Nuclear Fission and Fusion

This lesson covers nuclear fission and nuclear fusion as required by the AQA GCSE Physics specification (4.4.3). You need to understand how energy is released from the nucleus, the differences between fission and fusion, and how these processes are used in nuclear power stations and stars.

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Energy from the Nucleus

The nucleus of an atom contains a huge amount of energy. This energy can be released in two ways:

1. Nuclear fission — splitting a large nucleus into two smaller nuclei.
2. Nuclear fusion — joining two small nuclei together to form a larger nucleus.

Both processes release enormous amounts of energy — far more than any chemical reaction. This is because the nuclear forces holding the nucleus together are much stronger than the chemical bonds holding molecules together.

Exam Tip: Both fission and fusion release energy, but they work in opposite ways. Fission splits large nuclei; fusion joins small nuclei. A common exam question asks you to compare the two processes — make sure you know the key differences.

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Nuclear Fission

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei (called daughter nuclei or fission fragments), along with two or three neutrons and a large amount of energy.

Fission does not happen spontaneously for most heavy nuclei — it must be induced (started) by firing a neutron at the nucleus. When the neutron is absorbed, the nucleus becomes even more unstable and splits apart.

The Fission Process

1. A slow-moving neutron (called a thermal neutron) is fired at a large nucleus, such as uranium-235 or plutonium-239.
2. The neutron is absorbed by the nucleus, making it highly unstable.
3. The nucleus splits into two smaller daughter nuclei of roughly equal size.
4. Two or three neutrons are also released.
5. A large amount of energy is released (mainly as kinetic energy of the fission products).
6. Gamma radiation may also be emitted.

Fission of Uranium-235

When uranium-235 absorbs a neutron, it can split in many different ways. One possible fission reaction is:

U-235 + neutron --> Ba-141 + Kr-92 + 3 neutrons + energy

The exact daughter nuclei produced vary — there are many possible combinations, but the mass numbers and atomic numbers always balance.

graph TD
    A["Slow Neutron"] --> B["Uranium-235<br>Nucleus"]
    B --> C["Nucleus becomes<br>highly unstable"]
    C --> D["Splits into two<br>daughter nuclei"]
    C --> E["Releases 2 or 3<br>fast neutrons"]
    C --> F["Releases large<br>amount of energy"]
    C --> G["May emit<br>gamma radiation"]

    style A fill:#3498db,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#3498db,color:#fff
    style F fill:#f1c40f,color:#000
    style G fill:#9b59b6,color:#fff

Exam Tip: When describing fission, you must mention: (1) a neutron is absorbed by a large nucleus, (2) the nucleus splits into two smaller nuclei, (3) two or three neutrons are released, (4) energy is released. Missing any of these four points will lose marks.

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Chain Reactions

The neutrons released by one fission event can go on to be absorbed by other uranium-235 nuclei, causing them to undergo fission too. Each of these fissions releases more neutrons, which cause more fissions, and so on. This is called a chain reaction.

Conditions for a Chain Reaction

For a chain reaction to be sustained:

• There must be enough fissile material (a critical mass) so that the neutrons are likely to hit other nuclei before escaping.
• The neutrons must be slowed down (moderated) to increase the chance of absorption.

Controlled vs Uncontrolled Chain Reactions

	Controlled	Uncontrolled
Where	Nuclear power station	Nuclear weapon
Rate	Steady, constant rate	Extremely rapid
How	Control rods absorb excess neutrons	No control rods; all neutrons cause further fissions
Result	Steady release of heat energy	Massive, explosive release of energy

graph TD
    A["Neutron hits<br>U-235 nucleus"] --> B["Nucleus splits<br>+ 3 neutrons released"]
    B --> C["Neutron 1 hits<br>another U-235"]
    B --> D["Neutron 2 hits<br>another U-235"]
    B --> E["Neutron 3 absorbed<br>by control rod"]
    C --> F["Fission + 3 more<br>neutrons"]
    D --> G["Fission + 3 more<br>neutrons"]

    style A fill:#3498db,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#e74c3c,color:#fff
    style G fill:#e74c3c,color:#fff

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Nuclear Power Stations

Nuclear power stations use controlled fission to generate electricity. The key components are:

Component	Function
Fuel rods	Contain the fissile material (uranium-235 or plutonium-239)
Moderator	Slows down the fast neutrons so they can be absorbed by fuel nuclei. Usually water or graphite
Control rods	Absorb excess neutrons to control the rate of the chain reaction. Usually made of boron or cadmium. Lowered in to slow the reaction; raised to speed it up
Coolant	Carries the heat energy away from the reactor to the heat exchanger. Usually water or carbon dioxide gas
Heat exchanger	Transfers heat from the coolant to water, producing steam
Turbine	The steam turns the turbine
Generator	The turbine drives the generator, which produces electricity

How It Works

1. Fission of uranium-235 in the fuel rods releases heat energy.
2. The coolant absorbs the heat and carries it to the heat exchanger.
3. In the heat exchanger, the heat is transferred to water, producing steam.
4. The steam drives a turbine, which turns a generator.
5. The generator produces electricity.
6. Control rods are raised or lowered to maintain a steady rate of fission.

Exam Tip: Nuclear power stations do NOT use nuclear explosions to generate electricity. They use controlled fission, where the chain reaction is carefully managed so that, on average, exactly one neutron from each fission goes on to cause another fission. The control rods are the key to maintaining this balance.

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Nuclear Fusion

Nuclear fusion is the process in which two small, light nuclei join together (fuse) to form a single, larger nucleus, releasing a large amount of energy in the process.

Fusion is the process that powers the Sun and all other stars. In the Sun, hydrogen nuclei fuse together to form helium nuclei, releasing enormous amounts of energy.

The Fusion Process

For fusion to occur, the nuclei must overcome their natural electrostatic repulsion (both nuclei are positively charged, so they repel each other). This requires:

• Extremely high temperatures (millions of degrees Celsius) to give the nuclei enough kinetic energy to get close enough to fuse.
• Extremely high pressures to force the nuclei close together.

These conditions exist naturally in the cores of stars.

Fusion in Stars

The main fusion reaction in the Sun is the fusion of hydrogen into helium:

4 hydrogen nuclei (protons) --> 1 helium nucleus + 2 positrons + energy

This reaction releases a vast amount of energy, which is the source of the Sun's light and heat.

graph TD
    A["Two small nuclei<br>(e.g. hydrogen)"] --> B["Overcome electrostatic<br>repulsion"]
    B --> C["Extremely high<br>temperature needed<br>(millions of degrees)"]
    C --> D["Nuclei fuse together"]
    D --> E["Larger nucleus formed<br>(e.g. helium)"]
    D --> F["Huge amount of<br>energy released"]

    style A fill:#3498db,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#9b59b6,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#f1c40f,color:#000

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Comparing Fission and Fusion

Feature	Fission	Fusion
Process	Splitting a large nucleus	Joining two small nuclei
Nuclei involved	Heavy elements (e.g. U-235, Pu-239)	Light elements (e.g. hydrogen, deuterium)
Neutrons	Needed to start reaction; more released	Not needed to start reaction
Conditions	Needs a neutron to be absorbed	Needs extremely high temperature and pressure
Energy released	Very large	Even larger (per unit mass)
Where it occurs	Nuclear power stations, nuclear weapons	Stars, hydrogen bombs
Waste products	Radioactive waste (daughter nuclei)	Helium (not radioactive)
Currently used for power?	Yes	Not yet (research ongoing)

Exam Tip: Fusion releases more energy per unit mass than fission and produces less radioactive waste. However, fusion is extremely difficult to achieve on Earth because it requires temperatures of millions of degrees and pressures that are very hard to maintain. This is why we do not yet have fusion power stations, although research (such as the ITER project) is ongoing.

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Fusion in Stars: The Life Cycle Connection

The formation of elements heavier than hydrogen occurs through fusion in stars: