The worker was stopped and detained at the entrance of the Palo Verde Nuclear Generating Station, said U.S. Nuclear Regulatory Commission spokesman Victor Dricks.
Security officials then put the nuclear station on lockdown, prohibiting anyone from entering or leaving the facility.
Authorities described the device as a small capped pipe that contained suspicious residue.
Capt. Paul Chagolla with the Maricopa County Sheriff's Office said sheriff's officials have rendered the device safe and that investigators were interviewing the worker.
The plant was operating normally and there was no threat to the public, Palo Verde spokesman Jim McDonald said.