what is a "criticality" and "critical mass"?

Answer 1

Criticality : Physics.

a. pertaining to a state, value, or quantity at which one or more properties of a substance or system undergo a change.

b. (of fissionable material) having enough mass to sustain a chain reaction.

critical mass :

the amount of a given fissionable material necessary to sustain a chain reaction at a constant rate.


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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 its nuclear properties (e.g. the nuclear fission cross-section) and physical properties (in particular the density), its shape, and its enrichment. Surrounding fissionable material by a neutron reflector reduces the needed mass.

Answer 2

I believe that both things represent the same thing.

Critical mass will the the smallest mass of a fissionable material that will sustain a nuclear chain reaction at a constant level. If you have less than the critical mass, the nuclear reaction will happen but then will slowly die down.
If you keep it at a constant level absorbing neutrons preventing the nuclear reaction to runaway you have a controlled nuclear reactor.

If you add more fissionable material and do not absorbe the neutrons with control rods, etc, you have a nuclear explosion.

Answer 3

Hi. If you had a quantity of, say, plutonium and had many small separated pieces, the radiation would not be strong enough to start a chain reaction (explosion). Bringing more and more close together would increase the piece to piece effects of the radiation. At a certain point there would be enough plutonium close enough together (critical mass or criticality) that a self sustaining reaction would take place and you would not want to be around!