A failed electrical insulator at a switchyard at Exelon Generation Co.’s two-unit Byron nuclear plant in Lisle, Ill., has been blamed for the loss of offsite power on Monday morning that automatically shut down Unit 2 and forced Exelon to declare an “Unusual Event.”

Unit 2 shut down at about 10:18 a.m. CST, and smoke was seen rising from an onsite station transformer, though emergency responders found no evidence of fire, the Nuclear Regulatory Commission (NRC) reported later on Monday. Diesel generators were immediately deployed, and steam was released from the non-nuclear side of the plant to aid in the cooling process. The Unusual Event, the lowest of the four emergency classifications as established by the NRC, was terminated at 8 p.m. on Tuesday, following return of power to Unit 2.

Exelon said teams of “technical experts” had also completed repairs to the failed insulator on Tuesday but noted that a comprehensive investigation into the failed insulator continued.

“Our diesel generators performed as expected in providing continuous electricity to the unit during the Unusual Event. Plant teams will now focus on a safe and measured approach to returning Unit 2 to the electrical grid,” said Byron Station Site Vice President Tim Tulon, though he did not elaborate on when the plant could resume operations.

The NRC on Tuesday, meanwhile, initiated a special inspection to review how plant equipment responded to the loss of offsite power and evaluate the facts and circumstances surrounding the incident. The special inspection report is expected within 45 days.

Unit 2 was reported in “safe and stable shutdown condition. “There was a steam release from the non-nuclear side of the plant with trace amounts of tritium,” the agency said. “This type of steam release is used by nuclear power plants to release pressure in order to maintain the plant in a stable condition. Doses to the public from this type of release are significantly below even the most stringent Federal protective limits and, therefore, do not pose a risk to public health and safety.”

In a related story, Southern California Edison (SCE) on Tuesday reported it had begun a “precautionary shutdown” of Unit 3 at the San Onofre Nuclear Generating Station near San Clemente, Calif., because sensors had detected “a possible leak in one of the unit’s steam generator tubes.” The leak posed no imminent danger to the public or plant workers, and no release to the atmosphere had occurred.

San Onofre Unit 2 is currently offline for a planned maintenance, refueling, and technology upgrade outage. SCE said it had “ample reserve power” to meet customer needs while Unit 3 is offline, and noted it had informed the NRC of the possible leak.

Sources: POWERnews, Exelon, NRC, SCE