Hi, I'm a physician who deployed as part of the US military response, specifically, I was in the contingency planning cell. Assuming there is a hole in the wall, which is a curtain wall, not a containment wall, then it leads to work areas around and outside the base of the reactor containment vessel. The presumed hole is above the wet well and may or may not be on the side facing the containment pool.

If it is on the side of the containment pool, the wet well may not be covered by a floor (I have seen conflicting schematics on this). However, Unit 4's primary issues were with their spent rod pool. If that is involved, there are two concrete decks between the bottom of the pool and the space exposed by presumed hole.

So, my guess is the wet well may be exposed by that hole in the curtain wall, and may or may not have any damage, but the wet well is definitely failed in Unit 2, and I can see where they wouldn't want people whooping and hollering about wet wells. I'm sure the insides of that are a complete mess at this point. Even if this was a normal construction site, it would be a complete mess at this point.

I see others have mentioned there may be a truck access tunnel at this spot. Perhaps, but I'm unclear as to the orientation of the 4 sides, and that seems a bit irrelevant to the reactor.

Meanwhile, the NRC daily updates petered out long ago. I'm not planning any trips to Fukushima any time soon.

Update: here is a great presentation we got from Areva. Glad to see it's available to the public:

https://wikispooks.com/w/images/5/5d/Fukushima-areva.pdf