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Critical mass

The minimal amount of fissionable material needed to achieve a chain reaction. When a critical mass is reached, a sufficient number of neutrons produced by fission participate in further fission. This allows the fission reaction to run continuously. The critical mass depends on the type of fissile material, its concentration and the shape in which it is arranged. In the case of pure uranium 235, the critical mass would be a sphere of about 18 cm in diameter and weighing about 50 kilograms.