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Two rucksacks of U238 - a myth?

  1. Apr 8, 2014 #1


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    Is it true that two quantities of U238 approaching sufficiently close, and combining to form a critical mass will spontaneously explode?

    Does this apply to any known substance?
  2. jcsd
  3. Apr 8, 2014 #2
    You need U235 or P239. A chain reaction will take place where a neutron splits an atom, and this produces more neutrons, wich will split more atoms etc. The neutrons produced by splitting atoms don't have enough energy to split U238, so U238 won't work.

    The critical mass needed to sustain a chain reaction is 52 kg of Uranium-235 or 10 kg of plutonium-239. Since both are very heavy (around 20 tons/m^3), this will easily fit in a backpack.

    To get a sizable nuclear explosion, you need to bring the 2 pieces together much more rapidly than you could do it by hand. A primitive nuclear bomb would shoot one piece of uranium into another hollow piece using a gun barrel. An even better method is to compress a sphere of uranium or plutonium with explosives around it. This needs very precisely placed and timed explosions.

    If you bring two rucksacks together by hand, you'll only get an explosion large enough to fling the rucksacks away and stop the reaction. It will produce enough radiation to kill you however. This kind of thing has happened before. A big explosion was never produced. Google "criticality accident"
  4. Apr 8, 2014 #3


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    Obviously not, otherwise we would have blown ourselves to smithereens well before now.
  5. Apr 8, 2014 #4


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    Simply bringing together a critical mass will not explode, it will merely produce a large amount of heat and radiation until it is separated. In order to get a nuclear explosion, the critical mass must be assembled at explosive speeds.
  6. Apr 8, 2014 #5


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    Steam king: Even more obviously the connotation of any known substance was - if not true for U238 then true for any other substance.

    Thank you.

    I have proven that the problem I am currently working on is non-understandable at n, for all n.
  7. Apr 8, 2014 #6


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    The SL-1 reactor suffered a catastrophic accident brought about by a prompt criticality. The core reached 20 GW in just 4 ms, leading to a steam explosion.

    Would two rucksacks worth of U235 arranged in a critical mass produce enough energy to melt, or even worse explode, the material?
  8. Apr 8, 2014 #7


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    It depends on multiple factors; how much material you have, how close together they are, properties of the environment, the material itself, etc. If they were just critical they might only get just hot enough to be in thermal equilibrium and maintain their configuration. If sufficiently supercritical, the material could melt or the shock heating could cause it to rapidly shatter and disassemble.

    An interesting paper with descriptions of various criticality accidents can be found here: https://www.orau.org/PTP/Library/accidents/la-13638.pdf
  9. Aug 6, 2014 #8
    To quote Richard Rhodes in his "Twilight of the Bombs" (Chapter 16):

    "In 1986, when I helped the Nobel Laureate Louis W. Alvarez write his memoirs, Alvarez commented one day on the importance of keeping track of highly enriched uranium. Speaking of a quantity sufficient to form a critical mass, he told me, 'You can make a fairly high level nuclear explosion just by dropping one piece onto another by hand.'"

    That is, of course, what they were doing in the US with the Godiva experiments which wrecked the first Godiva machine, and the Soviet equivalents that killed at least three researchers in two different incidents at "Los Arzamas", though in the latter by radiation, not explosion.
  10. Aug 6, 2014 #9

    jim hardy

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    er, per LANL Lady Godiva was a pulse machine that deformed its uranium parts , warping them not exploding. Godiva II was somewhat sturdier.
  11. Aug 7, 2014 #10
    Unless the material has a very low spontaneous neutron rate.

    U-235 has a spontaneous fission rate of 0.16 fissions/sec-kg.
    A pure mass of U-235 weighing 60 kg produces only 9.6 fissions/sec.
    U-238 produces 35 times as many neutrons.
    U-234 produces 22 times as many neutrons.

    If you would somehow obtain U235 of >99% purity, you can produce nuclear explosion by assembling supercritical mass "by hand" (more like dropping one part onto another from some height, so that assembly takes much less than 0.1 second).

    U235 of >99% purity, to put it mildly, is hard to come by. For one, U234 is usually not removed by "typical" military production of HEU.
  12. Aug 7, 2014 #11
    What I haven't seen mentioned at all is a way to contain the energy. Explosions happen when the energy is too much to be contained in whatever is trying to restrain it. There is also no trigger mechanism which is essential to a nuclear bomb. You need to bring the two masses together quickly. The reason nuclear reactors don't explode is because they have control mechanisms, remove them and the nuclear fission reaction runs rampant and cannot be contained. But to answer the question, no I don't think it's possible. At best the two masses will go critical and produce huge amounts of heat energy. An explosion is highly unlikely
  13. Aug 7, 2014 #12

    What you are describing is essentially a trigger. This is how nuclear bombs are detonated. Bringing two subcritical masses together to create a critical mass or in this case for an explosion super critical mass because you want more neutrons to keep producing more power. You also need some sort of shell. Remember the danger behind nuclear bombs is not so much the radiation, it could take years to die from that given you're a good distance away from the initial impact, because radiation exposure will decrease with distance. The immediate danger comes from the extreme heat which will probably disintegrate anything in it's immediate vicinity and then the sound waves that come and bust up the earth and cause all kinds of destruction. Point is you need a trigger and some sort of container that won't be able to hold the energy generated
  14. Aug 7, 2014 #13

    jim hardy

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    If you would take a course in reactor physics you'd quickly learn that comparing a power reactor to a bomb is the analogy of comparing firewood to nitrocellulose.

    A reactor might suffer a steam explosion or even some self-disassembly, but that's a long way from a nuclear blast, as was demonstrated by the Borax tests .

    Let's drop the hyperbole.

    old jim
  15. Aug 7, 2014 #14
    I have taken reactor physics. Last semester actually. I was illustrating a point, mainly there is no trigger mechanism to cause an explosion. Not sure what you're suggesting by saying I need to take a reactor physics course. I know how reactors work, and I know how a bomb works. I even took the effort to explain the difference although not in great detail
  16. Aug 8, 2014 #15
    I am certain you are wrong. Trigger is not essential for fission bomb. What is essential is to have a highly supercritical mass.

    In implosion devices, the mass of fissile material is in highly compressed (-> highly supercritical) state for only a very short time. That's why *this particular kind* of implosion devices needs to have a source of neutrons - it needs to make sure the reaction will start during that very short time.

    Gun type devices can do without one.
  17. Aug 8, 2014 #16
    No, it's not what I'm describing.

    the "trigger" is usually a small fission device (usually of implosion type) used to ignite a fusion stage in hydrogen bomb.

    what I described is a theoretical setup how a crude, inefficient, fission device operable "by hand" (i.e. no explosives, using only muscle power and gravity) can be made using very pure U235. It is not of practical use - the "standard" implosion boms are cheaper, lighter, etc...
  18. Aug 8, 2014 #17

    The trigger does not have to be of an implosion type, the Fat-Man bomb used an implosion type trigger. It was the most difficult of the two bombs detonated to build. What you're describing is how a bullet style trigger works without the explosives needed to achieve the speed needed for an explosion.

    Now eventually they did improve on the implosion type triggers you are talking about by using fusion to increase fission rates. You took two different bombs and mixed them up. He was asking about fission explosions. A hydrogen bomb is a fusion explosion.

    Hydrogen bombs use the fission process as the trigger and then fusion is the secondary reaction. You are right though that it is incredibly inefficient to use the bullet type trigger, I read it had an efficiency of like 1.5% in the little man bomb. The fusion implosion bombs were more efficient at 17%. Lastly hydrogen bombs are more efficient and vastly more powerful than either of the two fission bomb designs.
  19. Aug 8, 2014 #18
    The above does not make any sense to me. What do you mean by word "trigger"?
  20. Aug 8, 2014 #19


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    A trigger in this context is a neutron source that can be initiated on a signal. Given the small window of time the rate needs to above 10^7 per sec or so.
  21. Aug 8, 2014 #20
    Then you are using incorrect terminology.

    That thing is called "initiator":
    """A modulated neutron initiator is a neutron source capable of producing a burst of neutrons on activation."""

    Ther term "trigger" describes a completely different thing:
    The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in "stages", with the detonation of each stage providing the energy to ignite the next stage. At a bare minimum, this implies a primary section which consists of a fission bomb (a "trigger"), and a secondary section which consists of fusion fuel. """
  22. Aug 8, 2014 #21

    I mean the thing that drives or initiates the reaction. I maybe using the wrong terminology as you've stated in the following post
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