Nuclear fission and fusion

    AQA
    GCSE

    Nuclear fission is the splitting of a large, unstable nucleus, typically Uranium-235 or Plutonium-239, into smaller daughter nuclei following the absorption of a slow-moving neutron. This process releases two or three high-speed neutrons and significant thermal energy, potentially sustaining a chain reaction if not regulated by control rods and moderators. Conversely, nuclear fusion requires the forcing together of light nuclei against strong electrostatic repulsion to form heavier nuclei, the primary energy mechanism in stars. Both processes demonstrate mass-energy equivalence, where a small decrease in mass results in a large release of energy according to E=mc².

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that a neutron is absorbed by a large, unstable nucleus (e.g., Uranium-235) to induce fission
    • Award 1 mark for identifying the products: two smaller 'daughter' nuclei, two or three neutrons, and gamma radiation
    • Credit responses that explain the chain reaction: released neutrons are absorbed by other unstable nuclei, triggering further fission events
    • Award 1 mark for defining fusion as the joining of two light nuclei to form a heavier nucleus, with some mass converted to energy
    • Candidates must specify that energy is released as kinetic energy of the fission products and neutrons, plus gamma rays

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that a neutron is absorbed by a large, unstable nucleus (e.g., Uranium-235) to induce fission
    • Award 1 mark for identifying the products: two smaller 'daughter' nuclei, two or three neutrons, and gamma radiation
    • Credit responses that explain the chain reaction: released neutrons are absorbed by other unstable nuclei, triggering further fission events
    • Award 1 mark for defining fusion as the joining of two light nuclei to form a heavier nucleus, with some mass converted to energy
    • Candidates must specify that energy is released as kinetic energy of the fission products and neutrons, plus gamma rays

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always use the term 'nucleus' and never 'atom' when describing nuclear processes; AQA examiners penalize this imprecision heavily
    • 💡When describing the chain reaction, explicitly link the 'released neutrons' from the first fission to the 'absorption' by subsequent nuclei
    • 💡For fusion, memorize the phrase 'mass is converted into energy' to explain the energy release, as this is a frequent 1-mark descriptor
    • 💡In 6-mark extended response questions comparing fission and fusion, structure your answer by: Process (Split vs Join) → Requirements (Neutron vs High Temp) → Products → Energy Source

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that the 'atom' splits rather than the 'nucleus' — this is a fundamental terminology error that often results in zero marks for that point
    • Describing the neutron as 'hitting' or 'colliding with' the nucleus instead of being 'absorbed' — the specific interaction is crucial for induced fission
    • Confusing the products of fission (daughter nuclei) with the emitted particles (neutrons), or failing to mention the release of neutrons required for a chain reaction
    • Assuming fusion occurs at low temperatures or pressures; failing to link the requirement for high temperature to overcoming electrostatic repulsion

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

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