Any explosion with sufficient power will produce a “mushroom” cloud the shape of the cloud is due to the force rebounding off the earth and shoving debris into the air. I very much doubt there was a “nuclear” explosion that included uncontrolled cascading fission - I do not doubt that there may have been an explosion involving nuclear material as debris due to the proximity of the explosion to that material. There were sufficient sources of flame and fuel for any number of explosions to occur.
there have been, in the past, bits of information that point to a nuclear explosion. For example, debris from the reactor is photographed being ejected. Determining where the object landed and back calculating, based on it's weight, how fast the object traveled along its trajectory when ejected renders force in excess of that generated by a hydrogen explosion. I wonder if a nuclear explosion would partly explain why the official radiation contamination map for Japan shows the entire nation is contaminated - could a hydrogen explosion do that?
