FUKUSHIMA REACTOR EXPLOSION SEEMS TO HAVE BEEN ON ROOF NOT INSIDE
I just came across this photo of the reactor and it is quite obvious the explosion was on the roof, not inside! You can see a clear pattern of a blast crater in the twisted beams. they point down and the lower walls have holes that they blew outwards. The explosion had to be from something on top of the building, exterior of the reactor, certainly not from inside the reactor room.
Good photo here: