Truckloads of dangerous waste (a few lorries’ worth a year per plant), which is only dangerous if you stand close to it in the next 10,000 years or so, something that can be mitigated by how one stores it. (Keeping it under water attenuates much of the radiation, for example.) Or, alternatively, it can be reprocessed to make more reactor fuel.

So it’s less than ideal, though one has to measure the quantity of waste and the risks against the certainty of health costs of fossil fuels (which are visible in cancer statistics even when everything is working perfectly).

Would you like to source that? ALL of the High Level nuclear Waste from the planet can be fit into a 21m cube

https://decarbonization.visualcapitalist.com/visualizing-all-the-nuclear-waste-in-the-world/

Medium level waste and low level waste can be deposited in normal surface landfills. LLW is normaly considered non radioactive in 50 years MLW 200. Fun fact there are entire cities that are more naturally radioactive than what we consider MLW. https://pmc.ncbi.nlm.nih.gov/articles/PMC6709356/

I'd like to make an edit here later on and specify the reason that we do not put it into a 21 m cube is because it would go super critical.

mountains?

Not much local(!) CO₂.