On September 24, 1996, the Comprehensive Nuclear-Test-Ban Treaty was opened for signature at the United Nations and was signed that day by 71 states, beginning with the United States and President Clinton. It prohibits any nuclear weapon test explosion or any other nuclear explosion, anywhere, in any environment. It has since been signed by 187 countries and ratified by 178.
It remains unfinished business. The treaty will enter into force 180 days after 44 named states have ratified it, and those 44 are the ones that had nuclear power or research reactors in 1996. As of now eight have not: China, Egypt, Iran, Israel, Russia, and the United States have signed but not ratified, and India, North Korea and Pakistan have not signed. Russia ratified in 2000. It withdrew in 2023.
The ban had been an aim since 1958 and had been blocked for forty years by the argument that testing was necessary to keep weapons safe and reliable. What broke the deadlock was the end of the Cold War, the French and Chinese test series of 1995 and 1996, which were deeply unpopular, and the review conference of the Non-Proliferation Treaty in 1995, where the extension of that treaty was linked to a promise of a test ban by 1996.
Verification is the part that had seemed impossible and was not. The treaty's monitoring system has 337 facilities, including 170 seismic stations, 11 hydroacoustic stations listening for underwater explosions, 60 infrasound stations for atmospheric ones, and 80 radionuclide stations that sample the air for xenon and particulates, of which 16 laboratories can analyse the samples. It was about 90 per cent complete by 2020 and has detected all six North Korean tests.
What the treaty has done is make testing expensive in reputation. The last nuclear test of the century was Pakistan's in May 1998, and since then only North Korea has tested: in 2006, 2009, 2013, twice in 2016 and once in 2017. Its 2017 test was estimated at about 250 kilotons, and it was detected by the network within minutes.
The American position has not changed since 1999, when the Senate refused to give its advice and consent, by 51 votes to 48, and the reasons were about verification and about whether the stockpile could be maintained without testing. Successive administrations have supported ratification and none has got it through. The United States has not tested since September 1992 and maintains its warheads under a programme of science and simulation rather than explosions.