An energy company says it's investigating why a reactor at a central Pennsylvania nuclear power plant unexpectedly shut down.

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.

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

The company says Unit 1 is operating normally.

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