The cloud of xenon-133, as projected by regression models. Both images via ZAMG
Kind of a big update here: I just received an email from Annika Thunborg, a spokeswoman for the Comprehensive Nuclear-Test-Ban Treaty Organization, which operates the global network of monitors that measure airborne radioactive material. According to Thunborg, Gerhard Wotawa, the ZAMG meteorologist who published the blog post referring to data that showed a potential radioactive gas release in North Korea, "isn't a radionuclide expert and misunderstood the data. There is so far no indication that there was a radioactive release from the North Korean test last week."
Nature has taken its post on the subject down completely, although I'm leaving it up here as an artifact. Wotawa's post is now also down. As of right now, it's not clear where Wotawa went wrong, and I'll update again when/if I get word. But for now, consider the following to be unfounded. TL;DR: There's no radioactive gas cloud confirming the North Korean nuclear test.
It's been about a week since North Korea conducted its latest successful nuclear test, and now it's calling for the "final destruction" of South Korea. None of that is good news, and now there's this: Austrian scientists say radioactive gases have been detected by sensors in Japan, and they are likely to have originated in North Korea.
That's not cause for worry in any immediate sense, as the radioactivity detected is too faint to be of harm to anyone. In other words, North Korea's test didn't release some radioactive death cloud that's going to sweep the world. In fact, the researchers note that if the release of xenon-133, a fission product of uranium and plutonium, came from North Korea, it would have come on February 15, three days after the earthquake caused by the test was first recorded. That, combined with the faint levels, would suggest that the test was underground and well-contained.
The animation above shows the results of a model tracing the measured gas in Japan backwards in time. As you can see, it appears that the gas was released over North Korea on February 15. Note that the model continues backward in time past the 15th (i.e. continues into Russia in the animation), but that's just the regression; if the cloud originated in North Korea, it then drifted eastward towards Japan.
The researchers say that while that sensor in Japan has relatively high background levels of xenon-133, the levels recently detected are high enough to suggest that they're the result of the nuclear test, and were released upon North Korea opening the underground test site.
"It was a small detection, just above the threshold," Gerhard Wotawa, a meteorologist with the Central Institute for Meteorology and Geodynamics in Vienna, told Nature. "But it was a clear detection."
Detecting radioactivity is a big deal because it helps confirm that the test was indeed nuclear. Unlike the above-ground muscle-flexing tests of the Cold War, an underground test of a device that's relatively small is hard to detect.
Unfortunately, while detecting the xenon-133 isotope suggests a test did happen, it's not enough data to assess what type of bomb North Korea has on its hands. Such an assessment would require measurements of xenon-135 and xenon-133m, which are also fission products of uranium and plutonium. Because those isotopes have much shorter half-lives, neither were able to be detected by the Japanese sensor.
Still, that a radioactive gas cloud appeared over North Korea at around the time of the test does suggest that Pyongyang wasn't just bluffing with some ridiculously-large conventional bomb or some other minute possibility, which is key for threat assessors and diplomats alike.