Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Form of energy released by fission bombs

  1. Aug 12, 2010 #1
    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 dont 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?
  2. jcsd
  3. Aug 12, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
  4. Aug 13, 2010 #3


    User Avatar
    Gold Member

    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?
  5. Aug 13, 2010 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
  6. Aug 14, 2010 #5
    Thanks Astronuc, that clears my questions.
  7. Aug 15, 2010 #6


    User Avatar
    Science Advisor


    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
  8. Aug 16, 2010 #7
    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).
    Last edited by a moderator: Apr 25, 2017
  9. Aug 16, 2010 #8


    User Avatar
    Science Advisor


    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.


    "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.


    Dr. Gregory Greenman
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook