What’s the smallest nuclear bomb you could make? Could you make a sort of “nuclear firecracker” with a tiny amount of fissile material?

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Mudassir Ali 8 months 1 Answer 96 views

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  1. There’s two basic types of nuclear weapons – gun assemblies where two subcritical pieces of Uranium 235 are smashed together to form a critical mass, and implosion devices where a subcritical sphere of Plutonium 239 is compressed by high explosives into a smaller, denser size that’s above critical (and accuracy is crucial – squishing a 5 inch diameter sphere of metal into a 3 inch diameter sphere and not being lopsided is difficult). And since we’re after “small”, U235 gun designs are ruled out as they take around 20kg of U235, while the minimum for implosion devices is somewhere around 4kg of Pu239.

    There’s a certain minimum amount of fissile material required to sustain a chain reaction that results in a nuclear detonation. Anything smaller than that results in a “fizzle”, or possibly just an “excursion event”, where a chain reaction gets started, but the generated heat disassembles the fissile material (usually without much damage – the vast majority of excursion events have happened indoors and didn’t even destroy the building, at worst just scattering radioactive debris around the room). So you’d end up with a radioactive mess, but none of the shock wave or thermal effect damage. At that point, rather than deal with the technical complexities of an implosion device, you’re better off just using a small amount of high explosives to scatter radioactive dust – still a viable attack at a large sports arena or similar.

    The smallest practical nuclear weapon ever tested was the Davy Crockett, which was small enough to be deployed by a mortar or recoilless rifle at a range of only 2,000 yards from the troops launching the device. It had a yield of only 10 to 20 tons of TNT (compared to 16 kilotons for the bomb dropped on Hiroshima. Although specific numbers remain classified, in the open literature it’s believed to be within 10% or so of the theoretical minimum for a nuclear fission device.

    The actual device is the black thing with fins, the shiny tube is the launcher:

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