Waterford — The Nuclear Regulatory Commission began a Special Inspection at Unit 2 of the Millstone Power Station Tuesday to review issues associated with an “Unusual Event” declared at the facility on Oct. 4.

The NRC announced that its two-member team arrived at the nuclear power plant Tuesday morning to begin the review.

Operators at the plant declared the "Unusual Event" — the lowest of four levels of emergency classification used by the NRC — about 9:30 a.m. on Oct. 4.

The declaration was based on reactor coolant system leakage of greater than 25 gallons per minute, the NRC said in a news release.

The leakage came from a relief valve on the plant's shutdown cooling system, which is used to provide cooling to the reactor when it is out of service.

The plant, owned by Dominion Resources, at the time was preparing for a refueling and maintenance outage.

In response to the leakage, the shutdown cooling system was isolated from the reactor coolant system, allowing repairs to be made and the return of the system to operational status, the NRC said.

The emergency declaration was terminated at 11 a.m. All the leakage was contained within other plant systems, as designed, the NRC said.

After conducting a preliminary assessment of the event, the NRC has determined that it meets the criteria for a Special Inspection.

Among other areas, the NRC team will review operator response to the event, equipment performance and immediate corrective actions.

“Our initial review of the event has raised questions regarding operator performance,” said Dan Dorman, NRC Region I administrator. “We have determined that the use of a Special Inspection is appropriate in this case to help the NRC better understand Dominion’s response to the event.”

A report documenting the team’s findings will be issued within 45 days of the conclusion of the inspection, the NRC said.

Ken Holt, spokesman for Millstone, said Dominion is conducting its own investigation "to ensure that we understand all the contributing factors in the event.

"We will be working with the special inspection team, and we look forward to any insights they may have to prevent this from ever happening again," he said.

The inspection could result in a finding of violations at the plant, he said.

j.benson@theday.com

Twitter: @BensonJudy