By Shadia Nasralla
VIENNA (Reuters) - Two months after what is widely believed to have been a nuclear test by North Korea, the radioactive particles to prove it have yet to be detected, suggesting the communist state is getting better at hiding the fallout, experts say.
Fifty-five days after North Korea's previous test in 2013, those particles, known as radionuclides, wafted over a monitoring station in Japan, proving what seismic evidence already suggested - that an atomic bomb had been detonated.
Now, 64 days after North Korea announced its latest test and seismic data appeared to confirm it, diplomats and nuclear analysts suspect final technical proof may never materialize.
"With every day that passes, it becomes less likely that we will get a conclusive analysis," said one diplomat who follows North Korea's nuclear program closely.
January's detonation followed three previous nuclear tests by North Korea, the last before January's was the 2013 one.
Back then, the Comprehensive Nuclear Test-Ban Treaty organisation (CTBTO), a body monitoring seismic and radionuclide data world wide, confirmed the nuclear bomb test as such by analyzing traces of the colorless, odorless noble gas xenon.
The CTBTO system can differentiate between chemical and nuclear explosions. It cannot differentiate between a standard nuclear bomb test and a hydrogen bomb test, a more powerful nuclear weapon that North Korea claimed to have used.
CTBTO head Lassina Zerbo and other diplomats have said seismic data did not indicate the test involved a hydrogen bomb.
"It can be assumed that North Korea is getting better at hiding the results of its nuclear tests, and sealing off the area where they are testing to prevent nuclides from leaking into the atmosphere," Mark Hibbs, nuclear expert at think tank Carnegie Endowment for International Peace, said.
Experts and diplomats caution that due to xenon's short nuclear half live it now might be too late to get such proof.
"Over 55 days after the announced nuclear test in the DPRK, several recent (...) radionuclide measurements could be consistent with either a release from a test or with other possible sources, and are therefore not conclusive," CTBTO's Zerbo said in an emailed statement about North Korea.
The CTBTO's analyses are based on a network of data centers across the globe which collect seismic and radionuclide data and which have in the past been the first points to pick up on North Korea testing a nuclear bomb.
Difficult weather patterns make such analyses more complex.
"The task may be even more difficult now if the North Koreans have been working to improve their nuclear explosion containment techniques," Daryl Kimball, executive director of the Arms Control Association, said in an emailed statement.
(Editing by Louise Ireland)