This is one section of our annual report, the full report can be found here.
We will be posting a section per day over the next week for ease of reading.
We established new theories for the meltdown at unit 2 based on the newer information from recent inspections. The possibility is high that molten fuel fell into the sump pit in unit 2’s pedestal area. This could have allowed the molten fuel to collect in the sump where the ongoing heat generation could focus it to burn down into the concrete basemat. This could have created a path for molten fuel to burn through or leak out through damage to the concrete basemat. (21)
Inspections of fuel debris found in the pedestal in 2018 and 2019 confirmed that the surface fuel debris contained low amounts of nuclear fuel. The bulk of the substance at the surface is assumed to be mostly melted metals and other structural materials. What the fuel debris below the surface of the debris pile might look like is unknown. Higher content fuel debris may be contained below this metal layer due to the stratification phenomenon.
NDF claims most of the fuel debris for unit 2 remains in the reactor vessel, citing the previous muon scan.
We explain in our 2016 report on the unit 2 muon scan, how that is unlikely, and that the NDF interpretation of the muon scan may be incorrect. (21)
This idea that the fuel containing debris remained inside the reactor vessel while the metal components dropped down into the pedestal defies the known stratification phenomenon in a nuclear fuel meltdown scenario. The stratification phenomenon is outlined in our earlier report of the unit 2 fuel debris investigations.(22) NDF does admit that there were multiple failure points where molten fuel dropped out of the reactor vessel into the pedestal. The conditions found in the pedestal including some structures left in identifiable condition, leans towards a slow melt through scenario where the fuel debris slowly drops, allowing it to burn down into the concrete basemat as we mention earlier.
Unit 2’s relatively air tight containment structure and low radiation levels in most of the reactor building has made unit 2 the first candidate for fuel debris removal. Before work to remove the fuel debris can begin, areas of the building that will be re-purposed need to be cleaned and unnecessary equipment removed. That process began in December 2019 (23)
TEPCO has collected a swipe sample off of the scope inspection equipment used inside unit 2 last year. The sample was sent to a 3rd party lab for chemical analysis. This should help understand the conditions inside containment and establish the fuel debris removal plan details. (24)
TEPCO plans to remove the small fuel debris from unit 2 first then scale up this work, as we outline earlier in this report. A plan to sample fuel debris before removal work begins is planned. This work does not have a scheduled start date.
A new plan for removing spent fuel from unit 2 has been established. Instead of removing the refueling floor level of the building, a platform and entry lock will be built into the side of unit 2’s refueling floor. (25) (26)
One of the factors leading to this new spent fuel removal plan at unit 2 was people moving back into the area near the plant. This new plan doesn’t involve removing the refueling floor walls and roof, reducing the dust potentially dispersed into the air and winds. (2)
Work to clean and investigate the refueling floor took place in 2018 and 2019. The radiation levels on the refueling floor have been reduced enough that some very limited human entry into this area may be allowed in the future if it is needed to install new equipment. (3) Spent fuel removal at unit 2 is currently expected to start in 2023 (2)
TEPCO confirmed the presence of microparticles of nuclear fuel in unit 2. Glass sphere type microparticles were found on the refueling floor. This type of microparticle was found by academic researchers far south of the evacuation zone, near Tsukuba Japan. The microparticles found inside unit 2’s containment had a different makeup.
Those were more crystalline with significant amounts of uranium and zirconium. The zirconium would be the direct result of melted fuel assemblies. The creation of these microparticles documents that temperatures inside the reactor exceeded 2500 Celsius. These findings and similar ones at units 1 and 3 show a clear source of the microparticles found in the environment and links them back to specific reactor units at the plant. (27)
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