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AQA GCSE Physics · Topic 4.4 Atomic structure · Physics only
Edexcel International GCSE Physics · Section 7 Radioactivity and particles
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.
In this lesson you'll learn to: describe nuclear fission and a chain reaction;
explain the job of each part of a reactor — fuel rods, control rods, the
moderator and the shielding; describe nuclear fusion using the words loss of
mass; explain the conditions fusion needs and why it is so difficult to achieve on Earth.
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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.
Edexcel onlyInside a nuclear fission reactor
| Part | What it does | Why that matters |
|---|---|---|
| Fuel rods | contain the uranium-235 | this is where the fission happens |
| Control rods | absorb neutrons | lowering them reduces the rate of fission; raising them increases it |
| Moderator (water or graphite) | slows down the neutrons | so they can be absorbed by the uranium fuel and cause more fission (fast neutrons are not absorbed) |
| Shielding (thick concrete) | absorbs the radiation, including gamma | protects people outside the reactor from irradiation |
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.
Edexcel onlySay “loss of mass”
Edexcel mark schemes want the words loss of mass, for fission and fusion. The products have slightly less mass than what went in, and that loss of mass is released as energy: E = mc², where c is the speed of light. Because c² is enormous, a tiny loss of mass gives a huge amount of energy.
Edexcel onlyThe conditions fusion needs
- Nuclei are positive, so they repel each other.
- A very high temperature gives the nuclei high kinetic energy, so they move fast enough to overcome the electrostatic repulsion and get close enough to fuse.
- A very high pressure (density) packs the nuclei close together, so collisions are more frequent.
Edexcel onlyWhy fusion is difficult to achieve on Earth
- It needs temperatures of millions of degrees and very high pressures, which are very hard to create.
- No material can hold anything that hot, so the hot gas (plasma) has to be held by strong magnetic fields.
- So far, fusion reactors have used more energy to heat and hold the fuel than they give out.
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 the energy comes from | a loss of mass | a loss of mass |
| 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.
Edexcel only“Explain the purpose of the moderator.”
The moderator slows down the neutrons, so they can be
absorbed by the uranium fuel and cause further fission.
Edexcel only“Explain why the reactor needs thick concrete shielding.”
The shielding absorbs the radiation (gamma) given out by the core, so it
protects people outside the reactor from irradiation.
Edexcel only“Explain what happens during nuclear fusion.”
Two light nuclei join to form a heavier nucleus. There is a
loss of mass, and this mass is released as energy.
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.
Two questions like the ones on pages 13 and 14 of your 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.
Exam-style question — inside the reactor
(a) A student says the moderator stops the chain reaction. Explain why the student is wrong. (2)
1. the moderator slows down the neutrons
2. so they are more likely to be absorbed by uranium nuclei, which keeps the chain reaction going (it is the control rods that absorb neutrons)
2. so they are more likely to be absorbed by uranium nuclei, which keeps the chain reaction going (it is the control rods that absorb neutrons)
(b) The control rods are raised out of the core. Explain the effect on the rate of fission. (2)
1. fewer neutrons are absorbed by the control rods
2. so more neutrons cause fission: the rate of fission increases
2. so more neutrons cause fission: the rate of fission increases
Exam-style question — fusion
(a) Explain where the energy released in fusion comes from. (2)
1. the new nucleus has less mass than the two nuclei that fused: a loss of mass
2. the lost mass is released as energy (E = mc²)
2. the lost mass is released as energy (E = mc²)
(b) Explain why fusion needs a very high temperature. (3)
1. nuclei are positive, so they repel each other
2. a high temperature gives them high kinetic energy
3. so they can overcome the electrostatic repulsion and get close enough to fuse
2. a high temperature gives them high kinetic energy
3. so they can overcome the electrostatic repulsion and get close enough to fuse
No mark: “fusion needs heat to start”. Every mark needs the reason: repulsion, kinetic energy, getting close enough.
📘 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 14 Edexcel iGCSE Physics Workbook · RADIOACTIVITY AND PARTICLES · pages 13 & 14
⬇ Free: page 14 (PDF)
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⬇ Free: pages 13 & 14 (PDF)
📄 Full workbook PDF · £2.99
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📄 Full workbook PDF · £2.99
Free sample = the page for this lesson. The full workbook covers the whole of Topic 4 Atomic Structure, with exam-style questions and worked answers.
Free sample = the pages for this lesson. The full workbook covers the whole of Section 7 Radioactivity and Particles, 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.
Watching is the easy bit — the keywords only stick once you've written the descriptions yourself. Pages 13 and 14 of the workbook are free: label your own reactor, then the fusion questions.
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