Washington, D.C. ? The U.S. Energy Department cannot meet its own post-Sept. 11 security standards to repel a terrorist force at the Fort Knox of uranium, a facility in Tennessee that stores an estimated 189 metric tons of bomb-grade material, agency officials acknowledged.

The material is stored in five masonry and wood frame buildings at the Y-12 facility, a key part of the nation’s nuclear weapons infrastructure at the Oak Ridge site near Knoxville.

The National Nuclear Security Administration, a part of the Energy Department, is building a secure facility to warehouse the material that is due to be completed in 2009. Until then, the Energy Department has given itself a waiver on meeting security requirements at the site.

At risk in an attack is terrorists gaining access to highly enriched uranium and then within minutes constructing a crude but powerful improvised nuclear device. It is believed such a device could have a yield equal to the Hiroshima atomic bomb.