Waterford - As the Unit 2 nuclear reactor at the Millstone Power Station begins its sixth day of a shutdown due to record high water temperatures in Long Island Sound, engineers at the plant are beginning to look at whether the plant could be operated safely with warmer water than the current license allows.

"It's a long-term project to analyze whether we could go above 75 degrees," said Ken Holt, spokesman for Millstone owner Dominion. "It may involve some changes in the equipment."

Unit 2 was shut down Sunday when the water temperature in the intake channel on Niantic Bay rose above 75 degrees, the maximum allowed by the plant's Nuclear Regulatory Commission license.

The water is used to cool the plant, and water over 75 degrees would not cool equipment sufficiently to keep it within the margin of safety set by the current license.

The plant would need approval from the NRC to change its license to allow it to use warmer water. Unit 2 is the only reactor in the country forced to shut down due to warm water temperatures.

Water used to cool Unit 3, the larger of the two operating reactors at the site, is drawn from a deeper, colder part of the bay, so did not have to shut down.

This is the first time the plant has been shut due to water temperature since it began operating in 1975. When the plant was designed, Holt said, a 75-degree maximum "looked like a good temperature to pick," based on what were then average temperatures for the Sound.

But since 1975, average temperatures in the estuary have been warming. While this is the first time Millstone has had to deal with this issue, it may not be the last.

"A plant is designed for the area where it is located, and the temperatures are higher now than 40 years ago when the plant was designed," he said. "We're taking steps to address it. We'd prefer to be online making electricity."

Nuclear power plants in other parts of the country with higher average water temperatures typically draw in larger volumes of cooling water than the Millstone plant, said Revis James, director of the generator research sector at the Electric Power Research Institute. The larger volumes offset the smaller temperature differential between the cooling water and the plant equipment temperatures.

Temperature data collected by environmental scientists at Millstone shows average temperatures of the waters around the plant have risen by 0.67 degrees per decade since 1976, Holt said. In 1976, the annual mean temperature was 51.6 degrees, compared to 53.4 degrees in 2009.

The warming trend in Long Island Sound was also noted in an article published this month in the journal "Marine and Coastal Fisheries: Dynamics, Management and Ecosystem Science." Authors Penny Howell, fisheries biologist at the state Department of Energy and Environmental Protection, and Peter Auster, research professor in the Marine Sciences Department at the University of Connecticut's Avery Point campus, note the warming trend seen at Millstone is consistent with the temperature data from DEEP water quality monitoring stations throughout the Sound.

Their article examines the impact of the warming water on the fish in the Sound.

James, of the Electric Power Research Institute, said that while there have been no other shutdowns of U.S. plants due to warm water, the situation has occurred in other countries. In 2003, for example, a heat wave forced the shutdown of several nuclear power plants in France.

Modifying plants to run with warmer water, extending intake channels into colder water or installing equipment to chill intake water when necessary would be very expensive and complex, James said. The financial impact of having to shut down periodically when water exceeds license limits may be less than the cost of the changes. Despite overall warming water trends, it is expected to remain a rare occurrence that the temperatures exceed license limits, James said.

Plant owners, he said, "are dealing with this as it happens."

Research is being conducted into finding new ways to cool plants with reduced use of water. That, however, is being motivated more by concerns about the future availability of cooling water than about warming water trends, James said.

j.benson@theday.com

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WATER TEMPERATURE, THEN AND NOW

Temperature data collected by environmental scientists at Millstone shows average temperatures of the waters around the plant have risen by .67 degrees per decade since 1976, Holt said. In 1976, the annual mean temperature was 51.6 degrees, compared to 53.4 degrees in 2009. The mean temperature in August 1976 was 67.3 degrees, compared to 69.8 degrees in August 2009. The next year, the temperature went still higher in August. In a 2010 annual environmental report, Millstone scientists noted that April and May of that year had the warmest temperatures on record, and that the mean August temperature was 70.3 degrees.

Last year, the mean August temperature was slightly lower, 69.9 degrees, while the annual mean for 2011 was 53.8 degrees.