A global monitoring system to detect nuclear explosions looks set to soon be put to the test.
North Korea has signaled it is gearing up to set off an underground nuclear device, its third, and experts who track such matters are filled with glee.
Pyongyang may give them an opportunity to assess whether the network really works.
The International Monitoring System (IMS) is a worldwide network designed to verify compliance with and detect and confirm violations of the Comprehensive Test Ban Treaty, which bans--without exception--all nuclear explosions.
And herein lies a problem. The CTBT, which was adopted at the U.N. General Assembly in 1996, has yet to take effect.
A core group of 44 countries must ratify the treaty before the global nuclear test ban can take effect. Of these, eight nations--the United States, China, India, North Korea, Egypt, Iran, Israel and Pakistan--have not ratified the treaty. Japan ratified it in 1997.
The IMS is already functioning.
Eventually, the verification regime will be set up at 337 sites in 89 countries. The equipment is designed to monitor acoustic vibrations in the Earth's crust and minute traces of radioactive particles released into the air following a nuclear explosion.
Nuclear experts say the network is capable of narrowing down a suspected testing site to a 1,000-kilometer square area if the yield is the equivalent of 1 kiloton of TNT, or about one-15th of the atomic explosion that leveled Hiroshima in 1945.
As of now, IMS monitoring stations have been established at around 80 percent of planned sites around the world, including 10 in Japan, ranging from Hokkaido in the far north to Okinawa at the nation's southern extremity.
Any activity suggesting a nuclear explosion has occurred is automatically relayed to the International Data Center in Vienna for analysis.
The staff in Japan working for the IMS also analyzes data recorded at its monitoring stations and reports its findings to the Japanese government.
But even if a nuclear explosion is detected, it would take several days for the authorities to announce their findings. This is because the monitoring network is not fully operational because the CTBT has yet to take effect.
The process of verification begins with measuring acoustic vibrations in the Earth's crust.
If vibrations show tell-tale signals of a nuclear explosion, researchers will try to detect traces of radioactive materials such as xenon and other rare gases in the atmosphere.
Equipment installed at a monitoring station in Takasaki, Gunma Prefecture, is said to be super-sensitive in detecting rare gases compared with sensors set up around nuclear power plants.
North Korea set off its underground nuclear explosion in 2009. At that time, experts could detect no radioactive particles.
But experts say the IMS is now better positioned to sense unusual activity, courtesy of a monitoring site in Ussuriysk in Russia’s Far East.
"The monitoring capability has been bolstered since the last test," said an official at the operating monitoring facilities for the IMS in Japan.
Scientists say measurements of radioactive materials in the atmosphere could pinpoint whether North Korea tested a uranium or plutonium device.
There is speculation that Pyongyang intends to use a uranium device, instead of one with plutonium, in its next test.
North Korea, slapped with international sanctions, is said to have suspended its production of plutonium and built a facility to enrich uranium, creating another headache for the global community.
If traces pointing to enriched uranium are detected, it would strongly suggest that North Korea is on the path to eventually developing a nuclear arsenal.
Scientists can figure out which device Pyongyang used based on the ratio of radioactive materials created in the process of nuclear fission. The ratio differs when plutonium and uranium are used.
The key is the amount of radioactive gases collected, according to Tetsuo Sawada, an assistant professor of nuclear engineering at the Tokyo Institute of Technology.
"If only fractions of samples were available, it would be extremely difficult for us to tell with certainty because precision analyses would not be possible with such a small amount," he said.
For example, xenon can be detected up to three weeks after a nuclear explosion.
Wind direction is also a major factor in forming an accurate picture.
Apart from the CTBT monitoring network, U.S. forces and the Self-Defense Forces would scramble to collect air samples from the Korean Peninsula to do their own independent analyses.
Tokyo regards the possibility of a third nuclear test by North Korea as a serious threat to national security.
In that event, scientists in Japan would analyze data collected by the Japan Meteorological Agency monitoring network of seismic activity.
When Pyongyang conducted its last nuclear test, on May 25, 2009, the agency’s network of about 200 monitoring stations registered acoustic vibrations in the Earth's crust that apparently were not the result of "natural seismic activity," a researcher said.
The explosion was estimated at magnitude-5.3 and centered less than 10 kilometers beneath the surface in northeastern North Korea, where its nuclear testing site is located.
The prime minister was quickly alerted. Japanese officials gave a news conference shortly after that.
Japan has since added 40 more monitoring stations to its network.
The Japan Meteorological Agency is not the only organization with sophisticated monitoring capabilities.
The National Research Institute for Earth Science and Disaster Prevention, a government-affiliated agency, operates a system called F-net (Full Range Seismograph Network of Japan), with 84 seismometers.
The institute releases its findings in real time.
(This story was compiled from reports by Yuki Takayama and Yu Kotsubo.)
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