Here is how the N Koreans could smuggle in an atom bomb or two.
First start with the Plutonium in one third critical masses in the shape of Pistons and put them into tungsten containers shaped like engine blocks, totally sealed off. Tungsten reflects neutrons and other subatomic particles back where they cam from and you can further dampen it by filling the blocks with nuclear dampening materials in fine granular form.
Second, you have it crated and tagged as a Chinese export to Mexico. On all the X-ray scans these parts will look just like normal engine blocks.
Third, when you get the blocks into Mexico, you smuggle them in like any other material that gets smuggled in by the ton across our border.
Fourth, when the material gets to its area of operation, you pack the plutonium into one of the piston shafts in three separate pieces so that it is still under critical mass. You rig explosives to the bottom of the piston shaft so that when the explosives go off, it will drive the plutonium into one mass, thus reaching critical mass and exploding.
Fifth, mushroom cloud over DC.
easy peasy
