The San Onofre nuclear power plant came under renewed scrutiny last week after a small radiation leak and the discovery of extensive tube damage.
The leak and the tube wear "at no point posed a danger to the community or to workers on site," said Jennifer Manfre, spokeswoman with Southern California Edison, which operates the facility.
But the incidents raised concern among environmental groups, which for years have kept a close eye on the plant near San Clemente following other safety problems.
"This should remind us that this is a radioactive factory that's sitting there cooking away, and things can go very wrong," said S. David Freeman, a former general manager of the Los Angeles Department of Water and Power who used to oversee nuclear power plants. "It's time we start transitioning to some other form of renewable resource."
Trouble began at San Onofre on Tuesday when a leak from a tube at one unit released a small amount of radiation, prompting operators to shut down the reactor.
On Thursday, at a second unit that was shut down for maintenance, nuclear regulation officials found extensive wear on tubes that carry radioactive water in a steam generator. The tubes were installed less than two years ago after they were delivered by the Japanese manufacturer of the generators, Mitsubishi Heavy Industries.
Two of the tubes showed more than 30% wall thinning, 69 had 20% thinning and more than 800 had 10% thinning, said Victor Dricks, spokesman for the Nuclear Regulatory Commission.
Manfre said the extent of the tube damage was not yet clear to Edison.
"So far we've had very preliminary testing," Manfre said. "We're looking to provide very accurate testing."
Both units were shut down Friday as officials conducted inspections. Manfre said it was too soon to know when the units would be working again or how much of the tubing needs to be replaced or repaired.
On Friday, a local newspaper reported a third incident involving a veteran worker at the plant who lost his balance while trying to retrieve a flashlight and tumbled into a reactor pool. The man reportedly did not suffer significant radiation exposure. Edison may review its procedures for working around the reactor pool, officials said.
Some critics saw the incidents as a sign of greater problems.
"To have this many failures is to have a real breakdown in quality assurance, maintenance and safety culture," said Dan Hirsch, president of the Committee to Bridge the Gap, which follows the nuclear industry.
In 2008, the plant received a string of citations over such issues as failed emergency generators, improperly wired batteries and falsified fire safety data, records show.
Early last year, the NRC issued its annual review of San Onofre, identifying improvements but noting that in the area of human performance, "corrective actions to date have not resulted in sustained and measurable improvement."
Officials with Edison said they would continue to inspect each unit in coming days to get a full assessment of this week's damage. Customer service or utility bills should not be affected by the incidents, Manfre said.