Adjective
Of less than critical importance
Having a numerical value less than some critical value
(physics) Having insufficient mass to sustain a chain reaction
a subcritical reactor
In the gun-type fission weapon two subcritical pieces of fuel are rapidly brought together. Source: Internet
Predetonation If two pieces of subcritical material are not brought together fast enough, nuclear predetonation can occur, whereby a smaller explosion than expected will blow the bulk of the material apart. Source: Internet
Implosion used explosives to crush a subcritical sphere of fissile material into a smaller and denser form. Source: Internet