Fukushima Unit 2 Muon Scan Not So Conclusive
TEPCO and IRID released a set of reports on the muon scan of unit 2. This is a follow up report to the preliminary scan results in June.
A claim is made in the new report that the majority of the melted fuel is in the bottom of the reactor pressure vessel but this claim is questionable upon our further review of the reports. You can find our previous reporting on the unit 2 muon scans at SimplyInfo.org.
This image below right is presented as matching the fuel estimates in red on the left. The dark portions of the image on the right clearly do not match up to the estimate on the left in any conclusive way.
Next we reviewed the up close view of the actual scan output used to justify the claim that most of the fuel is in the bottom of the RPV. When viewed without the wider view it looks like there must be some fuel in the bottom of the RPV and there likely is. Our research into existing meltdown literature and findings expects some amount of fuel residue to exist in the bottom of the RPV even if the bottom of the RPV fails catastrophically.
When you look at the same image with the entire scan view as seen below, the black area inside the RPV becomes less conclusive. This black band reaches far beyond containment and matches an area of interference documented on the earlier reviews of the scans. TEPCO also goes on to make an estimate of fuel volume in the lower portion of the RPV based on these questionable images. They provide numbers for the tons of material located in the sections of the RPV before the meltdown. They do not provide any justification for how they take the black spots in the image of the lower RPV and translate that to tons of melted materials and fuel.
The image below are results of the scan with the location of the core and RPV superimposed. In the previous review and this one they call this lower black band ground interference they claim is due to the angle of the muon scanner and the hill behind the reactor. This is the same swath as the black band on the other scan output.
Adding to the confusion is a piece of information we found in a 2015 NDF report on where they estimate the melted fuel to be. For unit 2 they make the following admission. “The photo of RPV pedestal inside taken from the opening confirms the structure of RPV lower region,
which shows that the vessel failure will be not so large” “Some molten fuel dropped to the RPV lower plenum and on the pedestal floor, and the remaining stays in the core(no fuel debris estimated outside the pedestal)”
This NDF report indicates they have obtained photos from inside the pedestal of the lower RPV and that it has some amount of failure. By that they estimate some of the fuel fell to the pedestal. They assumed most of the fuel would be found in the core region, the muon scans all seem to indicate this is not the case, including the previous scan done by Nagoya University.
At this point we consider the scans to be inconclusive either way on the question of fuel in the bottom of the RPV. Both scans of unit 2’s vessel showed little remaining in the core region.
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