Critical Mass
The minimum mass of fissile material needed to sustain a nuclear chain reaction.
Unit
kg
In depth
Below the critical mass too many neutrons escape through the surface before causing further fission. The value depends on enrichment, geometry and whether a neutron-reflecting shell surrounds the material, with a compact sphere requiring the least.
Examples in the real world
About 52 kg for a bare sphere of uranium-235, considerably less when surrounded by a reflector.