I don't know how much explosive it takes to initiate a nuclear chain reaction. I'm betting that particular item is a subject of deep secrecy as it would inform a weapons builder of what the design should be. What I mean is that the instantaneous density required for reaction is not unknown, heck, it's probably on the internet somewhere. If somebody mentioned the amount (and type) of explosive charge needed to get that density then reverse-engineering the geometry of the fissile mass would probably be the next step (speaking as a botanist here).

Be that as it may, if we look at the photos of the WWII devices and the hoisting elements needed to get them in place, I'll bet the entire bombs weighed under 2 tons. I seem to recall that there was 20 kg of plutonium in one of the bombs. Give a ton for the various non-explosive bomb parts and casing and that leaves perhaps a ton of chemical explosives. I'll bet it's much less than that, but so what?

No way that 2 tons of detonated explosive is going to be detectable around the world. So I think that your conjecture (initiating explosion is undetectable) makes sense.

Here's one way that the NK's could use conventional explosives to mimic a nuke: Use the highest-velocity explosive available and install LOTS of detonators throughout the explosive mass, all timed to explode simultaneously. If the ramp-up for the shockwave could be shortened to indistinguishable from nuke explosion with seismic sensors then NK makes the claim without the nuke and gets away with it.

And I'm betting that's why the airborne radionuclide sensors are being flown all over the place--to pick-up the characteristic decay products of the chain reaction so that two independent lines of analysis can be compared.

Too bad there isn't a spy on the ground, huh?

B