The time required to raise the value of k from 1 to the maximum value of 2 or so is called the reactivity insertion time, or simply insertion time. It is further complicated by the problem of subcritical neutron multiplication. This persistence of neutrons in subcritical masses further reduces the time window for assembly, and requires that the reactivity of the mass be increased from a value of Simply splitting a supercritical mass into two identical parts, and bringing the parts together rapidly is unlikely to succeed since neither part will have a sufficiently low k value, nor will the insertion time be rapid enough with achievable assembly speeds. By contriving a subcritical arrangement of fissionable material whose average density can be rapidly increased, we can bring about the sudden large increase in reactivity needed to create a powerful explosion. As a general guide, a suitable highly supercritical mass needs to be at least three times heavier than a mass of equal density and shape that is merely critical.
Implosion is capable of very short insertion times, gun assembly is much slower.