Southern California Edison can't restart the San Onofre nuclear plant, where a steam-generator leak released a small amount of radioactive steam, until problems are fixed, the U.S. Nuclear Regulatory Commission decided.

LOS ANGELES — The U.S. Nuclear Regulatory Commission (NRC), 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 already has 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.

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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 to operate.

The steam generators, manufactured by Mitsubishi Heavy Industries, were installed within the past 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.