The Lawrence Livermore National Laboratory just put out a bunch of movies of atomic bomb detonations. How does the nuclear reaction generate the destructive shockwave that can be seen? If it just creates "energy" how does that translate into a shockwave?

And how does that relate to chemical explosions? Are they explosions because they create a lot of heat, or because they create a lot of gas?

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    $\begingroup$ Note that Lawrence Livermore was not actually a person. The name refers to the national laboratory In Livermore, CA, named in honour if Ernest Lawrence. $\endgroup$ – Emilio Pisanty Mar 21 '17 at 23:36

Ultimately, the destruction we typically associate with explosions is due to overpressure (what you refer to as the "shockwave"): a large pressure differential that mechanically pushes and tears things apart.

An explosion can be produced by creating a relatively high concentration of pressure and/or energy. "High explosives" are defined as having a reaction front that moves faster than the speed of sound. Some chemical explosions have low "heat of explosion" and essentially just turn a solid into a gas very quickly, which creates the destructive overpressure. Other chemical explosions produce a lot of "energy" which heats the surrounding matter and which (thanks to Boyle's law) in turn creates the destructive overpressure.

A fusion explosion creates mostly "energy" in the form of high-frequency photons. So the bulk of a nuclear explosion is a "shockwave" of photons. (There's a small second shockwave created as the mass of the fusion device itself – minus a fraction of a percent turned into energy – becomes a high-velocity plasma. But in a vacuum that's not very exciting.) It's when that pulse of photons hits matter – e.g., an atmosphere – and heats that matter into a plasma, which then (thanks Boyle) creates the overpressure and the material shockwave travelling at the speed of sound that we can see.

  • $\begingroup$ Don't forget the neutrons... $\endgroup$ – Jon Custer Mar 21 '17 at 21:10
  • $\begingroup$ @JonCuster - Yep, I left those lumped into the "exploding" plasma of the fusion device itself. $\endgroup$ – feetwet Mar 21 '17 at 21:12
  • $\begingroup$ Well, a bunch come flying out at fairly high energies. But, yes, particularly in atmosphere it is all about the gammas and x-rays... $\endgroup$ – Jon Custer Mar 21 '17 at 21:31

How does the nuclear reaction generate the destructive shockwave that can be seen? If it just creates "energy" how does that translate into a shockwave?

A modern nuclear warhead, if it's not a "dial-a-yield" model, consumes basically all of its nuclear fuel within approximately one microsecond. During that microsecond, it emits however many kilotons or megatons of energy, mostly as high-energy photons.

The usual way to use a nuclear weapon is to explode it in the air above the target. Most of the photons emitted by the device will be absorbed by a huge volume of air, creating the incandescent "fireball" that you've seen in films.

Raising the temperature of air also raises its pressure. Within a microsecond, the pressure of that huge volume of air is raised high enough to flatten pretty much any above ground structure that wasn't vaporized by the direct radiation. It's the expansion of the heated air that drives the shock wave and the subsequent blast.

And how does that relate to chemical explosions?

"Explosion" usually just means that something flew to bits. "Detonation" and "Deflagration" are terms of art in chemistry that name specific ways of flying to bits. In a detonation, the reaction front moves through the fuel as a shock wave. In a deflagration, the reaction propagates by heat.

There is a chemical detonation that occurs when triggering a nuclear warhead, but if the physicists call the nuclear process "detonation", that's something I would not know about.


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