In partnership with the Department of Defense  , NNSA’s Defense Programs provides the research, development, secure transportation, and production activities necessary to support the U.S. nuclear weapons stockpile.
Following the end of the Cold War, the United States discontinued production of new nuclear warheads and voluntarily ended underground nuclear testing. Today, NNSA uses and oversees a wide-range of breakthrough science experiments, engineering audits and high-tech computer simulations, including extensive laboratory and flight tests of warhead components and subsystems, to keep the existing warheads reliable, secure and safe. Every year, the Secretary of Energy is able to certify the reliability of the stockpile without conducting an underground nuclear test. NNSA’s use of science-based research and development in the absence of underground nuclear testing in order to maintain the Department of Energy’s  portion of the nation’s nuclear deterrent is known as the Stockpile Stewardship Program.
NNSA’s nuclear weapons activities are carried out in a nationwide network of government-owned, contractor-operated national security laboratories, test site, and nuclear weapons production sites. These sites, collectively known as NNSA’s nuclear security enterprise, provide the necessary research, development and production capabilities needed to maintain the reliability, security and safety of the weapons stockpile.
Part of keeping the U.S. nuclear weapons stockpile safe and reliable includes working with the Department of Defense to maintain the quantity and quality of weapons necessary for U.S. national security needs. The New START Treaty, which was signed by President Obama and ratified by the US Senate in 2010, between the United States and Russian Federation, will cap the strategic deployed nuclear arsenals of each country at 1,550 warheads. As a result of these actions, the stockpile is the smallest it has been since the Eisenhower administration. NNSA’s Defense Programs is actively working to meet the reduced stockpile quantity levels by safely dismantling and disposing of those nuclear weapons that have been designated in excess of U.S. national security needs.
In addition to maintaining the actual weapons, NNSA is also focused on the security and safety of the weapons. Robust security protects weapons and weapons material both at each facility and through securely transporting materials and weapons between facilities and military locations. NNSA also strives to conduct operations in ways that are safe for the environment and the public.
As threats against the country evolve and become more and more unpredictable, and especially as the current weapons in the U.S. nuclear stockpile age and become increasingly difficult and expensive to maintain, NNSA is working now to transform itself and revitalize the entire nuclear weapons enterprise to be smaller, safer, more secure and more efficient. NNSA must be better able to quickly respond to technical problems in the stockpile and be able to rapidly respond to unforeseen national security needs.