Scientists define criticality as a measure of the ability of nuclear material to sustain a fission
chain reaction. If a system is subcritical, it cannot sustain a fission chain reaction. If a system
is supercritical, the fission chain reactions grow greatly. A system that is "critical" is the
bounding case - this means that it sustains a chain reaction with a constant rate.
The critical mass of fissile material depends on many factors:
- Purity of material
- Shape of material
- Density of material
- Temperature of material
- Surrounding materials
The "bare sphere critical mass" of weapons-grade uranium and weapons-grade plutonium is
approximately 52 kg and 10 kg (respectively).
The critical mass of fissile material informs the most fundamental question of nuclear
weapon design. The question which Scientists have asked for the first time at the beginning
of the Manhattan Project and which every nuclear weapon state has asked since then is:
how much fissile material is necessary for a nuclear weapon? The class of nuclear weapon
design (see section nuclear weapon design ) determines the amount which is needed. For a
gun-assembled design, more fissile material is necessary than for an implosion device, for
The International Atomic Energy Agency defines "significant quantities" of uranium and
plutonium as 25 kg and 8 kg (respectively). However, the figures which are below show that it is possible
to produce reasonable nuclear yields with much less nuclear material than this, even if the technical
capability is low.