The National Nuclear Security Administration (NNSA), moving to address security concerns at the California nuclear weapons laboratory, disclosed this week that another 20 percent of the plutonium and high-enriched uranium stored at Lawrence Livermore National Laboratory has been shipped to other Energy Department sites.

NNSA, the semi-autonomous DoE agency that oversees the department’s nuclear weapons complex, said that more than 55 percent of the lab’s weapons-usable nuclear materials have now been moved to the department’s Savannah River Site in South Carolina and its Y-12 plant at Oak Ridge, Tenn.

NNSA began its “de-inventory” initiative at Livermore in 2006 in response to concerns that the San Francisco Bay area lab is particularly vulnerable to terrorists. Livermore is located amid residential neighborhoods that could easily provide cover to an attacking force, allowing it to get close to the lab’s fences before they are detected.

Other DoE weapons sites are located in isolated, rural areas that would be harder to attack, though some security experts have suggested the Y-12 site also may have vulnerabilities.

NNSA plans to remove all high-risk nuclear materials from Livermore by 2012, two years earlier than its initial deadline of 2014. While some critics have questioned why NNSA cannot move faster at Livermore, the agency says some of the nuclear materials must be processed into safer, more stable forms so they meet federal shipping and storage safety standards.

The Livermore initiative is part of a broader NNSA plan to reduce the number of sites storing weapons-usable nuclear materials, which require hugely expensive security measures. NNSA plans to consolidate nuclear materials at five sites by 2012, and to significantly shrink the size of those sites by 2017 so security costs can be minimized.