Simplified schematic of a nuclear fission implosion weapon

Numbered parts:

Sequence of events in explosion:

Multiple detonators (2) simultaneously initiate detonation of high explosives (3).
As detonation progresses through high explosives (3), shaping of these charges transforms the explosive shock front to one that is spherically symmetric, travelling inward.
Explosive shock front compresses and transits the pusher (4) which facilitates transition of the shock wave from low-density high explosive to high-density core material.
Shock front in turn compresses the reflector (4), tamper (5), and fissile core (6) inward.
When compression of the fissile core (6) reaches optimum density, a neutron initiator (either in the center of the fissile core or outside the high explosive assembly) releases a burst of neutrons into the core.
The neutron burst initiates a fission chain reaction in the fissile core (6): a neutron splits a plutonium/uranium-235 atom, releasing perhaps two or three neutrons to do the same to other atoms, and so on; energy release increases geometrically.
Many neutrons escaping from the fissile core (6) are reflected back to it by the tamper (5) and reflector (4), improving the chain reaction.
The mass of the tamper (5) delays the fissile core (6) from expanding under the heat of the building energy release.
Neutrons from the chain reaction in the fissile core (6) cause transmutation of atoms in the uranium-235 tamper (5).
As the superheated core expands under the energy release, the chain reaction ends; the entire weapon is vaporized.
Total elapsed time: about 0.00002 seconds.