US-based utility Entergy is to decrease output at its 620MW Vermont Yankee nuclear power station while it repairs a leak.
The leak, which is allowing river water into the plant's condenser system, could take several weeks of planning to locate according to the company.
An increase in chloride concentration in the reactor water indicates that river water is finding its way into the main condenser used to cool steam from the turbine for reuse in the reactor.
Entergy said that there is no water leakage from reactor water to the outside environment.
Vermont Yankee station is located in Vernon Vermont, around 80 miles north of Hartford, Connecticut and entered service in 1972.
1MW powers around 1,000 homes in Vermont.