The company said the plant has been shut down in order to perform maintenance work on a valve in an area of the plant.
The system containing the valve is one of several that can provide cooling water to the reactor in the unlikely event normal systems became unavailable.
The Unit 2 at the Susquehanna plant continues to operate at full power.
The Susquehanna plant, located in Luzerne County about seven miles north of Berwick and is operated by PPL Susquehanna.
The nuclear facility is jointly owned by PPL Susquehanna and Allegheny Electric Cooperative.
