“Computer simulations show the melted fuel in Unit 1, whose core damage was the most extensive, has breached the bottom of the primary containment vessel and even partially eaten into its concrete foundation, coming within about 30 centimeters (one foot) of leaking into the ground.”
In a 2011 report Lawrence Livermore National Laboratory explains the total failures of the BWR Mark 1 reactor design when it is deprived of power sources. Without a source of power all of the cooling systems but one fail to operate. The remaining system (the RCIC) can only run for a short period of time before the torus system used to cool the water also overheats leaving the reactor with no way to cool itself. They estimate less than 10 hours from the time the RCIC cooling system fails to when fuel begins to melt down.
The report goes on to describe how difficult it is to locate melted fuel in a BWR Mark 1 reactor after a meltdown. They explain a process of following the CRD rail to the pedestal below the reactor, something TEPCO has tried at units 1 and 2 with no success. In both instances they were unable to run a scope into the pedestal due to severe damage inside containment.
Sandia National Lab also looked at the melt scenarios via computer modeling for Fukushima. The computer modeling used includes the ability to estimate the time needed for the melted core to burn through the drywell liner. This becomes critical as the edge of the drywell floor near the torus downcomers has no concrete containment structure backing it up. This is exactly at the point where the containment floor meets the containment wall.
“The model assumes an opening in the drywell liner occurs 15 minutes after debris first contacts the drywell wall”
The Sandia reports stop short of the computer modeling mentioned by Mr. Yamana, and does not look into the depth or spread of the melted fuel to this extent. These three reports while looking at different aspects of the meltdowns seem to agree on the severity.
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