Waterford — The Millstone 2 nuclear power plant, shut down Sept. 3 by a water leak, is now back in service, a federal official said Monday.

The leak was discovered Aug. 17 in a system that uses water from Long Island Sound to act as a coolant for the plant. Water leakage later reached unacceptable levels, and the plant had to be shut down while repair work was performed.

“The shutdown was necessary because the leakage exceeded the plant’s technical specifications,” said Neil Sheehan, a spokesman for the U.S. Nuclear Regulatory Commission.

The plant’s owner, Dominion, replaced a flange on the service water system, and NRC inspectors monitored the work to ensure it was done safely.

“They identified no concerns with respect to the repairs,” Sheehan said.

Sheehan said that by Monday morning, the plant was already back to operating at 98 percent of capacity.