Published: Aug 18, 2012 3:30 PM EDT
Updated: Aug 19, 2012 6:30 AM EDT

OAK RIDGE, Tenn. (AP) - Officials like to refer to the Y-12 National Security Complex as the Fort Knox for the highly enriched uranium used in nuclear weapons.

That's why an unprecedented incursion last month by an 82-year-old nun and two fellow protesters has critics mocking the notion that the weapons plant is secure.

The Department of Energy has called on the contractor that runs the sensitive facility just west of Knoxville to explain why it shouldn't be replaced.

Y-12 makes uranium parts for every warhead in the U.S. nuclear arsenal and is the nation's primary storehouse for bomb-grade uranium.

Officials insist that despite the more than two hours that the protesters went unchallenged on the facility, there was never any danger of them accessing sensitive materials.