Monitoring the Comprehensive Nuclear-Test-Ban Treaty

The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) monitors compliance with the treaty through seismic measurements, radioactivity measurements and special microphones in the oceans and the atmosphere. The Federal Office for Radiation Protection (BfS) participates in the verification by monitoring radioactivity and supports the German Federal Foreign Office in the technical evaluation and assessment of the data.

As an international network, the CTBTO is designed to detect secret nuclear weapons tests worldwide. Several dozens of networking monitoring stations worldwide can capture minute traces of radioactivity in the air. The Federal Office for Radiation Protection operates one of these stations for highly sensitive radioactivity measurements on Mt Schauinsland near Freiburg.

Seismic measurements can give a first indication of an underground nuclear weapons test. The radioactive noble gases resulting from a nuclear weapons test can leak into the atmosphere through the soil with a time lag. When this occurs, these gases can be detected by the highly sensitive radioactivity monitoring stations of the CTBTO and can clearly be attributed to a nuclear weapons test.

The treaty and its monitoring

The CTBTO, with its headquarters in Vienna, is currently setting up a worldwide monitoring system consisting of a network of 321 monitoring stations with the help of the signatory states. The network is able to detect, identify and also locate a nuclear explosion with high probability anywhere on Earth.

this system is based on

• 170 seismometers in the ground,
• 11 hydrophones in the oceans,
• 60 infrasound microphones in the atmosphere and
• 80 radionuclide monitoring stations for radioactivity in the air,

One of these radionuclide monitoring stations is Schauinsland Station of the BfS (Radionuclide Station RN33). The 80 radionuclide stations are supported by 16 radionuclide laboratories for quality assurance.

The importance of radioactivity measurements

The three techniques seismology, infrasound and hydroacoustics can promptly register and locate explosions with yields above 1 kiloton of trinitrotoluene (TNT) equivalent (unit of measurement for the energy released in an explosion). In a next step, the nuclear character of the explosion has to be unambiguously identified by radionuclide monitoring technology.

When an explosive device detonates, a large variety of fission products is produced. Most of the radionuclides formed in this manner do not occur in nature and also differ significantly in composition from the radioactivity released from nuclear power plants.

The area of release can be roughly localised by air mass analysis.

What is measured?

At all 80 planned radionuclide stations integrated in the fully operational measuring network, the air is sampled for traces of aerosol-bound gamma emitters. 40 of the 80 stations including Schauinsland Station additionally monitor the air for the radioactive isotopes of the noble gas xenon (xenon-131m, xenon-133, xenon-133m and xenon-135).

Radioactive noble gases were included in the measurement system as these can be vented into the atmosphere also by underground and covert tests and can thereby increase the risk of detection for potential treaty violators. In this context it is important that - by means of the measurements - a clear distinction can be made between radioactivity from civilian sources and the radioactivity from any possible nuclear weapons tests, which would constitute a treaty violation.

Analysis of the data

All measurement data are transmitted to the International Data Centre of the CTBTO in Vienna through a satellite-based communications system. There they are analysed, distributed to the signatory states and archived.