SEOUL – With North Korea appearing set to detonate an atomic device, the U.N. agency that detected its two previous tests says it is prepared to confirm an explosion when it takes place. But experts argue it may be difficult to establish whether the blast is nuclear.
The best indication of a test will be seismic tremors and gases released into the air, phenomena that the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty identified from previous testing. The Vienna-based organization’s most potent detection tools are more than 150 seismic stations across the globe. Although very small in yield, North Korea’s first atomic test in 2006 was picked up by the CTBTO, as was its second test in 2009.
Last week, Pyongyang warned that it is planning a third nuclear test to protest toughened international sanctions meant to punish it for firing a long-range rocket in December.
The threats have placed scientists and experts in South Korea on high alert, as any test is likely to aggravate tensions on the Korean Peninsula.
Seoul’s Defense Ministry said Tuesday it believes North Korea has nearly completed its nuclear test preparations, confirming satellite analysis last week by the U.S.-Korea Institute. Satellite images of the Punggye-ri site, where the previous two tests were conducted, show that the North Koreans may have been sealing a tunnel into a mountain where a nuclear device would be detonated.
In the event of such an underground nuclear test, both the CTBTO facilities and earthquake monitoring stations in South Korea can detect seismic tremors. But although this is a strong indication of a test, it is not an absolute confirmation.
Experts also note that artificial earthquakes, such as those created by nuclear explosions, rarely trigger the same wave patterns as natural quakes. North Korea could still try to deceive and give the impression that it exploded a nuclear device by simply exploding sophisticated conventional weapons that would trigger the same seismic waves produced by a nuclear test, said Chi Heoncheol at the Korea Institute of Geoscience and Mineral Resources.
By raising tensions this way, North Korea may hope to wrest concessions or aid in return for promises to scale back its unproven nuclear capability.
“Even if they bring truckloads of high-powered conventional explosives, put them (into an underground tunnel) and explode them, they will generate the same seismic wave and sound wave,” Chi said. The only difference is no radioactivity would be detected from the explosion of conventional weapons, he added.
The best course for scientists would be to collect air samples to look for increased radiation — but this process could take days. Even if the winds are favorable — and assuming Pyongyang conducts the test at Punggye-ri, in North Korea’s northeastern corner — it will take more than one day for airborne radioactive isotopes to reach South Korea, according to an official at the Nuclear Safety and Security Commission.
The official acknowledged it may be impossible for Seoul to confirm a test if the wind doesn’t blow southward or if North Korea plugs the underground tunnel so tightly that no radioactive gas escapes. Both South Korea and the CTBTO confirmed increased radiation levels following the North’s 2006 nuclear test but didn’t find anything in 2009.
CTBTO spokeswoman Annika Thunborg said that generally speaking, it is hard for those conducting nuclear tests to control the escape of noble gases — a clear indication of a nuclear test. With her organization’s extensive air sampling network, it is less dependent on wind direction than South Korea in identifying such traces.
If Pyongyang decides to conduct a so-called subcritical test, there would be no release of radioactivity at all — but that may be beyond North Korea’s expertise. Such an experiment requires a “very difficult technology” that only a few countries, such as the U.S., Russia and England, have acquired, said Whang Joo Ho of Kyung Hee University.
North Korea said its upcoming atomic explosion will be a “high-level” test and many analysts said that refers to a device made from highly enriched uranium, which gives the country a second source for manufacturing bombs in addition to plutonium.
Whether the North detonates a uranium- or plutonium-based device, there won’t be much difference in how easily scientists can detect the tests. Whang said a uranium-based test explosion also would mean that North Korea’s nuclear stockpile can continue to be enlarged at a time when there is no evidence of continued production of plutonium at its main Yongbyon nuclear complex.
According to analyst Shim Beom Chul at Korea Institute for Defense Analyses in Seoul, there is no way to determine when North Korea will conduct a nuclear test. U.S. spy satellites “can detect objects 15 cm in size on the ground, but they cannot detect what’s happening underground,” Shim said.
Click to enlarge