I rather glossed over the "hydrogen explosions" bit in reference to the disaster in Japan.
I have seen comments from some purported expert that the hydrogen was released as some reaction product of the fuel rod meltdown.
Since the fuel rods are INSIDE the reactor vessel, which is INSIDE the containment vessel, if that was the source of the hydrogen, then a visible hydrogen explosion would appear to be proof positive that the containment had been breached.
BUT ... nobody is reporting that containment has DEFINITELY been breached, so maybe the hydrogen came from somewhere else.
Interestingly, I used to work for a power generation company and I distinctly remember being warned repeatedly about not smoking in the generator hall. Since the whole place was burning tonnes of coal every hour, this was rather strange advice.
The generators are massive, and apparently there is significant gain to be made in efficiency by minimising shaft friction losses and armature drag.
The friction losses were addressed by using the very best fluid bearings - the "pump them up before you start the machine" kind that used high pressure oil to lift the armature shaft off the metal supports before spinning it.
The armature drag was addressed by filling the generator with, you guessed it, hydrogen. This is because it is lighter than the nitrogen and oxygen molecules that make up the largest part of ordinary air, and thus causes less drag as the armature pushes it out of the way.
The case of the generator is ever so slightly leaky (especially to hydrogen, which is much like a domestic rat - it can apparently escape through a hole smaller than itself). As a result, if ventilation ever fails, like say if a giant ten metre wave takes out the power to the fans, the generator hall is soon filled with a somewhat explosive mix of hydrogen and air.
Could this be the REAL source of the hydrogen involved in the explosions at Fukushima?
If it was, the explosions would have occurred in the generator hall - probably some distance from the reactors. Would that be enough explosive force, and close enough to the reactors to cause containment issues? Only the designers know.