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I Stupid question - how does fission bomb get initial neutron?

  1. May 30, 2017 #1
    OK, I understand the idea of neutron bombardment of fissile material causing the nuclei of this material to split, thereby releasing more than 1 neutron, which then go on to bombard other nuclei, such that if more than 1 neutron per reaction ends up causing further reactions, the rate of reactions grow exponentially. I also understand that this very key chain factor (or whatever it's called) depends on the shape of the material body, as it is more likely that neutrons will escape a small body than a large body, and that for a given shape type (typically a sphere as it is the most efficient), there is a minimum size (or alternatively mass) that yields this chain factor as 1, which when using the mass as the parameter for a spherical shape is called the "critical mass" - and that a bomb is set up such that there are wedges of the sphere that are kept separate, and thus with a net chain factor of less than 1, keeping the bomb from prematurely exploding, and then for the detonation the wedges are carefully shot together creating a single spherical shape that has enough mass together than the chain factor goes above 1, thereby yielding a quick exponentially growing chain reaction. I also know that to assist with keeping neutrons from escaping there is a coat of material outside of the sphere that helps to reflect neutrons that might otherwise escape, thereby making it possible for a lower critical mass than what would need to be without such a coat.

    But looking at Pu-239, the natural radioactivity is alpha decay, not neutron decay (something that doesn't even seem to be a natural form of radioactive decay). So I could see a body of Pu-239 shooting off alpha particles, but where does that initial neutron come from? Is it that although the standard radioactivity is via alpha particle release, a small portion will actually be via neutron release? Is it that an alpha particle will sometimes react with a Pu-239 nucleus and release that first neutron? Is that cosmic rays or whatever that are everywhere on Earth are always generating a few neutrons? Is it that a bomb has some special neutron source?
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  3. May 30, 2017 #2


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  4. May 31, 2017 #3
    That's a "gun-type" nuclear device. It is not used now, it's quite inefficient. Implosion devices are commonly used now, where a ball of fissile material is uniformly explosively compressed and achieves neutron multiplication factors of about 2.

    "Neutron" decay does exist. It's spontaneous fission. And in many cases, you would have hard time purifying your U or Pu to get rid of isotopes which have too high rate of spontaneous fission to have a usable nuclear device. This is what makes reactor-grade Pu almost unusable for bombs - it has Pu-240, which has high rate of SF, which would make nuclear-device predetonate and fizzle.

    "Gun-type" nuclear devices are much more susceptible to predetonation, since assembly time for them is not as fast as for implosion devices.

    Thus, there _are_ free neutrons from SF in the bomb all the time, a nuclear device can be made which would explode without special neutron source. However, having neutron source makes it more reliable, and yield more predictable.
  5. May 31, 2017 #4


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    It is impossible for plutonium. And the reason is the spontaneous fission you mentioned.

    Gun-type weapons only work with uranium, and even there the risk of an early neutron is high. Little Boy, the Hiroshima bomb, used it, but with ~10% risk of an early neutron reducing the yield. Today this design is not used any more. Well, maybe North Korea uses it, who knows. It is easier to build.
  6. May 31, 2017 #5
    With very pure U-235, the neutron flux is so low, "gun-type" device would work even with insertion velocities of some 10 m/s. No actual "gun" (accelerations to hundreds of m/s) necessary for assembly. This makes very pure U-235 especially worrisome material as proliferation concern.
  7. May 31, 2017 #6


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    You need ~98% purity until U-238 neutrons become less frequent than U-235 neutrons. Is there an application for kilograms of 98% enriched uranium?
  8. May 31, 2017 #7
    U-235 has a bare metal sphere critical mass of around 50 kg, and spontaneous fission rate of about 0,16 fissions/kg/s, meaning about 8 fissions/sec
    U-233 has a bare metal sphere critical mass of around 15 kg, and spontaneous fission rate of about 0,47 fissions/kg/s, meaning about 7 fissions/sec.
    So a U-233 core is as viable for low insertion speed explosion as a U-235 core... much more so, in fact.
    If you want to assemble a critical mass 1/3 that of an U-235 core in the same time, because your core has roughly 1/3 the volume, it has about 3-⅓ times the linear size and thus distance for insertion. Assuming same time, the velocity needed for insertion is also 3-⅓ times, and the kinetic energy to be conferred per unit mass is thus 9-⅓ times. Since the mass itself is 1/3 times that of U-235, the total kinetic energy needed is 243-⅓ times that needed for U-235, meaning less than 1/6.
    As we see above, the time needed is not quite equal, but you can see the logic.
    Who are likely to have produced or be producing large amounts of high purity U-233, suitable for low insertion speed explosion?
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