TEPCO sent the unnamed walker bot back into unit 2’s torus room for another inspection effort. This trip looked at a torus downcomer vents and the bellows joint on that vent. The downcomer vents inspected include the east side of the building and the south east corner of the reactor building. The south east vent is assumed to be the one facing the pedestal door inside containment.
This was followed by an additional inspection March 6th. They have now inspected half of the torus downcomer vent tubes. This south east tube is significant as the pedestal door is the known likely route for corium (melted fuel) to spread out on to the drywell floor inside containment.
The grouping of tubes inspected on the east side of the reactor building represents the group most likely to have experience corium burn through at these locations.
TEPCO focused on the condition of the bellows joint and also the drain tube for the sand pocket. Both are known failure points for the containment system. The sand pocket section faces the portion of the drywell where the concrete floor meets the steel liner. This small gap also happens to be one space where there is no concrete backing up the steel liner. It has an estimated burn through time that could be as rapid as 3.5 minutes under certain conditions.
Sand pocket below downcomer
On both inspections no obvious damage or leak paths was found. The lack of finding damage can be equally significant. With no damage in this area of the torus room it helps to rule out a significant burn through in this area. Unit 2 leaks somewhere below the 60cm water line on the drywell floor. Finding the leak path is critical to controlling radiation releases and towards future decommissioning work that will require filling the containment structure with water.
Large format images of the robot inspection can be found on our Flickr site here. The short video of the inspection work by TEPCO can be found on our Youtube channel The related TEPCO handout: U2_torusinspection_handouts_130305_01-e
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