Today I viewed and took screenshots of a large, non-explosive atmospheric release by the Daiichi plant.
These screenshots are not depicting naturally occurring fog. The 'fog' emanates from the reactor buildings and it seems to have particulates and gasses that stratify. At least, that is what my observations suggest.
You see in this sequence screenshots taken first at 1:20 and last at 2:51. The site becomes enveloped in the poisonous cocoon of its emissions: