In the immediate aftermath of North Korea's sixth nuclear test on Sept. 3, experts described the blast as six or seven times as powerful as the one that destroyed Hiroshima. That measure might not even come close. According to research site 38 North, the resulting 6.1 magnitude earthquake must have been caused by the release of roughly 250 kilotons, or a quarter megaton, of energy—equivalent to 17 times the strength of the atomic bomb dropped on Hiroshima, and very near the estimated maximum yield that can be contained at the Punggye-ri underground test site, report the Washington Post and Bloomberg. This assessment tops all others so far: US intelligence said the blast was 140 kilotons, while South Korea said it was 50 kilotons.
Pyongyang says it tested a hydrogen bomb that can be mounted on an intercontinental ballistic missile, but South Korea's Nuclear Safety and Security Commission has been unable to confirm that. Though the commission found the xenon-133 isotope in air samples collected after the nuclear test, other radioactive isotopes normally present during a nuclear explosion weren't detected, reports the AP. Tritium, traces of which are typically found after a hydrogen bomb is detonated, was one of those noticeably absent. That doesn't mean the North is misleading the international community, however. The North made changes "to make radioactivity less detectable from a distance" after a 2006 nuclear test sent xenon and krypton isotopes into the atmosphere, the AP notes.