Explain why nuclear fusion does not occur at low temperatures and pressures, and describe the role of the moderator in a nuclear fission reactor.

Pearson Edexcel International GCSE Physics (4PH1) — 7.3 Fission and fusion · Explain and Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Fusion requires nuclei to approach each other closely enough for the strong nuclear force to overcome the electrostatic repulsion between their protons. At low temperatures and pressures the kinetic energy of the nuclei is too small, so the Coulomb barrier cannot be breached.

In a fission reactor, the moderator slows down the fast neutrons produced by fission. By reducing their speed, the neutrons become thermal neutrons, which have a much higher probability of being captured by U‑235 nuclei and causing further fission.

Examiner tips

  • Use the terms ‘Coulomb barrier’, ‘kinetic energy’, ‘moderator’, ‘thermal neutrons’ and ‘U‑235’ to match the mark scheme.
  • Separate the two parts clearly: first explain fusion, then describe the moderator’s role.
  • Show the cause–effect relationship: low T/P → insufficient kinetic energy → cannot overcome repulsion; moderator → slows neutrons → increases absorption by U‑235.

Common mistakes

  • Confusing the role of the moderator with that of a reflector or stating that it increases neutron speed.
  • Using vague phrases like ‘high pressure’ without linking it to kinetic energy or Coulomb barrier.
  • Failing to mention U‑235 or the term ‘thermal neutrons’ when describing the moderator’s effect.

Mark scheme (4 marks)

  1. At low temperatures and pressures, the electrostatic repulsion between protons (in the two nuclei) cannot be overcome
  2. High temperatures and/or pressures give the nuclei sufficient kinetic energy to overcome this repulsion
  3. The moderator slows down (fast) neutrons produced during fission
  4. So that the slow (thermal) neutrons can be absorbed by uranium-235 nuclei and induce further fission

Key terms in this question

nuclear fusion · moderator · nuclear fission

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