sentences:  The Fukushima I nuclear accident was caused by a "beyond design basis event," the tsunami and associated earthquakes were more powerful than the plant was designed to accommodate, and the accident is directly due to the tsunami overflowing the too-low seawall.Since then, the possibility of unforeseen beyond design basis events has been a major concern for plant operators.There have been a number of nuclear whistleblowers, often nuclear engineers, who have identified safety concerns at nuclear power plants in the United States.