How is the chain reaction in a nuclear reactor kept safe, controlled and useful?
The function of the moderator, control rods and coolant in a thermal nuclear reactor, the safety features, and the handling and disposal of radioactive waste.
A focused answer to AQA A-Level Physics 3.8.1.8, covering the roles of the moderator, control rods and coolant in a thermal nuclear reactor, the safety features that prevent uncontrolled reactions, and the handling and disposal of radioactive waste.
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What this dot point is asking
AQA specification point 3.8.1.8 wants you to explain the roles of the moderator, control rods and coolant in a thermal nuclear reactor, describe the safety features that keep the chain reaction controlled, and discuss the handling and disposal of radioactive waste.
The moderator
The neutrons lose the most energy in collisions with nuclei of similar mass to themselves, which is why light nuclei (such as the hydrogen in water) make effective moderators. The neutrons reach thermal energies, in equilibrium with the moderator, before being captured by the fuel.
Control rods
The coolant
The coolant must not absorb too many neutrons and must transfer heat efficiently; common choices include pressurised water, carbon dioxide gas and, in some designs, liquid metal.
Safety features
Radioactive waste
Try this
Q1. State the function of the control rods in a nuclear reactor. [1 mark]
- Cue. They absorb neutrons to control the rate of the chain reaction.
Q2. Explain why a moderator is needed in a thermal reactor. [2 marks]
- Cue. It slows the fast fission neutrons to thermal speeds so they are more likely to be absorbed and cause further fission.
Q3. State the role of the coolant. [1 mark]
- Cue. To remove heat from the core to raise steam for the turbines.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20196 marksDescribe the functions of the moderator, the control rods and the coolant in a thermal nuclear reactor, and explain how the reactor is kept operating at a steady rate.Show worked answer →
The moderator (such as water or graphite) slows the fast neutrons released by fission to thermal speeds through repeated elastic collisions, so they are far more likely to be absorbed by uranium-235 and cause further fission.
The control rods (such as boron or cadmium) absorb neutrons. Lowering them removes more neutrons and slows the reaction; raising them speeds it up.
The coolant is pumped through the core to carry away the heat produced, which is used to raise steam to drive turbines.
To run steadily, the control rods are adjusted so that on average exactly one neutron from each fission goes on to cause another fission, keeping the reactor critical.
Markers reward the correct role of each component and explaining the critical condition (one neutron per fission) maintained by the control rods.
AQA 20214 marksExplain why spent fuel from a nuclear reactor is dangerous and describe how high-level radioactive waste is handled and stored.Show worked answer →
Spent fuel contains highly radioactive fission products and unused fuel with long half-lives, so it remains intensely ionising and hazardous to living tissue for thousands of years, and it also generates heat from continued decay.
It is first placed in cooling ponds of water for several years, which absorb the radiation and remove the decay heat. It is then sealed in robust containers, with long-term plans for deep underground geological storage in stable rock to isolate it from the environment.
Markers reward the long half-lives and continued activity making it hazardous, initial cooling in water ponds, and sealed long-term (geological) storage.
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Sources & how we know this
- AQA A-level Physics (7408) specification — AQA (2017)