The electrical fault that triggered Oyster Creek Generating Station to go offline and 22,000 Ocean County residents to be powerless last week has been identified and repaired, the Nuclear Regulatory Commission (NRC) stated in a report.

The loss of power was the result of an electrical ground caused by a tree contacting one transmission line, the report says. There also was an unrelated electrical circuit breaker coordinator problem that affected another transmission line running between the power plant and its connection to the electrical grid.

The transmission system operator repaired the problems, the report said.

Following the electrical fault, Oyster Creek declared an "Unusual Event" for approximately two hours and went offline for six days.

Two isolation condensers cooled the reactor and both emergency diesel generators automatically started and provided electrical power to safety equipment, the report said.

Due to the loss of offsite power, power to the reactor protection system also was interrupted, which caused the primary containment isolation, as expected, the report said. Plant operators utilized necessary equipment to control reactor pressure and level.

Exelon also initially reported to the NRC that the reactor building differential pressure indicated positive 0.25 inches at 3:57 a.m. and returned to its normal negative pressure at 4:34 a.m. on July 23.

It was determined that there was an instrument error due to a loss of instrument air. The containment pressure actually remained at its normal level throughout the Unusual Event, the NRC report said.

NRC’s resident inspectors responded to Oyster Creek and monitored the plant operator’s actions to stabilize the plant, restore offsite power and troubleshoot plant systems, the report said. The inspectors were assisted by regional specialists to review Oyster Creek’s actions to address equipment issues prior to restarting. 

A news release from Oyster Creek also stated that plant employees utilized the time offline to perform minor maintenance.

Oyster Creek returned to service on Saturday, July 28.

The Oyster Creek Generating Station is owned and operated by Exelon Corporation. Exelon is one of the largest electric utilities distributing electricity to approximately 5.4 million customers and natural gas to approximately 486,000 customers. The generating station is the oldest operating nuclear power plant in the country.