All nuclear operations have been halted at the Y-12 National Security Complex in Oak Ridge, a federal spokesman said Wednesday.

The unprecedented security “stand down” occurred after three activists breached the plant’s high-security area on Saturday and after an internal review by B&W Y-12 and the National Nuclear Security Administration found procedural violations, NNSA spokesman Steven Wyatt said.

“Once we found those, B&W decided it was prudent to stand down operations,” Wyatt said. B&W Y-12 manages and operates Y-12 for the NNSA.

The security stand down started Wednesday, and it is expected to end next week.

Wyatt declined to discuss details of the procedural violations, including the number and who committed them.

He said the nuclear operations that are being halted include a stockpile life-extension program, dismantlement and storage work, and movement of materials.

In addition to a temporary stop in nuclear operations, all special nuclear materials will be moved to vault-type facilities at Y-12, and contractor security personnel will undergo training and refresher instruction, according to a Wednesday afternoon press release from the NNSA.

Wyatt said the Highly Enriched Uranium Materials Facility, or HEUMF, stores most of the plant’s enriched uranium—bomb-grade uranium—but other Y-12 sites are used as well.

Officials are reviewing the impact of the security stand down on the nation’s naval nuclear program, Wyatt said.

He would not discuss whether anyone has been held accountable for Saturday’s security breach. In that incident, three anti-nuclear weapons activists allegedly sneaked through four fences at the Y-12 National Security Complex and into a high-security area before dawn Saturday and spray-painted messages and splashed human blood on the walls of the HEUMF before they were detained by security guards.

“It really would be not be appropriate for us to discuss disciplinary or employment information,” Wyatt said.

The NNSA release said the security stand down was ordered to address additional security training and execution deficiencies identified by the contractor after Saturday’s incident.

It said all nuclear materials at Y-12 are “in safe, secure storage, and we remain entirely confident in the security of Y-12’s facilities.”

Wyatt said there was a previous safety-related stand down at Y-12 in 1994 and an operational stand down after the Sept. 11, 2001, terrorist attacks.