Sea water flowing into Pilgrim Nuclear Power Station from Cape Cod Bay over the weekend reached an excessive temperature that forced the Plymouth plant to cut power and prepare for a rare shutdown, Pilgrim officials said Tuesday.
The operators of the station, which is about 35 miles south of Boston, cut power by 10 percent around 4 p.m. on Sunday to cool sea water pulled into the plant.
The plant resumed full power about 3½ hours later, and Lauren Burm, a spokeswoman for Entergy Corp., the plant’s owner, said, “the plant remained in stable condition the whole time, and there was never a threat to the public or the plant.”
Plant officials suggested that the elevated temperature — 75.09 degrees — was the result of a combination of tides and wind mixing water discharged from the plant with the water being drawn into its intake pipes.
But some scientists saw a potential sign of the impact of climate change in the spike in temperature, only the fourth time in the plant’s 43-year history that sea water flowing through its intake pipes exceeded the 75-degree limit set by the federal Nuclear Regulatory Commission, which oversees safety at the nation’s nuclear plants. The previous three times occurred in the summer of 2013.
Andrew Pershing, chief scientific officer of the Gulf of Maine Research Institute in Portland, said that the temperature of Cape Cod Bay has shown “a remarkably steady increase” and that Sunday’s incident should serve as a warning to the plant operators.
Between 1982 and 2014, the bay’s summer temperatures warmed by an average of about 3.5 degrees, five times the average warming rate of the planet’s oceans.
The bay’s summer temperature appears to be increasing even more in the most recent years. Between 2009 and 2013, the average temperature warmed more than 6 degrees.
The temperature of the bay was recorded at 66 degrees on Sunday, but on July 30, a federal gauge on the bay registered the temperature as 72.4 degrees, a record for that day and less than a degree below the highest temperature ever recorded on the bay, according to the National Oceanic and Atmospheric Administration.
“If the plants were designed to expect temperatures at a certain level, they’re going to see warmer temperatures,” he said. “They’re going to have to move their intake pipes into deeper water or shut down.”
But Dave Noyes, director of regulatory and performance improvement at Pilgrim, said the temperature surge was the result of “very specific conditions” involving winds and tides.
“I don’t expect this to happen more often,” Noyes said.
Officials at Pilgrim have been considering asking the Nuclear Regulatory Commission to allow the plant to raise the maximum allowable temperature of its sea water.
In 2012, Millstone Nuclear Power Plant in Connecticut shut down because of excessive sea-water temperatures, the only other plant to do so in the Northeast. Since then, after Millstone officials cited the increasing temperature of where it draws its water from in Long Island Sound, the commission allowed the plant to increase the temperature limit of its sea water from 75 degrees to 80 degrees.
If a plant’s water temperature exceeds federal limits for more than 24 hours, the commission requires it to shut down.
“We’re pursuing the temperature increase for economic reasons, so we wouldn’t have to reduce power under specific conditions,” Noyes said.
Critics of Pilgrim said that raising the temperature limit could compromise safety. They also cast doubt on Entergy’s explanation for last weekend’s power cut.
“We’ve always had tides and wind; it’s clear the water temperatures are increasing as a result of climate change,” said Mary Lampert, director of Pilgrim Watch, a group that has long sought to close the plant. “Climate change means these reactors cannot be considered reliable sources of energy, because when the water temperature increases, they have to be powered down.”
Nuclear Regulatory Commission officials said Pilgrim’s sea-water temperature limit was set when the plant was licensed in June 1972, and that Entergy has yet to file a request to raise the limit.
“The limit is important, because water temperatures above that can have a bearing on the heat-removal efficiency of the plant’s condenser,” said Neil Sheehan, a spokesman for the NRC. Power cuts have occurred for similar reasons at plants in other regions of the country, he said.
He added: “Water temperatures above that do not inherently pose any sort of increased safety risk, but their impact on the condenser’s cooling capabilities has not been analyzed.”
Sheehan said the commission would study whether climate change had played a role.