>>13892639It's not just about piling enough fissile material, you won't achieve much, you also need density, neutron reflectors and a specific geometric configuration.
Prompt criticality can be achieved in some nuclear reactors, this is what we believe happened in Chernobyl during the first explosion. But when prompt criticality is reached, some of the neutrons escape, others get absorbed by surrounding water/boron/graphite/else and the nuclear fuel eventually expands under heat and melts, losing its critical configuration. So even if you managed to reach criticality, it will self self regulate eventually.
In nuclear bombs a plutonium sphere is imploded onto itself using extremely well designed explosive lenses. You want to go from subcritical to critical as fast as you can otherwise the reaction will self regulate just like in nuclear reactors. This what we call a call a fizzle, the bomb explodes and the reaction happens, but it couldn't react fast enough to create a big boom before the plutonium sphere shatters/melts.
So that's the main difference, nuclear bombs are designed to have powerful prompt critical events, much more than what is physically possible in a reactor, which allows them to act as bombs. Also nuclear bombs aren't really efficient either, something like a few percents.
Only a few hundred grams of uranium-235 actually underwent fission and released energy, out of the 60 kg, because the remaining of the fissile material was blasted away by the prompt critical event as a self regulatory mechanism.
