The leak was detected Tuesday afternoon in the Unit 3 reactor at San Onofre Nuclear Generating Station, about 45 miles north of San Diego. The seaside plant was taken off line as a precaution while investigators tried to determine what happened.
While the leak wasn't large enough to require the plant to declare an emergency, any possible leak of radiation into the atmosphere is rare. Also concerning was that "many" tubes that carry pressurized radioactive water were damaged, according to a spokesman for the Nuclear Regulatory Commission.
The tubes are part of equipment that is virtually new, having been installed in 2010.
"The damage that they have found to many other tubes is unusual, and they are attempting to identify the reason," NRC spokesman Victor Dricks said.
News of the possible release of radioactivity was slow to emerge. Shortly after the incident, Southern California Edison issued a statement saying, "There has been no release to the atmosphere."
On Wednesday morning, however, Dricks said a small amount of radioactive gas "could have" escaped from a building that houses auxiliary equipment.
When asked, Southern California Edison spokesman Gil Alexander did not directly address why the plant used the language it did. He emphasized the relatively minor nature of the incident.
"I can't speak for the NRC but we would agree that there might have been an insignificant or extremely small release," Alexander said. He said the amount might not be detectable by monitors.
Dricks agreed, saying the radiation "would not pose a danger to anyone." The NRC was evaluating the plant's response to the leak, he said.
In November, nuclear watchdog and environmental groups criticized plant operators for taking more than an hour to notify the public of an ammonia leak in a storage tank that prompted the evacuation of some workers. There was no danger to the public, the company said at the time.
The Unit 3 reactor returned to full power in February 2011 after it was refueled and its two aging steam generators were replaced. The plant's other reactor, Unit 2, had similar work. The total retrofit cost more than $670 million.
Daniel Hirsch, who lectures on nuclear policy at the University of California, Santa Cruz, said he was concerned that the problem occurred with recently installed equipment.
"Edison has historically not been candid about the problems at San Onofre. That lack of transparency causes tremendous distrust and increases risk," Hirsch said.
"It makes one wonder about the quality assurance for the replacement equipment," he added. "This is not due to old equipment breaking but new equipment that wasn't up to snuff in the first place."
The latest leak occurred in one of thousands of tubes carrying radioactive water from the plant's reactor. The leak was initially estimated at a rate of 85 gallons a day an amount about half of what would require the plant to shut down. Alexander said the rate of the leak was "much less," but did not provide a figure.
Dricks said radioactive gas that leaked from that tube in the plant's steam generator was vented into the auxiliary building. The radiation was detected by monitors in that building, which is separate from the sealed structure that houses the reactor.
Because the auxiliary building is not sealed people come and go through doors it's possible radiation escaped into the atmosphere.
Each steam generator can contain as many as 16,000 tubes, each about 0.75 inch in diameter. The hot, pressurized water flowing through the tubes heats non-radioactive water outside the tubes. The resulting steam is used to turn turbines to make electricity.
According to the NRC, the tubes have an important safety role because they represent one of the primary barriers between the radioactive and non-radioactive sides of the plant. If a tube breaks, there is the potential that radioactivity from the system that pumps water through the reactor could escape into the atmosphere.
Alexander said he could not confirm any additional damage to other tubes, pending an inspection of the equipment.
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.
(Copyright 2012 by The Associated Press. All Rights Reserved.)