Today, Tepco has posted the results of the first water sample taken inside the unit #1 Primary Containment Vessel (PCV). We find results that come as more than a bit of a surprise. The interior water’s results have been compared to analyses of the water’s outside the PCV, in the basement of the unit #1 reactor building, taken in late September. First, the Cesium contamination level inside the PCV is half of the concentration found outside (35,000 Bq/cc vs. 74,000 Bq/cc). Thus, the interior water is significantly less contaminated than that in the outer reactor building. Second, the chloride level inside is ten times less than outside (19 part per million vs. 200ppm). This means that the salt concentration inside the PCV from the seawater used to cool unit #1 beginning at 8pm on March 12, 2011, is tremendously lower than outside. (Tepco; Kyodo News) This strongly suggests several things. First – the water inside the PCV is being recirculated more efficiently than that outside the robust containment walls. In other words, the recycled fresh water being injected into unit #1 is diluting the interior waters better than the exterior. Second – there can be no mixing of interior and exterior waters or the two sets of analyses would be essentially the same. While the concentration differences give us a better picture of the PCV’s interior environment, it raises a whole new set of questions as to what the actual water flow-path(s) through the building might be. Third – the interior water being lower in Cesium content than the exterior implies that the melted-then-re-solidified corium mostly remains inside the RPV itself. If the corium was mostly outside the RPV and heaped on the base-mat of the PCV – melted completely through the Reactor Pressure Vessel (RPV), as surmised by just about everyone in Japan – the interior waters should be massively higher in Cesium contamination than the exterior.
As regular readers of these updates can attest, I have long argued that if any of the three fuel cores actually melted through their RPV’s bottom head, it was unit #1. Today’s analyses put even that speculation into severe question. I have posted, on several occasions, that there was no melt-through of the unit #2 RPV. Unit #2 seems to have experienced core damage in the range of Three Mile Island in 1979, with most of the fuel cell melting and relocating to the RPV’s bottom head – but no melt-through. I have also written that I severely doubt melt-through speculations relative to unit #3 RPV. Unit #3 may well have experienced a total meltdown with full-core-relocation to the bottom head of the RPV, but once again, no melt-through. Given the differences with respect to the chemical make-up of the interior and exterior waters relative to the unit #1 PCV, and the fact that the highest radiation level inside the PCV is essentially parallel to the bottom head of the RPV (in today’s first update), I now believe it is possible that none of the reactors at Fukushima Daiichi experienced catastrophic RPV “melt-through”.