The Savannah River Site placed its H Area on a "phase II" security alert and personnel were told to remain in their offices or labs until further notice.
A nuclear site in South Carolina was reopened from more than two hours in lockdown on Monday, after a bomb-sniffing dog over-reacted and halted a delivery truck, the Department of Energy said in a statement.
No explosive residue or device was found on the truck after a search by law enforcement, the Savannah River Site said in a statement on its Facebook page.
The dog had barked at a truck that services vending machines, said an official, who spoke on condition of anonymity.
The facility, which was put in lockdown for a "potential security event" following the alarm, purifies highly enriched uranium and is part of the DoE's nuclear arm.
The Savannah River Site placed its H Area on a "phase II" security alert and personnel were told to remain in their offices or labs until further notice, according to an internal memo seen by Reuters, sent around 3:30 p.m. local time (1930 GMT).
The H Area is where highly enriched uranium is blended down to produce low enriched uranium for use in commercial reactors.
The all-clear was given and the plant returned to normal at 5:52 p.m., the site's latest Facebook post said.
Law enforcement agencies from South Carolina and Georgia had been called to the scene, the site said in an earlier statement.
The government-run plant was constructed in the 1950s to produce basic materials for nuclear weapons, according to a DoE website, and produced one third of US weapons-grade plutonium from 1953 to 1988.
Two of the Savannah River Site's 5 reactors have been deactivated. It now blends down enriched uranium for use in commercial reactors. It also is used to store spent fuel. About 36 million gallons (136 million liters) of radioactive liquid are stored there in 49 underground tanks.
© Thomson Reuters 2015