I’m not sure why you think those are not a confluence of smaller events or that something outside the design spec isn’t one of those factors. By “small,” I don’t mean trivial. I mean an event that by itself wouldn’t necessarily result in disaster. Perhaps I should have said “smaller” rather than “small.” With the O-rings, the cold and the pressure to launch on that particular day all created the confluence. With Fukushima, the earthquake knocked out main power for primary cooling. That would have been manageable except then the backup generators got destroyed by the tsunami. It was not a case of just a big earthquake, whether outside or inside the design spec, making the reactor building fall down and then radiation being released.