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Plutonium Bombs and Fallout

  1. Jun 29, 2011 #1
    Having followed a few threads due to Fukushima, and having read a lot of media reports, pdfs beyond number, scoured the Wikipedia pages for days, even cracked a book or two, I just realized I don't know the answer to a simple question. I was looking for a thread to ask it in, then realized that was selfish and short sited. Much like this opening paragraph is becoming.

    There may be more questions, other people may want to chime in, the whole thing could very well start a chain reaction. But enough of that.

    The query is this: Why and how do hydrogen bombs create radioactive cesium, iodine, and other dangerous fallout particles?

    For example

    I've read from multiple sources that the world is already lightly dusted with all the isotopes that come from a leaking reactor, or burning fuel pond. But why would a bomb that contains just plutonium create all the same radionuclides?
    And why so much of them? I thought a small part of the plutonium was turned into energy, thus the boom. Why and how and where did all the dangerous fallout come from?
  2. jcsd
  3. Jun 29, 2011 #2
  4. Jun 29, 2011 #3
    I understand that, I read that article.

    "The lasting radiation from a nuclear weapon is in part due to the neutron activation of the bomb itself and the surrounding material, in addition to fission products."

    How does that millisecond that the plutonium goes critical create all the long lived isotopes? The ones that take a long time to build up inside a reactor?
  5. Jun 29, 2011 #4
    In reactor fuel (which is low-enriched anyway) there is not much to activate. The isotopes you speak of are fission fragments, which subsequently decay. As only a few percent of the fissile Uranium or Plutonium actually fissions over the useful lifetime of a fuel element, there are not very many produced. Neutron activation makes radioactive isotopes in the walls of the reactor and the control rods and the water (tritium) and whatever else is inside.

    In a bomb, most of the material (which is mostly fissile (read: highly enriched) to begin with) does break up into fission fragments. So from the start it is much more efficient at making both fission fragments and neutrons. Add LiD (fusion!) and possibly a tamper made of uranium and suddenly you have oodles more neutrons and lots more fission products already, per unit mass, than in a reactor.

    And this is before the neutrons hit whatever is around the bomb!
  6. Jun 29, 2011 #5


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    An atomic bomb uses a fissile core, and the U-235 or Pu-239 fissions under a fast (prompt) neutron spectrum. The fission products include Te, I, Xe, Cs, Ba and Se, Br, Kr, Rb, Sr, . . .

    A hydrogen bomb uses a fissile trigger, so that produces fission products, and some configurations can use a tamper of depleted or natural U, and the U-238 fissions from fast neutrons, also producing fission products.

    One concept - http://en.wikipedia.org/wiki/Teller–Ulam_design
  7. Jun 29, 2011 #6


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    It doesn't. There are much fewer fission products created by a bomb than a reactor. A typical power reactor generates one kiloton TNT of energy every twenty minutes. However a bomb widely disperses all of the fission products into the environment, via the enormous explosion.
  8. Jun 29, 2011 #7


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    The state of the art in 1945 required a massive overdesign of the fission weapons used on Hiroshima (U235) and Nagasaki (Pu239). So the deposition of unfissioned U and Pu from those weapons would have been much greater than from a modern design optimized for fusion efficiency. Since the early designs were fission only they would also have generated relatively higher fission product yields in a smaller frallout footprint. The higher explosive level of a fussion weapon also scatters the fission products from the trigger more widely.
  9. Jun 30, 2011 #8
    In a thermonuclear (hydrogen) bomb, the only fission products (eg. 137Cs, 131I) come from (a) the fission of 239Pu in the bomb's primary (the fission "trigger" of a hydrogen bomb) and possibly from (b) the fast-neutron fission (with ~14 MeV fusion neutrons) of 238U in the "tertiary" stage of the bomb, the bomb's radiation case which can be made from 238U in order to add a whole bunch of nuclear yield (but it's relatively "dirty" fission yield as opposed to relatively clean fusion yield.)

    Neutron activation of environmental stuff doesn't create fission products. It might create other radionuclides, like 14C, 36Cl, radioactive Fe, etc, but not fission products.

    I've read from multiple sources that the world is already lightly dusted with all the isotopes that come from a leaking reactor, or burning fuel pond. But why would a bomb that contains just plutonium create all the same radionuclides?

    239Pu fission in a bomb creates fission products just like 239Pu fission in a reactor does. The distribution of fission products is a little bit different because the neutron spectrum is a bit different, but the general facts are the same.

    Well, be careful when reading and evaluating dubious anti-nuclear scientifically illiterate screed on the Internet. Keep your science and skepticism and critical thinking close at hand. Maybe - you know - it's not actually true at all. Can the source material be reviewed, verified or investigated further,

    There were a lot of bomb tests across the world over the last 70 years. They did release a good bit of radioactivity. We've stopped atmospheric testing, and that was a sensible decision.
  10. Jun 30, 2011 #9
    It's funny, because I don't read anti-nuke sites or literature. I guess I should investigate those arenas as these issues may have been discussed there before. But I don't enjoy discussions if they are not fact based.
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