Around 500 verification technology experts from 86 countries gathered in Vienna, Austria, for the International Scientific Studies (ISS) Conference from 10 to 12 June 2009. The ISS project’s aim is to examine the capability of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) to detect nuclear explosions. Amongst the most discussed topics were the findings of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) on the nuclear test announced by the Democratic People’s Republic of Korea (DPRK) on 25 May 2009.
Scientists compared the seismic findings on both the 2006 and 2009 DPRK announced nuclear tests. In 2009, 61 stations had registered the event, compared to 22 stations in 2006. This has allowed for a more precise assessment of the event’s characteristics, including its location and magnitude.
Verification technology experts such as Professor Paul Richards from the Lamont-Doherty Earth Observatory, Columbia University, USA, considered the scenario of a “bluff”, i.e. the creation of a nuclear explosion-like seismic signal using conventional explosives. While technically possible, he stated that it was highly implausible. As CTBTO seismic data have clearly indicated an explosion of a yield many times greater than that of 2006, it would have required several thousand tons of conventional explosives to be fired instantaneously. Richards explained that such a massive logistical undertaking would have been virtually impossible under the prevailing circumstances and would not have escaped detection.
The detection of radioactive noble gas, in particular xenon, could serve to corroborate the seismic findings. Contrary to the 2006 announced DPRK nuclear test, none of the CTBTO’s noble gas stations have detected xenon isotopes in a characteristic way that could be attributed to the DPRK event so far, even though the system is working well and the network’s density in the region is considerably higher than in 2006.
Through a method called Atmospheric Transport Modelling (ATM) using high-quality global meteorological data, it is possible to backtrack the three-dimensional travel path of an airborne particle back to its possible source region. Based on ATM calculations and the low level levels of xenon measured by the International Monitoring System (IMS), experts infer that less than one part per thousand of the radioactive noble gas produced by a nuclear explosion was vented.
Nor have CTBTO Member States using their own national technical means reported any such measurements. Given the relatively short half-life of radioactive xenon (between 8 hours and 11 days, depending on the isotope), it is unlikely that the IMS will detect or identify xenon from this event after several weeks.
Experts at the ISS discussed possible reasons for a so-called “containment” preventing the emission of noble gases into the atmosphere from an underground nuclear test. This depends on many factors, which vary strongly from case to case and are hard to predict. No data are publicly available that explain why this test was contained whereas the test in October 2006 was not.
The CTBT’s verification regime consists of three complementary elements: the 337 monitoring facilities of the International Monitoring System, the International Data Centre and, as the final verification measure, on-site inspections – challenge inspections involving up to 40 inspectors using a wide range of verification technologies. While the first two elements are already operating on a provisional basis, on-site inspections can only be invoked once the CTBT has entered into force.
A definite clarification of the nature of the DPRK event could only be brought about through an on-site inspection. Had the Treaty already been in force, the scientific evidence collected so far by the CTBTO on the DPRK event of 25 May 2009 would have provided a firm basis for a decision by the CTBTO’s future Executive Council to dispatch an on-site inspection.
On-site noble gas measurements would be one of many scientific methods used in an on-site inspection. Above the site of a nuclear explosion, concentrations of radioactive noble gases are orders of magnitude higher than what could be measured by IMS stations outside of the respective country’s borders. The noble gas xenon-133 could therefore be expected to be detectable for up to 150 days after the event. Such measurements could be conducted on sub-soil samples, thereby eliminating interfering background effects. An additional advantage of an on-site inspection would be the possibility to search for the radioactive noble gas argon-37, a very specific activation product from nuclear underground explosions, which IMS stations are not designed to detect.
The CTBT will enter into force once China, the DPRK, Egypt, India, Indonesia, Iran, Israel, Pakistan and the United States have ratified the Treaty, joining 148 other States that have already done so. The CTBTO conducted a full-fledged on-site inspection exercise at the former Soviet nuclear test site at Semipalatinsk, Kasachstan, in September 2008 – the Integrated Field Exercise 2008 (IFE08). This exercise provided many valuable lessons for future on-site inspections and demonstrated that the CTBTO is in principle capable of conducting such inspections.