Nuclear regulators said Thursday that extensive wear had been found on the tubes inside a unit at the San Onofre nuclear plant, where another unit was placed off-line after a leak earlier this week.
Dozens of tubes that carry radioactive water in a steam generator showed "many, many years" worth of wear, even though the tubing is only 22 months old, said Victor Dricks, a spokesman for the Nuclear Regulatory Commission.
Nearly 70 tubes, made from a metal alloy and formed into a U-shape, had 20% of their interior lining worn off, while hundreds more had 10% of the lining deteriorated. More than 9,000 tubes are in the generator.
Some of the tubes will require repair, Dricks said, while others will probably have to be replaced.
But officials for Southern California Edison, which operates the facility and is a majority owner, said that it was too premature to make any determination and that testing would continue.
The unit was off-line for a scheduled maintenance period of several months for technology upgrades and fuel replacement, said Gil Alexander, an Edison spokesman.
It is unclear why the tubes were showing so much wear.
The commission's findings follow a leak in a tube Tuesday, prompting operators to shut down a reactor. However, officials said the minuscule amount of radiation that leaked did not endanger the public.