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critical mass
The minimum mass of a fissionable material (generally 235U or 239Pu) that will initiate a chain reaction in a nuclear reactor or an atomic (fission) bomb. The critical mass of pure 239Pu is about 4.5 kg, and of 235U about 15 kg. The fissionable material of a fission bomb is divided into portions less than the critical mass; when brought together at the moment of detonation they exceed the critical mass and the high-energy chain reaction begins.
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