Guards stopped Roger William Hurd, 61, at the entrace to the Palo Verde Nuclear Generating Station when they spotted the bomb. He was detained about a half mile from the containment domes where the nuclear material is stored. Officials pulled his security clearances and placed the facility on lockdown.
"The guards were attentive and alert and took the appropriate action when they identified something suspicious," said Victor Dricks, a spokesman for the U.S. Nuclear Regulatory Commission, which will review the incident.
Doug Walters, senior director of security for the Nuclear Energy Institute, the industry's trade group, said Palo Verde's response was exactly as it should have been.
"We have a checkpoint for this reason," Walters said. "They were able to identify a suspicious item in the truck. I don't know what they could have done differently."
The Department of Homeland Security said there was no known terrorism link to the incident at Palo Verde. Hurd, of Goodyear, Ariz., told investigators he did not know how the bomb got in his truck and was released Friday afternoon.
Authorities described the device as a six-inch capped explosive made of galvanized pipe that contained suspicious residue. Tom Mangan, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said it was likely homemade.
"If this thing went off in the bed of the truck, it certainly would put a hole in it," Mangan said. "It was rather crude in construction, but it could certainly injure somebody."
Capt. Paul Chagolla with the Maricopa County Sheriff's Office said the pipe was not hidden. He said Hurd normally drove a motorcycle to work but was in a truck Friday because of the cool weather.
Sheriff's officials rendered the device safe.
Maricopa County Sheriff Joe Arpaio said investigators searched Hurd's home but found nothing helpful. Hurd was not arrested, and Arpaio said he expects Hurd to help with the investigation.
Nobody answered the door at Hurd's apartment in Goodyear. Messages left by The Associated Press at numbers listed for Hurd in Arizona and Hartsville, S.C., were not returned as of late Friday night.
Hurd worked as a procurement engineer, responsible for evaluating equipment purchases for the plant, Palo Verde officials said.
He had access to protected areas but had not been in any such area since Aug. 21, said Randy Edington, the chief nuclear officer for plant operator Arizona Public Service Co.
Edington said Hurd would have had access to the reactors but officials did not know the last time he would have been near the reactors.
The incident was considered an "unusual event" - the lowest of four emergencies the plant can declare, said Jim Melfi, an inspector with the U.S. Nuclear Regulatory Commission.
There was no threat to the public and the station operated normally Friday, McDonald said.
McDonald wouldn't say which company employed Hurd. Like everyone who has access to the plant, Hurd submitted to a background check.
Workers also must pass through two security checkpoints to get inside one of the plant's three containment domes, which house the radioactive nuclear material. One of the checkpoints includes an automated system that sniffs workers for the presence of bomb-making materials, McDonald said.
Palo Verde, operated by Arizona Public Service Co., is the nation's largest nuclear power plant both in size and capacity. Located in Wintersburg about 50 miles west of downtown Phoenix, the plant supplies electricity to about 4 million customers in Arizona, New Mexico, Texas and California.