50 USC 2401 - Establishment and mission

(a) Establishment 
There is established within the Department of Energy a separately organized agency to be known as the National Nuclear Security Administration (in this chapter referred to as the Administration).
(b) Mission 
The mission of the Administration shall be the following:
(1) To enhance United States national security through the military application of nuclear energy.
(2) To maintain and enhance the safety, reliability, and performance of the United States nuclear weapons stockpile, including the ability to design, produce, and test, in order to meet national security requirements.
(3) To provide the United States Navy with safe, militarily effective nuclear propulsion plants and to ensure the safe and reliable operation of those plants.
(4) To promote international nuclear safety and nonproliferation.
(5) To reduce global danger from weapons of mass destruction.
(6) To support United States leadership in science and technology.
(c) Operations and activities to be carried out consistent with certain principles 
In carrying out the mission of the Administration, the Administrator shall ensure that all operations and activities of the Administration are consistent with the principles of protecting the environment and safeguarding the safety and health of the public and of the workforce of the Administration.