Nuclear Fission

This lesson covers nuclear fission — the splitting of heavy nuclei to release energy — as required by the Edexcel GCSE Physics specification (1PH0), Topic 6: Radioactivity. You need to understand what fission is, how a chain reaction works, how nuclear power stations use controlled fission to generate electricity, and the advantages and disadvantages of nuclear power.

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What Is Nuclear Fission?

Nuclear fission is the process in which a heavy, unstable nucleus splits into two smaller nuclei (called daughter nuclei or fission fragments), releasing:

• Two or three neutrons
• A large amount of energy (as kinetic energy of the fragments and neutrons)
• Gamma radiation

Key Points

• Fission occurs in heavy nuclei — the two fuels used in nuclear power are uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu).
• Fission is triggered (induced) when the nucleus absorbs a slow-moving neutron (called a thermal neutron).
• The neutron is absorbed by the nucleus, making it even more unstable → the nucleus splits.
• The two daughter nuclei are not always the same — many different combinations of fission products are possible.

Exam Tip: Do not confuse fission with fusion. Fission = splitting a heavy nucleus. Fusion = joining light nuclei. Remember: "fission" sounds like "fissure" (a crack/split).

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The Fission Process — Step by Step

1. A slow-moving neutron is absorbed by a uranium-235 (or plutonium-239) nucleus.
2. The nucleus becomes unstable.
3. The nucleus splits into two smaller daughter nuclei (fission products).
4. Two or three neutrons are released.
5. A large amount of energy is released (as kinetic energy and gamma radiation).

graph LR
    A["Slow neutron"] --> B["U-235<br/>nucleus"]
    B --> C["Unstable<br/>nucleus"]
    C --> D["Daughter<br/>nucleus 1"]
    C --> E["Daughter<br/>nucleus 2"]
    C --> F["2 or 3<br/>neutrons"]
    C --> G["Energy<br/>(KE + gamma)"]

    style A fill:#3498db,color:#fff
    style B fill:#2c3e50,color:#fff
    style C fill:#c0392b,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#e67e22,color:#fff
    style F fill:#3498db,color:#fff
    style G fill:#f1c40f,color:#000

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Chain Reaction

The neutrons released by one fission event can go on to be absorbed by other uranium-235 nuclei, causing further fissions, which release more neutrons, which cause even more fissions, and so on. This self-sustaining process is called a chain reaction.

Uncontrolled Chain Reaction

• If the chain reaction is not controlled, the number of fissions increases exponentially in a very short time.
• This releases an enormous amount of energy almost instantaneously.
• This is the principle behind a nuclear weapon (atomic bomb).

Controlled Chain Reaction

• In a nuclear power station, the chain reaction is controlled so that, on average, exactly one neutron from each fission goes on to cause another fission.
• This produces a steady, constant output of energy that can be used to generate electricity.

graph TD
    A["One U-235 nucleus<br/>absorbs a neutron<br/>and splits"] --> B["Releases 2–3 neutrons"]
    B --> C["One neutron causes<br/>another fission"]
    B --> D["Excess neutrons<br/>absorbed by<br/>control rods"]
    C --> E["Releases 2–3<br/>more neutrons"]
    E --> F["Chain reaction<br/>continues at<br/>steady rate"]

    style A fill:#c0392b,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#7f8c8d,color:#fff
    style F fill:#27ae60,color:#fff

Exam Tip: In a controlled chain reaction, on average exactly ONE neutron from each fission event goes on to cause another fission. The rest are absorbed by control rods. This is a critical point — if more than one neutron causes further fission, the reaction accelerates; if fewer than one, the reaction dies out.

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Nuclear Power Stations

Nuclear power stations use the heat from controlled fission to generate electricity.

Key Components

Component	Role
Fuel rods	Contain the nuclear fuel (uranium-235 or plutonium-239). This is where fission occurs.
Control rods	Made of a material that absorbs neutrons (e.g., boron or cadmium). Raised or lowered into the reactor to control the rate of fission. Pushed in → absorb more neutrons → slows reaction. Pulled out → fewer neutrons absorbed → speeds up reaction.
Moderator	A material (usually water or graphite) that slows down the fast-moving neutrons released by fission to the correct speed for absorption by fuel nuclei. Fast neutrons are less likely to be absorbed; slow (thermal) neutrons are much more likely to cause fission.
Coolant	A fluid (often water) that circulates through the reactor, absorbing the heat generated by fission. The hot coolant is used to produce steam.
Heat exchanger	Transfers heat from the coolant to water in a secondary circuit, producing steam.
Steam turbine	The high-pressure steam turns a turbine.
Generator	The spinning turbine drives a generator, which converts kinetic energy into electrical energy.

Energy Transfer Chain

Nuclear energy (in fuel) → heat (kinetic energy of fission products) → steam (thermal energy in water) → kinetic energy (turbine spins) → electrical energy (generator)

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Advantages and Disadvantages of Nuclear Power

Advantages

Advantage	Detail
No greenhouse gases during operation	Nuclear fission does not produce CO₂ → does not contribute to climate change during electricity generation
Reliable	Produces electricity continuously (not dependent on weather, unlike wind or solar)
High energy density	A small amount of nuclear fuel produces a huge amount of energy — much more than the same mass of fossil fuel
Long operating life	A nuclear power station can operate for 40–60 years
Fuel availability	Uranium is relatively abundant and a small amount lasts a long time

Disadvantages

Disadvantage	Detail
Radioactive waste	Fission produces radioactive waste that remains dangerous for thousands of years — disposal is difficult and expensive
High building costs	Nuclear power stations are very expensive to build
Decommissioning costs	Taking a power station out of service is extremely expensive and time-consuming (can take decades)
Safety risks	Accidents (e.g., Chernobyl 1986, Fukushima 2011) can release radioactive material over wide areas
Nuclear proliferation	Nuclear technology can potentially be used to produce nuclear weapons
Non-renewable	Uranium is a finite resource (though supplies will last a long time)
Slow to build	Takes many years to plan and construct a nuclear power station

Exam Tip: "Evaluate" questions on nuclear power are common in the exam. You must give both advantages AND disadvantages, and then reach a conclusion. Do not simply list points — explain and compare them. For example: "Nuclear power does not produce CO₂, which helps combat climate change, but it does produce radioactive waste that is difficult to dispose of safely."

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Summary