Mystery surrounds detection of North Korea's nuclear test

Published 12 June 2009

Detecting radionuclide evidence in the form of radioactive gas is the “smoking gun” — proving that a nuclear explosion has occurred; seismologists say they are comfortable that explosion in North Korea two weeks ago was a nuclear test — but sensors have not been able to pick up radionuclide evidence

The meeting which began two days ago at Vienna’s Hofburg Palace, in which more than 800 scientists and diplomats participate, aimed to discuss nuclear weapons and world peace. The BBC’s Susan Watts reports that, officially, the conference aims to sum up how well science is doing in detecting, understanding, and warning the world’s politicians about an explosion anywhere in the world — and the likelihood that such an explosion might be a nuclear test.

Its tool is a scientific network that underpins the Comprehensive Nuclear-Test Ban Treaty (CTBT), which proponents argue has a key role in creating a world free of nuclear tests, and possibly, one day, a world free of nuclear weapons.

There was one thing everybody in the room wanted to know, however: Had the network of sensors picked up radionuclides from the North Korean explosion two weeks ago? Seismologists at the conference say they are comfortable that explosion was a nuclear test, but detecting radionuclide evidence in the form of radioactive gas is the “smoking gun”. The big news here is that they have not found that signal.

Watts writes that scientists do not really seem to know why. One delegate, an expert on radionuclide detection from Sweden, told the conference how well the network performed after North Korea’s nuclear test in 2006. Twelve days after that event the network picked up just a few hundreds of atoms of the noble gas Xenon 133 in Canada. He confessed to being “surprised” that this time round, so far, there has been nothing. He said he is sure the sensors are working properly. So why might there be no signal, and does it matter?

The eminent seismologist Professor Paul Richards from Columbia University implied it did not matter so much. The network includes a range of technologies — using seismic, infrasound, hydroacoustic, and radionuclide technologies precisely to give the world what he described as a “a quiver of arrows.” Thus if one arrow does not hit the target, then others will; if one detection set-up sees no nuclear signature, others will. And his personal view is that this was most likely a nuclear test.

So was there a deliberate attempt by the North Koreans to contain the explosion? Or was the explosion contained by accident? Some larger yield nuclear explosions can apparently “melt” the rock around them, so less noble gas seeps out. Attempts to explain the lack of a noble gas signal remain educated guesses at the moment. The official line here is that all this highlights the need for more countries to ratify the Treaty, so that it can come into force, thus allowing on-site inspection teams to move in to check out such tests.

In the meantime, scientists at the conference might be keeping their fingers crossed that something shows up soon, but they seem already to be resigned to the possibility that it may not.

Watts writes that those in Washington and elsewhere who see no value in treaties such as the CTBT may view this differently, perhaps as a vulnerability. The window of opportunity to detect noble gases from the May blast is closing. One more week and it will be too late. The material will be too widely dispersed or no longer radioactive enough to pick up.

In the end, the scientists here say their goal is to give the world the best data they have, and let politicians decide what to do about it. At least the data from this second north Korean blast reached key people soon enough that they were able to convene the Security Council on the same day. But the world waits to see what, if anything, will happen next.