Waterford — For the first time, record-breaking water temperatures in Long Island Sound have forced the shutdown of the Unit 2 nuclear reactor at the Millstone Power Station.

It appears to be the only nuclear power plant in the nation forced to shut down this summer due to high water temperatures, said Neil Sheehan, spokesman for the Nuclear Regulatory Commission.

This is the first time Unit 2 has been shut due to high water temperature since it began operations in 1975, according to Ken Holt, spokesman for Millstone owner Dominion.

The Unit 2 nuclear reactor at the Millstone Power Station will remain shut until the company is confident that the temperature of the water drawn into the plant from Long Island Sound will remain below 75 degrees, Holt said.

"We don't want to restart it and then have to shut it down again if the temperature spikes back up," he said.

The unit, the smaller of the two operating plants at the power station, was shut down on Sunday afternoon after the temperature of water from Long Island Sound used to cool the plant exceeded limits set by the plant's license.

Millstone was among plants that received emergency license amendments this summer related to the temperature of cooling water. Last week, Millstone received permission from the NRC to take an average of three temperature measurements for the water at the intake pipes for Unit 2, rather than using the single highest measure.

The plant's license states that it must be shut down when the water temperature is 75 degrees or higher over a 24-hour period. The water is used to cool instruments in the nuclear reactor building and the emergency diesel generators. Water that is too warm does not sufficiently cool the plant to keep it within the margin of safety.

Water temperatures in Long Island Sound this summer are the warmest on record. Nationwide, July air temperatures have been the hottest on record since record-keeping began in 1895, according to the National Oceanic and Atmospheric Administration, and are expected to continue to remain high through August.

Unit 2 produces 880 megawatts of electricity. Unit 3, which has water intake pipes that draw from a deeper, colder part of the Sound, is still at 100 percent capacity, producing 1,200 megawatts of electricity, Holt said.

Holt said federal, state and local officials have been notified of the shutdown, as well as ISO New England, the organization that oversees the regional power and transmission system.

Despite the Millstone shutdown, there has been sufficient capacity in the system both Sunday and Monday to meet normal power demands, Marcia Blomberg, ISO spokeswoman, said. The system had 2,600 megawatts of excess capacity on Monday, she added.

"We operate the system with a margin of reserve to allow for unplanned outages," she said.

Millstone crews are monitoring the water temperature to determine when it will be safe to restart the plant, which would take a couple of days. Holt said the company will wait unit it can be confident that the temperature will remain below 75 degrees.

He declined to discuss the financial implications of the shutdown on Dominion.

Sheehan said "it remains to be seen" whether the NRC will consider the issue of warming water temperatures and the long-term implications for nuclear power plants.

"We're always on the lookout for issues that may need further review for their generic implications," he said.

j.benson@theday.com