C.M. Glover for The New York Times

A reactor at the Millstone nuclear plant in Waterford, Conn., has shut down because of something that its 1960s designers never anticipated: the water in Long Island Sound was too warm to cool it.

Under the reactor’s safety rules, the cooling water can be no higher than 75 degrees. On Sunday afternoon, the water’s temperature soared to 76.7 degrees, prompting the operator, Dominion Power, to order the shutdown of the 880-megawatt reactor.

“Temperatures this summer are the warmest we’ve had since operations began here at Millstone,’’ said a spokesman for Dominion, Ken Holt. The plant’s first reactor, now retired, began operation in 1970.

The plant’s third reactor was still running on Monday, but engineers were watching temperature trends carefully out of concern that it, too, might have to shut down.

A spokeswoman for the regional grid control center, ISO-New England, said the shutdown had not impaired the functioning of the grid because generation has been more than sufficient. But in periods when industrial demand for electricity has been stronger, a reactor shutdown has sometimes forced grid operators to scramble.



The water from the sound is piped into the plant to absorb heat from pumps and other pieces of equipment. As the sound’s temperature inched upward this summer, Dominion Power received permission from the Nuclear Regulatory Commission to measure it at three locations instead of one and to calculate the average in the hope that it would be lower. That did not help on Sunday.

And higher water temperatures could lie ahead. The sound’s temperature usually does not peak until late August.

Eventually, engineers could change the Millford reactor’s intake pipe so it draws water from further below the surface, where temperatures are lower, Mr. Holt said. They could also sharpen their pencils and try to determine whether the plant can operate safely with cooling water above 75 degrees, but neither is a short-term project.

Cloud cover and the mixing of some cooler rainfall might also bring down temperatures, Mr. Holt suggested.

While some reactors in inland locations have had to reduce their power output or shut down because of warm cooling water in the past, it is unusual for coastal plants, nuclear industry officials say.

“We are evaluating our options for the future,’’ Mr. Holt said. “We don’t know, is this year an anomaly or is it the continuation of a longer trend?’

Power plants in the Midwest have also experienced problems as temperatures soared in recent weeks. In some cases, reactors shut down because the cooling water was too warm; in others, the ongoing drought had shrunken the body of water from which the cooling water is drawn, and the plant’s intake pipes were above the surface.

Last month the twin-unit Braidwood nuclear plant in Illinois needed special permission to keep operating because its cooling water pond reached 102 degrees as a result of low rainfall and high air temperatures. When Braidwood opened 26 years ago, it was designed to run at temperatures up to 98 degrees.