For those joining in the middle of this conversation, the NNSA (National Nuclear Security Administration) is the part of the Department of Energy that deals with nuclear weapons.
I don't see an explicit statement of mission at the NNSA website, although I would be surprised if a government agency didn't have such a group of words somewhere.
One can be obvious and say that the mission of NNSA is to maintain and certify the nuclear stockpile. But that doesn't tell us much of what I suspect tbw is asking about.
The mission of the nuclear weapons complex during the Cold War was to keep ahead of the Soviets. That meant more and better. When Gorbachev became First Secretary of the Communist Party, and it turned out that Ronald Reagan was a closet nuclear abolitionist, that got toned down. When the Soviet Union collapsed, the potential uses of nuclear weapons were no longer obvious.
The military never was all that crazy about nukes, either. The Air Force liked that they justified fancy missiles and bombers, but the other services tended to see them more as a liability to themselves. Too much standoff distance for the Army if you wanted to lob one at the other guy.