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AQA GCSE Physics · Topic 4.4 Atomic structure · Physics only
Nuclear Fission and Fusion
In this lesson you'll learn to: describe nuclear fission — how a neutron splits a
large nucleus and what is released; explain how a chain reaction works and how control
rods keep it steady in a reactor; describe nuclear fusion; and never mix the two up.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — split a nucleus
Don't read anything yet. Press Fire the neutron and watch what happens to the uranium-235 nucleus: what goes in, what comes out, and which keywords light up underneath. Then try Chain reaction, and finally Reactor, where you move the control rods and watch the power change.
Nuclear fission — splitting Uranium-235
▲ One neutron in; two smaller nuclei, 2 or 3 neutrons and gamma radiation out.
Now join two nuclei together
Fusion is the opposite. Fire two nuclei at each other at a low temperature, then turn the temperature up and fire again. Then switch to Hot dense plasma and change the temperature and pressure to see how the fusion rate changes.
Nuclear fusion — two nuclei, one collision
▲ Fusion needs very high temperature and pressure — that is why it happens in stars.
Seen them both? Now learn to describe them.
The exam asks you to describe fission, explain a chain reaction, say how control rods work, and describe fusion — using the right keywords. The notes below give you every one.
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Revision notes
1. Nuclear fission
Fission is the splitting of a large, unstable nucleus, such as uranium or plutonium. It very rarely happens on its own: usually the nucleus must first absorb a neutron.
⭐ Memorise this
A slow-moving neutron is absorbed by a large nucleus (such as uranium-235), making it unstable. It splits into two smaller nuclei of roughly equal size, and releases 2 or 3 neutrons, gamma radiation and energy.
The energy is released because the products have slightly less mass than what went in. All the fission products fly apart with kinetic energy.
2. Chain reactions
Each fission releases 2 or 3 neutrons. Each of those can be absorbed by another uranium nucleus and cause another fission, which releases more neutrons, and so on.
- In a nuclear reactor, the chain reaction is controlled, so energy is released at a steady rate.
- In a nuclear weapon, the chain reaction is uncontrolled, releasing a huge amount of energy almost instantly (an explosion).
💡 How control rods work
Control rods absorb neutrons. Lowering them into the reactor means fewer neutrons are left to be absorbed by uranium nuclei, so fewer fission events happen and the chain reaction stays steady instead of growing out of control.
3. Nuclear fusion
Fusion is the joining of two light nuclei (such as hydrogen) to form one heavier nucleus. It needs a very high temperature and pressure, because both nuclei are positive and repel each other. This is how stars, including the Sun, release energy.
⭐ Memorise this
In fusion, two light nuclei join to form a heavier nucleus. Some of the mass is converted into energy, which is transferred away as radiation.
4. Don't mix them up
| Fission — splits | Fusion — joins | |
|---|---|---|
| What happens | one large nucleus → two smaller ones | two small nuclei → one bigger one |
| What starts it | a nucleus absorbs a neutron | very high temperature and pressure |
| What is released | 2 or 3 neutrons, gamma radiation and energy | energy, as radiation |
| Where it happens | nuclear power stations | stars, such as the Sun |
💡 A trick to remember which is which
Fission sounds like fissure, a crack: it splits. Fusion sounds like fuse, as in fusing two things together: it joins.
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How to write each answer
These questions come up again and again. These are the sentences that get the marks — learn the shape, and use the keywords.
“Describe the process of nuclear fission.”
A slow-moving neutron is absorbed by a
large nucleus. The nucleus splits into
two smaller nuclei, releasing 2 or 3 neutrons,
gamma radiation and energy.
“Explain how a chain reaction happens.”
A neutron is absorbed by a uranium nucleus, which
splits (fission) and releases 2 or 3 neutrons. These are
absorbed by other uranium nuclei and cause further fission events.
“Describe the process of nuclear fusion.”
Two light nuclei join to form a
heavier nucleus. Some of the mass is
converted into energy, which is transferred as
radiation.
“Explain how control rods control the chain reaction.”
Control rods absorb neutrons, so fewer neutrons are
left to be absorbed by uranium nuclei, so fewer fission events happen.
the particles what happens what you get
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Worked example
This is the exam-style question from page 14 of the workbook. Cover the answer, try it, then check.
Exam-style question — a nuclear power station
A nuclear power station uses uranium-235 as its fuel.6 marks
(a) Name the particle that must be absorbed by a uranium-235 nucleus for fission to occur. (1)
Answer: a neutron (allow slow-moving / thermal neutron; not proton or electron)
(b) Describe what is released when a uranium-235 nucleus undergoes fission. (3)
1. two smaller nuclei (allow "two daughter nuclei")
2. two or three neutrons
3. gamma radiation and energy (allow energy alone)
2. two or three neutrons
3. gamma radiation and energy (allow energy alone)
(c) In a reactor, control rods are lowered into the core. Control rods absorb neutrons. Explain how this stops the chain reaction running out of control. (2)
1. so fewer neutrons are left to be absorbed by other uranium nuclei
2. so fewer fission events happen: the rate of the chain reaction falls and energy is released steadily
2. so fewer fission events happen: the rate of the chain reaction falls and energy is released steadily
Marking note for (c): the question already tells you control rods absorb neutrons, so repeating that scores nothing. Both marks are for the consequence: fewer free neutrons, therefore fewer fissions.
📘 Now do it in your workbook
Draw and label fission, a chain reaction and fusion, describe each one using the keywords, then try the 6-mark exam-style question — with worked answers at the back so you can mark your own.
AQA GCSE Physics Workbook · ATOMIC STRUCTURE · page 14Free sample = the page for this lesson. The full workbook covers the whole of Topic 4 Atomic Structure, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the notes and the free workbook page above cover every mark.
Now put it into practice
Watching is the easy bit — the keywords only stick once you've written the descriptions yourself. Page 14 of the workbook is free, and the full book covers every spec point in Topic 4 Atomic Structure with worked answers.
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