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.
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.
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.