Simplified schematic of a multistage thermonuclear weapon

Numbered parts:

Sequence of events in explosion:

STAGE 1: fission explosion

Multiple detonators (3) simultaneously initiate detonation of high explosives (4).
As detonation progresses through high explosives (4), 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 (5) which facilitates transition of the shock wave from low-density high explosive to high-density core material.
Shock front in turn compresses the reflector (5), tamper (6), and fissile core (7) inward.
When compression of the fissile core (7) 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 (7): 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 (7) are reflected back to it by the tamper (6) and reflector (5), improving the chain reaction.
The mass of the tamper (6) delays the fissile core (7) from expanding under the heat of the building energy release.
Neutrons from the chain reaction in the fissile core (7) cause transmutation of atoms in the uranium-235 tamper (6).
As the superheated core expands under the energy release, the chain reaction ends.

STAGE 2: fusion explosion

Gamma radiation from the fission explosion superheats the filler material (2), turning it into a plasma.
The vaporized filler material (2) is delayed from expanding outward by the bomb casing (1), increasing its tendency to compress the fusion pusher/tamper (9).
Compression reaches the fusion fuel (10), which has been partially protected from gamma radiation by the radiation shield (8).
Compression reaches the fissile sparkplug (11), compressing it to a super-critical mass.
Neutrons from the explosion of stage 1 reach the fissile sparkplug (11) through the channel in the radiation shield (8), initiating a fission chain reaction.
The sparkplug (11) explodes outward.
The fusion fuel (10) is now supercompressed between the fusion pusher/tamper (9) from without and the sparkplug (11) from within, turning it into a superheated plasma.
Lithium and deuterium nuclei collide in the fusion fuel (10) to produce tritium, and tritium and deuterium nuclei engage in fusion reactions: nuclei fuse by pairs into helium nuclei, producing a large energy release of gamma rays, neutrons, and heat.
The large release of neutrons from fusion in the fusion fuel (10) causes transmutation of uranium-235 atoms in the fusion pusher/tamper (9), releasing additional energy.
All reactions end as the superheated remnants expand under the energy release; the entire weapon is vaporized.
Total elapsed time: about 0.00002 seconds.