The nuclear power plant was shuttered on 12 August after record heat in July contributed to overheated water from Long Island Sound.
Millstone Power Station spokesman Ken Holt told Associated Press that the Unit 2 returned to 100% power on 25 August.
"The water temperature cooled sufficiently to support operations and that, combined with the weather forecast, has given us the confidence to restart," Holt said.
The water temperature, which was averaging 1.7 degrees above the 75-degree limit, has since dropped to 72 degrees.
Dominion Resources subsidiary Millstone has been allowed to use an average temperature of several readings, following an emergency license amendment last week by The US Nuclear Regulatory Commission, a federal agency
The nuclear power plant provides half of all power in Connecticut and 12% in New England.
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