Fears of further North Korean nuke tests

Nov 10 2011
VERTIC Blog >> Arms Control and Disarmament

Ryoji Sakai, London

Stanford Professor Siegfried Hecker, a regular visitor to North Korea, recently highlighted the possibility that the isolated nation might turn to further nuclear testing. The South Korean government also fear this possibility. Why might North Korea return to nuclear testing? And if they do, how easy will it be to detect?

North Korea’s nuclear ambitions are nothing new. Its nuclear programme began in the early 1960s, and the country conducted its first underground nuclear test in October 2006. Only achieving a yield of less than one kiloton, this test is widely considered to have been unsuccessful. Most nuclear weapon tests achieve a yield of at least ten kilotons. However, their second test, conducted on 25 May 2009, was believed to have been more successful. Various estimates suggest the yield of the explosion was around four kilotons, significantly larger than the yield of the first.

Although the international community has been investigating North Korea’s nuclear weapon programme for many years, its capacity remains highly ambiguous. While both tests were successfully detected, the techniques used could not reveal conclusive evidence regarding warhead design or test methodology. This leaves us wondering whether North Korea needs, or even wants, to conduct further nuclear tests.

Why might North Korea want to test again?
It is estimated that North Korea has enough fissile material for up to twelve nuclear weapons. Hecker believes that while the country has the capacity to build rudimentary plutonium weapons, these are not small enough to mount on a missile. Given this lack of technical confidence, Hecker argues that another nuclear test would appear technically attractive to North Korea if they want missiles equipped with nuclear weapons. He notes however that North Korea may be hesitant to resume testing, given the limited availability of fissile material and the probable international response.

From a different angle, South Koreans have raised concerns that the North might take provocative actions before next year’s presidential elections in South Korea and the United States. Kim Tae-Hyo, presidential secretary for national security strategy in Seoul, underscores North Korea’s economic motives behind another nuclear test. He has argued that the North might try to coerce the US and South Korea into providing sufficient economic aid by returning to nuclear weapon testing. Given North Korea’s technical and political motives, the possibility of another nuclear test cannot be dismissed.

How can we detect an underground nuclear test?
Both the 2006 and 2009 tests were conducted underground. These two tests were detected through a combination of seismic data, air sampling of radioactive material and satellite imagery. First, seismic monitoring networks detected a distinct pattern of seismic waves, which are easily differentiated from those created by an earthquake. The magnitude and decay of these waves allowed for calculations at to the explosive yield. Air sampling was also used to detect radioactive materials in the atmosphere which would not have been released from a conventional explosion.

Verification of the 2009 test highlighted at least two technical difficulties that give a rise to uncertainty in verification. First, air sampling after the event did not detect any trace of radioactive material associated with the nuclear test. There are at least two possible explanations for this. Firstly, it may be the case that North Korea ‘faked’ a nuclear test by using conventional explosives. The International Institute for Strategic Studies (IISS) dismisses this option because achieving such an explosion through a single detonation of conventional explosives is highly unlikely. Similarly, preparing for such a massive conventional explosion would have been a massive undertaking, which would have been easily detected by satellite imagery.

IISS offer a second, and more plausible explanation; the radioactive gasses had decayed before they were able to leak into the atmosphere. This lack of visible preparation and technical infeasibility makes it highly likely that the 2009 test was nuclear, rather than conventional. To definitively resolve this argument however, on-site inspection is necessary, and the possibility that the 2009 test was a conventional explosion cannot be ruled out entirely.

Another source of uncertainty is the difficulty in accurately calculating the explosive yield. To estimate how successful the 2009 test was, and to assess North Korea’s nuclear weapons capacity, it is essential to know how powerful the explosion was. The fact that the test was conducted underground makes such calculation difficult. For instance, while the U.S. Government estimated in June 2009 that the yield was “approximately a few kilotons”, the Russian Ministry of Defence calculated a yield of between 10 and 20 kilotons. This disagreement originates from the uncertainty in source depth, the volume of space left around the nuclear bomb and the geology of the test site. These factors can affect the amount of the explosive energy transmitted as seismic data, which is used to estimate the yield of the explosion. On-site inspections could collect such information, but it is unlikely that North Korea would admit inspectors to the test site.

Important research has been conducted which could alleviate some of this uncertainty. John Murphy and his colleagues at the Science Applications International Corporation (SAIC) conducted a study to provide a comprehensive analysis of the North Korea’s 2006 and 2009 nuclear tests. Part of the study is devoted to constraining estimates as to the depths of the explosions. The authors focused on the principle that particular seismic waves recorded from the explosions can provide the information necessary to identify source depths. Comparing the data observed at regional seismic stations against theoretical predictions, Murphy and his colleagues concluded that the 2006 test was conducted at a depth of about 200 metres while the 2009 test took place at a depth of about 550 metres. From this, they conclude that the yield of the 2009 test was 4.6 kilotons while that of the 2006 test 0.9 kilotons.

Another development in underground testing verification is the use of Global Positioning Systems (GPS). As a previous VERTIC Blog post noted, GPS could potentially be utilised for the verification system of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). While technical difficulties remain, when coupled with existing verification measures this technology could well augment the CTBT’s verification capabilities.

Uncertainty and verification
It is conceivable to argue that verification and clandestine nuclear testing could become a rat race. Speaking in the extreme, North Korea could dig deeper to conduct a third underground nuclear test if it were determined to hide it from scrutiny. From this standpoint, it could be a great challenge for the international community to confidently ascertain the true development of the North Korean nuclear weapon programme from further tests.

Nevertheless, it is too early to be dismissive of verification. In the end, North Korea’s nuclear tests were detected with reasonable certainty despite their abnormally low yields and locations deep under the ground. Furthermore, as seen above, verification techniques and technologies are constantly being improved. Although pessimistic views on the verification of underground nuclear testing may hold some elements of truth, they should be used as a way of improving verification capabilities. After all, verification cannot substitute for credible compliance or enforcement, and the level of acceptable uncertainty ultimately depends on what we want to verify. It is reasonable to say that the systems for detecting and measuring underground nuclear tests, including those of the CTBT, have been, and will continue to be effective enough to detect North Korean nuclear tests.

Last changed: Nov 10 2011 at 10:33 PM




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