Southern California Edison will receive two violations from the NRC for failing to identify a flawed design. The federal agency does not regulate Mitsubishi Heavy Industries, the builder of the plant.
Edison's chief nuclear officer, Pete Dietrich, said that it is not unusual for regulators to cite a plant's operator for problems caused by a vendor or contractor.
"Mitsubishi designed the system," Dietrich said. "Mitsubishi built the system. Mitsubishi's system failed. They are the experts. SCE was the customer."
The Nuclear Regulatory Commission said in a report that Southern California Edison failed to identify a flawed design for four replacement steam generators, which led to excessive wear to hundreds of tubes that carried radioactive water.
"There were opportunities to identify this error during the design of the replacement steam generators," the report concluded.
Edison announced in June it was shutting down the coastal plant between Los Angeles and San Diego for good, surrendering in a costly and drawn-out fight over whether the reactors were too damaged to operate safely.
The trouble began in January 2012, when a small radiation leak led to the discovery of the extensive damage to tubing. It never produced electricity again.
SCE has been fighting with Mitsubishi Heavy Industries, the Japan-based company that built the generators, over who gets the blame and pays the bill for the problems that centered on how steam flows inside the huge machines. Potentially, billions of dollars could be at stake in the dispute.
The federal agency does not regulate Mitsubishi. Separately, the NRC said in a statement it had issued a "notice of nonconformance" to Mitsubishi for problems associated with the design of the generators, which is an administrative step, not an enforcement action. It wasn't immediately clear if the step carried any possible fine.
Mitsubishi has said the generators "were designed and manufactured with SCE's full supervision and approval and in accordance with well-established and accepted codes and standards along with our own knowledge and experience plus that of outside experts."
In legal documents, the utility has claimed Mitsubishi Heavy Industries Ltd. and Mitsubishi Nuclear Energy Systems are liable for the defective generators, along with costs ranging from buying replacement power to the investment in the now-shuttered plant that the company earlier estimated at over $2 billion.
Edison said in a statement that the NRC has not imposed financial penalties.
A public forum hosted by the NRC on the decommissioning of the reactors at San Onofre for Thursday at 6 p.m. at the Omni La Costa Resort and Spa in Carlsbad.
The Associated Press and City News Service contributed to this report.