First, a specially shaped charge of chemical explosives surrounding a plutonium pit goes off.
From the economist.com
The plutonium pit was squeezed to increase its density by simultaneous detonation of the conventional explosives placed uniformly around the pit.
From the en.wikipedia.org
A hollow plutonium pit was the original plan for the 1945 Fat Man bomb, but there was not enough time to develop and test the implosion system for it.
From the en.wikipedia.org
Some of these strike plutonium or uranium nuclei in the primary's pit, initiating nuclear chain reaction.
From the en.wikipedia.org
In a warhead's detonation, a conventional explosive packaged around the pit compresses the plutonium inward, creating enough pressure for an atomic chain reaction.
From the usatoday.com
The Rocky Flats plant near Boulder, Colorado, was built in 1952 for pit production and consequently became the plutonium and beryllium fabrication facility.