The plant went on lockdown and into a heightened state of security.
Two people were seen arguing in the car and, when security officers approached, the bomb threat was made.
Authorities believe it was a domestic incident not related to the plant. No explosives were found.
The car was on a public road near the plant's entrance.
Police said two people were detained, and Ryan Raymer was arrested on an outstanding warrant of communicating threats.
A plant spokesman said neither person was an employee of the plant.