The U.S. Nuclear Regulatory Commission cited Southern California Edison and its equipment manufacturer, Mitsubishi Heavy Industries, for design flaws that led to the failure of the San Onofre plant in January 2012.
The commission determined inadequate computer modeling by Mitsubishi led to excessive wear in hundreds of generator tubes that carried radioactive water.
Edison released a statement saying Mitsubishi designed the system and is accountable for its failure to provide properly functioning steam generators.
But NRC spokesman Victor Dricks said Edison also holds responsibility.
"Mitsubishi and Edison have responsibility for the problems that developed with the steam generators," said Dricks. "Edison, because they are an NRC licensee and the NRC ultimately holds its licensees responsible for the actions of the vendors that they hire."
Extensive litigation is expected in the matter.
Edison has 10 days to notify the NRC whether it will accept the findings, and Mitsubishi has 30 days to respond.
Dricks said there’s no monetary fine associated with the documentation because the plant is shut down and won’t be operating again.
Damages related to the failed steam generators are estimated at $2 billion.