A radiological survey found no detectable contamination, and officials said contamination was not detected outside the affected facility.
Employees not involved in the response or shutdown operations were told to shelter in place, while vehicle entrances closed and rail, river and air traffic around the site were restricted.
The federal complex in Aiken County was built in the 1950s to produce tritium and plutonium for nuclear weapons and now handles radioactive waste disposal and environmental remediation.