AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Nuclear fission: The neutrons may go on to start a chain reaction.
Each fission releases two or three neutrons, and if at least one of them is absorbed by another large nucleus it causes a further fission, which releases more neutrons again.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 6 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Each fission releases two or three neutrons, and if at least one of them is absorbed by another large nucleus it causes a further fission, which releases more neutrons again. That self-sustaining sequence is a chain reaction. Whether it grows, holds steady or dies away depends on how many of the released neutrons, on average, go on to cause another fission. Exactly one gives a steady output; more than one and the number of fissions per second multiplies rapidly; fewer than one and the reaction fades. Not every neutron succeeds: many escape through the surface of the fuel, some are absorbed without causing fission, and fast neutrons must be slowed by the moderator before uranium-235 will readily absorb them.
What earns the marks
- At least one neutron from a fission being absorbed by another large nucleus.
- That nucleus then undergoing fission and releasing two or three more neutrons.
- The number of fissions per second growing when more than one neutron on average causes a further fission.
- A diagram in which the sequence continues to a second and then a third generation of fissions.
Common mistakes and how to put them right
Drawing neutrons that leave the first fission but never reach another nucleus.
Showing only one neutron released per fission, so the reaction cannot grow.
Saying the chain reaction is carried by the gamma rays or by the energy released.
Assuming every released neutron always causes a fission.
Describing the chain reaction without ever saying a neutron is absorbed.
Exam tips
- Draw at least two generations of fission; one split and some arrows is not yet a chain reaction.
- State what happens on average, not to a single neutron, when explaining whether the reaction grows.
- Mention escape and non-fission absorption if asked why a small piece of fuel will not sustain a chain reaction.
Try the lesson
The start of the Nuclear fission: The neutrons may go on to start a chain reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
In a nuclear reactor, what can the neutrons released during fission do?
Listen
Each fission releases two or three neutrons, and if at least one of them is absorbed by another large nucleus it causes a further fission, which releases more neutrons again. That self-sustaining sequence is a chain reaction. Whether it grows, holds steady or dies away depends on how many of the released neutrons, on average, go on to cause another fission. Exactly one gives a steady output; more than one and the number of fissions per second multiplies rapidly; fewer than one and the reaction fades. Not every neutron succeeds: many escape through the surface of the fuel, some are absorbed without causing fission, and fast neutrons must be slowed by the moderator before uranium-235 will readily absorb them. A strong answer earns marks for these points. at least one neutron from a fission being absorbed by another large nucleus. that nucleus then undergoing fission and releasing two or three more neutrons. the number of fissions per second growing when more than one neutron on average causes a further fission. a diagram in which the sequence continues to a second and then a third generation of fissions. Exam tip: Draw at least two generations of fission; one split and some arrows is not yet a chain reaction.
Key words
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chain reaction
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self-sustaining
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generations of fission
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neutron losses
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See every definition
- chain reaction
- A self-sustaining sequence in which at least one neutron from a fission is absorbed by another large nucleus, causing a further fission that releases more neutrons again.
- self-sustaining
- Describes a sequence in which the neutrons released by fission go on to cause further fissions, so the reaction keeps going.
- generations of fission
- The sequence continuing from a first fission to a second and then a third generation of fissions.
- neutron losses
- Neutrons that do not cause fission because many escape through the surface of the fuel or some are absorbed without causing fission.
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