The Unit 1 reactor at the Susquehanna nuclear power plant in Luzerne County, Pennsylvania, has returned to service.

Last week, the plant’s operator PPL Corp shut down this unit for a maintenance work on a valve, which is part of a backup system that can provide cooling water to the reactor in the unlikely event normal systems became unavailable.

PPL chief nuclear officer Timothy Rausch said that in addition to working on the valve, plant personnel did a number of other maintenance tasks that can only be done when the unit is shut down.

The Unit 2 at the power plant continues to operate at full power, the company said.

The Susquehanna plant is owned jointly by PPL and Allegheny Electric Cooperative.