supercritical

adjectives

(especially of fissionable material) able to sustain a chain reaction in such a manner that the rate of reaction increases
supercritical
is similar to
at or of a point at which a property or phenomenon suffers an abrupt change especially having enough mass to sustain a chain reaction
"a critical temperature of water is 100 degrees C--its boiling point at standard atmospheric pressure"; "critical mass"; "go critical"
critical