The Comprehensive Test Ban Treaty "bans all nuclear explosions in all environments, for military or civilian purposes", which presents quite a difficulty for American scientists developing a new nuclear warhead.
One of the assurances given by defense officials to Congress is that the new warhead will not have to undergo actual testing. Once developed, it would be used in the Trident missiles on submarines and eventually would replace warheads on the Air Force's missile arsenal, officials said. ...
Of overriding concern to members of Congress has been that the warhead be developed without the need for underground tests. The administration has sought to assure Congress that the design would not require such testing.
That's ridiculous, and especially so because America hasn't ratified the CTBT. We've got plenty of desert land to use for testing, and it's pretty foolish to not perform a full suite of tests on our new nuclear arsenal. Congress' insistence on this matter has weakened our national security substantially, not least because it almost forced the approval of a proposal based on an older warhead design that was tested in the 1980s rather than a new, untested technology.