A vehicle trying to enter a secure area at the Savannah River Site set off explosive detection devices, triggering an emergency response and lockdown at the nuclear reserve Tuesday morning.
The vehicle was extensively inspected, according to a Savannah River Site announcement, and was later cleared.
The incident happened at L-Area, where spent nuclear fuel is kept, people familiar with the matter said.
The site's emergency status website described it as a "security event," noting that all vehicles were being searched.
Entrances and exits at SRS, about 30 minutes south of Aiken, were closed off at one point.
Normal operations, officials said, have since resumed.