Someone steals Nuclear material and releases it over a large area. Or a design failure can lead to something about 20 times the size Of Chernobil and hundreds of times as deadly. Note Chernobil keep most of the radioactive material contained.
PS: Granted those are both vary unlikely, but if you want to talk worst case it can get really bad especially when you add in say a large earthquake on an unknown fault or a meter impact etc.
> Note Chernobil keep most of the radioactive material contained
And yet, compared to any other design still in use, it had virtually no containment. It had a concrete and steel lid for primary containment, but it's secondary containment was a corrugated tin roof, instead of the typical reinforced concrete that can take a jet impact (http://www.youtube.com/watch?v=25vlt7swhCM). On top of that, they disabled the safety systems deliberately because the reactor would normally not have allowed itself to be placed in the extremely unstable state that caused the disaster.
The Chernobyl accident can be compared to taking a car, replacing the crumple zones and roll cage with tin foil, cutting the brake lines, and then driving full speed into a wall in order to test the safety systems. Without using a crash test dummy. And then wondering why someone got hurt.
When people can do something as monumentally stupid and poorly engineered as Chernobyl and still keep most of the radioactive material contained, that really says something.
That's exactly right, which is why you don't replace your crumple zones and roll cages (primary and secondary containment) with tin foil. That way, even if your brake lines are cut, you still have something protecting you.
Modern reactors are designed so that they will be passively safe. Even if the safety systems get shut down and the reactor core turns into a radioactive puddle of metal, it shouldn't be able to lose containment.
If a meteor hits, you have bigger issues than nuclear fallout, like tsunamis, earthquakes, vaporization of water for an ocean strike that leads to weeks of rain, dust from a land strike cooling the planet, etc.
It's all a question of size. A 1Mt impact outside a city would probably not be that bad unless it hit a nuclear power plant or other nasty target. Probably a 1 in 1+ trillion risk but if all your asking is what's the worst that can happen it's on the list.
