On July 28 Tepco announced the unit 3 reactor muon scan preliminary results. The muon scan can detect masses of nuclear fuel if over at least a cubic meter.
Tepco declared that they will continue the muon scan, however this continued scan will certainly refine the data but not change much the results.
Tepco admitted finding no significant amount of fuel inside the unit 3 reactor vessel. These muon scans do not quantify possible fuel in the containment structure or in the base mat concrete of the building.
A recently done ROV inspection of the unit 3 containment structure, including the pedestal area below the reactor vessel, clearly located some melted fuel. However as the video and imagery published was limited and heavily edited, it is currently unclear how much fuel was found in that area.
Source : Tepco’s handout
TEPCO and IRID released a set of reports on the muon scan of unit 2, as a follow up report to the June preliminary scan results.
Tepco makes an assertion in the new report that the majority of the melted fuel is present in the bottom of the reactor pressure vessel but that assertion is quite questionable upon further review of the reports.
To justify the assertion that most of the fuel is in the bottom of the RPV, Tepco uses a close view of the actual scan output. Viewed without the wider view it seems there must be some fuel in the bottom of the RPV and there probably is.
When you look at the same image with the entire scan view, 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 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.
Existing meltdown literature and findings expect some amount of fuel residue to exist in the bottom of the RPV even if the bottom of the RPV fails.
Both scans of unit 2′s vessel showed little remaining in the core region.
At this point the scans are inconclusive either way on the question of fuel in the bottom of the RPV.