Simplified schematic of a multistage thermonuclear weapon

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.