NORTH Korea has been signalling that a third nuclear test is imminent, and speculation of a major advance has been fuelled by the assertion of its top military body, the National Defence Commission, that any test will be of a "higher level".

Numerous analysts believe this could point to the first-time test of a uranium device. The North's two previous tests in 2006 and 2009 used plutonium for fissile material.



The test will offer a rare chance to gauge where its nuclear program is headed, with most expert attention focused on what type of device is detonated and how.



Seismic monitors and ``sniffer'' planes capable of collecting radioactive evidence of the test will provide the forensic material for analysts to try to determine the yield and nature of the underground explosion.



"It's not that a uranium test would reflect any great technical achievement," said Mark Fitzpatrick, director of the non-proliferation program at the International Institute for Strategic Studies in London.



"But it would confirm what has long been suspected: that the North can produce weapons-grade uranium which doubles its pathways to building more bombs in the future,'' Fitzpatrick said.



A basic uranium bomb is no more potent than a basic plutonium one, but the uranium path holds various advantages for the North, which has substantial deposits of uranium ore.



"One alarm it sets off is that a uranium-enrichment program is very easy to hide," said Fitzpatrick.



"It doesn't need a reactor like plutonium, and can be carried out using centrifuge cascades in relatively small buildings that give off no heat and are hard to detect,'' he added.



North Korea revealed it was enriching uranium in 2010 when it allowed foreign experts to visit a centrifuge facility at its Yongbyon nuclear complex.



Many observers believe the North has long been enriching weapons-grade uranium at other secret facilities.



Another red flag raised by a uranium device relates to proliferation, according to Paul Carroll, program director at the Ploughshares Fund, a global security foundation based in California.



"Highly enriched uranium is the preferred currency of rogue states or terrorist groups," Carroll said.



"It's the easiest fissile material to make a crude bomb out of and the technical know-how and machinery for enriching uranium is more readily transferred and sold," he added.



Scientist and nuclear expert Siegfried Hecker, who was among those shown the Yongbyon enrichment facility in 2010, believes a uranium test is the most likely scenario given Pyongyang's stated desire to boost its nuclear arsenal.



Pyongyang has a very limited plutonium stockpile - enough Hecker estimates for four to eight bombs - and it shut down its only plutonium source, a five-megawatt reactor at Yongbyon, in 2007.



In an analysis published on Tuesday, in Foreign Policy magazine, Hecker held out the prospect of a multiple test in which separate plutonium and uranium devices are detonated simultaneously.



Digging into its precious stockpile to conduct a third plutonium detonation - like the 2006 and 2009 tests - would suggest Pyongyang had a miniaturised design that it was confident enough to try out.



A plutonium bomb requires less fissile material than a uranium version, making it easier to shrink to the size of a warhead that fits on a missile.



Proof that the North had mastered miniaturisation would be an alarming game changer - especially given the North's successful rocket launch in December which marked a major step forward in ballistic prowess.



"Pyongyang will almost certainly claim that the test was successful and will tout its sophistication," Hecker said.



"It will be difficult to distinguish truth from propaganda, but experience shows there is often a nugget of truth in North Korea's claims."



To some extent, Pyongyang has already boxed itself a corner by trumpeting the "higher level" scenario.



"While this statement is usefully ambiguous, it does set a baseline of expectation," said Hugh Chalmers, a nuclear research analyst at the Royal United Services Institute.



"If a new test turns out like the 2006 test, they will be very disappointed," Chalmers said.



The 2006, detonation was widely judged a failure by the international community with a yield estimated at less than one kiloton.

