The minimum amount concentrated fissionable material required to sustain a chain reaction. The exact mass of fissionable material needed to sustain a chain reaction varies according to the concentration (purity) and chemical form of the material, the particular fissionable isotope present, its geometrical properties, and its density. When pure fissionable materials are compressed by high explosives in implosion-type atomic weapons, the critical mass needed for a nuclear explosion is reduced.
The amount of nuclear fuel needed for a chain reaction to take place is the critical mass. For example, about 22 pounds of uranium-235 and 11 pounds of plutonium-239 are necessary for a chain reaction.
The amount of a fissionable material , at a given density and mixed in a specified way with other material, where on average one neutron caused by a fission reaction will go on to cause another fission.