SCE announced Friday that the wear was caused by a settling of support equipment along with vibrations.
SCE reported to the Nuclear Regulatory Commission last week that most of the wear in the tube wall was less than 20 percent, which is below the federal safety threshold of 35 percent. A tube over the limit must be plugged.
The cause of the wear on the tubes was vibration, which is unusual, and settling of the support structures, which sometimes occurs, according to an SCE spokesperson.
A leak from a tube forced a unit to be taken off line in January. The plant's other unit had already been closed for routine maintenance.
Neither has been returned to service.
City News Service contributed to this report.