They were wrong because they didn't take into account the possibility that backup power would be unavailable for so long. The backup generators and their switchgear were sited very poorly--behind a seawall that was overcome by the tsunami.
Please note that I'm not saying the reactor designs themselves weren't old and out of date. They were. But that wasn't the root cause of the problems. The stresses that these reactors have withstood due to the lack of backup power are far beyond any safety specification they were built to.
Please note that I'm not saying the reactor designs themselves weren't old and out of date. They were. But that wasn't the root cause of the problems. The stresses that these reactors have withstood due to the lack of backup power are far beyond any safety specification they were built to.