The big hazard of nuclear power has always been the fact that you wind up with an awful lot of highly radioactive material in one place. For that reason, nuclear power plants go to a lot of trouble to keep that highly radioactive material confined and contained. As of this writing, it is not clear that there has been a loss of containment in Japan, and it seems highly unlikely that a Chernobyl scale loss of containment could occur, but the current troubles do show that there are always more ways to go wrong than we expect.
I have generally been an advocate of nuclear power, but I have to admit that my faith has been shaken. Maybe these damned things are just too dangerous to tolerate.
The worst threat has always been that somebody with a nuclear weapon could set it off near a nuclear reactor. The resulting vaporization of the core could render hundreds of thousands of square miles uninhabitable and blight the futures of a generation of the hemisphere's children.