Radioactive materials not found in nature such as krypton-85 and xenon-135 are released after a nuclear test. Krypton-85 remains in the air for several decades and thus presents clear evidence of a nuclear test.

In October 2006, a U.S. WC-135 scout plane detected radioactive materials over the East Sea a couple of days after the North conducted its first nuclear test.

Military officials and experts assume that radioactive materials rapidly spread after the test last week due to strong winds, which prevented a scout plane from detecting krypton-85. Another hypothesis is that the North revamped its underground nuclear test facility to allow no radioactive sources to leak out, or the speed of the leak is so slow that it hinders detection.

Krypton-85, however, is bound to be leaked no matter how tight the facility is sealed, and a scout plane can detect radioactive material regardless of quantity, according to experts.

Domestic and overseas experts said last week’s nuclear test was as powerful as a four-kiloton bomb, but skeptics estimated just one kiloton. This explosive power can be demonstrated by detonating high explosives at an underground tunnel, raising speculation that the North could have conducted a bogus test.

A military source said, “The North’s second nuclear test was probably not a charade, considering the country’s previous nuclear test,” but added, “Unless krypton-85 is detected, it will be difficult for the international community to confirm the test.”