For the first time in its history, was forced to shut down Unit 2 on Sunday because the water in Long Island Sound was too warm.
The nuclear power plant uses water from Long Island Sound to cool “key safety components,” said Millstone Spokesman Ken Holt. However, if the water in Long Island Sound rises to 75 degrees, the Nuclear Regulatory Commission states the water is too warm to use, and mandates the plant be shut down, Holt said.
That happened for the first time on Sunday, after the water in Long Island Sound had an average temperature of 75 degrees for a 24-hour period for the first time in Millstone's history, Holt said. It is the first time Unit 2 has ever had to shut down for that reason, he said.
Water from Long Island Sound is used to cool a variety of systems at the power plant, including the emergency diesel generators, the control building air conditioner and water chillers, turbine plant component cooling water heat exchangers, charging pump coolers and reactor plant component water heat exchangers.
"By shutting down the unit you put yourself in a better position to be able to respond if you have an emergency," said Holt.
Unit 3 is still operational because it is a different design and it pumps in deeper water, Holt said. Deeper water is generally colder and the deeper water has not yet reached 75 degrees, he said.
However, that could change if the weather stays warm and the water continues to heat up, Holt said. Unit 2 will remain closed until the water temperature drops below 75 degrees and stays there, he said.
"It was a normal shut down. The plant is safe and ready to return to service once water temperatures are back at a level that would support operation," said Holt. "You don’t want to restart the unit and then have to shut it down again.”
The closure will hurt Millstone’s earnings, as it is producing no energy from Unit 2, Holt said. Millstone is trying to figure out a way that the NRC will allow them to have Unit 2 run with water temperatures above 75 degrees, he added. The NRC set the regulations governing water temperature back in the 1970s based on average sea water temperatures at that time.
Millstone's Unit 1 reactor was decommissioned in 1998.