Other nuclear plants have received permission to operate at higher water temperatures, said Diane Screnci, a spokeswoman at the NRC. Millstone will have to "provide a lot of calculations" demonstrating that safety-related equipment could operate with water at higher temperatures, she said.
Weather patterns such as the mild winter in 2011-2012 and little wind that keeps heat in on Long Island Sound were to blame for the unusually warm water, Robert Wilson, a professor at Stony Brook University's School of Marine and Atmospheric Sciences, said over the summer.
James O'Donnell, a professor of marine sciences at the University of Connecticut, said Long Island Sound water temperatures in the winter of 2012 ranged from nearly 48 degrees in January to about 40 degrees at the end of February.
In contrast, water temperatures this winter have been cooler, ranging from about 44 degrees in January to about 36 degrees this month. O'Donnell characterized it as a normal winter, which could lead to a cooler Long Island Sound this summer than in 2012.
The timing of the shutdown couldn't have been worse, because demand for power is typically highest in summer as businesses and homes rely on air conditioning.
"You want to be there providing electricity when people are using it the most," Holt said.
The partial nuclear plant shutdown due to excessively warm water was a first, said NRC spokesman Neil Sheehan. Chairwoman Allison Macfarlane has asked the agency's staff to look at impacts of climate change on nuclear plants, though it's focusing on plant changes following the 2011 Fukushima nuclear meltdowns in Japan, he said.
Major impacts from weather included an alert at a New Jersey nuclear plant in October as water from Superstorm Sandy rose outside the plant and threatened cooling equipment.
"We have seen some conditions that did not exist previously and we expect to learn from that going forward," Sheehan said.
Stephen Singer can be reached at https://www.twitter.com/SteveSinger10