SimplyInfo.org: Fukushima Unit 1’s Shocking Containment Inspection
What was found in this most recent round of inspections inside unit 1 was worse than any of our worst-case scenarios.
After much delay due to the pandemic and multiple technical challenges this inspection conducted by IRID, GE Hitachi, and TEPCO finally took place in recent weeks. The underwater ROV traversed inside the containment structure along the floor level from the X2 penetration hatch to the pedestal doorway on the side closest to the turbine building. This trip inspected just over half of the lower level of the containment structure. There are extended sections of both inspections missing from the videos provided by TEPCO. No explanation was given for the 25 and 45-minute gaps of missing video on one of the videos.
A pebble-like debris bed was seen near the X2 penetration that looks similar to what was found inside unit 2.
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. It is eroded or burned away roughly 1 meter up the pedestal wall and appears to be so not just in the pedestal doorway but across large sections of the pedestal wall. While there are other connections for the reactor vessel and concrete bioshield to tie into the outer containment structure, a significant portion of the weight of these two is supposed to be held up by the concrete pedestal.
The image above shows the concrete pedestal wall to the left of the pedestal doorway. A clear line shows where the concrete burned away about 1 meter tall leaving only the metal rebar. A debris bed can be seen below it. 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 piled on top in places.
The image above shows the pedestal doorway. Exposed rebar and missing concrete can be seen about 1 meter up the pedestal wall. A layer of debris can be seen in the pedestal doorway. The rebar further into the pedestal doorway appears to have slumped down into the fuel debris pile.
The image above shows the line where the concrete remains in the pedestal doorway showing a discolored and melted concrete surface. One current unknown is why the rebar was able to remain intact while the concrete burned or melted away.
The enhanced photo above shows the debris pile inside the pedestal. This doorway is approximately 6 feet (2 meters) tall. The debris pile appears to angle back and is at least the height of the doorway further inside. The debris pile closer to the door is a consistent 3 feet or 1 meter. It is unknown how much of this debris pile is fuel debris and how much is subsequent falling concrete. The findings of concrete pieces that appear to have fallen on top of the fuel debris bed raise concerns about the stability of the pedestal, bioshield, and reactor vessel.
The image above shows a jet deflector that sits over the downcomer tube for the suppression chamber torus. This is an unprotected route out of containment and has been a major concern that this was a path for molten fuel to escape containment. The jet deflector has a debris pile slightly less than halfway up the surface. This matches up with the estimates of the fuel debris pile on the containment floor. It also clearly indicates that fuel did escape containment into the torus tube and potentially into the torus room.
The image above of one of the jet deflectors near the pedestal doorway shows a solid fuel debris surface slightly less than half way up the height of the deflector. This surface appears to be fuel debris and does not have a layer of concrete rubble over the top. Multiple jet deflectors show this level of fuel debris intrusion. The smooth surface shows it was likely molten when it reached this area and cooled in this state.
The image above shows the second jet deflector near the pedestal doorway. It also shows fuel debris at a height of slightly less than halfway up. In this case it is more like 1/3 of the way up the height. This example shows the fuel debris pile has collapsed and is exposing layers of solidified debris with what appears to have been layers of gas or water that were trapped between them. Molten fuel debris generates a significant amount of gasses during the molten phase but this clear large stratification as it solidifies is not an expected phenomenon.
This jet deflector again shows solidified fuel debris slightly less than halfway up the jet deflector with a smooth surface. Multiple downcomer tubes from the containment structure suffered molten fuel debris intrusion. This seems to indicate the problem wasn’t isolated to one area of the containment structure.
The image above may be a close-up of the jet deflector in the previous image. It shows clear intrusion into the downcomer of the molten fuel debris.
The image above is near the jet deflector in the previous image. This shows a previously solidified smooth surface that cracked. This is quite different from fuel debris found in the other reactor units so far.
The inspection appears to have found a severe and abrupt meltdown and failure of the reactor vessel. The containment damage and volume of debris is significant along with the obvious escape of molten fuel from the containment structure. We will have follow-up reports on this as more analysis is done on the available information.
TEPCO report on the inspection – translated to English
Original TEPCO report in Japanese
https://www.dropbox.com/s/xl22cnyzbtsd5o4/u1_containment_inspection_rf_20220210_1.pdf?dl=0
Video’s provided by TEPCO for the inspections
https://www.tepco.co.jp/library/movie/detail-j.html?catid=107299&video_uuid=k593g02e
https://www.tepco.co.jp/library/movie/detail-j.html?catid=107299&video_uuid=ykq5a5n6
Watermarked images have been photo enhanced to bring out details for better analysis.
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Thank you for the detailed analysis. It is a bit surprising that significant concrete loss was observed for the pedestal wall (both inner and outer sides) while the concrete rebars did not seem to be melted away. The corium temperature must have dropped significantly (e.g. below 1500 C) after melting through the concrete cover.