Surely if the explosion was caused by a build up of hydrogen (as according to the government) it must have surely been produced from any water (or hydrogen sulphide) that was put into the vessel to cool it. {Normally temperatures are too high in reactors to use water even if it is refer to in diagrams - however I've just doubled checked the details and all Japan's reactors use light water - hydrogen dioxide {I can't do a subscript 2 here}}.

To split the Hydrogen from the Oxygen the tempertures would have been very high - so high that I'm not sure how some of the surrounding materials involved haven't decomposed.
This isn't exactly a new generation reactor after all.

{I'm no expert though}