Bay water used to cool reactor exceeded temperature limit Monday.

PLYMOUTH — A stretch of hot weather has again affected operations at Pilgrim Nuclear Power Station, marking the third summer out of the last four that the plant has been forced by excessive water temperatures to power down.

Temperatures of the Cape Cod Bay seawater used to cool the reactor and turbine exceeded the maximum allowed under federal standards Monday afternoon.

This is only the third summer in the plant’s 44-year history that excessively hot bay water has caused a slowdown at Pilgrim. Previous occurrences were in July 2013 and August 2015.

Pilgrim draws 500 million gallons from the bay every day to cool its systems through a network of thousands of tubes.

Its license caps the water intake temperature at 75 degrees Fahrenheit. It can be up to 30 degrees hotter when it is returned to the bay.

Depending on wind direction, some of the hot water exiting the plant may mingle with water being drawn in, raising its temperature, said Neil Sheehan, spokesman for the Nuclear Regulatory Commission — a scenario that likely occurred Monday.

“In other words, it’s a function of both the bay water temperature and other environmental conditions,” Sheehan said.

The excessive temperatures lasted for only one hour, but Pilgrim remained at 65 percent power until 2 a.m. Tuesday while it monitored bay intake.

Operators then reduced power further, to 50 percent, to perform a planned flushing of algae and other marine matter from the plant’s condenser.

Apparently the warmer water also had affected performance in the condenser, Sheehan said, which prompted operators to lower power even further to 40 percent. “The condenser is run under vacuum conditions to maximize efficiency,” Sheehan said. The warmer bay water can impair the condenser’s ability to cool steam generated by the reactor, he said.

Global warming also may play into warming bay waters.

An expert at the Gulf of Maine Institute said Cape Cod Bay, and more broadly the Gulf of Maine, have experienced ocean temperature increases since 1982 at six times the global average, and they have started to rise in even larger increments in recent years.

“Prior to 2009, the maximum average temperature in the bay never exceeded 70 degrees F,” wrote Andrew Pershing, the institute’s chief scientific officer, in an email. “Beginning in 2009, we started seeing temperatures that were more than 2 degrees F above that. At any given time, the temperature at a precise location like the power plant will be warmer or colder than the average, so it’s not surprising that they are increasingly encountering temperatures above the 75-degree F limit.”

Mary Lampert, director of the citizens group Pilgrim Watch, sees irony in the effect climate change is having on nuclear reactors.

“The nuclear industry incorrectly claims that nuclear power is the answer to climate change, but climate change brings warmer sea water temperature and this means that the reactor must shut down when the bay heats up,” Lampert wrote in an email. “On the other hand, when water temperatures get hot, truly clean sources of electricity — wind, solar, hydro, tide — operate just fine.”

Some plant owners have successfully applied to raise the federal 75-degree limit for intake water.

Millstone Nuclear Power Plant in Connecticut, which draws water from Long Island Sound, was the first in the Northeast to shut down because of rising water temperatures. Millstone has since successfully applied for an increase to 80 degrees for its intake water.

“Entergy has not to date applied for a license amendment that would allow for an increase in the intake water temperature limit,” Sheehan said.

David Lochbaum, director of the Nuclear Safety Project for the Union of Concerned Scientists, said it was probably not worth the cost to increase the maximum limit. The plant is due to shut down permanently in June 2019.

“Entergy would have to pay (at more than $200 per hour) for the NRC staffers reviewing their request,” wrote Lochbaum in an email. “So, it would cost Entergy a lot of money to increase the limit. The return on that investment is limited when the reactor only operates a few more years.”

Patrick O'Brien, spokesman for Entergy, said the company does not publicly release when the plant will return to full power since it is market-sensitive information.

— Follow Christine Legere on Twitter: @chrislegereCCT.