@ Anonymous Coward @ Ken Hagan
'Do we really know for a fact they didn't just pile up six thousand tons of TNT in an underground cave and blow it all up at once in order to bluff us?'
That is a possibility with these relatively (and I stress the word RELATIVELY) small yields. It's a fine art to distinguish one of these blasts from an earthquake when you don't know the geology of the region in any detail, let alone the cause.
The previous NK test vented some of its fission products into the atmosphere and they were picked up by the Americans and Japanese, so we know they did detonate a nuke before - we even know a little bit about how it was made and what sort of technology was being used.
So you can bet lots of planes with sticky bits of paper* hanging out of them are flying over the Sea of Japan right now in the hope of picking up something. The USAF has its Constant Phoenix which you can read about here: http://www.af.mil/information/factsheets/factsheet.asp?fsID=192
* Okay, really high tech sticky bits of paper if it makes you happy.
And Ken makes some good points.
'The WW2 weapons were quite possibly over-engineered to make sure they went off. North Korea's are quite possibly under-engineered to conserve their material. As a species, we have learned quite a lot about these bombs in the last 65 years, so it would be naive to assume that scientists in NK know less about these things than the pioneers of the Manhattan project.'
The Little Boy bomb was hugely over-engineered to guarantee that it would explode - it used more than 1 critical mass of uranium to make the big bang and a flash. The US was so confident the uranium cannon would work they didn't test it. The US built about 80 Little Boy type weapons in the immediate aftermath of WW2 until the superior implosion bombs started to be mass produced. The only country to replicate it was South Africa which built a handful of weapons in the 1980s and 90s before dismantling them with the fall of apartheid. SA wanted to avoid an obvious plutonium enrichment program and had plenty of experience with enriching U235 as a cover.
Fat Man was less over-engineered than Little Boy as it used a sub-critical mass of plutonium which was compressed into a super-critical mass by implosion. The US gradually reduced the amount of fissile material in its weapons by levitating the core (giving much more force to the implosion) and boosting the yield with deuterium / tritium. By the late 1950s the US had got a 0.5 kT fission weapon down to 23kg and just 28cm in diameter.
(1950s madness here: http://en.wikipedia.org/wiki/Davy_Crockett_(nuclear_device) )
But these were only made possible after a great deal of testing. It seems unlikely that NK could go to a miniaturised warhead so early on its own back. Although the really scary thought is that Pakistan *MIGHT* have provided them with one of their missile warhead designs (in turn obtained from the Chinese).
'Point 2: there has been considerable argument over the actual sizes of the Hiroshima and Nagasaki bombs over the years. Wikipedia says 13kt and 21kt respectively and those are at the bottom end of the scale. Historical estimates have been about double that, so a factor ot two uncertainty for the NK device is no big deal at this stage.'
The Hiroshima blast is the one open to most argument; Nagasaki's yield is pretty locked down as the US did immediate studies of the fireball and the radio isotopes in the plume. The figures for Nagasaki were also cross checked with the near identical data obtained from the Trinity explosion. No such work was done at Hiroshima so the estimated yield was back calculated from casualties and damage on the ground.