OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(h) basic structure of a fission reactor; components – fuel rods, control…
Full specification wording
Nuclear fission and fusion: (h) basic structure of a fission reactor; components – fuel rods, control rods and moderator
A fission reactor contains fuel rods, control rods and a moderator within a shielded core.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 8 of 18
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A fission reactor contains fuel rods, control rods and a moderator within a shielded core. Fuel rods hold fissile material such as uranium-235, often as uranium dioxide, and provide the nuclei that undergo induced fission. Control rods, made of a neutron-absorbing material such as boron or cadmium, are inserted or withdrawn to absorb neutrons and so regulate the rate of the chain reaction. The moderator, for example graphite or water, slows fast neutrons to thermal energies because slow neutrons are more likely to induce fission in uranium-235. A coolant transfers thermal energy to generate steam, and shielding absorbs radiation. The components work together to keep the reaction critical and controlled.
What earns the marks
- Identify fuel rods as the source of fissile nuclei, typically uranium-235, where fission occurs.
- Describe control rods as neutron absorbers, often boron or cadmium, used to control the rate of the chain reaction.
- Explain that the moderator slows fast neutrons to thermal energies so they can induce further fission.
- Recognise that the reactor is kept critical when on average one neutron per fission causes another fission.
- Note the roles of coolant and shielding in transferring energy and reducing radiation exposure.
Common mistakes and how to put them right
Confusing control rods with the moderator.
Put it right by stating control rods absorb neutrons while the moderator slows them.
Thinking the moderator absorbs neutrons.
Put it right by explaining it reduces neutron speed through collisions without removing them.
Believing fuel rods must be pure uranium metal.
Put it right by noting uranium dioxide ceramic pellets are commonly used.
Exam tips
- Name a specific material for each component, such as boron for control rods and graphite for the moderator.
- Explain the purpose of each component in terms of neutrons rather than just naming it.
- Use the term 'thermal neutron' when describing the effect of the moderator.
Try the lesson
The start of the Nuclear fission and fusion: (h) basic structure of a fission reactor; components – fuel rods, control rods and moderator lesson, made from checked teaching notes for this specification point.
Listen
A fission reactor contains fuel rods, control rods and a moderator within a shielded core. Fuel rods hold fissile material such as uranium-235, often as uranium dioxide, and provide the nuclei that undergo induced fission. Control rods, made of a neutron-absorbing material such as boron or cadmium, are inserted or withdrawn to absorb neutrons and so regulate the rate of the chain reaction. The moderator, for example graphite or water, slows fast neutrons to thermal energies because slow neutrons are more likely to induce fission in uranium-235. A coolant transfers thermal energy to generate steam, and shielding absorbs radiation. The components work together to keep the reaction critical and controlled. A strong answer earns marks for these points. Identify fuel rods as the source of fissile nuclei, typically uranium-235, where fission occurs. Describe control rods as neutron absorbers, often boron or cadmium, used to control the rate of the chain reaction. Explain that the moderator slows fast neutrons to thermal energies so they can induce further fission. Recognise that the reactor is kept critical when on average one neutron per fission causes another fission. A common slip: Confusing control rods with the moderator. Instead, stating control rods absorb neutrons while the moderator slows them. Exam tip: Name a specific material for each component, such as boron for control rods and graphite for the moderator.
Key words
Say what each word means before you turn the card.
Fuel rods
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Control rods
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Moderator
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Critical
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See every definition
- Fuel rods
- These hold fissile material such as uranium-235, often as uranium dioxide, and provide the nuclei that undergo induced fission.
- Control rods
- Made of a neutron-absorbing material such as boron or cadmium, these are inserted or withdrawn to absorb neutrons and so regulate the rate of the chain reaction.
- Moderator
- For example graphite or water, this slows fast neutrons to thermal energies because slow neutrons are more likely to induce fission in uranium-235.
- Critical
- The reactor is kept in this state when on average one neutron per fission causes another fission.
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