Anything that can come in with enough force and heat to damage the containment building, spent fuel storage, water pumps, electrical feeds may not actually cause a meltdown initially, but leads to events that do. What happened at Fukushima pretty much showed that trying to cool a reactor after a forced shutdown in all that mess isn't really possible. Not too long after the cooling stops, the temperature rises, hydrogen builds up, explosions start and the problem becomes even harder to address.
The whole fission reaction thing seems primitive, like the evolution of the gasoline engine. We lock-into a technology too early, over-capitalize on it, then are stuck making linear improvements over time, hoping to replace previous versions that weren't as good. Replacing existing systems is unlikely to happen, because the energy market isn't driven by safety issues, or worse-case scenarios, but by profit and best-case scenarios.
We should have put the brakes on early capitalization of nuclear power, and invested more in the science. If we spent more time and money developing a workable fusion reactor, we would have had something to show for it by now which would be much safer than fission.