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North Korea's Curious Nuclear Test

VIENNA—Researchers were scratching their heads earlier today at a meeting convened by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) over puzzling results from last month's nuclear test by North Korea. While the test produced a clearly recognizable seismic signal that was picked up by CTBTO's worldwide network of sensors, the organization's atmospheric detectors failed to pick up a whiff of the expected radionuclides in air. Even a deep underground test is usually expected to leak radionuclides, so their absence in this case caused quite a stir. Anders Ringbom of the Swedish Defense Research Agency in Stockholm says CTBTO's detectors for radioactive noble gases—a telltale signature of a nuclear test—can pick up a couple of hundred atoms from a cubic meter of air. On the lack of a signal, he said: "I was a little surprised, yes."

Some 400 scientists gathered here, CTBTO's home base, this week to discuss the results of a series of studies carried out by external researchers over the past year to test the capabilities of the system for detecting clandestine tests and to consider other scientific uses for the wealth of data collected. The system comprises 337 sensors across the globe looking for seismic signals, radionuclides, hydroacoustic signals in the oceans, and very low frequency infrasound in the air. Seismologists at the meeting say that the 25 May Korean test was an unmistakably man-made event and showed characteristics that make it almost certainly a nuclear rather than a chemical explosion. But the presence of radioactive xenon is considered the smoking gun for the nuclear nature of an explosion—and it wasn't detected.

It is possible to design a test to reduce the chances of radionuclides escaping into the air, for example, by carrying it out deep underground in particular types of rock. CTBTO chief Tibor Tóth says the North Korean case could be a combination of deliberate concealment and accident: Sometimes the underground structures and the melting and collapse caused by the explosion seals the fission products inside. The half-lives of the relevant xenon isotopes are measured in hours and days, so Ringbom says there remains about another week before the signal gets too faint. "The opportunity is not completely gone, but it's perhaps closing rapidly."

Some delegates pointed out that this uncertainty highlights the importance of bringing the treaty fully into force. One hundred forty-eight of its 180 signatories have so far ratified the treaty, but it requires all states with nuclear weapons or reactors to do so before it comes into force. Eight have not yet done so, including the United States. Once it is in force, members can call for a thorough on-site inspection to remove any doubt that a nuclear explosion has taken place.