Threatwatch: What the North Korean nuclear test means

Threatwatch is your early warning system for global dangers, from nuclear peril to deadly viral outbreaks. Debora MacKenzie highlights the threats to civilisation ? and suggests solutions

It was the biggest bang yet. At 0257 GMT on 12 February, a magnitude 5 tremor with its epicentre in North Korea shook a worldwide network of seismic monitors. Hours later, North Korea announced its third official underground nuclear test.

The Comprehensive Test Ban Treaty Organization (CTBTO) in Vienna, which runs the monitoring network, called the seismic signal "explosion-like" and declared it twice as big as North Korea's last test in 2009. That would make this bomb between 8 and 14 kilotonnes ? approaching the 15-kilotonne bomb dropped on Hiroshima, Japan, in 1945. The test brought condemnation from all major governments and the UN.

It was, however, no surprise. Satellites had detected activity at the site of North Korea's previous tests, in 2006 and 2009 ? and maybe, unsuccessfully, in 2010. A new test there seemed to be on the cards.

The CTBTO said the seismic data confirms that the location was "largely identical" to the previous tests. The similarity of the seismic signal to those suggests this was not a radically new design, such as a thermonuclear bomb, says the Institute for Science and International Security.

Blast-related disturbances in the ionosphere seen by GPS satellites are still being analysed, say researchers at Purdue University in West Lafayette, Indiana, and Ohio State University in Columbus.

Resolution pending

What can we expect now? Possibly another blast ? satellite images show two tunnels were built at the site. The UN Security Council is said to be planning another resolution tightening sanctions.

Paradoxically, though, the test might strengthen moves for the US to ratify the Comprehensive Test Ban Treaty. Lack of US ratification is preventing the treaty coming into force. Ratification was a goal during President Barack Obama's first term, but was shelved as opponents claimed a test ban cannot be verified. North Korea's test has demonstrated, again, that it can.

And the test poses two big questions. One is North Korea's claim that the device was "a smaller and light A-bomb unlike the previous ones". This means it might be delivered on a missile, such as the Nodong, with a range of 1300 kilometres. Siegfried Hecker, former head of the US weapons lab at Los Alamos, suspects this blast tested such a lighter device. However, unlike location and yield, its weight before detonation is unverifiable.

The other question, whether North Korea has switched from a plutonium bomb to one using highly enriched uranium (HEU), might be verifiable. A switch would be bad news: North Korea stopped making plutonium in 2008 and is thought to have enough for only eight to 10 weapons, but it has its own uranium and is making HEU. It is also alleged to be collaborating with Iran, which makes HEU too. Iran insists its uranium enrichment is peaceful, so cannot test a device itself.

Gaseous giveaway

Which material this week's blast used will only be clear if it emits gaseous fission products. They should reach CTBTO monitoring stations in days, unless the blast fused surrounding rocks and so prevented gases from escaping, as seems to have happened in 2009. South Korea said this morning that it hasn't detected anything as yet, but a circulation model at the Central Institute for Meteorology and Geodynamics in Vienna, Austria, predicts that relevant air masses should pass over monitoring stations in Russia and Japan today or tomorrow.

Beyond these immediate mysteries, any long-term solution must address why North Korea feels it needs nuclear weapons at all, says Hecker. He argues that this is because it regards the US as an existential threat. Talks could calm these fears.

Yet the US insists North Korea must give up nuclear weapons before it will talk. This insistence may make renewed talks ? and thus any change of direction in North Korea ? unlikely. "We have spent most of the past twelve years not talking to North Korea," says Joe Cirincione, head of the pro-disarmament Ploughshares Fund in San Francisco. "During that time they have conducted three nuclear tests and four missile tests. When we have talked to them, they haven't conducted any tests. They shut down their facilities. That should tell you something."

Both talks without conditions, and ratifying the test ban treaty, require the US to take the initiative. North Korea's nuclear ball is now in Washington's court.

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