Nuclear Fission

    OCR
    GCSE

    Nuclear fission is the splitting of a large, unstable nucleus, typically uranium-235 or plutonium-239, into two smaller daughter nuclei, accompanied by the release of energy and neutrons. This process is usually induced by the absorption of a slow-moving neutron, rendering the nucleus unstable. The reaction releases significant energy, primarily as the kinetic energy of fission fragments and gamma radiation. Additionally, the emission of two or three excess neutrons can trigger a chain reaction, which must be regulated in a nuclear reactor using control rods and a moderator to ensure safe energy generation.

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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 a neutron is absorbed by a large unstable nucleus (e.g., Uranium-235)
    • Award 1 mark for describing the nucleus splitting into two smaller daughter nuclei of roughly equal size
    • Credit responses that identify the release of 2-3 neutrons and energy in the form of gamma rays and kinetic energy
    • Award 1 mark for explaining the chain reaction: released neutrons are absorbed by other nuclei causing further fission
    • For control rods, award marks for stating they absorb surplus neutrons to regulate the rate of the reaction

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the splitting of the nucleus. Now, specify what triggers this event (neutron absorption)"
    • "Be careful with terminology: use 'daughter nuclei' instead of 'parts' or 'atoms' to secure the mark"
    • "You confused the moderator with control rods. Remember: Moderator = Speed, Control Rods = Rate (Number of neutrons)"
    • "Good description of the energy release. To improve, explicitly mention the kinetic energy of the fragments and gamma radiation"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating a neutron is absorbed by a large unstable nucleus (e.g., Uranium-235)
    • Award 1 mark for describing the nucleus splitting into two smaller daughter nuclei of roughly equal size
    • Credit responses that identify the release of 2-3 neutrons and energy in the form of gamma rays and kinetic energy
    • Award 1 mark for explaining the chain reaction: released neutrons are absorbed by other nuclei causing further fission
    • For control rods, award marks for stating they absorb surplus neutrons to regulate the rate of the reaction

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In 6-mark questions on nuclear reactors, structure your answer by component: Fuel (source), Moderator (slows neutrons), Control Rods (absorb neutrons)
    • 💡Memorise the mnemonic: Moderators Modify speed (slow down); Control rods Control rate (absorb neutrons)
    • 💡Ensure nuclear equations balance: the sum of mass numbers and atomic numbers on the left must equal those on the right exactly

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the roles of the moderator and control rods; students often incorrectly state control rods slow down neutrons
    • Describing the products of fission as 'atoms' rather than 'daughter nuclei' or 'smaller nuclei'
    • Omitting the initial absorption of a neutron when defining induced fission, describing it as a spontaneous split
    • Failing to conserve mass number or atomic number when completing nuclear equations

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Induced fission versus spontaneous fission
    Chain reactions and the role of neutrons
    Nuclear reactor components (moderator, control rods, shielding)
    Balancing nuclear equations
    Energy transfer and waste management

    Likely Command Words

    How questions on this topic are typically asked

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