Sorry, I am just joking, and I could not resist.
But Three Mile Island was definitely not prevented from being worse by "pure luck." Nor was it a minor accident.
But even thought the accident was very serious, it did not cause widespread damage. You basically need a Chernobyl to do that: Breach of the reactor vessel and explosion causing the fuel and other core material to go flying into the air. And trust me, the people operating Chernobyl that night certainly did just about everything they possibly could have to make sure that that accident happened...which is pretty much the only way to have an accident that bad.
The fact is that no form of steam plant is truly 100 percent safe. Properly designed, built, maintained, and operated nuclear plants, however, are pretty much the least bad of all the options IMHO. I'd rather have a nuclear plant in my back yard than a conventional one.