The huge difference is the lasting time. Few days or a week the water has gone and you can start with repairs/rebuilding. But after a nuclear disaster all is polluted for foreseeable future and not usable even if all infrastructure and buildings are intact and look safe.
Which is not true, if you look into for example Three Miles Island reactor meltdown, where from outside point of view, there is no difference and only 600 meters exclusion zone from the reactor.
Furthermore nuclear disasters killed dozens of people, while water related disasters killed thousands of people. Another reason to ban dams.
