On July 28, 2016, Tepco posted a detailed handout showing Unit #2’s previously-molten fuel (corium) is re-solidified and remains inside the reactor vessel (RPV). This conclusion was drawn as a result of muon imaging, with a dark “shadow” covering the interior of the RPV’s bottom head. (1) One obvious deduction is drawn by the Asahi Shimbun, which says that past speculations of a unit #2 melt-through are probably incorrect. (2) It says, “The latest finding negates past studies that have suggested that most of the nuclear fuel inside the reactor had melted through the vessel.” Unfortunately, the Asahi has been the only news outlet in Japan to make such a flat statement. Could the Asahi be incorrect?
On page four of the handout, we can see that most, if not all of the corium remains in the bottom head of the RPV. The image also shows that some of the damaged fuel is still in the core area, where it was located before the March, 2011, tsunami-spawned nuclear calamity. If we compare the unit #2 core barrel image with the March 19, 2015, image of unit #1, we see a drastic difference. (3) Unit #1’s core barrel showed brightly, indicating full meltdown and core relocation. The core was no longer there. However, the unit #2 image does not have a nearly as bright core barrel appearance. In fact, it is quite possible that a small-but-significant fraction of the original core remains in its original location. In fact, Tepco admits that at least 20 tons of material remains. Based on the relative density of the shadowing in the unit #2 core barrel region, with literally no brightness to indicate complete degradation from top to bottom, it is entirely possible that the maximum estimation of fuel (50 tons) remains in the core barrel region.
Where do these numbers come from?
Page six of the handout states that of the 210 tons of fuel and support structures that originally comprised the undamaged core, 20-50 tons remain in the core barrel area and “about 160 tons” is collected in the RPV’s bottom head. The inherently limited resolution with muon imaging compels an approximation of the respective masses. The 20-50 tons estimation leaves the door open for continuing speculation that as much as 14% of the core might possibly have worked its way through the bottom head and re-solidified on the base-mat beneath the RPV.
At this point we might ask… why leave the door of worst-case speculation open? If there had been any significant melt-through of the bottom head – and as much as 30 tons is pretty significant – the molten condition of the corium would possibly have cascaded through the breach. That there is a substantial pooling in the bottom head strongly suggests that if there were any melting-through, it must have been a mass much less than 30 tons. In fact, it suggests the possibility that there was no melt-through at all.
So why is Tepco leaving the door open for partial melt-through speculations?
Tepco has long-succumbed to Press and political pressure to accommodate worst-case scenario speculations. This is one time its “conservative” reporting has shown them to be timid and unwilling to draw a firm conclusion that flies in the face of their previous computer-based speculations. For more than three years, Tepco and the Nuclear Regulation Authority have made status reports that entirely cater to worst-case speculation, no matter how thin the evidence for such conjecture might be. In the case of the unit #2 muon image, worst case assumption ought to be rejected.
It is time for Tepco and the NRA to assert themselves and draw a conclusion that is most likely. When compelling evidence emerges that literally demands rejection of the low-probability worst-case scenario, there should be no room left for the worst-case scenario to perpetuate!
The Fukushima Daiichi Unit #2 muon scanning image virtually demands that we conclude that there was no compromise in the lower RPV head. It is likely that none of the corium, while still molten, made its way through the head or any of the penetrating control rod drive mechanisms (CRDM). To conclude otherwise make no sense to this reporter!