Acording to this Wikipedia article
The United States has stated that its ratification of the CTBT is
A: The conduct of a Science Based Stockpile Stewardship Program to
ensure a high level of confidence in the safety and reliability of
nuclear weapons in the active stockpile, including the conduct of a
broad range of effective and continuing experimental programs.
B: The maintenance of modern nuclear laboratory facilities and
programs in theoretical and exploratory nuclear technology which will
attract, retain, and ensure the continued application of our human
scientific resources to those programs on which continued progress in
nuclear technology depends.
C: The maintenance of the basic capability to resume nuclear test
activities prohibited by the CTBT should the United States cease to be
bound to adhere to this treaty.
D: Continuation of a comprehensive research and development program to
improve our treaty monitoring capabilities and operations.
E: The continuing development of a broad range of intelligence
gathering and analytical capabilities and operations to ensure
accurate and comprehensive information on worldwide nuclear arsenals,
nuclear weapons development programs, and related nuclear programs.
F: The understanding that if the President of the United States is
informed by the Secretary of Defense and the Secretary of Energy (DOE)
– advised by the Nuclear Weapons Council, the Directors of DOE's
nuclear weapons laboratories and the Commander of the U.S. Strategic
Command – that a high level of confidence in the safety or reliability
of a nuclear weapon type which the two Secretaries consider to be
critical to the U.S. nuclear deterrent could no longer be certified,
the President, in consultation with Congress, would be prepared to
withdraw from the CTBT under the standard "supreme national interests"
clause in order to conduct whatever testing might be required.