The article by Kyodo states that Nagoya's team conducted a total of 5 scans between April and July of 2014, of both units 2 & 5. The unit-5 reactor was shut down when the earthquake and tsunami caused the nuclear disaster in March of 2011, it and its sister unit #6 managed to retain one operational Emergency Diesel Generator following the tsunami that inundated the EDGs for units 1, 2, 3 and 4. This prevented units 5 & 6 from melting/exploding. Nagoya's tests were thus able to compare the condition of unit-2 to the condition of unit-5 which is known to still retain its core intact.
Side by side comparison between the data obtained from unit-2 and unit-5 is illustrated in the pdf linked in brackets above. Per that diagram there appears to be nothing left of unit-2's core. This was of course expected by all concerned, as TEPCO itself reported over a period of a year that units 1, 2 and 3 had all suffered total meltdowns based on other data and extrapolations from conditions at the facility. The muon scanning is intended to not just confirm what is already known, but to get an idea about what, if any, fuel still remains in the vessels for use in planning for the difficult task of accessing and removing the corium sat some point in the future.
It has also been long known that none of the three reactor vessels have been holding water since hours/days after the earthquake, but how much of the reactor vessel bottoms is gone is a question that will impact avenue of approach to the bulk of the corium. Nagoya's research team claims its muon-sensitive film can be used to help determine the condition of the reactor vessel bottoms and even to find the location of significant corium concentrations underground.
Radiation levels in the immediate vicinity of corium will be far too high to ever allow human workers to enter the containments to access it directly, but radiation hardened robots and remote control tools are being developed for that task. Once those new cantilevered buildings like the one at unit-4 have been built over the plants' ruins and succeeded in defueling their spent fuel pools, the remote control cranes will then be tasked with removing manageable pieces of corium through the reactor well openings on the level of the refueling floors and getting them safely into casks for isolation.
These coriums must be totally isolated from all imaginable human contact for a period of time longer than human beings have existed on this planet. There is no high level nuclear waste isolation facility capable of such isolation anywhere in the world at this point in time. Nuclear nations are actively seeking to create such a facility, but so far no nation has volunteered to host it by turning any region of its territory into a forever-sacrifice zone. It may be safely presumed such a facility will eventually be developed somewhere, at which point we modern humans will be tasked with creating some kind of warning to prevent future human civilizations, future [evolved] intelligent species and/or future visitors from elsewhere in the universe from ever digging into it just to see what kind of treasures it may be holding.
Now, I do not expect that such an accomplishment will be done in the lifetime of any humans alive today, but it is a very interesting conundrum our species faces, that absolutely MUST be done before our modern civilization ends and crumbles to dust (as is the inevitable way of things over deep time). Any good ideas out there on what such a warning to the distant future should look like? Remember, it's got to be something that WON'T be grave-robber enticing like the pyramids of Egypt were from the moment they were built. It's got to be very clear to any possible intelligent life forms from anywhere in the universe that what is interred within is deadly...
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