Waterford — Unit 2 at the Millstone Power Station will now be able to use cooling water up to 80 degrees, averting a repeat of the shutdown that happened two years ago when the water at the plant’s intake pipe in Long Island Sound exceeded its licensing limit of 75 degrees for the first time.

The Nuclear Regulatory Commission announced Monday that it has approved the increased temperature limit for Unit 2, the first time this has occurred at any of the 13 U.S. nuclear power plants located on marine waters, said NRC spokesman Neil Sheehan. License amendments for temperature increases have previously been allowed for plants located on rivers and lakes.

The 11-day shutdown in August 2012 was the first time a U.S. nuclear power plant had to shut down because of rising water temperatures. In response, the Nuclear Regulatory Commission ordered a review of climate change impacts on power plants nationwide.

After Unit 2 was shut down, forcing the plant to buy replacement power to supply the regional power grid, Millstone began preparing applications to the NRC to amend its license to use cooling water up to 80 degrees for both Unit 2 and Unit 3. Although Unit 3 did not have to shut down two years ago because its intake pipe draws water from a deeper, cooler portion of the Sound, the company nonetheless also wants the flexibility to use warmer water in the future if necessary, said Millstone spokesman Ken Holt. The NRC is expected to act on the application for Millstone 3 “in the near future,” Sheehan said.

Holt said that although the water temperature in the Sound did not exceed 75 degrees last summer, the company believes the conditions that occurred in 2012 are likely to recur.

“We’ve seen a long-term trend over the last 40 years of Long Island Sound temperatures steadily increasing,” Holt said. In 2012, the temperature in the Sound near the Unit 2 intake pipe was about 77 degrees, he said. The company believes the five-degree margin will be sufficient to allow it to continue operations without costly unplanned shutdowns, he said.

“We think we can stay well within the limit,” he said.

Millstone uses 1 billion to 2 billion gallons of water per day from the Sound to cool units 2 and 3. Unit 1 is shut down.

Holt said the only change made to the plants to accommodate the new limit was the replacement of old thermometers with new, more sensitive ones to more accurately measure the water temperatures.

Under the new limit, if the water temperature exceeds 80 degrees, the plant must be placed on “hot” standby within six hours and shut down within 30 hours.

The NRC made its decision after reviewing Millstone’s engineering analysis of how the higher temperatures would affect plant equipment. All electrical and ventilation systems that require cooling were also considered.

“They wanted to assure there was sufficient design margin in the equipment,” he said.

The NRC also considered how the higher temperatures would affect the back-up diesel generators the plant would depend on if there was an accident requiring emergency systems to run on generator power, Sheehan said.

“We wanted to be sure that up to 30 days after a severe accident the generators would still be able to dissipate heat at the higher water temperatures,” he said.

NRC inspectors at the power station will monitor the plant closely the first time it uses cooling water warmer than 75 degrees, Sheehan said.

“The resident inspectors will pay close attention when this change takes effect,” he said.

j.benson@theday.com