Nuclear physics
Critical Mass
The minimum amount of fissile material required to sustain a self-propagating nuclear chain reaction.
For a bare sphere of plutonium-239, the critical mass is about 10 kg; for uranium-235 it is about 52 kg. Critical mass is reduced by neutron reflectors (tampers) and by compression. Atomic bombs work by rapidly assembling a supercritical mass before the chain reaction blows itself apart.
Related terms
Nuclear Fission
The splitting of heavy atomic nuclei (typically uranium-235 or plutonium-239) into lighter fragments, releasing large amounts of energy.
Plutonium-239
A fissile isotope of plutonium produced from uranium-238 in nuclear reactors and used in most modern fission weapons.
Uranium-235
A fissile isotope of uranium that constitutes the explosive material in many fission weapons.
Atomic Bomb
A nuclear weapon whose energy comes from nuclear fission of uranium-235 or plutonium-239.