BERWICK, Pa. (AP) â?? An energy company says it's trying to figure out why a central Pennsylvania nuclear power plant reactor shut down unexpectedly.

PPL Corp. says Unit 2 at the Susquehanna nuclear facility near Berwick shut down early Sunday morning. The company says the reactor is safe and stable.

In a statement, Allentown-based PPL says the shutdown occurred during routine testing of a valve on the unit's main turbine system.

The company says the plant's other reactor is operating normally.

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