lightmysky.com/learn/science/fission-reactors-and-fusion-in-the-laboratory-mt_iTwgXI0iBy
source: https://lightmysky.com/learn/science/fission-reactors-and-fusion-in-the-laboratory-mt_iTwgXI0iBy.md
retrieved: 2026-09-12
---

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# Fission Reactors and Fusion in the Laboratory

A chain reaction is controlled by moderating neutrons and absorbing the surplus, while fusion needs temperatures high enough to beat the Coulomb barrier and confinement long enough to pay for itself. The physics of each explains the engineering.

Subject: Science · Area: Quantum & Modern Physics · Ages 20 to 21
Page: https://lightmysky.com/learn/science/fission-reactors-and-fusion-in-the-laboratory-mt_iTwgXI0iBy

## Ready when they can

- Explains the role of moderator, control rods and coolant in a reactor
- States the conditions fusion needs and why confinement time enters
- Compares the energy released per kilogram and the waste produced by each route

## Lesson: Splitting atoms and fusing them

Fire a neutron into uranium-235 and the nucleus wobbles and splits into two lighter fragments plus two or three neutrons. The menu varies: barium plus krypton one time, xenon plus strontium the next. Mass numbers balance, the 236 nucleons going in equal the fragments plus the neutrons, and the fragments weigh less than the starting nucleus, so the missing mass becomes energy.

Those fresh neutrons can split further nuclei, a chain reaction. Exactly one triggered fission per fission on average holds it steady; fewer dies out and more runs away. Water slows fast neutrons to keep splits likely, cadmium control rods swallow the surplus, and coolant ferries heat to power. Fermi's Chicago pile proved it in 1942.

Fusion builds up instead: light nuclei join into heavier ones, releasing energy. But nuclei repel electrically, so only brutal heat gives them the speed to touch. Heat alone is not enough: dense fuel must stay confined long enough to pay back, so temperature, density, and time enter together.

**Example.** Per kilogram fusion releases far more and leaves no long-lived actinide waste, yet it is harder to sustain. The Sun shows the scale: its proton-proton chain turns four protons into helium plus about 26 MeV, and even so the Sun loses only a tiny fraction of its mass over billions of years.

**Recap.** Fission splits heavy nuclei in a controlled chain, while fusion joins light ones that must stay hot, dense, and confined.

## Practice

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

## Needs first

- [Decay Modes, Decay Chains and Nuclear Reactions](https://lightmysky.com/learn/science/decay-modes-decay-chains-and-nuclear-reactions-mt_b_imidwJeQ)
- [Energy Released in Fission and Fusion](https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY)

## Opens up

- [Particle Accelerators, Detectors and Cross-Sections](https://lightmysky.com/learn/science/particle-accelerators-detectors-and-cross-sections-mt_euJtxtxXyV)