Nuclear test detection: photo from wikimedia

Was N. Korean nuclear test a dud?

James Acton of Vertic, an independent non-governmental organisation (NGO) in London that specialises in verification research, noted enormous discrepancies in the estimated size of the blast.

“I’ve heard from three different sources that it (the North Korean blast) was less than one kilotonne,” “If it turns out to be less than a kilotonne, it could look very much like a fizzle,” a bomb that failed to detonate properly and achieve a full chain reaction," said Acton, a nuclear physicist by training. Kahleej Times.

Russian Defence Minister Sergei Ivanov, however, has been quoted as saying that the nuclear device tested by North Korea ranged between five and 15 kilotons. That is the normal size of a successful test.

What data, besides seismic, can be used?

• In addition to seismic sensors run by national governments, the UN’s Comprehensive Nuclear-Test-Ban Treaty Organisation (CBTO) in Vienna also has a network of 189 seismic and hydroacoustic monitoring stations designed to detect nuclear tests.
• Radioactive particles and gases that can vent from an underground nuclear blast are also a telltale, providing clues as to the type of material (uranium or plutonium) that was used and to the size of the weapon.
• A third monitoring technique is to use satellites with ground-scanning radars, which record the topography of a test site before and after an event. Movement or subsidence of the soil is the sign of a big blast.

How do we tell if it was a nuclear explosion?

Like earthquakes, large explosions send out shockwaves that can be detected on seismographs. Big nuclear bombs make big waves, with clear signatures that make them fairly easy to detect, analyze and confirm that they were caused by splitting atoms. But smaller blasts - as North Korea's appears to have been - are trickier to break down. York Daily Record

A nuclear explosion has a more instant shockwave than a chemical one. The differences between regular bombs and a nuclear explosion are very fine and subtle, and you need time to analyse the signatures.

"People have different way of cross cutting the data and interpreting them,"
The CTBTO's stations are more extensive than those used by most countries. They monitor seismic events but also underwater data, radioactive particles in the air and radiowaves.
"Within 72 hours we will have full data. Then all this will be available to member states," said Lassina Zerbo, director of the International Data Center at the CTBTO, which is based in Vienna, Austria.

While the North Korean explosion was small, potentially complicating monitoring efforts, sensors in South Korea were likely close enough to categorize it as nuclear, if that is what is was, said Friedrich Steinhaeusler, professor of physics at Salzburg University.

A nuclear blast also gives off a clear signature - a clear graph of peaks and curves - that differentiates it from other kinds of shocks, he added.

"We'll have the confirmation soon," he said.

Additional reading can be found on Rueters.
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