It's not as uncommon as you might expect -- here is a list for the curious. And I don't mean to denegrate nuclear energy as a power source; it is vastly better than fossil fuels and safe when done correctly -- I have participated in the safe generation of nuclear power. But the ramifications of it being done incorrectly are severe to say the least, and everyone should be aware that we do commonly have issues with it, especially in aging facilities. We commonly extend plants decades beyond what their initial construction planned for.

Edited to say I just realized you said meltdowns, not radioactive leaks, which I do agree with. Sorry for confusion