What stops a nuclear bomb’s chain reaction?
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People on here are right that the bomb “blows itself apart,” but there is a detail that helps understand it a bit closer. If you could see the reaction in the fuel core in super slo-mo (like, on the scale of nanoseconds), you’d see that as the reaction spread through the core, the core itself heats up. As it heats, it expands. At some point, the expansion is large enough that the neutrons that spread from reaction to reaction won’t be able to find another atom to react with, on average, and the overall criticality of the system — the ability for one splitting atom to create splits in more than one other atoms — will drop. Once it dips too much, the reaction is over, and whatever energy you’ve gotten out is what you’re going to get.
All of this happens on the scale of tens of nanoseconds. A lot of bomb design is about trying to keep that core from expanding too much, because the longer you keep it together — and again, “longer” here means tens of nanoseconds — the more explosive energy you get. [Here is a diagram](http://blog.nuclearsecrecy.com/wp-content/uploads/2014/11/Glasstone-Snowplow-region.jpg) from the 1950s that illustrates the core expansion issue, from a once-classified manual about nuclear weapons, which illustrates the use of a heavy “tamper” (just a heavy metal around the core) that tries to slow down the expansion for a tiny bit of time (creating a “snowplow” region where the core expands into the tamper and squishes it).
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