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In 2010, two strange clouds of radioactive material drifted over the Far East. The clouds' make-up suggested that North Korea had tested two more nuclear bombs.
Seismic monitoring experts now say this was a false alarm. Any explosion was vanishingly small, they say, because seismometers should have picked it up ? and didn't. Other nuclear experts aren't so sure.
We need to know if there was a test. North Korea's own nuclear ambitions aside, the country has been accused of testing weapons for Iran.
And in the long run, the issue is whether science can reliably spot nuclear blasts. That will determine whether we can enforce the 1996 treaty banning nuclear tests: a treaty that Barack Obama vows to ratify, and that US Republicans oppose.
Nukes, nukes everywhere
The US and Russia have, between them, 18,000 nukes, and seven other countries possess a thousand-odd more. Global treaties aim, at the very least, to prevent any more countries arming themselves.
Although most countries approved the Comprehensive Test-Ban Treaty (CTBT) in 1996, it has yet to come into force, because eight nuclear states refuse to ratify it, including the US.
However, treaty members did build the International Monitoring System (IMS), which spots nuclear tests using seismometers, sound and radionuclide detectors. IMS seismometers picked up North Korean tests in 2006 and 2009.
On 13 and 15 May 2010, IMS stations in Russia and Japan, and another run by South Korea, detected clouds of barium, lanthanum and xenon isotopes in air from North Korea. Some had clearly been produced recently by nuclear fission.
Such clouds can stem from accidents at nuclear reactors. However, Lars-Erik De Geer of the Swedish Defence Research Agency in Stockholm realised that these clouds lacked other elements produced by accidents. But they did contain barium isotopes, only seen after accidents or nuclear tests.
Twin tests?
Other data was inconsistent with a single explosion. Putting it all together, De Geer deduced that North Korea carried out two tests, on 15 April and 11 May. Some nuclides leaked, and a few days later blast doors at the underground test site must have opened and released more (Science & Global Security, doi.org/jhg). Worryingly, though, any blasts did not get picked up by seismic monitors.
After analysing the nuclides, Gerhard Wotawa of the Central Institute for Meteorology and Geodynamics in Vienna, Austria, concluded that their masses suggested a 50-ton blast. While that is still big enough to amount to a useful bomb, it would only just have been detectable seismically, according to De Geer ? making the failure to spot any such signal less of a surprise.
Paul Richards of the Lamont-Doherty Earth Observatory in New York City disagrees. "We would have seen that," he says. He looked for shock waves similar to earlier tests in data from a Chinese seismic station near North Korea. There were none, even though his calculations show any explosion of more than a ton should have been detectable.
Or false alarms?
A 10-ton blast could be hidden by isolating it from the surrounding rock, but Richards says such a tiny test has little military value (Science & Global Security, DOI: 10.1080/08929882.2012.711183). He thinks De Geer's suspected tests are false alarms.
De Geer says North Korea might have tested small "hydronuclear" reactions that boost warheads. But Hans Kristensen of the Federation of American Scientists in Washington DC says a more likely explanation is that North Korea conducted the usual test on two occasions, and that both failed.
Failed tests might still emit nuclides, because they don't seal the surrounding rocks. That is what happened in the 2006 test. By contrast, the 2009 test was successful, and no nuclides were detected.
Monitoring works
Whatever really happened, the incident suggests the IMS is working. It detected a nuclear incident, involving a much smaller explosion than the system was originally expected to detect. "Existing verification would make it very difficult for a country to conceal a test [of] a militarily usable nuclear warhead," says Kristensen.
"There are fewer and fewer grounds for countries to refuse ratifying the CTBT by questioning the effectiveness of its verification," agrees De Geer.
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