Japan quakes

[josh1980]

Hydrogen is often used to cool the turbine generator. If it had been leaking(would anyone be surprised if some pipes burst?) then a spark would level the building. The turbine building is usually close to the containment building, but is not designed to be pressurized. I think that the building that is shown in the videos exploding was the turbine building.
The turbine buildings are often kept at a very slight vacuum so that any cracks in concrete(concrete is not always airtight and can be very porous), doors opening, etc sucks air into the turbine building. This provides the benefit of all air leaving the building only one way.. through HEPA filters. This is to help prevent any possible radioactive particles from escaping into the environment.

If a nuclear reactor gets hot enough the water will split back into hydrogen and oxygen(electrolysis). I'd expect that in an emergency if they have to vent the reactor, they vent it to the turbine building and then blow the air out through the HEPA filters into the environment. This would minimize the release to the environment. Of course, if you vent enough hydrogen and get a spark, you could level the building.

My guess is they vented to the turbine building and a spark ignited the hydrogen. You get a spectacular blast but it doesn't provide any indicator as to the status of the reactor.

One thing is for sure. If you destroy the turbine building, you'll likely never make any electricity again. The turbine generator is quite expensive . Of course, since core damage has likely occurred it's unlikely they'd even try to make electricity again anyway.

Edit:

One other thing i'd like to say...

The people at the plants are literally heroes. They're working hard despite possibly not knowing if their families are still alive, their homes are still there, etc. I'm sure some of them are grieving for lost ones and yet still expected to perform their job. Imagine the kind of stress on their shoulders right now. On top of that they're being exposed to large doses of radiation. It is possible that some of them may be exposed to lethal doses of radiation to protect the general public. Pray for the workers that may be sacrificing themselves.