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The Nuclear Regulatory Commission ordered the troubled San Onofre nuclear power plant in California to remain shut Tuesday until the plant's operator completes a full study of problems with critical safety equipment.
Edison International's (EIX) Southern California Edison utility, the plant's operator, agreed to determine the cause of premature wear found in several steam tubes, or pipes, and ensure that it has fixed the problem before it can restart the plant, the NRC said.
"Until we are satisfied that has been done, the plant will not be permitted to restart," Elmo Collins, the NRC's Region 4 administrator, said in a statement.
The NRC directive, contained in a document called a "confirmatory action letter," indicated that the equipment problems could be more complicated and difficult to resolve than earlier thought.
SoCal Edison said in a statement that it was "working with the NRC and abiding by its requirements."
Edison shut down one of two reactors at the San Onofre plant Jan. 31 after a steam tube ruptured and released radioactive steam. The NRC said the amount of radiation released from the reactor, called Unit 3, posed no harm to workers or the public.
About two weeks later, Edison said that several additional steam tubes had ruptured during pressure-testing. The NRC sent a special team of inspectors to the plant, in San Clemente, to oversee testing and work with SoCal Edison to determine the cause of the problem.
The plant's other nuclear unit, Unit 2, had been shut down for routine maintenance and refueling. Edison, which owns the plant with Sempra Energy's San Diego Gas & Electric utility.
At each unit, nearly 19,500 tubes carry hot, radioactive water and steam from pools of water that hold nuclear-fuel rods to the generators, which use the steam to produce electricity. The tubes are critical for keeping the plant's nuclear-fuel rods cool.
The steam tubes are contained in a chamber filled with cool water and must be strong enough to withstand pressure from the hot water inside and the cool water outside to ensure that radiation doesn't escape.
The tubes are components of four steam generators that Edison and Sempra bought from Mitsubishi Heavy Industries Ltd. (7011.TO) and installed in 2009 and 2010 at a cost of $800 million.
The NRC said that the premature wear of the tubes appeared to be caused by vibrating and rubbing against nearby tubes and against support structures inside the steam generators. The agency said SoCal Edison was still working to determine what was causing the vibrating and rubbing.
Nuclear watchdog group Friends of the Earth demanded that SoCal Edison publicly release details about what it has found out about the cause of the steam tube ruptures.
"Californians deserve the truth about what's happening at San Onofre," Damon Moglen, director of the group's the climate and energy project, said in a statement.
The NRC said that premature wear similar to that found in several steam tubes in Unit 3 was found in several steam tubes in Unit 2 and that 186 of these tubes had been plugged, or taken out of service. The agency said ongoing testing of steam tubes at Unit 3 would help determine how to handle steam tubes at Unit 2 that showed similar signs of wear.
SoCal Edison and California's grid operator have been working to line up backup power generation for the summer, in the event that the San Onofre plant, which has 2,200 megawatts of electricity generating capacity, remains offline for an extended period.
The utility and the California Independent System Operator have been considering beefing up existing transmission lines to be able to ship more power from other areas of the state, upgrading existing power plants to generate more electricity and asking customers to conserve electricity, among other actions.
-By Cassandra Sweet, Dow Jones Newswires; 415-439-6468; firstname.lastname@example.org