K323 / 2027
Radioactivity overview

Lesson 7 of 7

Fission and fusion

  • I can explain fission and fusion and relate suitable nuclear-fuel processes to energy release.Syllabus

    Syllabus K323, 20(j). Explain fission and fusion and relate suitable nuclear-fuel processes to energy release.

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In a nuclear reactor, what starts the fission of a uranium-235 nucleus?

  1. Heat from burning fuel, which splits the nucleus apart.
  2. A neutron absorbed by the nucleus, often one released by an earlier fission.
  3. A proton fired at the nucleus from an accelerator in the reactor.
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A neutron absorbed by the nucleus, often one released by an earlier fission.

When a uranium-235 nucleus absorbs a slow neutron it becomes unstable and splits, releasing energy and two or three more neutrons. Those neutrons can cause further fissions.

Fission splits a heavy nucleus. Fusion joins light nuclei. Suitable reactions involving nuclear fuels release energy.

A nucleus contains protons and usually neutrons. Nuclear changes differ from ordinary chemical burning, which rearranges electrons and chemical bonds.

Nuclear fission
A heavy nucleus splits into smaller nuclei. In a suitable fuel reaction, energy and usually neutrons are released. An incoming neutron can initiate a fuel-fission event, though an external neutron is not part of the definition of every possible fission event.
Nuclear fusion
Light nuclei join to form a heavier nucleus. Suitable light-nucleus fuel reactions release energy. Fusion changes the nuclei; it is not simply mixing two substances together.

Two ways suitable nuclear fuels release energy

The large shapes represent whole nuclei, without displaying their nucleon counts. Sizes and arrows are schematic. These examples show the processes, not a calculation of reaction energy.

Fission: a heavy nucleus splits

A uranium fuel nucleus splits into smaller nuclei and releases neutronsAn incoming neutron initiates one illustrative uranium-235 fuel-fission event. The heavy nucleus splits into two smaller nuclei and releases neutrons. The large outlined shapes represent entire nuclei; individual proton and neutron counts are not drawn or assigned to the fragments. The three emitted neutron symbols illustrate one possible outcome, not a fixed neutron number for every fission. Released energy is carried by moving products and radiation. No reaction energy or chain reaction is specified, and this initiating neutron does not define every possible fission process.Uranium-235 fuel exampleHeavynucleusnNeutronFissionSmallernucleusSmallernucleusNeutrons releasednnnMoving products and radiationcarry released energy.

A neutron can initiate a suitable fuel-fission event. The daughter nuclei and emitted particles carry energy away; different fission events can have different products.

Fusion: light nuclei join

Deuterium and tritium fuse, forming a helium nucleus and an emitted neutronTwo light hydrogen fuel nuclei, deuterium and tritium, join under suitable fusion conditions. The products are a heavier helium nucleus and a neutron, which carry released energy. The large outlined shapes represent whole nuclei, not individual nucleons or particle counts. Arrows indicate the process and products rather than measured trajectories or speeds. No numerical reaction energy is specified, and the example does not claim that every arbitrary joining of nuclei releases energy.Light hydrogen nucleiDeuteriumTritiumFusionHeliumnNucleusNeutronReleased energy is carriedby the products.

This suitable light-nucleus reaction releases energy. The change is in the nuclei; it is not ordinary chemical burning or simply mixing substances.

These are qualitative nuclear-fuel examples. The nucleus shapes represent whole nuclei rather than countable nucleons. In both suitable reactions, energy is carried away by the products and radiation.

Follow the released energy

Reaction products can move rapidly and radiation can carry energy away. Energy transferred from those products to surrounding material can increase its internal energy. This is the connection between a nuclear reaction and a useful energy supply.

Both suitable fission and fusion reactions can release energy. The choice of nuclei matters: arbitrary splitting or joining of any chosen nuclei is not guaranteed to release energy.

For electricity-generation context, nuclear fuel as an energy resource connects a heat supply to a turbine and generator. The nuclear reaction is the source of the released energy, while the generator transfers mechanical energy electrically.