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Nuclear bomb chain reaction

  1. Dec 7, 2008 #1
    Do Uranium-235 nuclei ever undergo fission spontaneously? If not how does a nuclear bomb actually work? I understand that two pieces of Uranium (which are subcritical) are driven togther by a chemical explosion and this initiates the chain reaction; however, if nuclei can not undergo fission spontaneously, then where does the first neutron come from?

    p.s. I am not planning to make one
  2. jcsd
  3. Dec 7, 2008 #2


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    U235 does fission spontaneously, but the neutrons produced escape from the material.
    If the size is large enough, more neutrons induce further fission than escape.
    Don't try this at (my) home.
  4. Dec 7, 2008 #3


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    The rate of spontaneous fission is relatively low for U-235, so one can introduce a somewhat stronger neutron source. Note that there are two subcritical masses involved.


    Implosion systems have an advantage in increasing the density of the fissile material which increases the probability of fissioning, i.e. under high pressure compression, the critical mass decreases.
  5. Dec 7, 2008 #4


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    Note that nuclear bombs do not rely on spontaneous fission to trigger the chain reaction. In fact, that would be downright dangerous! Of course, that's the reason for maintaining subcriticality until the weapon is ready to use.

    The first neutron comes from a material known as an initiator, which sits in a small cavity at the center of the subcritical mass. Generally this is a thin foil of some light isotope (e.g. various Li or Be-8) with a strong propensity for alpha decay. The implosion and/or assembly process induces the release of a free neutron, which starts things rolling.
  6. Dec 7, 2008 #5

    Vanadium 50

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    I don't know what they use for an initiator, but I know it's not Be-8, which is incredibly unstable: it must have a half-life around 10-17 seconds. I suspect Be-9 in conjunction with an alpha source is more likely.
  7. Dec 7, 2008 #6


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    The 1945 devices used Polonium + (stable) Berylium as an initiator
    Modern devices use an accelerator and a metal target (eg lithium hydride)
  8. Oct 15, 2010 #7
    I know that you've said that U-235 spontaneously fissions, though at a very low rate. What exactly is the rate (per minute/hour/day)?

    Was spontaneous fission what was relied on for the initial neutrons to operation of Fermi's Chicago Pile 1, the first nuclear reactor?
  9. Oct 15, 2010 #8


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    For U-235, probability of SF = 7.0E-9 % Multiply that by the decay constant and the number of atoms in 1 gram to get a specific activity.

    I would expect that CP-1 used a PoBe or RaBe n-source.

    The construction of the pile was monitored, and criticality was achieved with thermalized neutrons.
  10. Oct 15, 2010 #9
    From http://en.wikipedia.org/wiki/Neutron_source
    "Neutrons are produced when alpha particles impinge upon any of several low atomic weight isotopes including isotopes of lithium, beryllium, carbon and oxygen. This nuclear reaction can be used to construct a neutron source by intermixing a radioisotope that emits alpha particles such as radium or polonium with a low atomic weight isotope, usually in the form of a mixture of powders of the two materials. Typical emission rates for alpha reaction neutron sources range from 1×106 to 1×108 neutrons per second. As an example, a representative alpha-beryllium neutron source can be expected to produce approximately 30 neutrons for every one million alpha particles. The useful lifetime for these types of sources is highly variable, depending upon the half-life of the radioisotope that emits the alpha particles. The size and cost of these neutron sources are also comparable to spontaneous fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe), or americium-lithium (AmLi). The neutron initiators of early nuclear weapons used a polonium-beryllium layers separated by nickel and gold until a neutron pulse was desired."

    Natural beryllium has a very low energy threshold (2 to 3 MeV) for the photonuclear γ,n reaction, so a high α,n reaction yield is not unexpected.

    Bob S
  11. Oct 16, 2010 #10


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    Neutron sources for modern commerical reactors include Cf-252 for primary sources used to start the first cycle, and Sb-Be secondary (photonuclear γ,n) sources which are used in subsequent startups until sufficient transuranics are produced to give sufficient SF neutrons.
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