SEOUL/VIENNA – As nations strove Thursday to verify North Korea’s claim that it had detonated a hydrogen bomb, Japan’s nuclear regulator said there were no radioactive materials in airborne samples collected by Air Self-Defense Force jets on Wednesday, the day of the blast.
Airborne radioactive particles could determine whether the North detonated a hydrogen bomb, but confirmation may take time. It was almost two months before isotopes were detected after the North’s previous nuclear test, in 2013.
Seismic monitoring stations around the world detected a tremor on Wednesday morning that the U.S. Geological Survey measured at a magnitude of 5.1.
Its seismic characteristics led experts to quickly conclude that North Korea had probably conducted a fourth nuclear test.
South Korea’s spy agency said the estimated explosive yield from the blast was much smaller than that of even a failed hydrogen bomb detonation.
South Korean lawmaker Lee Cheol-woo said he was told by the National Intelligence Service that an estimated explosive yield of 6.0 kilotons had been detected.
That was smaller than the estimated yield of 7.9 kilotons reported after the North’s 2013 test, he said, and only a fraction of the hundreds of kilotons that a successful H-bomb test would usually yield.Even a failed H-bomb detonation typically yields tens of kilotons, the NIS told Lee, who sits on the parliament’s intelligence committee.
The detection of airborne radioactive particles 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 a regular atomic bomb and would mark a significant advance for North Korea.
Another possibility is that the detonation was not a nuclear device at all but a conventional high-yield explosive.
Following the North’s 2013 nuclear test, it was 55 days before radioactive xenon gas was detected at a monitoring station in Japan, about 1,000 km 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.
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 bomb and would also require the presence of lithium.
“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.
There can be added detection challenges if the underground blast was completely contained, although that would be rare.