Verification of the CTBT

Detecting a nuclear weapon test explosion has become so effective that no would-be cheater could be confident that a nuclear explosion that would adversely affect  U.S. security would escape detection, thereby providing a reliable deterrent against cheating.

Verification capabilities will be even greater with the CTBT in force, which would include its global verification and monitoring network, as well as the option of short-notice on-site inspections (OSI), performed by the CTBTO. Through these mechanisms, the CTBT gives the U.S. tools otherwise unavailable to resolve compliance concerns and respond effectively to potential nuclear testing by other states.
The CTBT establishes a far-reaching International Monitoring System (IMS) to detect potential nuclear explosions using four technologies – seismic, hydroacoustic, radionuclide, and infrasound, including a new array of highly-capable “noble gas” monitoring stations that can detect minute amounts of the radioactive gases emitted by underground explosions into the atmosphere. More than 80% of the planned test ban monitoring network is operational and nearly an additional 10% is being built.
The CTBT provides for monitoring stations inside Russia, China, and other sensitive locations, including some places where the United States could not gain access on its own. (Click here to explore the CTBTO’s interactive map of IMS stations.)
The International Data Center (IDC) based in Vienna collects and analyzes information from the IMS and disseminates the raw and processed data to member states for their own evaluation.

The treaty also allows the United States and other member states to monitor CTBT compliance by using their own, highly-sophisticated satellites and other national technical means. Tens of thousands of high-quality civilian seismic stations around the world provide further detection capabilities.
Today, nuclear test explosion monitoring capabilities are excellent and getting better. During the Senate debate on the CTBT in 1999, some critics claimed that the IMS could only monitor for underground explosions at yields at or above one kiloton TNT equivalent. In reality, actual nuclear test explosion monitoring capabilities were much better and have only improved since then.
A 2002 National Academy of Sciences (NAS) panel determined that “underground nuclear explosions can be reliably detected and can be identified as explosions, using IMS data down to a yield of 0.1 kilotons (100 tons) in hard rock if conducted anywhere in Europe, Asia, North Africa and North America.”
Advances in regional seismology have provided additional confidence. For some locations (such as Russia’s former nuclear test site at Novaya Zemlya) the use of new seismic arrays and regionally located seismic stations has lowered the detection threshold to below 0.01 kilotons (10 tons).
North Korea’s nuclear tests in 2006 and 2009 demonstrated that the CTBT verification system is working well and can detect very small explosions. When North Korea conducted its first nuclear weapon test explosion in October 2006, the extensive network of sensors detected the relatively low-yield (0.6 kiloton) blast at 31 seismic stations in Asia, Australia, Europe, and North America.
No would-be cheater could be confident that a nuclear explosion that could possibly threaten U.S. security would escape detection. Although very low-yield nuclear tests cannot be detected with absolute certainty, countries that might be able to carry out such clandestine testing already possess advanced nuclear weapons and could add little, with additional testing, to the threats they already pose to the United States. Countries of lesser nuclear test experience and/or design sophistication would be unable to conceal tests in the numbers and yields required to develop advanced warheads. It is strongly in the U.S. interest to stop all other states from nuclear testing.
The CTBT would provide, for the first time, the option of short-notice, on-site inspections in response to signs of a suspicious event. In the event of a suspected nuclear explosion that cannot be resolved by remote sensing means, CTBT states may call for a short-notice, on-site inspection (OSI) of a suspected test site—an important deterrent against potential clandestine nuclear testing.
During the Senate debate in 1999, some critics complained that because the CTBT requires 30 of 51 nations on its Executive Council to agree to an on-site inspection, states unfriendly to the United States could block such inspections. In reality, the CTBT’s on-site inspection provisions were established to balance the need for rapid response to a suspected test against the possibility of “frivolous or abusive” inspections. The approval of 30 out of 51 members of the CTBTO’s Executive Council was designed to give nations like the United States confidence that OSIs would be approved as needed, but not by a small minority with questionable motives.
To protect national security interests unrelated to the inspections, states are allowed to restrict access to parts of the inspection area no larger than four square kilometers or a total of no more than 50 sq. km. However, if an inspected state restricts access it must provide alternative ways for the inspection team to carry out its mission. If the bar for inspections had been set much lower, or if no allowances had been made for unrelated national security interests, one could imagine that there might be concerns in the Senate that CTBT on-site inspections could unduly infringe on U.S. sovereignty.
The CTBT Bans all nuclear test explosions. Some CTBT critics erroneously believe that because the treaty does not define a "nuclear test explosion,"  some countries, such as Russia, consider hydronuclear tests (which are in fact very low-yield nuclear explosions) to be a "permitted" activity under the treaty.
In reality, the negotiating record of the treaty is clear that all nuclear explosions (any energy yield from a self-sustaining chain reaction) are prohibited by the CTBT. The Russian government made it clear when it ratified the CTBT in 2000 that: “Qualitative modernization of nuclear weapons is only possible through full-scale and hydronuclear tests with the emission of fissile energy, the carrying out of which directly contradicts the CTBT.” In other words, the CTBT establishes a “zero-yield”  worldwide prohibition on nuclear test explosions.