Well, if a dam goes it's certainly true that a lot of people get killed if they happen to be in the way of the water. But I guess there's a couple of differences between a disaster like that, and a potential nuclear one. One is that once the water has gone, it's gone, but a nuclear disaster could leave damage lasting decades. And the second is the insidious nature of the health risks, both in their nature and in the potential for very long term health issues.
And I can't help but suspect that radiation risks to health have a rather special meaning to the Japanese, given Hiroshima and Nagasaki. I know that's a very different situation, but radiation dangers are radiation dangers and it'd be astonishing if those events didn't raise a few special demons in the national psyche. I'd suspect that the only other people on earth that might really understand quite how that feels would be those living in the vicinity of Chernobyl.
Perhaps I'm wrong, but I suspect the notion of radiation raises particular demons for the Japanese people. I suspect that the spectre of radiation elevates the psychological impact of events a notch or two above the impact of an "ordinary" natural disaster, if there is such a thing, even a really bad one like this appears to be.