What stops a nuclear bomb’s chain reaction?
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There are 2 (primary) types of Nuclear bombs: fusion and fission (I’m ignoring neutron bombs or dirty bombs).
Fission bombs are propagated until the fissionable material is too diluted by the distance between the nuclei of the atoms for the chain reaction to continue. In essence the bomb runs out of fuel because it has blown the material too far apart for the neutrons to hit the other fissionable material.
Fusion bombs can be much more complicated than I’m laying out here and can actually be multiple layers, but leaving aside engineering complications, the bomb continues to explode until the small atoms no longer have the energy to fuse together. Unlike in fission there is a huge energy barrier to fusion, you can keep fusible material in very close contact without any real chance of producing fusion. The energy barrier to induce fusion for less ideal fusion material (like the air, thank your preferred deity of choice – I suggest Tod) is great enough that fusion quickly hits a net loss of energy point soon after the initial blast.
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