Some things decay quickly and some very slowly, forming a sort of chain where the events might happen fast-slow-fast-fast-superfast-superslow-slow-fast-medium-f

Some things decay quickly and some very slowly, forming a sort of chain where the events might happen fast-slow-fast-fast-superfast-superslow-slow-fast-medium-fast-slow, or something like that. So these fast-decaying things disappear quickly but are also being constantly created by things spontaneously decaying into them. The point of a nuclear reactor is to increase the overall rates of decay, so you end up with disproportionately large amounts of the short-lived isotopes as opposed to what you would find in nature. https://en.wikipedia.org/wiki/Decay_chain https://en.wikipedia.org/wiki/Decay_chain If you scroll down you see thorium series, neptunium series, uranium series, and so on. You can see the half-life at each step.