Video of the explosion itself:
http://www.bbc.co.uk/news/world-asia-pacific-12720219
Now, to my untrained eye, the explosion was 'shaped'. It clearly has an upward blast, with some lateral blasts. But not a lot of diagonal blast. My uneducated guess there is that it was an air-mix explosion, contained within the rectangular building. That would explain the shape and resultant destruction of the building. If it had been a core explosion, I would expect a more uniform shape? (Reactors are domes usually?). You can see the panels of the building flying off in specific directions.