Before digging deeper the video I saw seemed to be a chemical explosion. Certainly doesn't look nuclear, no flash, and it doesn't look like a diesel tank going up, which would produce a rich red flame and thick black smoke. The later thick black smoke may be indeed coming from diesel tanks afire but they don't seem to be the point of origin, or of immediate concern.

Knowing a little about reactors, I'll have to refresh my memory and knowledge base, much of it from researching TMI, I'm leaning toward a hydrogen buildup and explosion in the containment as the cause. Which would cause the surrounding building, but not necessarily the reactor vessel or plumbing, to disintegrate.

A large volume of hydrogen and air going up would look like that, a colorless blast with a moderately soft brissance. Looking at the video I note that the ability to see materials near the blast center moving makes me think neither conventional or nuclear explosive are involved. both them move so fast that the human eye, even most cameras, can't catch the action. Going off you see it as things being there, then not. Not like in the movies, which tend to be the special effects fallback, small charges under bottles of naphtha.

The question is; how much damage was done to the reactor vessel, plumbing, pumps? The reactor itself isn't capable of creating a true nuclear explosion but a meltdown isn't impossible and, based on TMI and several other explosions at nuclear plants, a hydrogen explosion, isn't improbable.

Given the Japanese engineers at the site were talking about releasing 'vapor' I don't think it is unlikely that the vapor was, at least in part, hydrogen gas and that they released it, hoping to limit the spread, into the surrounding building, which isn't designed to absorb the abuse of an explosion, where it was sparked by some unknown source. The frame of the building remaining supports this and is, to my way of thinking, a good sign.

In these cases it is always better to get information from credible sources, even outside my own brilliant analysis, and avoid pronouncements that are not clearly backed up by evidence.