AQA A-Level Physics Paper 2, June 2025: Question 29

AQA A-Level Physics • Nuclear Physics

Roles and Functions of Materials in Thermal Nuclear Reactors

What this question tests

This question assesses AO1 knowledge of thermal nuclear reactor core design and fuel cycle safety, specifically:

The distinction between a moderator (slowing neutrons down) and control rods (absorbing neutrons).
The properties and roles of water as a coolant and moderator in thermal reactors (such as PWRs).
Post-fission waste handling, including the use of cooling ponds to absorb radiation and residual heat.

Question 29: Identifying the Non-Use of Water

Multiple Choice Analysis • [1 Mark]

✅ Correct Answer: A

Option A: "to absorb neutrons released from fission"

Water is explicitly chosen as a moderator because it has a very low neutron absorption cross-section. If water absorbed the fission neutrons, the chain reaction would quickly quench and stop. Neutron absorption is the role of control rods (typically made of boron or cadmium), not water.

Option	Role in Nuclear Industry	Is it a real use of water?
A	Absorb neutrons released from fission	NO – Absorbing neutrons is done by control rods (boron/cadmium). Moderating materials must intentionally avoid absorbing neutrons.
B	Cool a nuclear reactor	YES – Pressurised water or boiling water acts as the primary coolant, removing thermal energy from the fuel rods to turn turbines.
C	Shield radiation produced by nuclear waste	YES – Spent fuel rods are stored in deep cooling ponds where water acts as an effective radiation shield (especially against gamma rays and beta decay) and keeps the spent fuel cool.
D	Slow neutrons to thermal speeds	YES – In Pressurised Water Reactors (PWRs), light water acts as the moderator. Protons in H₂O have a mass similar to neutrons, allowing efficient momentum transfer through elastic collisions.

💡 Key Knowledge: Reactor Components

Moderator (e.g. Water, Heavy Water, Graphite): Must have light nuclei (mass close to neutron mass) to maximise kinetic energy transfer per elastic collision, and a low probability of neutron absorption. Slows 2 MeV fast neutrons down to ~0.025 eV (thermal speeds).
Coolant (e.g. Water, CO₂, Liquid Na): High specific heat capacity, non-corrosive, flows past fuel elements to extract heat.
Control Rods (e.g. Boron, Cadmium): High absorption cross-section to absorb excess neutrons and maintain a multiplication factor k = 1 (critical).
Spent Fuel Ponds: Water absorbs decay heat from highly active short-lived fission products and shields workers from gamma radiation.

🧠 Exam Technique: "NOT" Questions

Circle the negative prompt: The word not changes the objective. You are looking for the false statement.
True / False labeling: Write 'T' or 'F' beside each letter. B is true, C is true, D is true; A is false. Therefore, A is the correct option.
Precision in terminology: Always distinguish carefully between slowing down (elastic scattering) and absorbing (capture/capture-decay).

❌ Common Misconceptions & Pitfalls

Confusing Moderator and Control Rod Actions: Students often conflate "slowing down" with "stopping/absorbing". Moderation requires collisions that do not capture the neutron.
Forgetting Cooling Ponds (Option C): Some students believe only lead or concrete can shield nuclear waste. However, spent fuel rods are submerged under meters of water immediately after removal from the core for both thermal cooling and shielding.
Overlooking Heavy Water vs Light Water: While light water absorbs a tiny fraction of neutrons (leading to the requirement of slightly enriched uranium in PWRs), absorbing neutrons is a detrimental side-effect, never a purposeful use.

Mark Scheme Reference: Question 29 • Answer: A • Assessment Objective: AO1 (Recall and understanding of reactor design and function).

AQA A-Level Physics Paper 2, June 2025: Question 29

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