SEOUL/VIENNA In the days and weeks ahead, nuclear experts will be hunting for airborne radioactive particles that could shed light on North Korea's assertion that it tested a hydrogen bomb, but drawing an independent conclusion could prove lengthy and difficult.
Seismic monitoring stations operated by governments around the world detected an earthquake on Wednesday morning that the U.S. Geological Survey measured at a magnitude of 5.1.
The location of the quake, near a known North Korean nuclear test site, and its seismic characteristics led experts to quickly conclude that North Korea had probably conducted a fourth nuclear test. Pyongyang then announced it had done so.
But it is the detection of airborne radioactive particles that will give clues as to the type of device that was set off and whether it was a hydrogen bomb, which is more powerful than an atomic bomb and would mark a technological advance for North Korea. [nL3N14Q2DH]
Another possibility is that it was not a nuclear device at all but a conventional high-yield explosive.
Following the North's last nuclear test, in 2013, it was 55 days before radioactive xenon gas was detected at a monitoring station in Japan, located about 1,000 km (600 miles) from the test site, which pointed to a nuclear blast by Pyongyang.
"What I would say at this point is that it's very consistent with what the world saw in 2013, which was a declared nuclear test, largely deemed to be a nuclear test," Randy Bell, head of the International Data Centre at the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), told reporters in Vienna.
"To go further into detail to try to ascertain some very particular nature, such as whether this indicated nuclear or non-nuclear, or a particular type of nuclear, is not appropriate at this stage. The seismic data alone would not provide that sort of insight," he said.
Though not confirmed independently, the seismic characteristics of Wednesday's blast at 10:00 a.m. Pyongyang time (0130 GMT) led the Japanese government and other experts to conclude that North Korea had indeed conducted a nuclear test.
Proving that the blast was a hydrogen bomb would depend on the presence of the hydrogen isotope tritium, which would set it apart from a fission atomic bomb and which in turn would require the presence of lithium.
South Korean intelligence officials and analysts are doubtful that the test was of a full-fledged hydrogen device, which would be expected to produce a much greater yield than the reported 6 kilotonnes.
By comparison, the blast produced by the weapon dropped on Hiroshima in 1945 measured 13 kilotonnes, according to the CTBTO, the global body set up to monitor a planned ban on nuclear testing.
The first U.S. hydrogen bomb was equivalent to 10 megatons, nearly 1,000 times more powerful than the Hiroshima bomb, and the Soviet Union in 1961 set off what is known to be the most powerful bomb in history, with a 50-megaton yield.
"If they find lithium, then it's definitely a hydrogen bomb test, but if it's only xenon ... then you're not going to know," a member of South Korea's parliamentary intelligence committee, Shin Kyung-min, said, quoting from a briefing by the country's spy agency chief.
The U.S. Air Force is likely to fly aircraft to try to detect gases in the air, a South Korean lawmaker said.
There can be added detection challenges if the underground blast was completely contained, although that would be rare.
(Additional reporting by Ju-min Park in SEOUL; Editing by Tony Munroe and Mike Collett-White)
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