Detection and Analysis of the DPRK Nuclear Test in February 2013
On 12 February 2013, the DPRK announced that it had conducted an underground nuclear test. Even before the public announcement, seismic sensors of the CTBT's International Monitoring System (IMS) had alerted the international community to the event, and analysis of the apparent test had started. This third nuclear test (following one each in 2006 and 2009) was in clear breach of UN Security Council resolutions 1718 (2006), 1874 (2009) and 2087 (2013) which demand North Korea not to conduct any further nuclear tests.
Scientists at Geoscience Australia, using data from IMS (and national) seismic monitoring stations (reference Figure 1) identified a suspicious seismic event occurring at approximately 13:58 hours AEST. This event was subsequently confirmed to be an explosive event with an initial estimated magnitude of 4.9 occurring in the vicinity of the P'unggye nuclear test site in north-eastern DPRK (the site of the declared 2006 and 2009 tests). This was almost certainly the DPRK's third test given its explosive-like characteristics, proximity to the P'unggye test site and its shallow depth (refer Figure 1). This was consistent with the DPRK's announcement later that afternoon.
The estimated magnitude corresponds to a likely nuclear explosive yield of 5 kilotons (ranging from 3 to 13 kilotons). This is greater than the 2009 test (1–4.6 kilotons) and is many times larger than the first DPRK nuclear test (less than 1 kiloton) in 2006. It was nevertheless quite small compared to most nuclear tests conducted by other countries in the past.
The IMS employs other sensing technologies to determine whether an event is a nuclear explosion, including detection of radioactive gases and particles emitted by a nuclear explosion as well as infrasound and hydroacoustic monitoring.
The event was detected globally by 94 seismic and two infrasound IMS stations. It was more strongly observed than the 2009 and 2006 tests. The clear detection of all DPRK tests by seismic stations, including the much smaller 2006 test, has demonstrated how seismic monitoring will provide a strong underpinning for verification of the CTBT when it enters into force.
However, conclusive confirmation of a nuclear test would need collection, analysis and confirmation of relevant radionuclides, in particular radioxenon, that are associated with nuclear explosive events, if and when this is available. The IMS currently has 66 radionuclide stations in operation across the globe, of which 30 are able to detect noble gases such as xenon. Assuming the availability/collection of the radionuclides and prevailing weather conditions, radionuclide data may be available in days or take up to several weeks.
Figure 1: Initial location and error ellipse of the event as obtained by Geoscience Australia using the 26 IMS stations indicated on the globe. Note that the error ellipse covers the P'unggye test. © Commonwealth of Australia (Geoscience Australia) 2013. This product is released under the Creative Commons Attribution 3.0 Australia Licence.
A detection of xenon isotopes that could be attributed to the event was made at the radionuclide station in Takasaki, Japan, located at around 1,000 kilometers from the DPRK test site, 55 days after the event's occurrence. Lower levels were picked up at another station in Ussuriysk, Russia. In particular, the radioisotopes xenon-131m and xenon-133 were observed. Analysis of the ratio of these isotopes showed consistency with a nuclear fission event occurring more than 50 days before the detection (nuclear fission can occur in both nuclear explosions and nuclear energy production). While not clear proof of a nuclear explosion, these detections coincide very well with the seismic detection of the explosion event occurring on 12 February 2013.
After the 2006 test, radioactive xenon was detected by a CTBT radionuclide monitoring station in Canada, supporting the assessment that the 2006 test was a nuclear explosion. However, no radionuclides were detected following the 2009 test. This suggests that radionuclides generated by that explosion were largely contained underground – whether by chance or effective design is not clear.
Of significance in 2013, is the DPRK's claim of successfully testing 'a smaller and light A-bomb unlike the previous ones, yet with great explosive power.' There has been open source conjecture on whether highly enriched uranium (HEU) or plutonium was used in the device. In the 2006 and 2009 tests, it is widely considered that plutonium was used. If timely radionuclide observations were available, it might have been possible to determine, based on radioxenon isotope ratios, whether a uranium or plutonium based device had been used. However, such data needs to be collected within around six hours from the time of detonation. On this occasion, that was not possible.
Open source reporting suggests that it is more difficult to construct a compact device using uranium, than for plutonium. For this reason alone, it may be presumed that plutonium was used. In any case, the available information does not tell us anything to substantiate the DPRK's claim to have constructed and tested a miniaturised device, irrespective of whether HEU or plutonium was used.
In order to clarify whether a nuclear explosion has occurred in cases such as the 2009 test where no relevant radionuclides were observed, the CTBT provides for the conduct of an on-site inspection (OSI). But this is not possible before the Treaty enters into force – and entry into force requires ratification by all Annex 2 countries, including the DPRK. Although the 2009 test does not appear to have vented radioactive particles or gases in quantities sufficient to be detected at a distance, on-site investigation could be expected to find such evidence. Noble gases should rise to the surface over time, even from a well contained nuclear explosion, and be detectable. An inspection team would also look for other evidence of test activity at or below the ground surface. Drilling could also take place to obtain samples at an explosion site. Drilling samples could provide evidence of whether HEU or plutonium was used.