As part of the recent roadmap document TEPCO explained efforts that have begun to find a suitable method to plug the downcomer pipes in the containment structures. The containment (drywell) connects to the torus (wetwell) by a set of 8 downcomer pipes. In order to eventually hope to flood the containment structure to remove whatever fuel remains in the containment structure they will need to prevent leaks into the torus and torus room. This area is assumed to be a leak route where water already leaks out.
TEPCO’s prime contractors (Hitachi & Toshiba) have both mentioned the need for this work to be developed as has IRID. From what is published here it sounds like Toshiba may be leading up the research effort on this task.
The diagram above show the area of the downcomer (in yellow) where they want to attempt to plug the pipe. The method being investigated right now involves various types of “plastic grout” to inject and plug up the pipe. It must be able to hold under considerable pressure and seat itself while being installed in an area with running water. Material tests to test for compression and similar properties are being conducted.
Plastic grout sample injected into sample tube.
Tests on sample material.
The challenges with the idea of flooding containment to retrieve the remains of the melted fuel is that it currently assumes the downcomers to be the main leak path without considering other routes. It also assumes the bulk of the fuel is still located in the pedestal of the containment structure. This may not be the case. Without more confirmation this effort may be missing clues towards more viable solutions.
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