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Learn5054Paper 1 · Multiple ChoiceNuclear physicsFission and fusion
Nuclear physics · Subtopic 4 of 4

Fission and Fusion

In fission, a heavy nucleus such as uranium-235 absorbs a neutron and splits into two smaller nuclei, releasing energy and two or three neutrons that can split more nuclei: a chain reaction. In fusion, light nuclei join to make a heavier one, releasing even more energy per kilogram; it powers the Sun. Both convert a little mass into a lot of energy.

Cambridge 5054 Paper 1Cambridge 5054 Paper 2Cambridge 0625 Papers 2, 4Cambridge 0625 Papers 1, 3Edexcel 4PH1 Paper 1PEdexcel 4PH1 Paper 2P≈ 30 minRevision 5 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Describe nuclear fission and explain a chain reaction.
  • Describe how a nuclear reactor controls the chain reaction.
  • Describe nuclear fusion and why it needs very high temperatures.
  • Balance nuclear equations for fission and fusion.
Pause and think

A uranium nucleus splits and releases 33 neutrons; each splits another nucleus, releasing 33 more, and so on. After 1010 steps, how many fissions happen at once?

Section 2 of 14

Core idea

Start hereThe Sun and a power station

A nuclear power station splits heavy uranium nuclei; the Sun joins light hydrogen nuclei. Opposite processes, both releasing huge energy from tiny amounts of matter.

The big idea

Fission: U-235 absorbs a neutron, splits, releases energy and 2–3 neutrons (chain reaction). Fusion: light nuclei join at extreme temperature and pressure.

1. Fission and chain reactions

A chain reaction
nU-235grows rapidly
Each fission releases neutrons that can split more nuclei. In a reactor, control rods absorb some neutrons so that on average exactly one goes on to cause another fission.
A controlled chain reaction

Remove the control rods and the reaction grows uncontrollably.

Worked exampleBalancing a fission equation

 92235U+01n→ 56141Ba+3692Kr+x 01n^{235}_{\ 92}\text{U}+{}^1_0n\to{}^{141}_{\ 56}\text{Ba}+{}^{92}_{36}\text{Kr}+x\,{}^1_0n.

Goal
xx.
Strategy
Balance nucleon numbers.
  1. 235+1=141+92+x235+1=141+92+x
    Top numbers.
  2. x=3x=3
    Solve.
Answer3 neutrons.
✓ CheckProtons: 92=56+3692=56+36 ✓.

2. Fusion

  • Needs huge temperature and pressure to overcome repulsion between positive nuclei.
  • Powers stars; not yet practical on Earth.
Common misconceptionFission vs fusion Fission = split (heavy); fusion = fuse (light).
Pause and think

What if control rods were removed?

Summary

  • Fission splits heavy; fusion joins light.
  • Rods and moderator control the chain.
Key words
Chain reaction
Each fission triggering more.
HistoryMeitner on a winter walk. In December 19381938, Lise Meitner and her nephew Otto Frisch, walking in the snow in Sweden, worked out that uranium nuclei were splitting in two, and calculated the huge energy released. Frisch borrowed the word "fission" from biology, where cells divide.
Fun factThe Sun loses 4 million tonnes a second. Every second, the Sun fuses about 600600 million tonnes of hydrogen into helium, and about 44 million tonnes of mass is converted into energy. It's so big that it can keep this up for another 55 billion years.
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