The discovery came two days after a tube leak prompted operators to shut down the reactor near San Diego as a precaution. Regulators say the unusual wear was found on dozens of virtually new tubes that carry radioactive water at the plant.
A Nuclear Regulatory Commission spokesman says a small amount of radioactive gas "could have" escaped in the leak earlier this week.
The station is run by Southern California Edison. A spokesman says the amount leaked would have been "extremely small" and possibly not detectable by monitors.
Both the company and federal regulators say the release would not have posed a safety risk for the public. Seven-million people live in a 50-mile radius of the power plant.