Still no hope for Fukushima Diaiichi
https://www.simplyinfo.org/?p=20320 (Broken link?)
The area around the pedestal doorway was a significant part of this initial effort to inspect inside unit 1. What was found is worse than any of the computer modeling, existing research, or even our worst-case scenario estimates. Clear visual evidence was found and confirmed by TEPCO that the concrete pedestal is essentially gone, except for the metal rebar.
TEPCO stated that they are unable to differentiate between melted fuel, fuel combined with reactor and building structures and additional concrete debris that can be seen
https://web.archive.org/web/20220212180720/https://www.simplyinfo.org/?p=20320 (This is an archived version as it seems simplyinfo.org is down, or gone forever. (13/03/2022))
Looks like reactor 1 is going to crumble if something isn't done soon, and I doubt it will. It has taken 11 years just to get a robot down there to have a look. These are the first pictures from that part of the reactor. Basically, the concrete pedestal is the part that supports the reactor pressure vessel, with all the fuel rods, control rods and bits and bobs. If the pedestal collapses the reactor pressure vessel, along with what is left of the fuel will fall and contact the water. It will make it hundreds of times more difficult to try and remove the molten fuel on the floor as the reactor pressure vessel will be sitting on top of it all, along with what other debris that falls too. This is a diagram of reactor 1. I have indicated the pedestal in green. The videos are worth a watch just to see how bad it really is. One thing that comes to mind as to what would cause the concrete to erode and possibly not the rebar is Hydrochloric acid, or a similar acid. Without knowing what type of concrete was used and what type of metal/alloy the rebar was, I can't say for sure, but if an aggregate concrete was used this could explain the 'pebble-like debris'. Is it possible that during the meltdown something reacted with the water to turn it to acid?