A nuclear excursion or criticality accident is possible and has happened several dozen times since 1945; this may be another such accident. Nuclear explosion? I’d like to see what isotopes are present if they are claiming that outcome. As for the “mushroom” cloud, no, that is not evidence of a nuclear yield, at least not with the size observed.
To achieve a supercritical explosion, it would require a supercritical mass of sufficient enrichment. It is highly unlikely and unprecedented to have a “nuclear explosion” from a nuclear fuel source. I call bullshit. Criticality explosion perhaps, but not a “nuclear explosion” in the common parlance.
Maybe some yellowcake.
So, was there a reactor? I don't think there's a single Japanese record left from that period that discusses the existence of a reactor, and there were certainly few witnesses to the whole program ~ since they all got killed in an "accident" in Manchuria, or a US Air Force bombing run on Japanese facilities there.
But let's say for just a second that there WAS a reactor and the Japanese have been hiding its existence for years through the simple expedient of explaining anomalous radiation readings as coming from radioactive rocks tossed up by volcanos OR material created at Nagasaki or Hiroshima.
Now, thanks to this nuclear power plant problem they can start blaming all of these readings on the power plant ~ which should pretty well cover all the anomalous reports in the North!
Why I bring this possibility up is that the Japanese could very well have built some working bombs and stashed them in their R&D facilities in what is now North Korea ~ if they had a reactor!
Think about it ~ every "test" the NK have done has had UNUSUAL results totally unexpected given Russian, American, British, French, Chinese, Pakistani and Indian experience in the matter.
If the Japanese did devise some sort of atom bomb during WWII and left them in Korea it's really long overdue for them to 'fess up!
“In the Reactor 3 explosion, there was a flicker of fire, then a vertical, black smoke up the reactor building. A hydrogen explosion does not produce such a black smoke.”
In my experience black smoke is caused by incompletely oxidized carbon in the exhaust column (i.e. soot). While yes Hydrogen does not generate black smoke, I can imagine any number of carbon based fuels in the building that could have been ignited by the hydrogen explosion.