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Extensive work has taken place at unit 1, focused on the refueling floor and an attempt to conduct a new containment inspection. After it was discovered that work to cut an access hole into a containment hatch door was kicking up radioactive dust,
all work stopped. This containment inspection has now been put on indefinite hold while the issue is under review. (10)
This inspection was initially announced in March of 2019 along with photos of the swimming robot that would be used. (11)
The suspended investigation work would use a boat type robot to collect more samples of fine debris from the containment floor. This robot would have the ability to collect samples near the pedestal doorway.(3) Earlier containment inspections at unit 1 collected samples of what looked like coarse beige sand. TEPCO claimed this material did not contain reactor fuel but did not clarify what the material was or how it was created. NDF assumes molten fuel fell into the pedestal then reacted with the inner concrete pedestal wall. NDF suggests the need for an internal inspection of the pedestal.(3)
They also make the assumption that some of the fuel may have left the pedestal. Our research shows that it is highly likely at least some of the fuel debris relocated to the torus room floor and the torus tube, outside of containment. (12)
After the dust incident stopped work, TEPCO installed a new dust monitor in the reactor well. This was intended to detect any radioactive dust leaking from inside containment out through the reactor well to the environment. A few months after this installation they put the entire project on hold.(13)
A major inspection of the reactor well area took place in 2019. This included sending a pair of wifi enabled robots in between the dislodged slabs of the reactor well concrete. Photos and radiation readings were obtained to help understand the condition in this area. (14)
(Top) Radiation readings at the center of the containment cap were generally higher than other areas of the reactor well. This has caused suspicion that the reactor cap may have leaked at a pipe connection in the center of the cap. (15)
(Bottom) The containment cap was inspected during this work. This area appears to have experienced high heat as most of the paint is no longer there. TEPCO had previously admitted the gasket for the containment cap was failed. Diagram of a BWR reactor containment cap area.
(Top) Unit 1 containment cap flange and bottom of reactor well
(Bottom) Panorama of unit 1 containment cap, dislodged concrete reactor well slab above. This cap was painted yellow during normal pre-disaster operation.
(Top) Unit 1 containment cap bolting array
(Bottom) Unit 1 containment cap flange seat
Photo above; Unit 1 containment cap and flange prior to the disaster on a routine inspection.
The cap itself appears intact and no highly unusual structural damage was found in the cap areas inspected. Information like this will help establish new containment and fuel debris removal concepts. This also helps narrow down the events that took place during the meltdown. (16)
Work to begin removing debris on the refueling floor level began in 2018 but has progressed slowly. (3) The potential to disperse alpha radiation containing dusts from the reactor building has been a major cause for concern that has generated multiple delays.
Portions of the refueling floor have been cleaned of debris. The section over the spent fuel pool remains untouched.
Spent fuel pool inspections were conducted in the latter half of 2019. The work looked at the condition of the fuel gate between the spent fuel pool and reactor well, the general condition of the fuel and fuel racks, and any debris that may prevent further work. A floating “air mattress” type protector will be rolled out onto the surface of the water then inflated to prevent debris from
falling into the pool while they do more debris removal work. (17)
(Top) Debris on top of spent fuel racks in the unit 1 spent fuel pool taken in 2019.
(Bottom) Cables and other debris hanging above the spent fuel pool at unit 1.
Eventual spent fuel removal work at unit 1 and the other reactors is dependent on securing interim storage space such as dry cask storage. (3) Spent fuel removal at unit 1 is currently expected to start in 2023. (2)
Due to concerns about people moving back to the areas near the plant site, additional measures are being taken to prevent alpha dust dispersion. TEPCO hopes to have this new cover installed by 2021.(2)
Unit 1 cover concept for debris removal and spent fuel removal (photo TEPCO)
The new cover appears to use the base framework of the current partially disassembled cover building. Once this new cover is installed, including an air handling system, work will begin to cut apart and remove the cranes and other debris.
Before the new cover building is installed, some preparatory work will take place. A series of steel support beams will be put under the damaged cranes to keep them from falling into the spent fuel pool.
A support cover will be installed over the fuel gate between the spent fuel pool and the reactor well. The fuel gate keeps water in the spent fuel pool when the reactor well is drained of water, as it currently is. A robotic cutting arm will be used in
certain places to remove hanging cables and other small obstructions. This is all planned for completion by December of 2020.
Diagram of steel support installation for cranes. (below)
(Top) Steel and mortar supports for cranes.
(Bottom) Diagram showing the robotic cutting arm and an area of cables
(Top) Diagram showing the fuel gate protection platform.
(Bottom) Diagram showing the installation process for the floating air mattress type protector on the spent fuel pool.
Plans for eventual fuel debris removal work inside unit 1 are currently hindered by high radiation levels in the reactor building near the hatch locations that could be used for this work. Radiation levels near the X-6 hatch are still 630 mSv/hr. (3) A process to lower the radiation levels or an alternative location will be required.
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