It’s a chain reaction. You take a few heavy atoms with large, unstable nuclei, and you spray some fast moving neutrons. One of them will randomly hit a random one of the atoms, causing its nucleus to break, which will release even more fast moving neutrons that can collide with other atoms and so on. The rate of the fission can be controlled, if you want it faster you inject more neutrons, if you want it slower you put something like heavy water to damp and slow down the neutrons
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