IGCSE Physics / Fission and fusion
Chain reactions revision notes
Edexcel International GCSE Physics (4PH1), chapter 25: Fission and fusion, specification point 7.20. Free lesson.
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A chain reaction happens when neutrons released by one fission cause more uranium-235 nuclei to split. Whether the number of fissions grows or stays steady determines whether the reaction is uncontrolled or controlled.
How a chain reaction starts
A fission is when a uranium-235 nucleus splits after absorbing a neutron. It releases energy and two or three more neutrons. These neutrons can split other uranium-235 nuclei, which then release still more neutrons. The reaction keeps itself going, so it is called a chain reaction. Each fission takes in one neutron but releases more than one. Each round of fission happens in a tiny fraction of a second.
Why an uncontrolled reaction grows
If every neutron from a fission causes another fission, the number of fissions triples each round when each fission releases three neutrons. The number quickly becomes very large, so energy is released almost all at once. This is an uncontrolled chain reaction. Starting with one fission, the rounds give 1, 3, 9, 27, then 81 fissions. An uncontrolled chain reaction is used in a nuclear bomb.
How a controlled reaction stays steady
In a controlled reaction, on average one neutron from each fission causes another fission. The number of fissions stays steady, so energy is released at a steady rate. This is needed in a nuclear reactor in a power station. Not every neutron in a reactor causes another fission. The other neutrons are absorbed or escape from the fuel without hitting a nucleus. If every neutron caused another fission, the number would grow each round and the reaction would become uncontrolled.
Describe how to set up a chain reaction
For a three-mark description, state what happens to the first neutron, what the fission releases, and how the released neutrons lead to further fissions.