• Question: is there a way to make a working scale model of a nuclear bomb?or do they all have to be massive?

    Asked by mattheworgan to Arttu, Ceri, James_M, Monica, Philip on 13 Jun 2011.
    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 13 Jun 2011:

      They all have to be massive. The bomb needs a chain reaction in which neutrons that are produced when one nucleus splits hit other nuclei making them split and so on. If the amount of material is less than the so called critical mass, the neutrons can escape and a chain reaction stops.

    • Photo: James M Monk

      James M Monk answered on 13 Jun 2011:

      I disagree with Arttu a bit here, since I think it ought to be possible to design a “model” nuclear bomb, but quite difficult to do and since there is no reason to do it (conventional explosives do the same job) no one has.

      The outer layer of a thermonuclear bomb is depleted uranium-238. In normal circumstances U238 is completely safe – it even gets used in some particle detectors because it is extremely dense. A fusion bomb, however, uses the chain reaction of the fusion of hydrogen (actually deuterium) to produce an extremely large number of very high energy neutrons. These neutrons induce fission in the normally inert U238, which goes crackers and releases a ludicrous amount of energy. This is the reason that a fusion bomb can be so much larger than a fission bomb – there is almost no upper limit to how big it can be.

      To initiate the fusion reaction in the deuterium, a “normal” fission bomb of plutonium is used, which as Arttu says has to have a minimum size in order to reach chain reaction. The reason a plutonium bomb is used is that very high temperatures and pressures are needed to initiate fusion. If one were able to reach such high temperatures/pressures by other means then the overall scale of the bomb could be reduced to any size you liked.

      Research is ongoing into using lasers to create the high pressures on small pellets of deuterium, which is effectively a very small (and controlled) nuclear fusion bomb. The aim of the research is to generate clean electricity, not bombs. More speculatively, perhaps a nuclear isomer could be used as a trigger for a fusion bomb of arbitrary size.

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