[This fact sheet has been updated; see current version.]
Key Point: The CTBT is a “zero-yield” treaty.
This means that the agreement prohibits all nuclear explosions that produce a self-sustaining, supercritical chain reaction of any kind whether for weapons or peaceful purposes. The decision not to include a specific definition of scope in the Treaty was a deliberate decision by the negotiating parties, including the United States, made to ensure that no loopholes were created by including a highly technical and specific list of what specific activities were and were not permitted under the Treaty. A thorough review of the history of the Treaty negotiation process, as well as statements by world leaders and the negotiators of the agreement, shows that all states understand and accept the CTBT as a “zero-yield” treaty.
At the time the Treaty opened for signature, all parties understood that the Treaty was a “zero-yield” treaty as advocated by the United States in the negotiations. The United States led the efforts to ensure the Treaty was a “zero-yield” treaty, after the parties had negotiated for years over possible low levels of testing that might be allowed under the agreement. The evolution of this position, along with public statements by national leaders, confirmed that all parties understood that the CTBT was and is, in fact, a “zero-yield” treaty. For example, former Russian President Boris Yeltsin confirmed publicly in 1996 that the Treaty banned nuclear explosions of any magnitude. [For more official P-5 statements on the scope of the CTBT, click here.]
The United States decided, at the outset of negotiations on a CTBT, that it was unnecessary and potentially problematic to seek to include a definition in the Treaty text of “a nuclear weapon test explosion or any other nuclear explosion.” Other governments participating in the negotiations agreed. Moreover, Treaty negotiators followed the precedent of the 1963 Limited Test Ban Treaty (LTBT) that used this same approach, and it remains effective to this day. The LTBT has been in force for nearly fifty years, and the language has never been at issue in its implementation.
The United States and other CTBT signatories have a clear understanding of what is prohibited by the CTBT. Once the Treaty enters into force, we will have the opportunity to hold other States Parties accountable to this standard of Treaty obligations when judging compliance. Under the CTBT, supercritical hydronuclear tests (which produce a self-sustaining fission chain reaction) are banned by the Treaty, but subcritical hydrodynamic experiments, which do not produce a self-sustaining fission chain reaction, are permitted. These decisions were made to ensure that the Treaty did, in fact, ban all nuclear weapons testing, but permitted the United States to conduct activities essential to maintaining a safe, secure and effective nuclear arsenal without explosive nuclear tests. Thus, the Treaty does not prohibit subcritical experiments to help ensure the continued safety and reliability of nuclear weapons.