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The Atomic Bomb

  1. Nov 23, 2003 #1
    I'm doing a report on the Manhattan Project and there are a few things I don't understand about the bombs.
    (1) In the Uranium model (Little Boy), there are two separate containers of Uranium. They get shot together and attain supercritical mass. Looking at the diagram on this page:http://www.nuc.berkeley.edu/neutronics/todd/nuc.bomb.html#IV.A [Broken], I don't see where the neutron initiator is in the Uranium model. If I'm not mistaken, once supercritical mass is achieved, there's still needs to be a neutron initiator to get the chain reaction going.
    (2) There's also talk about how the two Uranium pieces needed to brought together quick enough so that spontaneous fission doesn't occur and fizzle the bomb out. I don't understand why the period of time where the one piece is shot into the other is particularly susceptible to spontaneous fission. Wouldn't spontaneous fission be possible in the plane ride up there and where it was being stored in the first place?
    (3) At what level are each of the two Uranium pieces at while they're separated (not yet critical, critical...)?
    (4) How come neutrons that are reflected back from the neutron deflector in either the Uranium or the Plutonium models don't bring about a premature fission?
    (5) When the bomb eventually explodes, is the fission controlled or is controlled nuclear fission only used for testing in labs and stuff?
    (6) Why are neutron initiators needed in the first place? Why not just successfully get the fissionable material to supercritical mass and then spontaneous fission set off the chain reaction?

    I'm a little confused. Thanks!
    (...and sorry if this is the wrong forum.)
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Nov 23, 2003 #2
    1. The gun barrel barrel type bomb didn't need a neutron source because, unlike Plutonium, Uranium can fission without the need of a nuetron initiator.
    2. I think the U-238 neutron shield was used for just such a reason; to prevent accidential spontaneous fission (the fizzle kind, not an all out explosion). Supercritical mass is needed to make the now one piece (due to the fusing of both peices of uranium) fission uncontrollably
    3. Both pieces are sub-critical, put them together and they are critical but not uncontrollable. If fused together quickly, they are supercritical....I think...
    4. I don't know about this one.
    5. The fission of an atomic bomb is uncontrolled. Controlled fission is used in nuclear power plants.
    6. You don't have initiators with Uranium bombs. But Plutonium doesn't have fast enough neutrons to initiate a reaction on its own so it needs a neutron source that gives off faster neutrons.

    It's not everything but I hope it helps.
  4. Nov 24, 2003 #3
    While I do not think a gun barrel bomb requires an initiator, one would help jumpstart the chain reaction.

    What you want to avoid is a (relatively) slow collision between the subcritical components. Otherwise the incomplete fission would reduce the explosive yield. What they mean by spontaneous fission is an explosion triggered by neutrons from spontaneous fission (rather than an initiator). This would reduce the fission mass confinement before an acceptable level of chain reaction has been completed.

    If the masses aren't close enough, the early chain reaction will generate enough heat and explosive force to separate the unreacted uranium, thus choking off the exponential progression of the chain reaction. With the mass in a smaller volume, the exponential progression proceeds at a faster rate. This allows for less required fissionable material for a given explosive yield.

    This problem is so pronouced in plutonium type bombs that implosion of a sphere of plutonium is the only way to get the required mass into the critical volume fast enough to prevent a fizzled explosion.

    I presume by level you mean subcritical, critical, supercritical.

    Subcritical is the answer.

    Reflectors aren't perfect reflectors, even if they were, the lack of critical mass would mean the number of neutrons present wouldn't be enough tip the chain reaction into the required exponential curve.

    Reasons for reflectors:

    Think about a fission based explosion in terms of exponential chain reaction - each neutron triggering two more neutrons, in an exponential progression.

    Now, realize that many of the neutrons will not hit another nucleus, thus be wasted. The more material they have to travel thru the higher the chance of colliding with another nucleus. The more fissionable material you can get into a fixed volume, the higher the chance that a neutron will hit another nucleus, rather than escape. Using a reflector (U238/Lead), will enable you to get a second chance with the neutrons that have left the fissionable mass, by reflecting them back into the fissionable mass. This will reduce the critical mass needed for a successful nuclear explosion.

    A second benefit of reflectors is to confinement. The mass of the reflector helps keep the fissionable material in a more confined space, until the exponential progression has exhaused most fissionable fuel.

    Define controlled.

    The bombmaker would consider the exponential chain reaction, such that the majority of the fissionable material is converted into chain reaction breakdown products, giving a maximum energy yield, to be controlled.

    A nuclear reactor officer on a sub would consider an exponential chain reaction a very bad thing, therefore uncontrolled.

    More efficient use of fissionable material - exponential reaction rates (first derivative) are slow, at first (relatively), progressing to more and more rapid. Since slow reactions can cause the fissionable material lose confinement (due to explosively dissipation) before the exponential progression has consumed an acceptable amount of the fissionable material, this is undesireable.
    What the initiator does is start the reaction a little further along the exponential curve, into the more rapid portion of the reaction. This allows for less required fissionable material for a given explosive yield.
    Last edited by a moderator: May 1, 2017
  5. Nov 24, 2003 #4


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    In the context of fission (reactors or bombs), mu is the average number of neutrons per fission.

    Added note, I thought is was nu, not mu.
  6. Nov 25, 2003 #5
    If you are referring to my tagline, the question (koan) is about a thousand years old, so the context is probably not nuclear physics.
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