Energy Release and Chain Reactions Revision Notes | GCSE AQA Higher Physics
In a nuclear power station, a controlled chain reaction heats water to make steam, which drives turbines connected to generators. Free GCSE Physics notes.
Updated 20 Mar 2026 · Revise Right Team
Energy Release and Chain Reactions is part of Nuclear fission and fusion (physics only) in GCSE AQA Higher Physics.
4 min
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23
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GCSE AQA Higher
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What this lesson covers
In a nuclear power station, a controlled chain reaction heats water to make steam, which drives turbines connected to generators. Free GCSE Physics notes.
Explain how nuclear fission releases energy and produces kinetic energy in fission products
Describe how neutrons from fission can start chain reactions
Compare controlled chain reactions in nuclear reactors with uncontrolled reactions in nuclear weapons
Describe that fission releases energy and produces kinetic energy in the fission products
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Use this lesson to stay inside Nuclear fission and fusion (physics only) while connecting the detail back to the wider Atomic structure topic.
Atomic structure
Nuclear fission and fusion (physics only)
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Energy Release and Chain Reactions before testing yourself from memory.
1. The Nuclear Split - What Happens When Atoms Break Apart
Picture the heaviest atoms in nature - uranium and plutonium. These massive nuclei are like overcrowded buses, barely holding together. When a slow neutron hits them, they can't cope with the extra passenger and split violently in two.
This splitting process releases enormous amounts of energy - millions of times more than chemical reactions. But here's the crucial part: it also releases more neutrons, and these neutrons can hit other nuclei, causing them to split too.
The fission products move away with kinetic energy, and energy is released in the process.
2. The Chain Reaction - How One Split Leads to Many
Here's where nuclear fission becomes truly powerful. Each uranium-235 nucleus that splits releases 2-3 neutrons. These neutrons can then hit other uranium-235 nuclei, causing them to split and release even more neutrons.
Think of it like a domino effect: neutrons from one fission can hit other nuclei and cause more fissions.
But there's a catch - not every neutron causes another fission. Some neutrons escape the material, others are absorbed by non-fissile nuclei, and some have the wrong energy. For a chain reaction to continue, you need enough fissile material in the right shape.
3. Controlled vs Uncontrolled - The Difference Between Power and Destruction
The same chain reaction that can destroy cities can also generate electricity safely. The difference lies entirely in control - how fast the reaction proceeds and whether it can be stopped.
In a nuclear reactor, the chain reaction is carefully controlled. Control rods made of neutron-absorbing materials like boron or cadmium can be inserted between the fuel rods. When inserted, they absorb neutrons and slow down the reaction. When withdrawn, more neutrons are available and the reaction speeds up.
In a nuclear weapon, the chain reaction is uncontrolled, so the energy is released very rapidly.
4. The Energy Equation - Calculating Nuclear Power
Nuclear reactions release energy because the products have less mass than the reactants. This 'missing' mass is converted to energy according to Einstein's famous equation. Let's see how to calculate this energy release.
Worked example: Explaining Energy Release in Fission
A large nucleus splits into smaller nuclei. — [M1] for identifying fission correctly
Some mass is converted into energy. — [M1] for explaining that energy is released
Energy is released during the fission reaction. — [M1] for linking fission to energy release
Fission releases energy and can also release neutrons. — [A1] for a clear final explanation
Answer: Fission releases energy.
This might seem like a tiny amount of energy, but remember - this is from just one nucleus! In 1 gram of uranium-235, there are about 2.6 × 10²¹ nuclei. If they all underwent fission, the total energy would be enormous.
5. Nuclear Reactors in Practice - How We Harness the Chain Reaction
Modern nuclear power stations use the controlled chain reaction to generate electricity. The reactor core contains fuel rods of enriched uranium, control rods to absorb neutrons, and a moderator to slow down neutrons to the right speed for fission.
The heat generated by fission is used to boil water, creating steam that drives turbines connected to generators. This is exactly the same principle as coal or gas power stations - the only difference is the heat source.
Nuclear physics questions often test your understanding of the chain reaction concept and energy calculations. Here's how to approach them systematically and avoid common mistakes.
**Trap**: Many students think nuclear reactors and nuclear weapons work completely differently. Actually, they both use the same chain reaction - the only difference is whether it's controlled or uncontrolled.
Worked example: Exam-Style Chain Reaction Question
Explain why a chain reaction can occur in uranium-235 and how this is controlled in a nuclear reactor. [6 marks]
When uranium-235 undergoes fission, it splits into two smaller nuclei and releases 2-3 neutrons — [B1] for describing fission process
These neutrons can go on to hit other uranium-235 nuclei, causing them to undergo fission and release more neutrons — [B1] for explaining how chain reaction starts
This creates a self-sustaining reaction where each fission event can cause multiple further fission events — [B1] for explaining self-sustaining nature
In nuclear reactors, control rods made of neutron-absorbing materials are used — [B1] for identifying control rods
When control rods are inserted, they absorb neutrons and slow down the chain reaction — [B1] for explaining mechanism of control
This allows the reaction rate to be controlled and prevents an explosive uncontrolled reaction — [B1] for explaining the safety aspect
Answer: Chain reactions occur because fission neutrons cause further fission events. Control rods absorb neutrons to regulate the reaction rate and prevent dangerous uncontrolled reactions.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
How does a nuclear power station generate electricity?
In a nuclear power station, a controlled chain reaction in the reactor core heats water to make steam, which drives turbines connected to generators. The core contains fuel rods of enriched uranium, control rods to absorb neutrons and a moderator to slow down neutrons.
What are control rods made of?
Control rods are made of neutron-absorbing materials like boron or cadmium. They can be inserted into or withdrawn from a nuclear reactor core to control the rate of the chain reaction.
What is a moderator in a nuclear reactor?
A moderator is the part of the reactor core that slows down neutrons to the right speed for fission. The core also contains fuel rods of enriched uranium and control rods to absorb neutrons.
How do control rods control a nuclear reactor?
Control rods are made of neutron-absorbing materials like boron or cadmium. When inserted between the fuel rods, they absorb neutrons and slow down the chain reaction; when withdrawn, more neutrons are available and the reaction speeds up.
Why does nuclear fission release energy?
Nuclear fission releases energy because the products have less mass than the original nucleus, and this 'missing' mass is converted to energy. The fission products move away with kinetic energy, and the energy released is millions of times more than chemical reactions.
What is the difference between a nuclear reactor and a nuclear weapon?
A nuclear reactor and a nuclear weapon both use the same chain reaction; the difference is control. In a reactor, control rods keep the chain reaction controlled, while in a nuclear weapon the chain reaction is uncontrolled, so the energy is released very rapidly.
Why doesn't every neutron cause another fission?
Not every neutron causes another fission because some escape the material, others are absorbed by non-fissile nuclei, and some have the wrong energy. Enough fissile material is needed to maintain a self-sustaining chain reaction.
Why is enough fissile material needed for a chain reaction?
Enough fissile material is needed to maintain a self-sustaining chain reaction. If there is not enough material, too many neutrons escape without causing fission, so the chain reaction stops.
What happens when control rods are pulled out?
When control rods are withdrawn, more neutrons are available and the chain reaction speeds up. When they are inserted, they absorb neutrons and slow the reaction down.
How many nuclei are in 1 gram of uranium-235?
There are about 2.6 × 10²¹ nuclei in 1 gram of uranium-235. If they all underwent fission, the total energy released would be enormous, since each fission of a single nucleus releases energy and 2–3 neutrons.
What is inside a nuclear reactor core?
A nuclear reactor core contains fuel rods of enriched uranium, control rods to absorb neutrons, and a moderator to slow neutrons down to the right speed for fission. The heat from fission boils water to make steam.
Why do some neutrons not cause fission?
Not every neutron causes another fission: some escape the material, others are absorbed by non-fissile nuclei, and some have the wrong energy. A chain reaction needs enough fissile material in the right shape.
What happens to the fission products after fission?
The fission products move away with kinetic energy after fission, and energy is released in the process. The total mass of the products is less than the original nucleus.
How is a nuclear power station like a coal power station?
A nuclear power station works on exactly the same principle as a coal or gas power station: heat boils water, creating steam that drives turbines connected to generators. The only difference is the heat source, which is the controlled fission chain reaction.
How do you answer a 6-mark chain reaction question?
A full chain reaction answer describes uranium-235 splitting and releasing 2–3 neutrons, explains that these neutrons cause further fissions, notes that this is self-sustaining, then describes control rods absorbing neutrons to regulate the rate and prevent an uncontrolled reaction.