TEPCO announced that the next phase of containment inspections inside unit 1 will finally take place in early 2022. This next phase of inspections had been delayed due to technical challenges, concerns of radioactive dust releases, then due to the pandemic. As preparation work takes place, more details about these inspections have been revealed as we explain later in this report. You can read more about unit 1’s extensive history in our reports here.
Some of the explanatory materials provide new insight into the true situation inside unit 1. We have speculated since 2011 that melted fuel (aka corium) flowed out of the pedestal of unit 1, across the containment floor, and burned through the thin connection edge of the containment structure. Our corium experiments can be found here. Diagrams included in this most recent report show that the entities tasked with decommissioning the damaged reactors think so too. Diagrams originally from NDF, the decommissioning authority, and IRID, the main agency tasked with research, show the same.
The graphic below shows a significant pile of melted fuel in the pedestal and along the outer edge of the containment structure. The right side graphic shows melted fuel a significant way up to the lower edge of the downcomer tubes that route into the torus tube. Evidence of fuel debris inside the torus tube was found in earlier inspections along with our early findings that appeared to show fuel debris under the water in the torus room outside of containment.
This side view of the same area shows what IRID and TEPCO assume to be the situation inside unit 1’s containment as they prepare for the upcoming inspections.
The red color is solidified previously melted fuel, The brown layer is the sediment layer and the blue layer is standing water inside containment. The sediment layer on the right side where inspections have already taken place is significantly lower than the sediment bed directly adjacent to the assumed location of the fuel debris. The depth difference appears to be 3 times as much adjacent to the melted fuel locations.
The debris bed on the shallow right side was roughly 4-10 inches deep based on TEPCO estimates in 2017. If the left side adjacent to the solidified fuel is 3 times the depth, it would be 12-30 inches in depth. What exactly this debris bed is and how it developed had caused head-scratching for years. A TEPCO report in 2017 showed it contained stainless steel, materials related to shielding, cabling, and some low levels of reactor-based radioactive isotopes. If this material contained additional substances or not was a bit ambiguous. There had been some initial assumptions this was pulverized concrete. There may have been some involvement of the concrete structures into this debris pile through mechanical destruction or molten corium concrete interaction, but TEPCO provides insufficient data to confirm or rule this out.
Another 2017 report gave some rough estimates of the depth of the known parts of the debris bed. The known parts from an earlier set of inspections would be the general area marked by the A in the above graphic. This area is roughly 4 – 10 inches in depth. The left side adjacent to the solidified fuel would then be about 12 – 30 inches in depth. The deposits closer to the pedestal opening were close to 1 meter deep on a 2017 inspection. TEPCO could not determine if there was any solidified fuel beneath. They assumed the debris bed was providing extensive shielding that would prevent the detection of any layer of solidified fuel. This debris bed appears to reside 1/3 of the way up the downcomer cover. It is likely some amount of it has entered the torus tube and potentially the torus room.
All of this tracks with our early estimates of fuel spread, melt through, and fuel stratification. Our series of corium experiments estimated this behavior. The individual experiment using glass closely matches this scenario.
The upcoming inspections include a total of 6 ROV units. The remote operational vehicles are not true robots as each one has a control tether. The biggest concern with these units is having one become stranded, preventing the introduction of future ROV units to continue inspections. Each ROV unit has an assigned task. Due to internal equipment inside containment, a series of rings will be placed by the first ROV to help guide the ROV units and prevent entanglement.
ROV-A will attempt to traverse the south direction to the pedestal doorway.
ROV-A2 will attempt to enter the pedestal to capture imagery of the conditions and potential fuel location.
ROV units B to E each have similar tasks tied to characterizing the fuel debris and sediments.
Each ROV has about an 80-hour high radiation tolerance. They will be introduced by the level of risk with ROV-A2 going into the pedestal last due to the high risk. Preparation work begins in January. The entire series of inspections are currently scheduled to take 10 months to complete. We will release new reports as work takes place, check our website at Simplyinfo.org, or follow us on Twitter or Facebook for the latest news on Fukushima Daiichi.
Our machine translation of the TEPCO report can be found here:
The original report in Japanese can be found here:
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