Critical Mass
The minimum mass of fissile material required to sustain a continuous self-sustaining nuclear chain reaction.
Governing formula
Neutron Leakage Rate = Neutron Production Rate (k = 1)
SI unit
kg (Kilograms)
In depth
Critical mass depends on material purity, density, shape, and surrounding neutron reflectors. If mass is subcritical, too many neutrons escape from the surface before triggering subsequent fissions. In supercritical mass (k > 1), fission rates escalate exponentially.
Examples in the real world
A bare sphere of pure Uranium-235 possessing a critical mass of approximately 52 kilograms.