--- title: "Nuclear Fission and Fusion" description: "Fission splits a heavy nucleus and can release neutrons that sustain a chain reaction. Control rods absorb neutrons; moderators slow them in thermal reactors. Fusion joins light nuclei. Useful fusion " 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Nuclear Fission and Fusion Fission splits a heavy nucleus and can release neutrons that sustain a chain reaction. Control rods absorb neutrons; moderators slow them in thermal reactors. Fusion joins light nuclei. Useful fusion requires high temperature, suitable fuel density and enough confinement time. 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 how neutrons sustain a fission chain and distinguishes control rods from the moderator in a thermal reactor - Explains how high temperature, fuel density and confinement time help make fusion reactions useful - Compares fission and fusion inputs and products, and explains why either can leave radioactive material ## Lesson: Splitting heavy, joining light Fission splits a heavy nucleus into lighter nuclei. Uranium-235 can absorb a neutron and split, releasing energy and usually two or three neutrons. Some released neutrons cause further splits, forming a chain reaction. Others escape or are absorbed without fission. The chain stays steady when each split causes one further split on average. It grows when that average exceeds one. Some nuclei also split spontaneously. *(drawing: This shows an event sequence, not particle geometry. Not every released neutron causes another split.)* Control rods absorb neutrons. Inserting them farther leaves fewer neutrons for fission and lowers fission power, with other conditions fixed. Withdrawing them can raise power. In a thermal reactor, a moderator slows neutrons, making further uranium-235 fissions more likely. Fast-neutron reactors use a different design without a moderator. Coolant carries heat away; that is a different job from regulating the chain. Fusion joins light nuclei and can release energy. Hydrogen nuclei are positive and repel electrically. Higher temperature makes close encounters more likely. Useful fusion also needs suitable fuel density and enough confinement time. Gravity keeps dense fuel together in stars. On Earth, experiments use magnetic confinement or briefly compress fuel with lasers. They do not need a star's exact conditions. Higher temperature raises nuclei's speeds, making close encounters more likely. Greater fuel density means more nuclei in a given volume, so encounters happen more often. Longer confinement keeps fuel and energy from escaping too quickly, allowing more fusion reactions before conditions are lost. *(drawing: Fusion joins light nuclei. Its products depend on which nuclei react.)* Spent fission fuel contains radioactive fragments, unused fuel and other heavy nuclei. Their lifetimes vary; some need isolation for thousands of years or longer. One studied fusion reaction joins deuterium and tritium, hydrogen isotopes, producing stable helium and a fast neutron. Tritium is radioactive, and neutrons can make reactor materials radioactive. Fusion waste depends on the fuel and materials; it is not always absent or short-lived. Use the previous stop's distinction: shielding reduces irradiation, while containment limits radioactive material escaping and spreading contamination. Stopping a fission chain does not stop radioactive decay in the fuel. Decay still produces radiation and heat, so cooling and radiation protection remain necessary after shutdown. Fission power stations use controlled chains to provide heat for electricity generation. Fusion experiments also release energy. Energy released in fuel is different from net electricity supplied by a whole plant. A fusion plant must sustain suitable fuel conditions, collect heat, protect materials and run equipment. Its electricity output must exceed the electricity used by the whole plant. One fusion event or hot fuel does not demonstrate this. **Recap.** Fission splits heavy nuclei; fusion joins light ones. Control, cooling, radiation protection and whole-plant energy costs matter. *This lesson, question bank and flashcards were written for this dataset. OpenStax fission, fusion and nuclear-energy material informed the factual study.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Irradiation, Contamination and Dose](https://lightmysky.com/learn/science/irradiation-contamination-and-dose-mt_WyxR7ZVEhi) ## Optional connections - [Where Elements Come From](https://lightmysky.com/learn/science/where-elements-come-from-mt_KYx0m4OyZv) ## 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)