Fukushima Unit 2 Fuel Debris Inspection Results
TEPCO completed another inspection inside unit 2 containment this week. The newest work involved attempting to move and grasp pieces of fuel debris inside the pedestal.
TEPCO again refused to make temperature and radiation data from the inspection available. The Guardian erroneously claimed readings from another unit 2 containment inspection as a “spike in radiation at the plant”. Years later TEPCO is still using this as their standard excuse to not make data public. Temperature and radiation data are critical towards understanding the type and condition of the fuel debris.
The inspection confirmed that pebble shaped pieces of debris on the cable tray could be moved. What was described as “clay like” debris in the same area could not be moved, indicating it is solidified material. There was also an attempt to dislodge fuel debris clinging to the remains of the upper catwalk floor. This was unable to dislodge any significant amount of the material clinging to the catwalk.
Small pieces of debris were easily moved and were not adhered to each other. Some larger pieces of debris appeared to have reflective portions of metal within them. It is not clear what metals could be subjected to a meltdown and 8 years of wet, radioactive conditions and not oxidize. Areas with what was described as “clay like” appearance were solidified. In these areas fine cloudy silt could be seen suspended in the water on the surface. This could cause significant amounts of fine radioactive debris to leave the containment structure along with the water that leaks out of the unit.
Most of these fuel debris substances appear to be brown corium. The radiation levels in this area on previous inspections indicate that the fuel debris in the top layer may not contain significant amounts of actual fuel. Known behaviors during a meltdown cause the various materials to stratify with the heaviest materials sinking to the bottom of the molten mass. Uranium from the fuel would be the heaviest material, indicating it likely fell into the pedestal first. Fuel debris containing most of the uranium would be at the lowest depth of the piles on the pedestal floor. It is also assumed that this material burned down or through the concrete basemat of the building.
The total time for the inspection work took 8 hours. A total of 6 locations on the cable tray were inspected with the grasping device. 5 of the 6 attempts resulted in being able to move the material at the surface. No samples were taken during the work, another entry to remove a fuel debris sample is planned for April.
This inspection shows that the loose pebble like debris could easily be removed with some form of scooping equipment. The solidified layers below will require cutting equipment. As removal works further down into the layers of solidified fuel debris, the risk of a criticality event increases. Higher concentrations of uranium and plutonium in these deeper layers would be more likely to set off a criticality event or spontaneous fission. Either could cause dangerous conditions at the plant. A criticality could potentially cause dangerous conditions beyond the plant boundaries.
Images below show some of the inspection work.
Small pebble like porous debris picked up by the probe at the cable tray location in the pedestal.
Solid debris adhered to the catwalk in the upper pedestal. Fine silt like debris can be seen suspended in the water puddled on the substance.
The probe picks up a piece of debris that appears to have a white or shiny metallic consistency.
Probe picks up the metallic piece of debris.
The probe digs into a pile of pebble like brown fuel debris.
The probe picks up small pieces of fuel debris.
The probe inspects a solid flat section of fuel debris.
The probe picks up a large piece of fuel debris. The edges appear to be porous, the color variegated.
A different angle of the large piece of fuel debris.
The same large piece of fuel debris from another angle.
The large piece of debris released from the probe.
A solid section of fuel debris.
Loose section of small fuel debris.
Small pieces of fuel debris break away when touched with the probe.
Solidified fuel debris on the catwalk in the upper pedestal.
The probe was able to dislodge a small piece of debris. This appears to be the catwalk location.
This also appears to be the catwalk location.
TEPCO handouts on this work:
https://www7.tepco.co.jp/wp-content/uploads/handouts_190214_01-e.pdf
https://www4.tepco.co.jp/decommission/information/newsrelease/reference/pdf/2019/1h/rf_20190213_1.pdf
https://www7.tepco.co.jp/wp-content/uploads/handouts_190213_01_e-1.pdf
TEPCO video of the probe:
https://www4.tepco.co.jp/library/movie/detail-j.html?catid=107299&video_uuid=yq53a9f4
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