What does North Korea’s warhead test tell us about its nuclear capabilities?

  • Author: Hugh Chalmers
  • Source: Royal United Services Institute
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  • Published: 12 February 2013
  • Filed: 13 February 2013
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Passengers at Seoul Railway station watch a television broadcast reporting on North Korea’s nuclear test (Photo: Getty)

Passengers at Seoul Railway station watch a television broadcast reporting on North Korea’s nuclear test (Photo: Getty)

Seismic signals detected around the world indicate that North Korea has successfully conducted its third nuclear test, and taken a step towards a more potent nuclear deterrent. But without more information, it is very hard to say just how big this step is.

In what it described as a ‘self-defensive measure’, at around 0300 GMT on 12 February, North Korea successfully conducted its much-anticipated third nuclear test, following those in 2006 and 2009. Seismic signals emanating from the region, which does not normally display much seismic activity, indicated that a man-made explosion of magnitude 4.9 originated from the mountainous nuclear test range in the North of the country.

Seismic monitoring suggests that North Korea’s nuclear weapons can now achieve a yield of approximately 7kT, an explosion equivalent to 7,000 tons of TNT; an improvement on previous designs, but far smaller than the yields achievable by the standard bombs of the established Nuclear Weapon States. However, without access to further information about the test, the international community will remain in the dark as to the size, variety, and deliverability of North Korea’s nuclear arsenal.

A ‘higher level’ nuclear arsenal?

The first such test under the new leader Kim Jong-Un has not come as a great surprise. Since the North threatened a ‘higher level’ nuclear test last month in response to UN Security Council Resolutions, satellite imagery of the test range had shown a flurry of activity. Despite this, the test has nonetheless sent shockwaves through the international community, which has been quick to condemn the test.

Since its first test in 2006, North Korea has maintained a rudimentary nuclear weapon capability, albeit one restricted by its poor explosive performance in previous tests, a limited supply of plutonium fuel (having closed its plutonium production reactor), and no credible delivery mechanisms.  However it was revealed in 2010 that North Korea had constructed a uranium enrichment facility, opening up a new source of nuclear weapon material. Today’s test, claimed to be that of a ‘miniaturised device’, combined with North Korea’s successful rocket test at the end of last year exacerbates fears that North Korea is now one step closer to a credible, deliverable and scalable nuclear capability. Unfortunately, with only seismic data to go on, it is difficult to assess whether today’s test represents a significant technical improvement to the North’s arsenal of nuclear weapons.

The explosive yield

In terms of explosive power, a quick look at the seismic signals does indicate that this test was far more successful than either of North Korea’s two earlier tests. The Comprehensive Test Ban Treaty Organisation (CTBTO), which maintains a network of seismic monitoring stations around the world, observed a seismic event with a magnitude of 4.9. By itself, this figure does not suggest much about the actual yield of the tested device. However, this is certainly higher than the seismic shocks produced by previous tests, and fulfils the North’s promise to conduct a provocation of a ‘higher level’.

Furthermore, a preliminary analysis of the seismic signal and the known geological characteristics of the region can produce an estimate of the device’s yield. Three Norwegian scientists that have studied the region’s geology suggest that there is a simple relationship between the yield of the test and the magnitude of the seismic waves produced. If their relationship between test yield and seismic magnitude in North Korea is applied to the seismic signals detected, then that suggests that the test produced a yield of approximately 7.3kT. An alternative figure given by the South Korean Ministry of Defence puts the yield between 6 and 7kT. In either case, this is an improvement on the estimated yields of the North’s 2006 and 2009 tests (thought to be less than 1kT and between 2-6kT respectively).

The warhead design

Seismic data cannot provide any indication as to whether the device used plutonium or uranium. From North Korea’s perspective, each option has its advantages and disadvantages. While Pyongyang has a limited supply of plutonium, it is more conducive to compact warhead designs and, therefore, fitting onto missiles. Similarly, while uranium is plentiful in North Korea, it is relatively unfamiliar to North Korean weapon designers, and is not so well suited to miniaturisation. It is also possible that North Korea tried to ’boost’ the test by igniting nuclear fusion (the reaction that powers the sun) among light atoms in the centre of the device. However, if such an attempt was made, the yield measured suggests that it was not successful.

To determine exactly what type of device was tested requires access to the radioactive products of the nuclear explosion, as the types of particles produced, and their relative quantities, are an indicator of the materials used in the explosion. But without access to the test site, the international community will have to rely on any radioactive particles vented from the underground testing chamber by the force of the explosion. Once airborne, these particles will travel on the prevailing winds to the south-east, where they could be picked up by ground-based radiological monitoring stations in Japan within approximately 24 hours. Once detected, preliminary tests on these particles could confirm the nuclear nature of the explosion within another 24 hours, and with more in-depth analysis, an indication as to the type of device tested could be given a few days later. We may therefore have more clarity before mid-February.

By describing their deterrent as now ‘diversified’, the state-run Korean Central News Agency (KCNA) may be suggesting that the device drew upon North Korea’s plentiful uranium resources, rather than their limited stocks of plutonium. However, the international community may ultimately be unable to conclude either way. If North Korea buried their test far enough underground and carefully sealed the chamber, it is possible no radioactive products will escape to be detected. Indeed, KCNA reported that the test has not had any ‘adverse effect to the surrounding ecological environment’. And even if radioactive particles escaped and are collected for analysis, there is no guarantee that even this analysis will produce conclusive evidence of one warhead design over another.

A step forward for North Korea, but how far?

Similarly, although North Korea described the tested device as ‘miniaturised’, without visible evidence of the device’s dimensions there is no way to confirm this. While the North has never before described its test devices in such a manner, it has many reasons to give this impression.

Ultimately, the true potency of North Korea’s nuclear weapon programme is determined not by the precise yield of its warheads, but by the credibility of the threat they present. And this, on the whole, depends more upon the North’s ability to deliver a number of nuclear warheads to a number of different targets, rather than the specific yield of these warheads. Simply put, the size, sustainability, and deliverability of its arsenal is more consequential than the yield of individual warheads. While the seismic signals created by today’s test may refine our estimates as to the potential yield of North Korea’s nuclear weapons, unless the international community can similarly draw upon radiological signals, they will remain uncertain as to the deliverability and potential size of North Korea’s emerging nuclear forces.

Source: Royal United Services Institute (United Kingdom)

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