LOS ANGELES — Forty-five days after a radioactive water leak prompted a Southern California utility to shut down a nuclear reactor, investigators Friday sought to pinpoint why tubing in the plant eroded at an alarming rate while the prospect of an extended repair job raised questions about summertime power supply.
Tests on massive steam generators at the troubled Unit 3 reactor at the San Onofre Nuclear Generating Station, which was shut down as a precaution after the leak on Jan. 31, revealed seven alloy tubes that carry radioactive water are in danger of rupturing under high pressure. Traces of radiation escaped during the January leak, but officials said there was no danger to workers or neighbors.
Unusual wear has been found on hundreds of similar tubes that carry radioactive water at its twin, Unit 2, which was shut down earlier this year for routine maintenance, leaving questions about the integrity of equipment the company installed at the two reactors in a multimillion-dollar makeover in 2009 and 2010.
A special team of federal investigators was dispatched to the seaside site, about 45 miles north of San Diego, on Thursday to focus on Unit 3. While gradual tube wear is common in steam generators over time, no one knows why so many state-of-the-art tubes in relatively new equipment have degraded so quickly.
There are nearly 20,000 steam generator tubes in each of the two reactors. After tests, the company said a total of 321 tubes will be plugged and taken out of service at the two reactors, well within the margin to allow them to continue to operate.
No date has been set to restart the reactors.
The testing "is designed to help us understand the potential safety implications and significance of this situation," Ron Litzinger, president of plant operator Southern California Edison, said in a statement.
The company has finished surveying the wall thickness of the tubes but has yet to determine the cause of the unusual wear, which could range from debris circulating within the generators to a manufacturing or design defect.
The steam generators were manufactured by Japan-based Mitsubishi Heavy Industries, according to company officials.
Inside a steam generator, hot, pressurized water flowing through bundles of tubes heats a bath of non-radioactive water surrounding them. The resulting steam is used to turn turbines to make electricity.
The tubes are one of the vital barriers between the radioactive and non-radioactive sides of the plant, according to the NRC. If a tube breaks, there is the potential that radioactivity from the system that pumps water through the reactor could escape into the atmosphere.
Serious leaks also can drain cooling water from a reactor, said David Lochbaum, director of the nuclear safety project for Union of Concerned Scientists.
"If there is a tube leak, the water that is leaking out is more likely to reach the environment. If there is any radioactivity in that water, it's more likely to get places it shouldn't be," he said.
Nineteen percent of all power used by SCE customers comes from nuclear generation.
A spokeswoman for the agency that operates the state's wholesale power system, the California Independent System Operator, said the San Diego and Los Angeles areas could see rotating power outages this summer if both reactors remain off line. The agency is taking steps to prevent those shortages.
"It's all about balancing supply and demand," said ISO spokeswoman Stephanie McCorkle. "You have to have a certain amount of plant (power) generation where the heavily populated areas of California are."
An Edison statement said the utility welcomes the NRC inspection team, which is expected to begin work Monday.
The plant is owned by 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.