Nuclear Fusion

This lesson covers nuclear fusion — the joining of light nuclei to release energy — as required by the Edexcel GCSE Physics specification (1PH0), Topic 6: Radioactivity. You need to understand what fusion is, where it occurs naturally, why it is difficult to achieve on Earth, and how it compares to fission.

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What Is Nuclear Fusion?

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

Key Points

• Fusion involves light nuclei — typically isotopes of hydrogen:
  • Deuterium (²₁H) — hydrogen with 1 proton and 1 neutron
  • Tritium (³₁H) — hydrogen with 1 proton and 2 neutrons
• The combined mass of the products is slightly less than the total mass of the original nuclei — this "missing mass" has been converted into energy.
• Fusion releases a very large amount of energy — even more per kilogram of fuel than fission.

Example Fusion Reaction

When deuterium and tritium fuse:

²₁H + ³₁H → ⁴₂He + ¹₀n + energy

• Two hydrogen isotopes combine to form helium-4 and a neutron.
• A huge amount of energy is released.

Exam Tip: Do not confuse fusion with fission. Fusion = joining LIGHT nuclei together. Fission = splitting a HEAVY nucleus apart. Both release energy, but by opposite mechanisms. Remember: "fusion" sounds like "fuse together."

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Where Does Fusion Occur Naturally?

Fusion is the process that powers the Sun and all stars.

Fusion in the Sun

• The core of the Sun is at a temperature of about 15 million °C and under immense pressure due to gravity.
• Under these extreme conditions, hydrogen nuclei (protons) fuse together through a series of reactions to form helium nuclei.
• Each fusion reaction releases energy — this is the source of the Sun's light and heat.
• The energy released by fusion in the core works its way outward and is eventually emitted as electromagnetic radiation (including visible light).

Why Extreme Conditions Are Needed

Atomic nuclei are positively charged (they contain protons). When two positive charges approach each other, they experience electrostatic repulsion — they push each other apart.

To overcome this repulsion and force nuclei close enough to fuse, you need:

1. Extremely high temperature — gives the nuclei enough kinetic energy to overcome the electrostatic repulsion. The nuclei must be moving so fast that they can get close enough for the strong nuclear force to take over and bind them together.
2. Extremely high pressure — forces the nuclei close together, increasing the chance of collision and fusion.

The combination of very high temperature and very high pressure is required. In the Sun, gravity provides the pressure; the nuclear reactions themselves maintain the temperature.

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Why Is Fusion Difficult to Achieve on Earth?

Scientists have been trying to develop controlled fusion on Earth for decades, but it remains extremely challenging:

The Key Problems

Challenge	Explanation
Extreme temperature	Temperatures of around 100 million °C (or more) are needed — hotter than the core of the Sun. At these temperatures, matter exists as plasma (a gas of charged particles).
Containing the plasma	No physical container can withstand 100 million °C — the plasma would melt any material. Instead, magnetic fields are used to contain the plasma (in devices called tokamaks). This is technically very difficult.
Maintaining conditions	Keeping the plasma at the right temperature, density and stability for long enough for useful fusion to occur is extremely difficult. Instabilities in the plasma tend to disrupt the process.
Energy input vs output	Currently, the energy required to heat and contain the plasma is often greater than the energy released by fusion. Achieving "net energy gain" (more energy out than in) is the key milestone.
Engineering challenges	The neutrons produced by fusion reactions cause radiation damage to the reactor walls, which must be regularly replaced.

Exam Tip: If asked why fusion is difficult on Earth, focus on two key points: (1) extremely high temperatures are needed to overcome electrostatic repulsion, and (2) no physical material can contain the plasma, so magnetic confinement is required and this is technically very challenging.

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Fusion vs Fission — Comparison

Feature	Fission	Fusion
Process	Splitting a heavy nucleus	Joining two light nuclei
Nuclei involved	Heavy (e.g., U-235, Pu-239)	Light (e.g., hydrogen isotopes)
Products	Two daughter nuclei + 2–3 neutrons	One heavier nucleus + neutron(s)
Energy released per kg	Large	Very large (more than fission per kg of fuel)
Fuel availability	Uranium — limited supply	Hydrogen (from water) — virtually unlimited
Radioactive waste	Produces long-lived radioactive waste	Produces little long-lived radioactive waste
CO₂ emissions	None during operation	None
Currently used for power?	Yes — nuclear power stations worldwide	Not yet — still under development
Chain reaction?	Yes — controlled in reactors	No chain reaction needed
Conditions required	Relatively moderate	Extreme temperature and pressure
Risk of explosion/meltdown	Possible (e.g., Chernobyl)	Very low — reaction stops if conditions are not maintained

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Potential Advantages of Fusion Power

If scientists and engineers can successfully develop controlled fusion, the potential benefits are enormous:

Advantage	Detail
Abundant fuel	Deuterium can be extracted from seawater — virtually unlimited supply. Tritium can be bred from lithium.
No long-lived radioactive waste	The main product is helium (non-radioactive). Some radioactive waste is produced from neutron activation of reactor materials, but it has a much shorter half-life than fission waste.
No CO₂ emissions	Fusion does not produce greenhouse gases during operation.
High energy output	A small amount of fuel produces an enormous amount of energy.
Inherently safer	If conditions are disrupted, the reaction simply stops — there is no risk of a runaway chain reaction or meltdown (unlike fission).
No risk of nuclear proliferation	Fusion technology cannot easily be used to make nuclear weapons.

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Current Fusion Research

Several major international projects are working towards achieving practical fusion power:

• ITER (International Thermonuclear Experimental Reactor) — a large tokamak being built in France, due to begin fusion experiments. ITER aims to produce 10 times more energy than it consumes.
• JET (Joint European Torus) — a tokamak in the UK that has achieved record-breaking fusion energy output.
• Various private companies are also pursuing alternative fusion reactor designs.

Exam Tip: You do not need to know the names of specific fusion projects for the exam, but you should be aware that fusion is not yet a practical energy source — it is still under development. If asked "why don't we use fusion for power yet?", explain the technical difficulties: extreme temperatures, plasma containment, and achieving net energy gain.

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Summary