Why do fuel rods have to be cooled for so long after being not actively used anymore?
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The heat produced by nuclear fuel rods isn’t like heat produced by coal or natural gas. There isn’t something being *done to* them to get them to release heat… as with fossil fuels when you burn them.
The heat produced by nuclear fuel rods is because just them existing creates heat. They’re actively breaking down as atoms, and each time one atom breaks down it’s a little bit of heat, times bajillion breakdowns happening at once because there are **so** many atoms in the rods.
So, you need to keep them cool until enough breakdowns have happened over time that they aren’t making so much heat anymore.
They cool down over time because each time an atom breaks down, that’s one fewer atom making that heat. They’ll continue to be warm for decades, but I believe a few months or years gets them past the point of lighting themselves on fire out of nowhere.
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