Critical Mass

Although an average of two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction - some escape from the reaction vessel - some interact with non-fissile atomic nuclei. If the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.

The point where the chain reaction can become self-sustaining is referred to as critical mass.

The critical mass is the smallest mass of fissile material required for a nuclear chain reaction to be maintained. The output from a critical mass is steady because there is an equilibrium between the number of neutrons produced and those causing fissions.

The critical mass of a fissile (able to undergo fission) material depends upon:

its nuclear properties (e.g. the nuclear fission cross-section),
its density,
its shape (mass to surface area ratio needs to be as large as possible - so a sphere produces the 'best shape' for a low critical mass)
and its enrichment (what percentage of its atoms are thosoe of the fissile isotope)
surrounding the fissionable material with a suitable neutron "reflector" (beryllium) - the loss of neutrons can reduced and the critical mass can be reduced. (By using a neutron reflector, only about 11 pounds (5 kilograms) of nearly pure or weapon grade plutonium 239 or about 33 pounds (15 kilograms) uranium 235 is needed to achieve critical mass).

A subcritical mass is a mass of fissile material that does not have the ability to sustain a fission reaction. The population of neutrons introduced to a subcritical assembly will exponentially decrease, typically rapidly as they escape from the fissile material faster than they can be produced by fissions.

A supercritical mass is one where there is an increasing rate of fission. The material may settle into equilibrium (i. e. become critical again) at an elevated temperature or power level..