critical mass noun
: an amount of material (such as plutonium) that is large enough to allow a nuclear reaction to occur
: the size, number, or amount of something that is needed to cause a particular result
: a size, number, or amount large enough to produce a particular result <the critical mass of activity needed for a retail store>
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critical mass noun (Concise Encyclopedia)
Minimum amount of a given fissionable material necessary to achieve a self-sustaining nuclear chain reaction under specified conditions. Critical mass depends on several factors, including the kind of fissionable material used, its concentration and purity, and the composition and geometry of the surrounding reaction system.
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