DPRK Announces Fourth Nuclear Weapon Test

Posted by () on Jan 06 2016
VERTIC Blog >> Verification and Monitoring

The Preparatory Commission for the Comprehensive Test-Ban Treaty Organisation (CTBTO) has announced that its International Monitoring System (IMS) detected an ‘unusual seismic event in the Democratic People’s Republic of Korea (DPRK)’ at 01:30 UTC on 6 January 2016. The DPRK’s state-run news agency, KCNA, has issued an official announcement claiming this event was the country’s first test of a hydrogen bomb.

Was it a nuclear test?
The CTBTO has announced that its experts are analysing the data they possess, and will provide information through a dedicated webpage. The CTBT detection regime consists of an international network of three different waveform technologies—seismic, hydroacoustic and infrasound—together with global monitoring of radionuclide aerosols and noble gases in order to detect, trace and identify nuclear events down to 1 kT TNT equivalent. Final confirmation of whether the event was caused by a nuclear explosion will need to wait for further information from this system, and from other ad-hoc measurements made by states.

The clearest indication of a nuclear explosion would be from the detection of radioactive isotopes of xenon. Previous tests by the DPRK, including the last test in 2013, were confirmed by detecting such isotopes – in particular the isotopes 131m-Xe and 133-Xe.

These isotopes, along with 133m-Xe and 135-Xe, are produced in fission reactions and exhibit suitable half-lives and radiation emissions to be detected in the atmosphere at low levels at great distances from the release site. According to the website of the CTBTO, the ‘ratio of the detected xenon isotopes’ identified in 2013 were ‘consistent with a nuclear fission event occurring more than 50 days before the detection’. This information corresponds with the DPRK’s initial announcement in February of that year, 55 days before the xenon was detected, that it had carried out a nuclear test. The detections in 2013 were registered at the Japanese radionuclide station in Takasaki on 8 and 9 April. This location is over 600 miles away from the DPRK’s nuclear test site. Lower levels were detected at Russia’s Ussuriysk station from April 12 to 14.

These radionuclide stations are likely to play a key role in detecting any xenon releases from the suspected test. By analysing the ratio of any xenon isotopes detected at these stations, and by tracing their origin through Atmospheric Transport Modelling (or ATM, which models three-dimensional dispersal patterns according to local climate factors), the CTBTO may be able to better characterise the event.

However, this is not guaranteed. After an underground explosion, radioactive noble gases can seep through layers of rock and sediment until they reach the air, or they can be released by human activity at the test site. However, determining the site where the radioactive gas originated from through atmospheric sampling is not easy: xenon isotopes are not only produced by nuclear explosions but also by nuclear reactors and by medical isotope production. To confirm an explosive nuclear test, the CTBT will have to discriminate between any radioactive xenon released by an explosion and this ‘background’ radiation.

Further uncertainty surrounds the expected amount of noble gas released from underground nuclear explosions. These explosions create large quantities of xenon but the amount released into the atmosphere depends on the geological and containment characteristics of the detonation site. It is expected that only a fraction of the xenon generated will escape, especially if efforts are made to ensure the explosion is well contained.

If so, what type of nuclear device was it?
The majority of radioactive xenon released in the immediate aftermath of a test will be a direct product of the explosion itself. If the isotopic ratios of such directly-produced xenon can be measured, it may be possible to determine whether the weapon was based on weapons-grade plutonium or highly-enriched uranium. However, time is working against the CTBTO. These directly-produced isotopes are quickly overwhelmed by xenon isotopes released by other radioactive products (in particular, the decay products of radioactive iodine). These indirect xenon isotopes do not give a clear indication of the materials used to create the explosion. The race is now on to sniff out as many radioactive products from this event as possible.

Last changed: Jan 06 2016 at 4:21 PM

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