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Wish to understand atomic weapons

  1. May 20, 2006 #1
    I also wish to understand atomic weapons...

    The TV said Iran can build a 'device'...

    I read here at the forum that Nuclear plants are limited to 5% pure fuel...

    I assume an H-bomb is more than 5% pure fuel...

    Is the media just trying to spook us?

    Also if my research is correct..it takes huge amounts of raw material to even make a mini a-bomb...

    ...please explain the various states such as critical mass...sub critical...super critical...etc...

  2. jcsd
  3. May 20, 2006 #2


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    A good book to get hold of would be 'The Los Alamos Primer' which contains the original lecture notes annoted by Robert Serber himself. I've read this book before, but unfortunatly forgot most of the material. However, I think this is Astronuc's field of experties.

    As for the 5% pure limit, I believe that is a saftey precaution to help prevent a massive chain reaction (as in chernobyl).

  4. May 20, 2006 #3
    I have the movie 'Trinity and Beyond'

    which is very fascinating...man has unleashed the ultimate power in the universe it appears...

    Kinda spooky...kinda awesome
  5. May 20, 2006 #4


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    Er... Please don't use a "movie" as a valid source, or even something you use to base your opinion on here in the physics section of the forum.

  6. May 20, 2006 #5
    ok...found the book

  7. May 20, 2006 #6


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    Purity has to go from around 5% to 80% to make weapons grade U235.
  8. May 20, 2006 #7


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    Note that an undergraduate physics major designed a "creditable" nuclear bomb. It really isn't that hard to do. Getting the materials, bulding and testing one is still a major project for a government (and not an individual). However, nobody should doubt that a government (like Iran) could implement a successful program to build nuclear weapons if they feel threatened.

    For a specific example see

    #pragma irony
    Of course, I don't see how Iran could possibly feel threatened by any of the actions of our current administration. Why would they go to the trouble and expense of building a "device"?
    Last edited: May 20, 2006
  9. May 20, 2006 #8
    Religion/Politics seem always to feel threatened...

    thats the fuel for their egocentric self centered

    Last edited: May 20, 2006
  10. May 20, 2006 #9
    in the movie H-bombs have liquid fuel...

    and i am sure its not Drain-O :surprised
  11. May 20, 2006 #10
    uh oh...that was philosophy...me in trouble again..(?)

  12. May 20, 2006 #11


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    One certainly needs more than 5% U-235, the naturally abundant fissile isotope of uranium, to produce a nuclear weapon. One preferably tries to make the u (in metal form) as close to 100% as possible, but because that is prohibitively expensive 80-90% is usually sufficient.

    Natural uranium (from the mine) has about 0.71% U-235, 99.245% U-238, with some U-234. It is used in CANDU Heavy Water Reactors without enrichment, so the Iranian could use CANDUs.

    Light Water Reactors (LWRs in the West and Russian VVERs) require enriched uranium). The current licensed limit (i.e. authorized by varies national governments) is 5.0%. Some research reactors use 20% or higher.

    The CANDU and LWR fuel is in the form of UO2 encapsulated in long tubes of a Zr-alloy and sealed at both ends. The water coolant serves to cool the fuel and moderate it. The CANDUs use heavy (deuterated) water, so there is less moderation than in LWRs. The LWRs require higher enrichment to overcome the absorption of neutrons by H in the water (deuterium is used in heavy water to avoid this matter).

    Anyway - getting back to nuclear weapons. Uranium could be used in gun type similar to Little Boy (http://en.wikipedia.org/wiki/Little_Boy).

    More modern nuclear weapons use an implosive geometry, and preferebly Pu 239, because Pu-239 has a higher cross-section for prompt neutrons and allows a smaller critical mass. Pu-239 is used in 'triggers' for thermonuclear weapons.

    Pu-239 is produced by virtue of neutron capture in U-238, which becomes U-239, and then through two successive beta decays becomes Pu-239. Pu then must be separated chemically from the U, purified and formed into a metal of the appropriate geometry.

    So far the discussion concerning Iran is focused on the potential to enrich uranium to greater than 5%, which they could do in theory.

    For every kg of U-235, one need ~140 kg of natural U. Uranium bearing ores contain about 2-3% uranium or, so one needs tons of ore from which to extract uranium. Iran apparently has its own deposits, so it makes sense for them to develop nuclear power, IF they do it for peaceful purposes.
  13. May 20, 2006 #12


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    Deuritium is an isotope of hydrogen.

    What does implosive geometry mean?
  14. May 20, 2006 #13


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    There is a pretty good book for the semi-layman that i read last year that a professor recommended to me. It was written by a guy who actually is/use to be a high school teacher but the book was great nontheless. He had written a series of kind of "physics topics for dumbies" if you wish books that were used in some university lecture courses. I don't remember the name of the book series though! It was something like "A not too complicated look into nuclear energy" or "Not too serious look into nuclear energy" or something. I liked it because it wasnt really a science text book but it went a little beyond just normal reading material because each topic had a few questions like textbooks do. One chapter talked about E=mc^2 and then had questions in the end on usinig such a formula. Neat book, can't remember the damn name though
  15. May 21, 2006 #14


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    It means that you assemble a critical mass from subcritical components at the time of detonation. You should be aware that additional details may not be suitable for public discussion.
  16. May 21, 2006 #15


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    That sounds a lot more complicated—why is it done? Is it more fruitful?
    Oh poo, does that mean I'm going to have to work hard to get the answer instead of ask you? frowny face --> :frown:
  17. May 21, 2006 #16


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    What would happen if you had the critical mass of a fissile material?

  18. May 21, 2006 #17


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    Basically it is a round geometry, essentially spherical, that allows a sub-critical geometry to be compressed into a critical geometry very rapidly.

    It is the principal behind the "Fat Man" bomb dropped on Nagasaki during World War II. http://en.wikipedia.org/wiki/Fat_man

    As Tide indicated, the details are absolutely not appropriate for public discussion.

    There was an unfortunate accident back in 1945 involving Louis Slotkin and a critical mass assembly. The assembly consisted of two hemisphere of uranium (mostly U-235). With one hemisphere fixed, Slotkin would lower (holding it with two screwdrivers) the upper hemisphere toward it while monitoring the activity with a radiation detector. Unfortunately, one time, the screwdrivers slipped and the upper hemisphere combined with the lower in a critical mass - and it went critical. Slotkin stuck his hands in and pried apart the assembly, but in doing do he received a massive dose of neutrons and gamma-rays.

    quotes from http://www.gettingit.com/article/165

    This is pretty much as it is described in other sources.
  19. May 21, 2006 #18
    please explain in laymans terms 'it went critical'


    ps:are u saying that if Louis had not 'pried it apart' it would have detonated ?
    Last edited: May 21, 2006
  20. May 21, 2006 #19


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    Critical means that the number of neutrons increases exponentially with time. Had Slotkin not separated the assembly, it would not necessarily have exploded, but it would have produced a lot of radiation. I believe one or two others were also severely irradiated, and IIRC one died.

    Most likely the critical assembly was not 'prompt' critical (i.e. subsequent fissions are induced entirely by fast neutrons released from fissioned atoms), otherwise it would have 'exploded', but the yield would have been relatively low. It was more likely critical with some delayed neutron component, which is what allows us to control nuclear reactions in nuclear reactors. There are mitigating factors such as neutron leakage from the assembly and the fact that as energy is produced in the metal critical assembly, it would heat (or melt) and thermally expand which would cause it to go subcritical.

    Nuclear weapons are designed to be prompt critical, and that is facilitated by increasing the density of the fissile material.
  21. May 21, 2006 #20
    When you say compressed, what kind of pressures are we talking about? Having recently read about the diamond anvil and it obtaining in the order of 10^6 Atm of pressure, i wondered what mass of fissile material would be required to acheive criticallity at those kinds of conditions. Maybe im too curious lol. I lack the knowlage to do the calculations myself.

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