it is legal to own a nuclear device in the US, providing you get all the federal permits from the regulatory agencies...just as flamethrowers are legal because they are agricultural equipment under the control of the US state department's dept. of agriculture.
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not really. if you can get all the right permits(nuclear fuel, explosives, etc.) from all the different regulatory agencies you could in legal theory own a perfectly legal nuke, so long as you had no intention of detonating it, which would be a felony.