It’s something else entirely. When an atom “splits” (mainly a radioactive one since they are unstable and more prone to splitting already) it throws neutrons out which have a chance to hit other atoms which will make them split and continue the process in very rapid succession. These multiple rapid splits of atoms amount to trillions of atoms being split almost instantly.
Imagine an atom splitting being a fire cracker. One isn’t bad at all, but trillions all at once is absolutely devastating.
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