I apologize for taking up a topic already discussed thoroughly on this thread, but there's one thing I've been trying to find an answer to, thus far with no success:
As far as I understand, the US plants with Mark I containments were at some point of time refitted with what is called a "hardened venting", mentioned e.g. here, on page 10
. Similar arrangements were made in the Swedish/Finnish BWR:s during the 1990:s (see here
, page 10). Would anyone have any idea, whether the plants in Fukushima Dai-ichi had anything equivalent, or was the venting of containment steam attempted e.g. through pipe lines designed just for changing the atmosphere from air to nitrogen under normal pressure/temperature conditions?