The entire decommissioning plan has hinged on this one thing, the ability to flood the containment structures of the reactors with water. This technique was done at Three Mile Island after the partial meltdown of the reactor core at that nuclear plant. The situation at TMI was vastly different than Fukushima Daiichi. At TMI the reactor vessel didn’t fail, it could still hold water. This enabled the fuel to be retrieved through the refueling bay.
IRID and METI have been working under the assumption that containment flooding would be possible even while both entities expressed serious reservations about the possibility. METI early on expressed concerns that even if the structures could be made water tight, filling them with water may cause them to structurally fail.
It now appears that the plan has officially turned away from attempting to flood containment and instead look for ways to retrieve fuel from containment without the ability to flood the structure with water. This will be much more difficult and will require even more new technologies. IRID has previously directed a series of validation tests on containment sealing options including plugging the torus downcomer tubes with plastic polymers. Neither agency explained clearly what finally caused them to scrap the containment flooding plans but there must be some overwhelming evidence that this isn’t possible.
Mainichi reports the change of tactic and explains the newest theory on how they will attempt to remove the estimated 450 tons of melted fuel in each unit’s containment structure.
“As a result, a hole in the side of the containment vessel made of concrete that is about two meters thick and put the unmanned conveyor equipped with a camera and cutting apparatus for molten fuel at the tip of the arm from there, to the containment vessel bottom by remote control it was supposed to consider how carry out Grasp the molten fuel fell.”
So far no additional details about how they will prepare remote controlled equipment to do this task or prevent other problems such as criticality risks has been explained. More information is likely to come out as validation work begins or IRID requests more input for ideas.
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