Describe the differences between nuclear fission and nuclear fusion. In your answer, include what happens to the nuclei involved and the conditions needed for each process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei and one or more neutrons; it is triggered when a neutron is absorbed by the nucleus. Nuclear fusion is the joining of two light nuclei to form a heavier nucleus; it requires extremely high temperatures (and pressure) to overcome the electrostatic repulsion between the nuclei. Both processes release large amounts of energy, with fusion releasing more energy per kilogram of fuel than fission.
Examiner tips
- Use the exact terms ‘heavy nucleus’, ‘light nuclei’, ‘neutron absorption’, ‘high temperature and pressure’
- Show the direction of the reaction (splitting vs joining)
- Mention the energy release and compare per kilogram
Common mistakes
- Confusing the direction of the reactions (saying fusion splits nuclei or fission joins them)
- Omitting the requirement of a neutron for fission or the temperature/pressure for fusion
- Failing to state that both processes release energy
Mark scheme (5 marks)
- Nuclear fission involves a large/heavy nucleus splitting into two smaller nuclei (and neutrons)
- Nuclear fusion involves two small/light nuclei joining together to form a larger nucleus
- Fission requires a neutron to be absorbed by (or fired at) the nucleus / fission is triggered by a neutron
- Fusion requires extremely high temperatures (and pressure) to overcome the electrostatic repulsion between nuclei
- Both processes release large amounts of energy / fusion releases more energy per kilogram of fuel than fission
Key terms in this question
nuclear fission · nuclear fusion
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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