[Vecr]

Asteroid impact? Is the reaction even going to be critical at that point? Assuming you mean something smaller like a meteoroid, I think most US plants would either be okay, or you have other problems to worry about (like if any people are within multiple miles of the plant in every direction).

[c4mpute]

There would be an intermediate size asteroid that would be able to penetrate the containment and damage the core. That same size asteroid wouldn't be much of a worry if it just hit a field next to a town. On the other hand, that same asteroid could topple over some skyscraper and kill a few thousand people, which is more than the aforementioned nuclear-asteroid accident would.

[quacked]

I was just listing things that could affect the structure of the core. The real question about nuclear disasters isn't whether or not humanity can respond to them given functioning society, it's what happens when a society collapses (extended power outage, etc.) and then something affects the fissile material. Fukushima took a lot of people to clean up.