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Chain reaction
Intermediate
Neutrons from fission trigger further fission.
Neutrons from one fission cause further fissions; if each fission produces on average k > 1 following fissions, it grows exponentially. Critical mass is needed.
k = 1 for a steady reactor.
Moderated neutrons are more effective.
In more detail
A chain reaction occurs when each fission releases neutrons that cause further fissions. If one neutron per fission causes another, the reaction is self-sustaining.
Real-world uses
- Reactors are designed to keep the chain reaction steady.
- Uncontrolled chains are used in nuclear weapons.
- Reactor designers control the chain reaction so that each fission releases just enough neutrons to sustain it steadily.
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Frequently asked questions
What is criticality?
A state in which the chain reaction is self-sustaining at a steady rate.
How is it controlled?
By absorbing neutrons with control rods.
What makes a chain reaction self-sustaining?
On average, at least one neutron from each fission must cause another fission, which is the criticality condition.
Related topics in Atomic & Nuclear Physics
Nuclear reactions · Q-value · Nuclear fission · Nuclear reactors · Nuclear weapons · Nuclear fusion