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As pointed out before, it orginated inside the reactor vessel but exploded outside: how did it get there? Did it pass the shell by diffusion or was it released by the engineers?

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up vote 2 down vote accepted

They released the steam into the space between the reactor pressure container and the outer wall (sheet metal, deliberately to work as a kind of rupture seal?). Maybe they hoped that some of this substances would condense there when the steam cools down? But because there was a lot of Hydrogen in the steam, explosions blew away the sheet metal.

The reason for the ignition of the hydogen/air mixture is not known. Maybe the temperature of some hot parts of the pressure vessel did that.

Edit Inside the reactor vessel the hydrogen cannot explode, there is no oxygen available. And: All this was dealt already in two treads. Search for Fukushima.

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What are the reasons for releasing the gas inward the building rather than outwards, needlessly exposing staff to radiation? – artistoex Mar 17 '11 at 18:18
Where do You know from that there was some staff there? Even without radioactivity, from steam (!) alone everybody there would have been killed imediately. Concerning "reason" I wrote a assumption. – Georg Mar 17 '11 at 18:24
It was just a naive assumption on my part. – artistoex Mar 17 '11 at 19:08
Oh, and thanks, thank you for answering all my questions :-) – artistoex Mar 17 '11 at 21:50
@George @artistoex Engineers released the steam in order to regulate the pressure inside the reactor chamber. See: – Arkive Mar 18 '11 at 0:12

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