The disclosure came two days after a tube leak at the plant's other unit prompted operators to shut down the reactor as a precaution. A tiny amount of radiation could have escaped, but officials say workers and the public were not endangered.

The problems at Unit 2 were discovered during inspections of a steam generator, after the plant was taken off-line for maintenance and refueling. That equipment was replaced recently in both units of the twin-reactor plant, which is located about 45 miles north of San Diego.

"The amount of wear that we are seeing on these tubes is unusual for a new steam generator," Nuclear Regulatory Commission spokesman Victor Dricks said.

Plant operator Southern California Edison did not immediately comment.

The plant is owned by Southern California Edison, San Diego Gas & Electric and the City of Riverside. Southern California Edison serves nearly 14 million residents with electricity in Central and Southern California.