Terry Johnson, a spokesman for the Tennessee Valley Authority, which operates the plant, said a valve that regulates water flow into a steam generator unexpectedly closed Wednesday, leading operators to manually shut down the unit, the Knoxville (Tenn.) News Sentinel reported Thursday.

Johnson said workers are investigating the cause of the incident.

The pressurized-water reactor at the plant utilizes a nuclear reaction that heats up water under high pressure in a reactor vessel. The water then travels through steam generator tubes that boil feedwater to produce steam. The steam then turns the electricity-generating turbines.

The valve that closed was designed to control feedwater flow into the steam generator.