Simplified schematic of a nuclear fission implosion weapon
- bomb casing
- conventional high explosion
- pusher (aluminum, others) and reflector (beryllium, tungsten)
- tamper (uranium-238)
- fissile core (plutonium or uranium-235)
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.
© 2001 by Wm. Robert Johnston.
Last modified 30 June 2001.
Return to Home. Return to Nuclear Weapons.