DPRK nuclear test

Posted by Andreas Persbo (andreas.persbo) on Feb 14 2013
VERTIC Blog >> Verification and Monitoring

By Hassan Elbahtimy with Andreas Persbo

On 12 February 2013, the DPRK announced that it had conducted its third nuclear test. Hours before the announcement, however, data started to flow from various monitoring stations indicating seismic activity in North Korea. This gave considerable credibility to the assertion that the DPRK conducted an explosive test possibly of a nuclear nature.

Testing of nuclear weapons was a salient feature of the nuclear age until the 1990s. The North Korean tests bring doubts to confident assertions that nuclear testing can be comfortably written off as something of the past.

Testing is, strictly speaking, not necessary for the construction of a nuclear weapon. Indeed, the first weapon ever used (‘Little Boy’) was an untested gun-assembled design. There are many more examples. It is assumed that Israel has developed nuclear weapons. The Israeli government has never denied this allegation. Yet, Israel has probably never conducted a nuclear weapons test. Also, Sweden, while drawing up final plans for its weapons programme, decided in 1962 that nuclear testing was desired, but not necessary, to develop an independent nuclear arsenal. A study by the US National Academy of Sciences explains that, “without testing, South Africa produced six modernized lighter U-235 gun-type weapons.” In other words, the test ban is not a major constraint on states seeking to acquire nuclear weapons.

However, countries can conduct tests for various other reasons. States wanting to develop high-yield, reliable, economical and lightweight nuclear weapons for missile delivery need to conduct both assessment trials and safety trials. Thus, while a state may be able to develop a functional design without testing, they may face challenges should they want a safe missile-delivered weapon. Naturally, military planners would need to have some assurance that the weapon will work when deployed, that it won’t go off by accident or rough handling, and would want to have a rather precise idea of the destructive capability of the device.

Nuclear tests leave behind several footprints that can be studied, analysed and therefore used for verification purposes. One of these footprints is the seismic impact of the test. Nuclear tests are ultimately explosions caused by man-induced nuclear reaction either by fission, fusion or some combination of both. Nuclear explosions are followed by the rapid release of vast amount of energy. Depending on the environment or the medium in which the nuclear detonation took place, shockwaves produced by the sudden release of energy can be detected, collected and analysed.

Seismic stations are particularly equipped to detect underground tests and it was through these stations that early data about the DPRK started to flow. National seismic stations run by the US, South Korea and Japan among in addition to international networks part of a global CTBT verification regime also detected the North Korean activity.

Analysis of seismic waves can provide indicators for the nature of the explosion, its strength and its location. For example, It can indicate whether the seismic activity due to a natural phenomenon or the result of man-made explosion. Most of the time, just glancing at the waveform data is enough. An explosion has an instant impact on the surrounding environment, a sharp initial wave, that then subsides. An earthquake, on the other hand, builds up in intensity and then subsides. Signal strength depends on many factors: the yield, the depth of emplacement, and the nature of the geological structure surrounding the device.

The strength of the tremor provides indications to the possible yield of the explosion. Calculating yield can be a challenging endeavour and depends on converting data reflecting shockwave activity into an equivalent of explosive charge calculated in tonnes of TNT. As shockwaves travel through various geological formations to reach sensors, geological factors need to be taken into consideration to arrive at an accurate calculation of yield. This is particularly challenging in the North Korean case due to lack of sufficient information about the geology of the site. Information available on the North Korean activity show a seismic activity with an estimated magnitude five. Various sites have put the yield estimate as low as seven kiloton, and as high as 20. The explosion could have been more, but is not likely to have been less, powerful than that.

However seismic date cannot on its own establish whether that explosion is the result of a conventional ordinance or one that involved a nuclear reaction. Additional means are needed to reach such a conclusion. Analysis of this seismic data traces back the shockwaves to the location where DPRK’s two previous tests in 2006 and 2009 took place. It should also be noted that it would be extremely challenging to detonate a very large conventional explosion instantly. The explosives would need to be transported to the site, requiring hundreds, possibly thousands, of truckloads, all channeled into an often rather narrow tunnel. That kind of activity would be difficult to hide from watching surveillance satellites. Then the explosives would need to be wired in a way that ensures that it goes off at exactly the same moment. While not beyond the realm of possibility, that in itself would be an exceptionally challenging task. Finally, why would anyone having joined the CTBT want to fake non-compliance?

While the seismic footprint provides important indicators, it is radionuclide analysis that can provide more conclusive evidence about the nature of a detected explosion and particularly whether it is of a nuclear nature. This can be established through detecting certain isotopes that result from the nuclear reaction. If conducted in the atmosphere certain fission products can be detected but this is not the case when the test or explosion is buried and contained underground. In this case however, certain noble gases that are also produced during the reaction can seep through and leak into the airspace. Because of their nature as noble materials they don’t readily react with the environment. Of these Xenon is of particular importance given its relatively longer half life than Krypton, another material produced in the reaction, for example. Air currents, wind and meteorological conditions more generally play a big role in the direction and rate of Xenon’s dispersal. Different Xenon isotopes are also produced from nuclear related activities including reactor and accelerator operations and therefore form background noise that needs to be factored in to correctly establish its source.

As radionucleides drift with the wind, the collection and analysis of this data takes more time than seismic detection: days, or possibly weeks. Information on the radionuclide footprint of the North Korean activity is yet to appear. The verification regime of the CTBT involves radionuclide stations and labs dispersed around the world half of which are equipped with noble gas detection and analysis capabilities that are needed to verify the nuclear nature of any underground explosion. The United States also has its own national technical means and previously released information from its air sampling systems in the region that were particularly relevant to 2006 North Korean test.

What added value could this test bring to the DPRK? The test may well have domestic or international political reasons. What is more important perhaps, a third nuclear test can be helpful to further advance the DPRK’s bomb making capabilities. The previous two tests seem to have left considerable room for design improvement considering their relatively low explosive yield. A DPRK test can therefore test design enhancements that could increase bomb yield. It is also possible that the DPRK might need to experiment with lighter or smaller nuclear devices that can be successfully mated with a delivery vehicle and particularly a ballistic missile. The official DPRK statement explicitly highlighted these factors by stating that the new test used a ‘smaller and light’ device compared to those used in previous tests.

The DPRK is not sitting on unlimited reserves of bomb grade fissile materials. On the contrary, the production of nuclear fuel suitable for bomb making is a challenging task. Particularly so for a country like the DPRK which has been under the crunch of international sanctions for many years. Moreover, North Korea’s main plutonium producing reactor was shut down in 2007. With an estimated plutonium inventory in the range of 24-42 Kg, enough for about ten charges, one would expect this stock to be used with due diligence and economy unless the DPRK has other sources for fissile materials.

Plutonium is widely considered to be the source of fissile material for the previously tested devices in 2006 and 2009. However, the DPRK has also taken steps toward mastering uranium enrichment. The scale and efficiency of uranium production capabilities available to the DPRK can only be guessed. A visit by Siegfried Hecker in 2010 to North Korea’s enrichment plant provides evidence that the DPRK started to achieve considerable progress in uranium enrichment technology. However, absent full IAEA inspections, no credible estimates exit on whether the DPRK is capable of producing enriched uranium with sufficient purity and quantities for bomb making.

Can the recent test help us identify whether uranium was used rather plutonium? Ultimately, one way to differentiate can be through analysis of radioactive materials leaking from the explosion and mainly isotopes of radioactive Xenon. While some argue that plutonium based and uranium based designs produce different isotopic compositions of Xenon, others doubt that this can be accurately detected to differentiate between plutonium and uranium designs. Either way, it might be too early to put this to the test now. Without access to the test site, materials used for any such analysis has to be collected by air samplers, taking into consideration various meteorological factors, in a process that takes longer than the collection and analysis of seismic data.

Last changed: Feb 14 2013 at 8:11 PM




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