From my understanding, it’s considered clean by comparison to fossil fuels in that it’s easier to contain the byproducts that nuclear produces since they are primarily solid and liquid waste compared to the additional gaseous waste that fossil fuels make. The problem is in the potency of the pollution, the length of time that it needs to be stored, and while I said that it was fairly easy to store, that doesn’t stop storage facilities from doing what the lowest bidder tends to do which is perform below the minimum expectation. The quantity of the waste is typically lower by tonnage compared to fossil fuels and a good chunk of nuclear waste is in the form of contaminated safety equipment that has reached the end of it’s expected life cycle.
Something has to go horribly wrong for dangerous nuclear contamination to happen, but that’s not to say that things can and have gone horribly wrong.
Hydro is kinda devastating to any environment it touches if the scale is large enough. I think that it’s usually pretty good on a micro scale that doesn’t have to throttle waterways and flood others.