By which route/medium/mechanism should the majority of total air releases from the plant have escaped from unit2?
Several weeks ago I heard that the contamination in the Namie / Iitate region to the NW of Fukushima Daiichi was linked mostly to emissions from unit 2. Researchers reviewing SPEEDI predictions realized that the rain and snow that brought down most of the radioactivity there must have picked up the radioactivity at Fukushima Daiichi around the time the suppression chamber blew up, according to a late night documentary I watched on NHK.
The suppression chamber explosion in unit 2 on the morning of March 15 sounded like bad news at the time as it was a clear breach of the containment. The presence of the containment supposedly was the reason why Fukushima was not going to be another Chernobyl (the main reason I decided that same day to get out of Japan for a while were the #4 SFP problems though).
The reactor pressure vessel vents into the containment when pressure gets too high and it does that through the water in the torus, which acts as a fairly effective scrubber, catching most of the radioisotopes other than noble gases. Consequently, when TEPCO had difficulties venting the containment in unit 2 later on and pressure far exceeded design limits, leading to the suppression chamber bursting, there must have been a lot of volatile substances in the torus already.
I would be very interested in getting a clearer picture of exactly what events at Fukushima Daiichi match up with the various spikes in radioactivity in these charts, especially on March 15 and 16:
I have no clear image either what path any release would take from the torus. Would the radioactivity have to climb up through the stairwells between the floors? Radioactivity inside the #2 reactor building did not seem to be vastly different from levels in #1 when TEPCO first sent in robots and then people.
We do know that one panel was blown off the side walls near the #2 SFP.
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