The 3 big Nuclear disasters circumstances.3 Mile Island: was the first and can be blamed on poor 1950's - 1960's type design. The cooling water system malfunctioned and a partial meltdown occurred though all in all it wasn't near as bad as the other two. Simply this occurred because of bad design a similar incident is very unlikely today because of redundancy in safety systems.
Chernobyl: This was a Class A F up. The operators did a test and actually jumped out some safety systems on a poorly designed Russian system. Huge disaster. I was living in Europe at the time and it was a super big deal. This should have never happened. A better design, with redundancy, would have easily prevented it.
Fukashima: Class A F up. Super dumb. Unexpected tsunami flooded the station and backup generators. So dumb because there was ample geological evidence of prior tsunamis in the past. Simply having the generators installed in a hardened building would have easily prevented this. They simply couldn’t keep a bunch of electrical pumps going due to water infiltration which is a problem so easily solved.