Form of energy released by fission bombs

[Sakha]

Atomic bombs are really powerful and destructive devices. I understand that during a fission reaction, the parent isotope splits in 2 lower mass isotopes with high kinetic energies due to the binding energy that were released. But I don't understand how this becomes into the destructive effects of the bombs (the mushroom cloud and the enormous pressures).

Is it that the child isotopes with high kinetic energies (high temperatures) 'warm' up the air around really fast to cause the explosion?

 

[Astronuc]

A fission of a U-235 or Pu-239 nucleus produces about 200 MeV, but about 170 MeV goes to kinetic energy of fission products, and the rest goes to gamma rays, beta particles, and neutrons. 1 ev ~ 11605 K, so 170 MeV ~ 2 trillion K. However, not all fissile material fissions, and the fissions do not all occur simultaneously. Suffice it to say the core heats up within several microseconds to several 100 million K. The solid becomes a super hot plasma.

In addition, the gamma radiation, brehmsstrahlung and x-rays, and UV and visible light heat up the surrounding environment to several thousands K. When solids and liquids are heated to thousands K, the vaporize and rapidly expand. Hence the explosion and shock wave.

 

[mheslep]

A fission of a U-235 or Pu-239 nucleus produces about 200 MeV, but about 170 MeV goes to kinetic energy of fission products, and the rest goes to gamma rays, beta particles, and neutrons. 1 ev ~ 11605 K, so 170 MeV ~ 2 trillion K.

As those high MeV fission products impact surrounding matter, wouldn't they trigger X-rays, and thus convert a significant portion of the kinetic energy to electromagnetic which can escape out in front of the shock wave?

 

[Astronuc]

As those high MeV fission products impact surrounding matter, wouldn't they trigger X-rays, and thus convert a significant portion of the kinetic energy to electromagnetic which can escape out in front of the shock wave?

The fission products will knock other atoms, and certainly X-rays will be produced as the fission products and ions recover electrons, but a lot is also visible light and UV. There is also a portion of energy in gamma-rays. At the instant of detonation, the fissons occur in the mass so the X-rays are mostly absorbed in the metal (pit and housing). As the bomb vaporizes, the density decreases and the range of X-rays increases. Basically, the X-rays and gamma rays then scatter off electrons in the atmosphere and ionize atoms, and irradiate the surrounding area.

 

[Sakha]

Thanks Astronuc, that clears my questions.

 

[Morbius]

Is it that the child isotopes with high kinetic energies (high temperatures) 'warm' up the air around really fast to cause the explosion?

Sakha,

That's basically correct. Of the 200 MeV / fission reaction released in fission; about
165 MeV is in the form of kinetic energy of the daughters.

The device turns into a plasma - like a piece of a star. However, whereas a star
has lots of gravity to keep it from exploding, a nuclear device doesn't. So you get
an explosion.

The mushroom is caused by the hot air rising so fast - because it is buoyant in the
gravity / atmosphere. You wouldn't get a mushroom cloud if you exploded the bomb
in space. In space, there's nothing special about any particular direction - hence there
is no "up".

BTW - I'm a physicist with Lawrence Livermore National Laboratory.

Dr. Gregory Greenman

 

[mugaliens]

The mushroom is caused by the hot air rising so fast - because it is buoyant in the
gravity / atmosphere. You wouldn't get a mushroom cloud if you exploded the bomb
in space. In space, there's nothing special about any particular direction - hence there
is no "up".

In part. However, part of the anatomy of how that mushroom cloud appears is due to the reflection of the shock wave off the ground, ocean surface, and ocean bottom.

For example, the first British bomb test occurred at 8,000 feet. As you'll see in http://www.youtube.com/watch?v=yOwH55lnA8M", it still forms a mushroom cloud, but it takes much longer for that cloud to develop, and the stem of the mushroom is truncated at first so that it looked more like a jellyfish than a mushroom cloud.

Very different cloud shapes were formed by Hardtack Umbrella, 8 kt at 150 ft depth, and Hardtack Oak, 8.9 MT (depth unknown, but at Eniwetok).

 

[Morbius]

In part. However, part of the anatomy of how that mushroom cloud appears is due to the reflection of the shock wave off the ground, ocean surface, and ocean bottom.

mugaliens,

When you have shock waves running around, they will affect the shape of a lot of things,
buildings, roadways, ...and yes mushroom clouds.

However, the shock wave is not necessary to the formation of the mushroom cloud.

It reminds me of my high school chemistry class. Someone asked the teacher why only
nuclear bombs made mushroom clouds. The teacher said that wasn't true. He then went
to the science lab storeroom and came back with some chemicals he dumped in a pile on the
front lab table. He stood back and wacked the pile with a meter stick.

There was a flash, and the smoke formed a very nice mushroom cloud as it rose to the
ceiling. Mushroom cloud, but no shock wave.

http://en.wikipedia.org/wiki/Mushroom_cloud

"Mushroom clouds form as a result of the sudden formation of a large mass of hot, low-density gases near the ground creating a Rayleigh–Taylor instability. The mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges, forming a vortex ring and drawing up a column of additional smoke and debris in the center to form its "stem". The mass of gas eventually reaches an altitude where it is no longer of lower density than the surrounding air and disperses, the debris drawn upward from the ground scattering and drifting back down"

If you look close enough at any Rayleigh-Taylor unstable surface, you will see the
characteristic mushroom shape.

http://en.wikipedia.org/wiki/Rayleigh–Taylor_instability

Dr. Gregory Greenman