In short, it doesn’t.
Take a nuclear reactor for example. It splits atoms slowly, and in a controlled manner, and all we get is heat, no explosion.
A nuclear bomb on the other hand is based around chain reactions. 1 atom splits 2, that atom splits 4 others, that one splits 8, etc. This happens incredibly quickly so that the energy released from the previous atom splitting hasn’t “moved,” much yet, so when the next split releases energy it piles on top, creating a massive increase in heat and pressure. When that pressure is released, it becomes an explosion.
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