140 KT is one the smaller scale of a city destroyer if the delivery vehicle is accurate enough. As a comparison the Trident III carries the W-76 a 100 KT weapon. The question is how did the Norks achieve this. Booster fission devices where one places fusion material in the middle of the fission warhead can produce these types of yield. The fusion that occurs releases more neutrons resulting in more “burning” of the fission fuel boosting the power. Another option is the Layered Cake design developed by the Russians. This design employs alternating layers of fission/fusion materials. Or the Norks tested a small true thermonuclear device; the two stage Teller-Ulam design. That type of weapon can be scaled to whatever yield you want.
“As a comparison the Trident III carries the W-76 a 100 KT weapon”
Yeah, but it carries 10 of them. It could carry 14.
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.