Um, I thought Fukushima was not caused by any single point of failure either: it was caused by an earthquake and a tsunami happening together (admittedly, the earthquake caused the tsunami). The earthquake caused the reactor to automatically shut down, which would have been fine, except then they got hit with a tsunami that topped their seawall and flooded the basement, shutting down the emergency generator, which disabled the pumps which cooled the reactor.
The earthquake didn't cause any damage. The single point of failure was the seawall being breached. The rest was a zipper of each failure causing the next. The tragedy is this zipper could have been halted at many places.
For example, the hydgrogen overpressure in the reactor (caused by previous failures) was vented through an overpressure valve into a pipe. The pipe exited into the enclosed reactor building, which lead to an explosion.
The pipe should have been vented to the exterior. The cost of that would have been insignificant, like numerous other zipper-stops.
Deepwater Horizon was a big zipper, too, none of which would have been costly to prevent.
Also, the emergency generators should have been put on platforms so they wouldn't flood. Critical machinery should have been located far enough away from the reactor so they could be worked on without fear of radiation.
