A modern nuclear powerplant is incapable of exploding, at worst it can meltdown and leak radiation in the vicinity. This can of course have very bad results, but check Fukushima - more people died from the evacuation, and from a fire caused by a fuel tank in another city due to the natural disasters that caused the meltdown, than did from radiation. A radiation leak is easy to detect (and all areas near nuclear power plants in semi-developed countries have automated collection of radiation), and isn't an immediate death sentence.

Meanwhile when a dam breaks, immediately everyone and everything in its path will be obliterated. Vastly more people have died from dam failures than even the worst of the worst of nuclear power plant incidents, Chernobyl, where staggering incompetence met horrific design flaws and bugs. Something like that is impossible to happen today.