It remains unclear how long the San Onofre nuclear power plant will remain closed after the U.S. Nuclear Regulatory Commission prohibited Southern California Edison from restarting the plant until problems recently discovered are thoroughly understood and fixed.

The plant has already been shut down for two months, the longest in San Onofre's history, after a tube leak in one of the plant's steam generators released a small amount of radioactive steam. Since then, unusual wear has been found on hundreds of tubes that carry radioactive water.

Neither regulators nor Edison have said when they believe the plant will reopen. San Onofre supplies about 2,200 megawatts of power, or enough electricity to serve of 1.4 million homes. State officials are already working on contingency plans to avoid power outages during the summer months if the plant remains out of commission.

Until now, the cause of the tube problems has been a mystery. But in a letter federal regulators sent to Edison on Tuesday, officials said tubes were vibrating and rubbing against support structures and against adjacent tubes.

According to the NRC, the tubes in Unit 3 were rubbing against each other and against the support structure, while those in Unit 2 were rubbing against the support structure but not against each other. Commission spokeswoman Lara Uselding said the root cause of the issue is still unclear.

The NRC's letter prohibits Edison from restarting the plant until regulators feel the problems have been addressed and San Onofre is safe to operate.

“Until we are satisfied that has been done, the plant will not be permitted to restart,” NRC Region IV Administrator Elmo E. Collins said in a statement.

Although the NRC has ordered nuclear plants shut before, the move is considered a significant step signaling that the plant is unsafe to operate.

The problems are perplexing because the steam generators were installed within the last two years at a price tag of $671 million to be paid by Edison and San Diego Gas & Electric customers through higher rates. The new equipment was intended to last until the plant's license expires in 2022.

Officials said the wear on the tubes was unusual for such new equipment.

The situation posed a safety threat for two reasons. The rupture of tubes can release radiation. Moreover, if many tubes rupture at one time, it could compromise the cooling system of the reactor’s core.

In Unit 3, a total of eight tubes failed pressure tests performed in recent weeks. In Unit 2, 192 tubes--about 1% of the unit’s total--showed signs of wear and were taken out of service.

Typically, a steam generator can operate at full power with about 8% of its tubes plugged, although some have operated with 30% plugged, Uselding said.

Under the order issued Tuesday, before the plant can return to service, Edison must determine what is causing the tubes in Unit 3 to rub against each other and act to ensure the same thing does not happen in Unit 2. It also must complete pressure testing of tubes with possibly excessive wear and plug those that show too much deterioration, and it must develop a schedule of added inspections once the plant returns to service.

Edison said it is committed to meeting the NRC’s requirements.

“Our number one priority is, and always has been, the health and safety of the public and our employees,” said SCE President Ron Litzinger in a statement. “The utility will only bring the units on line when we and the NRC are satisfied that it is safe to do so.”

Mitsubishi Heavy Industries, which built the generators, released the following statement Tuesday: "As the manufacturer of the steam generators, we are currently supporting our customer, Southern California Edison, in the thorough and systematic inspection and analysis of the steam generators in order to identify and understand the root cause of the situation."

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-- Abby Sewell

Photo: San Onofre Nuclear Generating Station at San Onofre State Beach. Credit: David McNew/Getty Images