2) could the ‘fizzle’ (less than intended blast) be caused by purity problem of the Pu?

1/2 kiloton blast releases roughly 30-40 grams of fission products, 4 kilotons release roughly 200 grams of fission products, and a WW2 size blast about 1,000 grams.

Altho the fission products cause acute contamination, (some lasting for 100+ years), the Pu is a hazard for many multiples of its ~30K year halflife.

The ideal target for a small atomic weapon is a “civilian” atomic plant.

Each reactor (there may be 1 to maybe 8 reactors at a site) contains fallout equivalent to detonating 1,000 hiroshima-sized bombs.

Upwind of a city, industrial area, or agriculteral zone, the civilian power plant makes an excellent high leverage target for a terror weapon.

and a successful blast means a lot more than 200grams of element soup.

If the NK device carried 5 kilograms of plutonium, and it were detonated in the atmosphere, it would disperse somewhere around 4,970 grams of plutonium.

Interestingly, if it went to the full yield of 4 kilotons, rather than 1/2 kiloton, it would still disperse some 4,770 grams of Pu into the biosphere.

That fizzle came within 200 grams of Pu fissioning (only a few 10s of billionths of a second) from going full-tilt-boogie.

If the North Koreans can only produce a fizzle through testing, what does that say about their ability to field a credible nuclear weapon? Chances are a fizzle under test conditions will yield nothing when traveling at a few hundred kph on the tip of a Scud.

But in the sense of a “failure” to detonate a fully effective, massively destructive atomic weapon, that “failure” would be a misleading description.

”… even a 1 kiloton explosive would still be a terrifying weapon. Recall that the 1995 Oklahoma City explosion involved only a few tons of ANFO. A 1 kiloton (1,000 ton TNT equivalent) bomb could kill people in an area of about one square mile and would partially destroy a much larger area. Most of these deaths would be from fire or from the prompt nuclear radiation.”

Calling what was likely a “fizzle”, a “dud” or a “failure” is a major error, leading to underestimating what a terrible thing has occurred.

A fizzle is simply a detonation of lower yield than designed. It does not mean a dud. Wrong words leads to wrong thinking.

The 1/2 kiloton NK fizzle would kill with immediate radiation over a 2 square kilometer area.

The area in the red ring in this image of Manhattan is the >500 REM lethal zone. The yellow arrows show the World Trade Center for scale purposes:

Also, I’ve read reports that the seismologists don’t even have much information about the geology of the site, which is essential for analyzing the seismic information.

On the other hand, they could probably do several modeling runs with different geologies and see how much they differed.

But so far, there’s not enough in the open literature on either seismics or radionuclides for us to do more than speculate.

It doesn’t sound like Garwin and van Hippel have any more than we have.

On a more relevant note: I am still puzzled as to why the possibility of a decoupling cavern has been totally ignored.

Is there something “obvious” I am missing (except brains possibly)?