Waterford — The Nuclear Regulatory Commission has notified officials at Millstone Power Station about a preliminary finding of a safety violation involving Unit 3’s turbine-driving auxiliary feedwater pump.

In a letter dated Thursday, the NRC said it has issued a preliminary finding of low-to-moderate safety significance after a special inspection completed on July 21. The finding is categorized as a “white,” second-lowest on the four-tier scale of safety violations.

Millstone owner Dominion has 10 days to respond to the NRC about whether it will accept the finding or present additional information. Neil Sheehan, spokesman for the NRC, said that if the finding is finalized, Millstone will be put under additional oversight for a year or more. In addition, a team of NRC inspectors will visit the plant to ensure that the Millstone staff has successfully evaluated the root cause, made corrections and ensured that no other plant systems are affected. The auxiliary pump is considered an important component of safety systems that cool the plant when it is shut down.

If the finding is finalized, no fines would be levied, Sheehan said, but Dominion would be charged by the NRC for the additional inspections.

The special inspection of the pump was done in response to three overspeed trips of the pump in November, December and January, the NRC said in its letter.

Millstone spokesman Ken Holt said the problem was caused by an incorrect bearing in the pump since August 2013. The bearing was mislabeled by the manufacturer, Holt said, but plant staff did not identify the mistake until January. The part is key to reducing friction in the pump, he said.

Dominion officials have not yet decided on their response to the NRC, Holt said. Millstone last had a “white” finding in 2011.

In addition, the NRC also notified Millstone Thursday about a preliminary finding of a “severity level 3” violation involving its power line outage detection system. Both nuclear power plants at the site were shut down for several days on May 25 due to loss of off-site power from Connecticut Light & Power.

Unit 3 did not produce power for 11 days after the shutdown, and Unit 2 was restarted after four days. While the two power plants both produce electricity, both also receive power that runs play systems. It was the first time both plants had shut down simultaneously, and it was termed by the NRC to be an “unusual event.”

Sheehan said the NRC concluded after a special inspection that Millstone staff “could have taken steps that would have avoided the cascade of issues that led to both units going off line.”

Holt said the company has not yet determined its response to the preliminary level 3 finding.

He added that the company is working with CL&P on the power lines that service the power station “so that what happened on May 25 will not happen again.”

j.benson@theday.com

Twitter: @BensonJudy