Here is what likely happened as described in my trusty nuclear engineering text book Intro to Nuclear Engineering 3rd edition Lamarsh:
The explosion would not have happened if proper cooling was in place. The uncovered fuel began to melt and lead to the initiation of various exothermic chemical reactions between the molten material and the water steam mixture, some of which produce hydrogen. That hydrogen along with other gases were released to the reactor building during controlled venting by the operators to control containment building pressure which somehow ignited. Keep in mind the reactor building is just a weather structure. It is not intended as a barrier. The hardened containment building within the reactor building was not damaged and is intact. It looks bad, but containment is still in place. I wish they would properly explain what is going on!!
Here is an excerpt for hydrogen production explanation provided by Dr Josef Oehmen (see the link below and here) Note: his explanation does not include core melt.
The problem is that at the high temperatures that the core had reached at this stage, water molecules can “disassociate” into oxygen and hydrogen – an explosive mixture. And it did explode, outside the third containment, damaging the reactor building around.
The Japanese are doing all the right things...evacuations, monitoring, etc. We should applaud and recognize how well the reactors and procedures are working despite such a huge event. Keep in mind that the refinery has been burning for three days with heavy black smoke plumes up to 3000ft into the air and thousands of gallons of spilled oil. Which one has a more significant environmental and human effect?
Excellent explanation of the overall scenario here