North Korea's first two nuclear tests are believed to have been plutonium-based devices. But there has been active speculation amongst experts that this latest test might involve a device based upon uranium.
Determining this will depend upon the identification of any leakage of gases during the test itself.
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There is no point in developing a working nuclear device if it cannot be weaponised”
Third time unlucky?
North Korea's nuclear tests
Mark Fitzpatrick, Director of the Non-Proliferation and Disarmament Programme at the London-based International Institute for Strategic Studies, suggests that one tell-tale sign that this might be a highly-enriched uranium (HEU) device would be the detection of xenon or other noble gases in the atmosphere.
James Acton says "there is a significant chance that there will be leakage of gases, but there is no guarantee that material will be detected". There were leaks after Pyongyang's first nuclear test, he notes, but none were detected after the second.
A device based upon highly-enriched uranium would be a very significant development.
As Mark Hibbs, also of the Carnegie, notes: "In the past North Korea had no choice but to deplete its small and finite inventory of plutonium to test nuclear weapons.
Leaders and demonstrators in South Korea and Japan condemn North Korea's actions
"Today and in the future," he says, "an unchecked and growing enrichment capability in North Korea is a game-changer because it will allow Pyongyang to indefinitely stockpile highly-enriched uranium fuel for an ever-larger nuclear weapons arsenal."
SEOUL, South Korea—South Korean and U.S. troops began naval drills Monday in a show of force partly directed at North Korea amid signs that Pyongyang will soon carry out a threat to conduct its third atomic test.
The region is also seeing a boost in diplomatic activity focused on North Korea's announcement last month that it would conduct a nuclear test to protest international sanctions toughened over Pyongyang's long-range rocket launch in December.
Pyongyang's two previous nuclear tests, in 2006 and 2009, both occurred after it was slapped with increased sanctions for similar rocket launches. The U.S., South Korea and other countries have urged North Korea to scrap its nuclear test plans or face grave consequences. North Korea's state media said Sunday that at a high-level Workers' Party meeting, leader Kim Jong Un issued "important" guidelines meant to bolster the army and protect national sovereignty. North Korea didn't elaborate, but Kim's guidelines likely refer to a nuclear test and suggest that Pyongyang appears to have completed formal procedural steps and is preparing to conduct a nuclear test soon, according to South Korean analyst Hong Hyun-ik.