In the morning of July 28, 2012 three activists from the "Transform Now Plowshares" group gained access to the Highly Enriched Uranium Materials Facility (HEUMF) building, which is part of the Y-12 Complex in Oak Ridge. The activists were able to penetrate the facility's security perimeter and spray-paint a message on the exterior of the HEUMF building. There are conflicting accounts of how much time the activists were able to spend inside of the security perimeter before being arrested by the security guards and whether the intrusion was properly detected by the Perimeter Intrusion Detection and Assessment System (PIDAS).

On August 1, 2012, the contractor that operates the Y-12 facility, B&W, ordered a security stand-down, during which ""all special nuclear materials will be moved to vault-type facilities on site, all nuclear operations will be halted, and contractor security personnel will undergo training and refresher instruction." The stand-down is expected to last about a week.

HEUMF is a new facility that was open in January 2010.