Published online 16 June 2009 | Nature | doi:10.1038/news.2009.575

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North Korea's ignoble blast

Key gas signature of nuclear testing has not materialized.

Station RN16Radionuclide station RN16 detected North Korea's first test.CTBTO

A global network of sensors designed to verify nuclear testing has failed to pick up radioactive gases from North Korea's nuclear blast.

The lack of gases, which are created in large amounts during a normal nuclear detonation, has puzzled non-proliferation experts and scientists around the world. But few doubt the nuclear nature of the test, which the US government reported as "a few kilotons" in size on Monday.

North Korea conducted what it claims was its second nuclear test on 25 May (see 'North Korea's bigger blast'). Within seconds, a global network of seismographs had detected the shock wave from the blast. The seismographs are operated by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), a Vienna-based body that would enforce a global ban on nuclear testing if enough nations were to sign up to the treaty. The CTBTO seismographs showed that the tremors caused by the explosion were of magnitude 4.5, far larger than the nation's first nuclear test in October 2006.

Missing signature

The seismic signature of the test strongly suggested that the blast was man made, but the CTBTO hoped to use a follow-up set of measurements to verify its nuclear nature. During nuclear fission, plutonium and uranium generate many lighter fission products, such as xenon isotopes. Unlike other nuclear debris, xenon, an unreactive noble gas, can filter out through fissures in the rock after an underground test. Once in the atmosphere, plumes of xenon isotopes can be blown for thousands of miles. In 2006, for example, a CTBTO station in Yellowknife, Canada, detected traces of xenon-133 two weeks after North Korea's first test.

“If we didn't measure it, it's unlikely that anyone outside of North Korea's borders did.”

Lassina Zerbo
CTBTO

So far, however, no monitoring stations have detected this test's xenon signature, according to Lassina Zerbo, the director of the International Data Centre at the CTBTO. What's more, the likelihood of spotting anything is dropping fast because some xenon isotopes rapidly decay to levels that would make the signature appear similar to that generated by civilian nuclear power plants. This long after the blast, he says, "there is very little chance that we will pick up anything".

Zerbo points out that the CTBTO network is far more complete in 2009 than it was in 2006 and that all stations were operational at the time of the test. "If we didn't measure it, it's unlikely that anyone outside of North Korea's borders did," he says.

Airtight argument

The lack of isotopes has become an interesting puzzle for proliferation researchers. It could mean that the North Koreans used conventional explosives to mimic a nuclear test. Such a mock test would be unusual, although not unprecedented. In the 1980s the United States government set off several multi-kilotonne chemical explosions to test how various weapons and communication systems would respond to a nuclear blast.

But few think that the North Koreans would be capable of moving the thousands of tonnes of explosives necessary without detection, much less detonating them simultaneously underground. "I just don't buy it," says James Acton, a physicist at the Carnegie Endowment for International Peace in Washington DC.

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Another possibility is that the North Koreans carefully sealed the nuclear test tunnel so that the gases could not escape. But doing this is no mean feat, says Harold Agnew, a former director of Los Alamos National Laboratory in New Mexico who presided over numerous underground nuclear tests. "It's a real art," he says.

Agnew adds that in his experience it's not particularly unusual for nuclear tests to release no xenon. "Stuff gets trapped," he says. Acton suggests that, if the blast were big enough, it could have liquidized the surrounding rock and created an airtight chamber that would hold the gas. That could explain why the smaller 2006 test released xenon, whereas the larger test did not.

Jeffrey Lewis, an arms-control expert at the New America Foundation in Washington DC, says that the xenon detection would have been "icing on the cake". But he says that the seismic signal was strong enough to leave analysts with few doubts about the nuclear nature of the blast. 

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  • #60611

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