This proposed explanation seems quite plausible but in retrospect, even more worrisome to me.
If the hydrogen gas were leaking at the drywell head, then that leak was inside the concrete containment, and so presumably was at least part of the explosion. I am wondering about the likelihood that the concrete shield plug(s) were blown off the top of the secondary (concrete wall) containment.
Although, in retrospect, a lay person would have to conclude that any hydrogen arising from a reaction involving the fuel rods would have to arise from within the reactor vessel, I had considered it got from there to the building's interior by a controlled venting process, not leakage from the drywell head, then concrete containment.
One must assume, for this mechanism to be the cause, that the concrete shield plug would not be fitted tightly enough to prevent leakage of H2. It also seems a reasonable assumption, in retrospect, that the concentration of H2 might be significantly higher under than over the concrete shield plug.
Has anyone made an assessment of the probability that the H2 explosions at Units 1, 2, and 3 blew the concrete containment plugs off like the cork on a bottle of cheap champaign?
If the diagram is accurate, it also gives a bit more insight of a hypothetical course any molten material might take from the floor of the SFP to the building's exterior.
Also, in the videos of the explosion of unit 3, I seem to remember something large and heavy falling from the vertical plume just to the side, and that someone pointed out that the blast at (unit 3?) was more vertically directed, like a cannon shooting straight upward. I have to review that video again.
Yes -- here --
Something large and heavy (who knows what), curls off to the left and comes down over the left hand tower in the video. What are the odds if it were the plug, it might be reasonably intact and visible among the debris on the ground. Back to the satellite images . . .
Hmmm. No way to tell, but it gives pause to think what might have fallen through the roof of the building in front of unit 3, doesn't it?
Here's a link to the video:
I understand this is, again, all speculative, but I am compelled to try to understand what my eyes are telling me.