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Critical mass
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Critical mass

The critical mass of fissile material is the amount needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon the nuclear (eg the nuclear fission cross-section) and physical properties of the material, its geometry (shape), and its purity, as well as whether it is surrounded by a neutron reflector. In the case of a sphere surrounded by a neutron reflector the critical mass is about 15 kg for uranium-235 and 10 kg for plutonium-239. Bare-sphere critical masses of some other isotopes whose half-lives exceed 100 years are compiled in the following table.

Until detonation is desired, a nuclear weapon must consist of a number of separate pieces, each below the critical size either because they are too small or unfavorably shaped. To produce detonation, the fissile material must be brought together rapidly, usually by conventional explosives.

See also Nuclear weapon design.


The term is also used figuratively, meaning something like "be sufficient to work properly", especially when a sufficiently large amount is needed to cause growth.


For the car-free environment event, see Critical Mass.

For the English Anarcho Environmentalist pressure group founded in 1984, see Critical Mass (pressure group) .