AQA A-Level Physics Paper 2, June 2025: Question 30

1 mark · Easy difficulty · Multiple Choice

Identify the purpose of a moderator in a thermal nuclear reactor.

Practise this question

Question

Multiple-choice question 30: The purpose of a moderator in a thermal nuclear reactor is to: A absorb neutrons to reduce the reaction rate, B add neutrons to start the chain reaction, C remove heat from the reactor, D slow neutrons to thermal speeds. Each option has a selection box next to it.

Mark scheme

Show the mark scheme Mark scheme table row for question 30: shows answer key 'D', description 'slow neutrons to thermal speeds.', and assessment objective 'AO1'.

How to answer it

Thermal Nuclear Reactors: Function of the Moderator

📌 What this question tests

This question assesses knowledge of thermal nuclear reactor design (Specification Section 3.8.1.4: Nuclear fission & reactor core components). Specifically, it tests your recall of the distinct role of the moderator in comparison to control rods and coolant.

Question 30 Analysis

Multiple Choice [1 Mark] — AO1 (Recall & Understanding)

✅ Correct Answer: D

slow neutrons to thermal speeds.

Fission neutrons are released with high kinetic energy (around 2 MeV, travelling at roughly 10⁷ m s⁻¹). Uranium-235 nuclei have a far higher probability of capturing slow-moving neutrons (around 0.025 eV to 1 eV, travelling at ~2000 m s⁻¹). The moderator's job is to reduce the speed of these neutrons via elastic collisions until they are in thermal equilibrium with the core.

💡 Key Reactor Components & Roles

  • Moderator (Water / Graphite): Slows fast fission neutrons down to thermal energies through elastic collisions with light nuclei (like ¹H or ¹²C) without absorbing them.
  • Control Rods (Boron / Cadmium): Absorb neutrons to maintain a steady reaction rate (criticality: 1 neutron per fission causes a subsequent fission) or shut down the reactor. (Matches option A).
  • Coolant (Water / CO₂ / Liquid Na): Removes heat generated by the core and transfers it via a heat exchanger to produce steam for turbines. (Matches option C).
  • Neutron Source: Often an artificial emitter (e.g. Americium-Beryllium) used to supply initial neutrons to start the reaction safely. (Matches option B).

🧠 Exam Technique: Elimination Strategy

Every incorrect distractor in this question accurately describes a different part of the reactor system:

  • Option A describes the control rods.
  • Option B describes a startup neutron source.
  • Option C describes the coolant.

By mapping each option to its designated reactor component, only D matches the moderator.

❌ Common Errors & Confusions

  • Confusing Moderator and Control Rods: In everyday English, "to moderate" means to slow down, control, or reduce an amount. Students often falsely infer that a moderator "moderates" (reduces) the rate of reaction by absorbing neutrons (Option A).
  • Misunderstanding "Thermal": A "thermal neutron" does not mean a "hot" neutron. It means a neutron whose kinetic energy is comparable to the thermal agitation energy of the surrounding material at that temperature (around 3kT/2 ≈ 0.025 eV at 300 K).
  • Absorption vs. Collision: A good moderator must not absorb neutrons; it must undergo purely elastic collisions with minimal neutron capture cross-section.

📐 Elastic Collisions in the Moderator

Why are light nuclei used as moderators?

  1. In an elastic collision between an incoming particle of mass m and a stationary target of mass M, kinetic energy transferred is maximal when:
    m ≈ M
  2. Because a neutron has a mass of approximately 1 u, nuclei with low atomic mass numbers (e.g. ¹H in water, ²H in heavy water, or ¹²C in graphite) extract the most kinetic energy per collision.
  3. Heavy nuclei (like ²³⁸U) cause neutrons to simply bounce off with almost no loss in speed, making them useless as moderators.
Mark Scheme Reference: Question 30 | Answer: D | AO1 (Recall of reactor design principles) | 1 mark

Topics

Physics · 3.8 Nuclear physics (A-level only)

Question and mark scheme from the AQA A-Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.