Describe the process of nuclear fission and explain why it releases a large amount of energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nuclear fission is used in power stations as an alternative to burning fossil fuels. In a fission reaction, a large unstable nucleus splits into smaller nuclei after absorbing a neutron.
Model answer (5 marks)
A large unstable nucleus, such as U‑235, absorbs a neutron. The nucleus becomes highly excited and splits into two smaller daughter nuclei (and sometimes a third very light fragment). Two or three neutrons are released during the split. These free neutrons can be captured by other U‑235 nuclei, triggering further fission reactions and producing a self‑sustaining chain reaction. The energy released is large because the total mass of the fission products and neutrons is less than the mass of the original nucleus plus the captured neutron; the mass defect is converted into kinetic energy and radiation according to E=mc².
Examiner tips
- Use the exact sequence: neutron capture → split → neutron release → chain reaction → mass defect → energy release
- Mention the mass defect explicitly and relate it to E=mc²
- Keep the answer concise, covering all five points
Common mistakes
- Omitting the chain‑reaction step
- Failing to state that the released neutrons are free and can cause further fission
- Not mentioning the mass defect or E=mc² as the source of energy
Mark scheme (5 marks)
- A large unstable nucleus (e.g. uranium-235) absorbs a neutron
- The nucleus splits into two smaller (daughter/fragment) nuclei
- Two or three neutrons are released / emitted during the split
- These released neutrons can trigger further fission reactions, causing a chain reaction
- The energy released is large because the products have less mass than the original nucleus and neutron / mass is converted into energy
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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