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Success, failure or bluff? Scientists pore over data
(AFP)

9 October 2006


PARIS - Scientists took a dour wait-and-see attitude after North Korea claimed to have successfully conducted a nuclear test on Monday.

Only careful analysis of data returned by seismic or atmospheric sensors will say whether the blast was a success or a damp squib, they said.

Nor could they rule out the possibility of a scam, in which North Korea blew up a huge stock of conventional explosives to bolster its claim to have joined the nuclear club.

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.

The Korea Earthquake Research Centre in South Korea said there was a 3.58-magnitude tremor from North Korea’s North Hamgyong province that translated into the equivalent of 800 tonnes (0.8 of a kilotonne) of TNT.

But Russian Defence Minister Sergei Ivanov, quoted by the ITAR-TASS news agency, said the strength was five to 15 kilotonnes. By comparison, “Little Boy,” the US atomic bomb, which destroyed Hiroshima during World War II released the equivalent of around 12,500 tonnes of TNT.

“I’ve heard from three different sources that it (the North Korean blast) was less than one kilotonne,” said Acton, a nuclear physicist by training.

“This (the Russian figure) is not a difference of 10 or 20 percent (in the yield). It’s huge. We should wait to see if that Russian statement is confirmed,” he said.

Acton said that going for a 15-kilotonne yield was “the natural size” for a country trying to test a nuclear weapon. Paradoxically, it is easier to make and test a Hiroshima-sized arm of this size rather than to make a smaller one, which requires mastery of important miniaturisation techniques.

“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, Acton told AFP.

Possibilities

Another theoretical possibility is that North Korea stashed lots of TNT underground and blew it up.

“It is possible to tell the difference between a conventional explosion and a nuclear test,” said Acton. “The differences are very fine and subtle, and you need time to analyse the signatures.”

Bruno Seignier, in charge of the analysis and monitoring department at France’s Atomic Energy Commission (CEA), said a nuclear explosion “has a more instant shockwave than a chemical one.”

He said that “in a small (seismic) event”, picking out such differences would take time.

“The analysis is complicated because the energy that radiates out is weak compared with the subterranean background noise picked up by detectors. You really have to make a very detailed analysis when you look into such an event.”

As for the scenario of a hoax, Acton cautioned that to detonate a huge quantity of TNT to simulate a nuclear blast was in itself quite difficult, as it entails digging a large cavity underground—which would be visible to spy satellites—and requires detonators to be triggered all at the same time.

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.

The body is not qualified to make public statements on the nature of the incidents registered by its monitoring systems, and therefore did not confirm whether or not a nuclear explosion had taken place as claimed by North Korea.

However, the raw data has been passed on to the organisation’s 176 member states and to 770 institutions around the world.

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.

Sniffer planes and ground sensors can be used to monitor this airborne evidence. In the case of a totally sealed site, nothing may emerge, though.

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.

 

 



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