IRID has published a new plan to complete phase 2 of the robot containment inspection of unit 1 at Fukushima Daiichi. The initial plan involved dropping the shape changing robot down to the basement level of the containment structure. Due to brackish water and small debris in the water they have opted to not attempt to drop the shape changing robot or another swimming robot down into the lower level.
Instead they have equipped a shape changing robot with a retractable drop down camera to look down into the lower levels. The location selected to do this is near the pedestal doorway near the area of the steam pipes. This area is likely to provide new insight into the location of the melted fuel. Two concepts are considered likely for a meltdown in a BWR Mark 1 type reactor. One involves a slow release of molten fuel to the pedestal below. In this case it is assumed the molten fuel begins to burn down through the pedestal floor and stays mostly within the pedestal. The second concept is a fast release of molten fuel from the reactor. In this case the molten fuel flows like hot lava and spreads out of the pedestal across the containment room floor. This inspection should provide enough information to confirm if either of these two scenarios took place. You can read more about these two concepts and see our series of hands on corium experiments here.
As the work eventually takes place to do this second phase inspection it is worth keeping in mind that work to scan the reactor vessel has already taken place. A muon scan found no fuel inside unit 1’s reactor vessel back in March of 2015. If this new robot inspection finds no fuel debris in the pedestal or on the containment floor it will raise serious questions about the location of the melted fuel in unit 1.
IRID document in Japanese http://irid.or.jp/wp-content/uploads/2016/01/20160128.pdf
Machine translation to English
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