THE BLAST AT BLDG 3
OK, this fits. The fuel handling machine went ballistic.
The camera angle for the video is from the south-southwest (green arrow).
The blast is from the southeast corner (over the SFP3)
The large heavy debris in the video does a back flip over the tower, coming down to the left of the tower, and smacking the north west corner of Bldg 3.
I again propose that the heavy debris was the green fuel handling machine, and I cannot dismiss that if it were hooked onto a single spent fuel rod assembly that was in the process of being uploaded for dry cask storage when the quake hit, that the FHM it took one with it on its ballistic trajectory (dark green parabola).
Refute it!
here is the video again
http://video.ft.com/v/825918290001/Fukushima-nuclear-plant-explosion
And here is a shot showing the direction of the wind after the blast, which also fits.
https://www.physicsforums.com/attachment.php?attachmentid=33860&d=1301658258