Explain why a controlled chain reaction in a nuclear fission reactor requires the use of a moderator.

IB DP Physics Standard Level (2023 syllabus) — E.4 Fission · Explain · 4 marks · View as Markdown

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

In a nuclear fission reactor, uranium-235 nuclei undergo fission when struck by neutrons, releasing energy and further neutrons. Without careful control, the reaction rate can increase uncontrollably.

Model answer (4 marks)

Fast neutrons produced in fission have high kinetic energy and a low probability of inducing further fission in U‑235. U‑235 undergoes fission most readily with slow (thermal) neutrons. A moderator slows the fast neutrons by elastic collisions with light nuclei, reducing their kinetic energy through repeated collisions. The resulting thermal neutrons have a high fission cross‑section, allowing a controlled chain reaction at a steady rate, maintaining criticality of approximately one fission per neutron on average.

Examiner tips

State that fast neutrons have low fission cross‑section for U‑235
Explain that a moderator slows them to thermal energies
Show how thermal neutrons sustain a controlled chain reaction
Use the term ‘criticality’ to demonstrate understanding

Common mistakes

Confusing the role of the moderator with that of control rods
Claiming that fast neutrons are needed for a chain reaction
Using vague terms such as ‘slow down’ without mentioning elastic collisions

Mark scheme (4 marks)

Neutrons released by fission are fast (high kinetic energy) neutrons
Fast neutrons have a low probability of causing further fission in uranium-235 / U-235 undergoes fission most readily with slow (thermal) neutrons
The moderator slows neutrons down by (elastic) collisions with moderator nuclei / neutrons lose kinetic energy through repeated collisions
This produces slow (thermal) neutrons that sustain a controlled chain reaction at a steady rate / maintaining a criticality of approximately one fission per neutron on average

Key terms in this question

moderator · chain reaction