International A Level (IAL)PhysicsEdexcelRevision Notes5. Thermodynamics, Radiation, Oscillations & CosmologyNuclear Fusion & FissionNuclear Fusion

Nuclear Fusion (Edexcel International A Level (IAL) Physics): Revision Note

Exam code: YPH11

Katie M

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Katie M

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Nuclear Fusion

  • Fusion is defined as:

Small nuclides that combine together to make larger nuclei, releasing energy

  • Low mass nuclei, such as hydrogen and helium, can undergo fusion and release energy

  • On Earth, research is focused on achieving the deuterium-tritium (D-T) reaction

  • This involves fusing a deuterium nucleus and a tritium nucleus together to produce a helium nucleus and a neutron

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Deuterium-tritium fusion

A nuclear fusion reaction, where deuterium and tritium combine to form helium, releasing a neutron and energy.
The fusion of deuterium and tritium nuclei to form a helium nucleus and a neutron, with the release of energy

Conditions for nuclear fusion

  • For two nuclei to fuse, both nuclei must have high kinetic energy

  • This is because nuclei must be able to get close enough to fuse

  • However, two forces acting within the nuclei make this difficult to achieve

    • Electrostatic repulsion 

      • Protons inside the nuclei are positively charged, which means that they electrostatically repel one another

    • Strong nuclear force

      • The strong nuclear force, which binds nucleons together, acts at very short distances within nuclei

      • Therefore, nuclei must get very close together for the strong nuclear force to take effect

  • It takes a great deal of energy to overcome the electrostatic force, hence, fusion can only be achieved in an extremely hot, dense environment, such as the core of a star

Fusion products

  • When two hydrogen nuclei (protons) fuse, a deuterium nucleus is produced

    • A positron and an electron neutrino are also produced as one of the protons converts into a neutron through beta-plus decay

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  • In the centres of stars, four hydrogen nuclei open parentheses straight H presubscript 1 presuperscript 1 close parentheses fuse to produce a helium nucleus open parentheses He presubscript 2 presuperscript 4 close parentheses, plus the release of energy

    • This provides fuel for the star to continue burning

The proton-proton chain

The proton-proton fusion process in five stages Stage 1: Two protons fuse Stage 2: One proton changes into a neutron, as in beta-plus decay, to leave a deuterium nucleus Stage 3: Another proton joins on, making a nucleus of helium-3 Stage 4: Two helium-3 nuclei fuse Stage 5: Two protons break off, leaving a helium-4 nucleus
The proton-proton chain involves a series of nuclear reactions which produces a helium nucleus from the fusion of four protons

Examiner Tips and Tricks

In the fusion process, the mass of the new, heavier nucleus is less than the mass of the constituent parts of the nuclei fused together, as some mass is converted into energy. 

Not all of this energy is used as binding energy for the new, larger nucleus, so energy will be released from this reaction. The binding energy per nucleon afterwards is higher than at the start. 

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    Katie M

    Author: Katie M

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    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.