SEOUL: South Korean nuclear experts said Thursday they had been unable to detect any radioactive fallout from North Korea's nuclear test, confounding efforts to determine the nature of the device.
South Korean warships and air force planes equipped with highly sensitive detection devices were deployed after Tuesday's test to try and collect any traces of radioactive fallout.
The South's state-run Nuclear Safety and Security Commission said it had so far analysed eight atmospheric samples collected on land, sea and air.
"No radioactive isotope has been found yet," it said in a statement.
Their main target is traces of xenon gases released in the detonation that could be analysed to determine what fissile material was used.
"We are analysing samples and xenon has not been found yet," the commission statement said.
Experts are keen to discover whether the North switched from plutonium -- used in 2006 and 2009 -- to a new and self-sustaining nuclear weaponisation programme using highly enriched uranium.
If the underground test was well contained, it is quite possible there would be little or no radioactive seepage into the atmosphere.
And even if some gases did escape, scientists stress there is a large amount of luck involved in collecting them.
No xenon gases were detected after the North's 2009 test.
As well as the military detectors, the commission said there were 122 automated devices across South Korea that were continually capturing and analysing air samples.
The detection effort is running on a very tight deadline. Xenon-133m, a metastable isotope needed to pin down the type of weapon, has a half-life of just over two days.