Fatigue was the basic cause of the problem, but the hairline cracks characteristic of fatigue are very difficult to detect, and were frequently missed, until they became critical.
For example, sensors placed in a bridge could monitor the health of the concrete and steel, then send an alert if one of the sensors picks up a hairline crack.
The challenge comes about three weeks after contractors cutting a hole in the building to replace the reactor's lid noticed a long hairline crack on one side of the opening.
