Between 1945 and 1996, the world witnessed as many as 2,000 nuclear explosion tests; since then there have been six tests in all — two by India, one by Pakistan, and three by North Korea.

What happened in 1996?

The Comprehensive Test Ban Treaty (CTBT), which seeks to fully halt critical nuclear tests, opened for signature. The treaty can come into force only after all 44 nuclear weapon states have signed and ratified it, which hasn’t happened yet.

Guess who refused to sign?

India, Pakistan and North Korea. India sees the CTBT as no different from the nuclear Non-Proliferation Treaty (NPT), which it vehemently opposes as discriminatory. Secondly, India wants to use the CTBT as a bargaining chip to gain concessions elsewhere.

The US, China, Israel, Egypt and Iran have signed but not ratified the treaty. And now, with the nuclear sabre-rattling in the Russia-Ukraine war, the US appears unlikely to ratify it any time soon.

But the Comprehensive Test Ban Treaty Organisation (CTBTO), which was set up to bring the treaty into force, remains hopeful.

Moreover, the work-in-progress treaty has achieved a key objective — preventing countries from conducting any further nuclear tests. The CTBTO credits this to its ability to detect any nuclear test anywhere — on ground, underground, atmosphere, or underwater.

Interestingly, the organisation’s network of sensors and sensing technologies have useful spin-offs for industry and society. For example, they find application in monsoon forecast, tsunami warning, tracking whale movements, and research in radio nuclides.

The CTBTO’s International Monitoring System (IMS) runs over 300 ‘monitoring stations’ around the world, including many in some of the “most remote and inhospitable environments”.

The IMS rests on four pillars — seismic, infrasound, hydroacoustic and radionuclides.


With two seismic networks — primary (50 stations) and auxiliary (120 stations) — the IMS can detect any vibration on ground. The primary network consists of seismic array stations that can determine the type of seismic wave and its origin or cause. The auxiliary seismic stations supplement the work of the primary stations.

Typically a seismic event generates two types of waves — body waves (P and S waves) and surface waves (Rayleigh and Love waves), which differ in speed, direction and medium of propagation. Body waves travel through earth’s depths, surface waves move along the surface. IMS seismic network can detect both types of waves.


Audible sound frequency is 20-20,000 Hz; infrasound is below 4 Hz. Infrasonic waves cause minute changes in atmospheric pressure, which are measured by micro-barometers. Infrasound travels long distances, hence it is useful in detecting atmospheric nuclear explosions. The IMS infrasound monitoring system has 60 array stations in 35 countries. Each array has four or more elements arranged in geometric patterns, a meteorological station, a central processing facility, and a communication system for data transmission.


Hydroacoustic technology is used to measure changes in water pressure caused by sound waves. Hydroacoustic data can pinpoint the location of a nuclear explosion underwater, near the ocean surface, or near a coastline. Sound propagates efficiently through water but, at one level in the water, sound travel is slower but particularly efficient: the ‘sound fixing and ranging channel’ or SOFAR, at about 1,000 m depth. “Hydroacoustic monitoring makes use of the unique phenomenon of sound waves being trapped in that layer,” says CTBTO. The 11 IMS hydroacoustic stations keep a ear on all the oceans and provide tsunami warnings.


The presence of radionuclides — isotopes of elements that undergo radioactive decay — is the clinching evidence of a nuclear explosion. Isotopes of noble gases — xenon, in particular — are produced only by nuclear fission; hence, radionuclides are a ‘smoking gun’. The IMS has 80 radionuclide stations and 16 radionuclide laboratories.

All the stations of IMS generate a lot of data daily, which is sent to its international data centre (IDC) in Vienna. The IDC today is a massive repository of data, which serve as fantastic raw material for scientific research.

The CTBT is not born yet, says Dr Robert Floyd, Executive Secretary of the CTBTO, “but it is already a success.” Except for North Korea, no country has since dared to conduct a nuclear test, because detection is inevitable and would lead to consequences.

(The writer was in Vienna at the invitation of CTBTO to attend its science conference SnT 2023)