Monday, the wonk world was briefly aflutter with the news reported by the Chosun Ilbo: a radionuclide detection station in South Korea had caught a strong whiff of xenon, whose radioactive isotopes are the products of nuclear fission.

The Chosun’s correspondent even suggested that the xenon spike, which took place in mid-May, might have something to do with North Korea’s announcement of an unspecified achievement in fusion technology:

On May 14, two days after the North’s announcement, air analysis of KINS’s radiation detection station in Geojin, Gangwon Province showed about eight times as much xenon as in ordinary times, a government official said. “Authorities concerned have concentrated on analyzing this,” he added.

(Readers may recall that the alleged fusion breakthrough supposedly took place in mid-April.)

Perhaps needless to say, a fusion reaction doesn’t produce fission products. Chosun quoted an unnamed expert who seemed to be suggesting that North Korea might have tested a boosted device — a fission bomb with a small touch of fusion — which brings to mind some of Jeff’s recent speculations. But the South Korean government shortly ruled out a nuclear test, observing that there had been no unusual seismic activity.

Let’s also observe that there was no announcement. Pyongyang has never been shy about sharing the news of a nuclear test with its own people or with the world. That’s sort of the point: to demonstrate capability. And, as I observed back in February, the North Korean government has lately become much more concerned with signaling its ability to deliver food and consumer goods than big rockets and earth-shaking blasts.

Where the Xenon Came From

There are a handful of other ways that some excess radioactive xenon could have found its way to South Korea’s skies.

One possibility is the production of medical isotopes. But it doesn’t appear that there’s a facility big enough and close enough to have caused the spike.

A second possibility is a reprocessing facility, like North Korea’s “radiochemistry laboratory” at Yongbyon. Pyongyang claims to have finished its last reprocessing campaign there last year, but it’s possible that they were bluffing until recently.

Unfortunately for this theory, even if the plant had been operating in May 2010, the spent fuel would have aged too much to release much xenon. The half-lives of its radioisotopes are counted in hours or days. So scratch that idea.

The last important possibility is a nuclear reactor of one sort or another. It seems that whenever a reactor is started up or the pressure vessel is opened for refueling, gases escape, including xenon. A typical light-water reactor is refueled annually. And given all the power reactors across the Far East, that probably happens around there with some regularity.

All that’s really required to explain the unusual reading at Geojin is for a reactor startup or opening to have occurred within hundreds of miles in the previous week or so. Heck, if you want a specific candidate, Japan’s Monju breeder reactor was restarted on May 6.