AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Factors affecting the choice of materials for the moderator, control rods…
Full specification wording
Induced fission (A-level only): Factors affecting the choice of materials for the moderator, control rods and coolant. Examples of materials used for these functions.
Each reactor component has a distinct function, so material choice depends on different physical and chemical factors.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Each reactor component has a distinct function, so material choice depends on different physical and chemical factors. A moderator must slow neutrons without absorbing them, so light nuclei with low neutron absorption are preferred; water, heavy water and graphite are common examples. Control rods must absorb neutrons strongly to regulate the rate of fission; boron, cadmium and hafnium are used because they have large neutron absorption cross-sections. A coolant must transfer thermal energy efficiently from the core to a heat exchanger, remain stable at high temperature and ideally have low neutron absorption; carbon dioxide, water and liquid sodium are examples. Material choice also involves cost, corrosion resistance, melting point and induced radioactivity.
What earns the marks
- Identify the function of the moderator as slowing fast neutrons while minimising neutron absorption, and give water, heavy water or graphite as examples.
- Identify the function of control rods as absorbing neutrons to control the fission rate, and give boron, cadmium or hafnium as examples.
- Identify the function of the coolant as transferring thermal energy from the core, and give carbon dioxide, water or liquid sodium as examples.
- Explain that a moderator should contain light nuclei so elastic collisions remove neutron energy efficiently.
- Explain that control-rod materials need a high neutron absorption cross-section and must be able to be inserted and withdrawn safely.
- Discuss additional factors such as melting point, corrosion resistance, thermal conductivity, cost and neutron absorption when selecting coolant and structural materials.
Common mistakes and how to put them right
Confusing the roles of moderator and control rod.
Put it right by stating that the moderator slows neutrons whereas control rods absorb them to reduce the fission rate.
Choosing a moderator solely because it is light without considering neutron absorption.
Put it right by noting that a good moderator must also have a low neutron absorption cross-section.
Assuming any liquid can act as a coolant without considering induced radioactivity or chemical reaction with the core.
Put it right by discussing stability, corrosion and activation.
Exam tips
- Structure answers by component: state the function first, then the required material property, then a named example.
- Use comparative language such as higher neutron absorption cross-section or lower mass number to justify each choice.
- Link each material property to a consequence for reactor operation, for example controlling the chain reaction or removing thermal energy safely.
Try the lesson
The start of the Induced fission (A-level only): Factors affecting the choice of materials for the moderator, control rods and coolant. Examples of materials used for these functions. lesson, made from checked teaching notes for this specification point.
Have a go first
Which material is commonly used as the moderator in a thermal nuclear reactor?
Listen
Each reactor component has a distinct function, so material choice depends on different physical and chemical factors. A moderator must slow neutrons without absorbing them, so light nuclei with low neutron absorption are preferred; water, heavy water and graphite are common examples. Control rods must absorb neutrons strongly to regulate the rate of fission; boron, cadmium and hafnium are used because they have large neutron absorption cross-sections. A coolant must transfer thermal energy efficiently from the core to a heat exchanger, remain stable at high temperature and ideally have low neutron absorption; carbon dioxide, water and liquid sodium are examples. Material choice also involves cost, corrosion resistance, melting point and induced radioactivity. A strong answer earns marks for these points. Identify the function of the moderator as slowing fast neutrons while minimising neutron absorption, and give water, heavy water or graphite as examples. Identify the function of control rods as absorbing neutrons to control the fission rate, and give boron, cadmium or hafnium as examples. Identify the function of the coolant as transferring thermal energy from the core, and give carbon dioxide, water or liquid sodium as examples. Explain that a moderator should contain light nuclei so elastic collisions remove neutron energy efficiently. A common slip: Confusing the roles of moderator and control rod. Instead, stating that the moderator slows neutrons whereas control rods absorb them to reduce the fission rate. Exam tip: Structure answers by component: state the function first, then the required material property, then a named example.
Key words
Say what each word means before you turn the card.
Moderator
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Control rods
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Coolant
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Neutron absorption cross-section
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See every definition
- Moderator
- A component that slows fast neutrons while minimising neutron absorption, and should contain light nuclei so elastic collisions remove neutron energy efficiently.
- Control rods
- Components that absorb neutrons strongly to regulate the rate of fission, needing a high neutron absorption cross-section and the ability to be inserted and withdrawn safely.
- Coolant
- A substance that transfers thermal energy efficiently from the core to a heat exchanger, remains stable at high temperature and ideally has low neutron absorption.
- Neutron absorption cross-section
- A property that must be high in control-rod materials and low in moderators and ideally coolants, relating material choice to each component's function.
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