Good post.
Here is an excellent (unclassified) report used by the CIA to determine yield:
http://www.dtic.mil/dtic/tr/fulltext/u2/a223490.pdf
The claims by the Norks are highly suspect as the depth of the blast and the rock density are unknown. That’s why it’s called a heavily censored explosive yield.
I have seen the sniffing reports done by the Chinese and no radioactive markers have been released so far that are beyond background radiation.
If, if the 6.3 magnitude holds and that is unlikely, there is an absolute upper yield of around 600 kilotons. Looks like the magnitude will probably settle in at 5.9 so the upper theoretical yield would be no more than about 370 kilotons.
Guessing at the depth of the blast is difficult. Given that the blast cavern has already likely collapsed, with no surface emission the depth was likely at least 300 meters.
The rock density is fairly soft in that area allowing manual mining historically. So, the blast yield would likely be in the 110 to 160 kiloton range.
That is still a significant improvement for the Norks.
Boosted fission is very likely. The Nork’s main problem is acquiring refined beryllium. They have the ore but lack the very tricky refining technology. Even the Chinese have to buy it.
Without a significant beryllium neutron reflector, air blasts would be very inefficient compared to an underground test.
Very interesting: thank you!