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Q & A: Fission criticality

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Most recent answer: 10/22/2007
Q:
I am not quite clear on why you need a critical mass of a fissionable substance for fission to proceed. Why cant a gram of U235 sustain fission but 56kg can?
- jake (age 14)
central memorial highschool, canada
A:
When one nucleus falls apart it sends out a few neutrons. These can trigger fission in other nuclei, if they hit them. In a small piece of U235, the neutrons are unlikely to hit another nucleus before they leave. In a bigger piece, they have much more chance of hitting another nucleus. Once the average number of new fissions made by the neutrons from each previous fission gets bigger than one, the chain reaction grows until it blows the material apart.


mike w

(published on 10/22/2007)

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