Compare nuclear fission and nuclear fusion in terms of the nuclei involved and the conditions required for each process to occur.

Eduqas GCSE Physics — 9.4 Nuclear fission and fusion · Compare · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Fission:
1. A heavy nucleus (e.g. ²³⁵U) splits into two lighter fragments.
2. Triggered by absorption of a neutron.

Fusion:
1. Two light nuclei (e.g. ²H + ³He) combine to form a heavier nucleus.
2. Requires extremely high temperatures (and/or pressures) so the nuclei can overcome their electrostatic repulsion and get close enough to fuse.

Examiner tips

Use the word ‘heavy’ for fission and ‘light’ for fusion. Mention the neutron trigger for fission. State high temperature/pressure for fusion. Keep the answer concise and use the exact terminology.

Common mistakes

Confusing the direction of the reaction (splitting vs joining). Forgetting to mention the neutron in fission. Omitting the temperature/pressure requirement for fusion.

Mark scheme (4 marks)

Fission involves a large/heavy nucleus splitting into two smaller nuclei (fragments), whereas fusion involves two small/light nuclei joining together.
Fission requires a neutron to be absorbed by the heavy nucleus (to trigger the split).
Fusion requires extremely high temperatures (and/or pressures) to occur.
High temperatures are needed for fusion because the nuclei must overcome the electrostatic/repulsive force between them (so they can get close enough to fuse).

Key terms in this question

nuclear fission · nuclear fusion