I'd wager their impact will be less than that of climage change if we look a few hundred years into the future.

Regardless, both those disasters were more or less man-made. Chernobyl was caused by serious design flaws, and (as far as I know) people who refused to admit something was wrong until it was too late. Fukushima was built below the line indicated by stones setup to warn about tsunamis. I don't think there's any doubt such considerations need to be taken into account when building the next-generation nuclear reactors.