Nuclear operations were temporarily halted at a Tennessee complex that stores and processes uranium after three protesters were able to intrude into a high-security area over the weekend.
The Y-12 National Security Complex said all nuclear material is safe.
The temporary stand-down was expected to end by next week. Special nuclear material will be moved to vaults on site, and contractor security personnel will undergo training and refresher instruction.
The Oak Ridge nuclear complex regularly gets protesters and activists to the site around the August anniversaries of the atomic bombings of Hiroshima and Nagasaki during World War II.
According to its website, Y-12 maintains the safety, security and effectiveness of the U.S. nuclear weapons stockpile. It also provides nuclear fuel for the Navy and for research reactors worldwide.