Just because you can use the same fuel twice (or even 3, 10, 100, 1000 times) doesn’t mean it’s renewable. You can’t do that ad infinitum, at a certain point there will simply not be fissile material left in the spent fuel to use, and no more will ever be formed on earth (at least not in any meaningful quantities, I’m sure eventually fusion experiments or something may start spitting out uranium, but by the point we can do that at any meaningful scale we’d clearly have fusion pretty well figured out so fission would be a moot point.) What is here is what we have to work with until we start mining asteroids or what have you.
And on a much bigger timescale (and I’m talking the absolute mind-breaking cosmic sort of timescale that’s really kind of not worth mentioning because it’s so far out from our human frame of reference so I’m only mentioning it because it’s really cool to think about,) eventually all of the fissile material that exists in the universe today will cease to be all on its own whether or not we do anything with it as it decays into more stable elements. A bit more will be created along the way thanks to neutron stars and supernovae and such, but even that slows down and stops as we inch closer to the heat death of the universe.
On the other hand, given enough time and the right conditions, every ounce of carbon on this planet- plants, animals, plankton, plastic happy meal toys, could potentially become fossil fuels, coal, oil, natural gas, etc. and can keep going around the carbon cycle. Unfortunately that’s a process that can take thousands or millions of years and we use fossil fuels much faster than they can be replenished. Even if earth becomes uninhabitable and we have to all up and leave, the carbon in our bodies and whatever we take with us will end up in a new carbon cycle on whatever new rock we end up on, and can potentially keep going as long as we do.
Nuclear is cleaner than fossil fuels but is in no way renewable, no matter what tricks we use to get the most use out of it, there is a finite supply.
Fossil fuels are dirty and not really renewable on a useable timescale for us.
Renewables like solar, wind, and hydro will be a viable option as long as the sun is shining, the earth has an atmosphere, and there is liquid water. Technically there will come a day when those also run out on us but those are also kind of prerequisites for us to live here, so kind of a moot point.
And geothermal will potentially be an option as long as the Earth’s core stays hot, so hundreds of millions if not billions or tens of billions of years, which puts us in the sort of timescale where we have to worry about things like the sun turning into a red giant and engulfing the inner planets, so we’ll have bigger things to worry about than keeping the lights on.
