The four tubes failed Thursday at the San Onofre coastal plant in northern San Diego County, Southern California Edison announced. Three had failed Wednesday.The utility shut down the plant's Unit 3 reactor and began testing samples from thousands of tubes in its steam generators on Jan. 31 after a leak was found. Traces of radiation escaped during the leak, but officials said there was no danger to workers or neighbors.A spokeswoman for the power plant said safety is the top priority and the reactor wouldn't be back on line until the problems are solved. The Nuclear Regulatory Commission said Thursday it was sending a special team of inspectors to try to determine why the metal tubes, which were installed only a few years ago, have become frail enough to pose a risk of leaks."This is a significant issue," said NRC spokeswoman Lara Uselding. "A tube rupture is really the concern.""This is not supposed to happen," said San Diego State University professor Dr. Murray Jennex, who help build the plant. "It [is] unusual for so many to break." "The San Onofre nuclear generating station is safe. Both units are currently safely shut down," Uselding said. Inside a steam generator, hot, pressurized water flowing through bundles of tubes heats non-radioactive water surrounding them. The resulting steam is used to turn turbines to make electricity.The tubes are one of the primary 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.An Edison statement said the utility welcomes the NRC Augmented Inspection Team, which is expected to begin work Monday.Investigators already have been looking into excessive wear on tubes in Unit 3 and its twin, Unit 2, which has been off line for maintenance and refueling. In a $670 million overhaul, two huge steam generators, each containing 9,700 tubes, were replaced in Unit 2 in fall 2009 and a year later in Unit 3.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."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.