Feds: San Onofre nuclear plant can't reopen until problems fixed San Onofre Nuclear Generating Station at San Onofre State Beach. David McNew/Getty Images / March 27, 2012 By Abby Sewell March 27, 2012, 2:55 p.m. The U.S. Nuclear Regulatory Commission, citing serious concerns about equipment failures at the San Onofre nuclear plant, on Tuesday prohibited plant operator Southern California Edison from restarting the plant until the problems 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. Neither regulators nor Edison have said when they believe the plant will reopen. Since then, unusual wear has been found on hundreds of tubes. 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 it's still unclear what is causing the rubbing. The NRC's letter prohibits Edison from restarting the plant until regulators feel the problems have been addressed and San Onofre is safe 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. The steam generators, manufactured by Mitsubishi Heavy Industries, were installed within the last two years. Officials said the wear on the tubes was unusual for such new equipment and poses safety concerns because of the potential release of radiation if a tube ruptures and because they carry water that helps to cool the reactor core. 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 take action 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.