The potential for a really low yield test is also relatively small. Regardless of how much Pu-239 one uses for a fission bomb, it ultimately has to be compressed into a critical mass. If you decrease the initial mass of Pu-239, you have to increase the amount of force compressing the pit.

Neither scenario is complete impossible, but seems highly unlikely in context. Getting a sub-kiloton yield on a pre-announced test just leaves too much ambiguity as to technical capabilities. I think it would be safer to chalk this one up to failure, but remain suspicious.

I honestly wouldn’t put too much stock into the Norks transfering their nukes to anyone, especially (*gasp*) terrorists. No states have transfered nuclear weapons since 1945 because they are just too much a liability in another person’s hands.

‘The percentage of optimal isotope for a decent bomb is lots higher than the percentage for fuel, but I’m more familiar with the older case of U235/U238.’

The plutonium would almost certainly have come from the 50MWe Yongbyon reactor which is a copy of the British Magnox reactors of the 1950s. It is probably a copy of the first large reactor at Calder Hall, the plans of which were declassified a long time ago.

The Magnox stations allow on-line refuelling which not only boosts their up-time, but allows fuel elements to be irradiated for relatively short periods of time; ensuring that there is relatively little Pu240 in the recovered nuclear fuel which spits out neutrons and ups the critical mass.

The alternative is that NK has got a uranium cascade up and working which could spin uranium up to bomb grade. Technically that is more complicated than separating plutonium from uranium which is relatively straightforward – especially now that the Purex process has been declassified for many years now.

A cannon-style Little Boy bomb is possible, but only if NK has made bomb grade U235. Whilst a plutonium cannon is theoretically possib, it requires some insane muzzle velocities to overcome the risk of predetonation from stray neutrons coming out of Pu240.

The second drawback is that a cannon needs more than a critical mass of material; if NK has as little fissile material as we suspect, than a uranium cannon seems very wasteful; it’d be better spent putting a subcritical mass into an implosion device. Only two countries have ever built them – the US with Little Boy and South Africa which ‘needed’ a number of bombs in a hurry.

What makes me suspect that it is an implosion bomb is that it didn’t work. The uranium cannon, whilst terribly inefficient, is a dead cert. Fire it and you end up with more than a critical mass – after that Dr. Einstein is in control and you end up with 12 kT.

Here? Hmmm it looks like a case of predetonation in an implosion bomb. We should know the fissile material fairly soon now that the US is reporting that it has found fission products in atmospheric samples.

They wanted to prove they had nukes. They wanted to make it inescapably clear. They’ve only been blustering and threatening about them since I was in middle school (am now in graduate school). That they couldn’t even manage a kiloton of yield suggests strongly that their technical capabilities are not up to par.

They may be able to manage it next time — but I think the results of this test are not nearly as ugly to us as it might have been if they’d been successful. It doesn’t look good that we didn’t do a single worthwhile thing to stop them, but we get another chance nonetheless.