critical mass

The critical mass is 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 kilograms, and of 235U about 15 kilograms. 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.