7.8 Nuclear Fusion

A free GCSE Physics lesson for AQA (8463) from OBA School: joining light nuclei, where the Sun’s energy comes from, why fusion needs extreme conditions, and the challenge of fusion power. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe nuclear fusion and explain that some mass is converted into energy
• Explain why the Sun and other stars release energy by fusion
• Explain why fusion needs very high temperatures and pressures
• Describe the challenges of building a fusion reactor and compare fusion with fission

Key words

• Nuclear fusion: Two light nuclei join to form a single heavier nucleus, releasing energy.
• Plasma: A very hot gas of bare nuclei and free electrons (the atoms have lost their electrons).
• Deuterium: Hydrogen-2, ²₁H: one proton and one neutron. Found naturally in sea water.
• Tritium: Hydrogen-3, ³₁H: one proton and two neutrons. Radioactive.
• Electrostatic repulsion: The force pushing two positive nuclei apart.

What is fusion?

In nuclear fusion two light nuclei join together to make one heavier nucleus. The mass of the products is slightly less than the mass of the nuclei that fused. This “missing” mass has been converted into energy, which is carried away as kinetic energy of the products and as radiation.

Worked example: checking a fusion equation and the mass difference

²₁H + ³₁H → ⁴₂He + ¹₀n top: 2 + 3 = 4 + 1 ✓ bottom: 1 + 1 = 2 + 0 ✓
masses (in atomic mass units): before 2.0141 + 3.0160 = 5.0301; after 4.0026 + 1.0087 = 5.0113
mass lost = 5.0301 − 5.0113 = 0.0188 units = 0.0188 ÷ 5.0301 × 100 = 0.37% of the starting mass

Answer: about 0.4% of the mass is converted into energy

Key fact: Fusion releases energy only for light nuclei (up to about the mass of iron). Making heavier nuclei by fusion takes in energy instead.

Fusion in the Sun

The Sun is mostly hydrogen. In its core the temperature is about 15 million °C and gravity squeezes the plasma to a huge pressure and density. Hydrogen nuclei (protons) collide fast enough to fuse, in several steps, into helium. Every second the Sun converts about 4 million tonnes of mass into energy. This energy reaches us as light and other electromagnetic radiation.

Key fact: All stars on the main part of their lives are powered by fusing hydrogen into helium. Heavier elements up to iron are made by fusion in older, hotter stars.

Why fusion is hard

All nuclei are positive, so two nuclei repel each other. Only if they are moving very fast can they get close enough to fuse. For nuclei to move that fast the plasma must be at an extremely high temperature, and it must be at a high pressure / density so that collisions happen often enough.

On Earth the pressure in the Sun’s core cannot be copied, so fusion reactors must be even hotter: about 100 million °C. In a doughnut-shaped reactor (a tokamak) a huge electric current and other heaters raise the temperature, and strong magnetic fields hold the plasma away from the walls. If the plasma touched the walls it would cool and fusion would stop (and the walls would be damaged).

Exam tip: A useful fusion reactor must give out more energy than it takes in to heat and hold the plasma. Experimental reactors have only just reached this for short times, so fusion power stations are still being developed.

Fusion compared with fission

• What happens; Fission a large nucleus splits into two smaller ones; Fusion two light nuclei join into one
• Fuel; Fission uranium-235 or plutonium-239: mined, found in only some places; Fusion deuterium (from sea water) and tritium (made from lithium)
• Waste; Fission radioactive fragments with long half-lives; Fusion helium, not radioactive; reactor parts become mildly radioactive
• Conditions; Fission a chain reaction starts easily at normal temperatures; Fusion needs about 100 million °C and a magnetic field to hold the plasma
• Status; Fission used in power stations for over 60 years; Fusion experimental; not yet producing electricity

Worked example: comparing energy in and out

An experimental reactor uses 50 MJ to heat its plasma and the fusion releases 20 MJ.
energy out ÷ energy in = 20 ÷ 50 = 0.4

Answer: less than 1, so it takes in more energy than it gives out – not yet useful

Common mistakes

• Mixing up fission and fusion: f-u-sion is joining (fusing) together.
• Saying fusion needs a high temperature “to give the nuclei energy to stick”. The point is to make them fast enough to overcome the repulsion between positive charges.
• Saying mass is “destroyed” in fusion. A little mass is converted into energy; mass number is still conserved.
• Thinking the plasma is held in a container. It is held away from the walls by a magnetic field.

Revision questions

Try these, then check your answers below each one.

Question 1. What is nuclear fusion? (Choose from: Two light nuclei joining to form a heavier nucleus, A heavy nucleus splitting into two smaller nuclei, A nucleus emitting an alpha particle, An electron joining a nucleus.)

Answer: Two light nuclei joining to form a heavier nucleus

Working: Fusion joins two light nuclei into one heavier nucleus, releasing energy.

Question 2. Why must nuclei be at a very high temperature for fusion to happen? (Choose from: They must move fast enough to overcome the repulsion between them, Heat makes the nuclei bigger, The nuclei must first be melted, High temperature removes their neutrons.)

Answer: They must move fast enough to overcome the repulsion between them

Working: Nuclei repel each other. Only very fast nuclei get close enough to fuse, and high temperature means high speed.

Question 3. Complete the fusion reaction: ²₁H + ³₁H → ⁴₂He + ?<br/>Name the missing particle.

Answer: neutron

Working: Top: 2 + 3 = 4 + 1; bottom: 1 + 1 = 2 + 0. A particle with A = 1 and Z = 0 is a neutron.

Related lessons

• Previous lesson: 7.7 Nuclear Fission
• Next lesson: 7.9 Nuclear Issues
• In this chapter: 7.1 Atoms and Radiation
• In this chapter: 7.2 The Discovery of the Nucleus
• In this chapter: 7.3 Changes in the Nucleus

Continue in the full course

This lesson is section 7.8 (AGCPH0708) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.