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No melt-troughs occurred with Fukushima Daiichi units #2 & #3!
Virtual confirmation – No melt-troughs occurred with Fukushima Daiichi units #2 & #3!
(June 28, 2017) The Tokyo Electric Power Company (Tepco) has told the Japan Atomic Industrial Forum that thermal data with units #1, 2, & 3 at Fukushima Daiichi has shed new light on where the formerly molten fuel debris (corium) may be located. (1) The logic behind the process of compiling the data seems sound, but Tepco’s conclusions seem at odds with the data. More importantly, one key data-point tends to utterly collapse any notion of a unit #2 or #3 melt-through; i.e. the absence of fission product isotope Xenon-135 in the atmosphere of both PCVs!
We might point out that for several years this reporter has disagreed with the consensus opinion that there were catastrophic melt-throughs of the bottom heads of the three reactor pressure vessels (RPV). (see - http://www.hiroshimasyndrome.com/fukushima-melt-throughs-fact-or-fiction.html) We admit to the possibility of molten corium seepage through some of the control rod drive mechanism (CRDM) housings for unit #1, but entirely reject the same for units #2 & 3. Muon tomography for unit #2 has essentially confirmed our opinion for unit #2 (2). On the other hand, the images with unit #1 are inconclusive because the RPV bottom head could not be captured in the field of “vision” for the device (3), and the muon image for unit #3 is currently being generated.
Tepco says that their relatively recent reduction in coolant flow to the three vessels has revealed where solidified corium might be presently located. Tepco reduced cooling water flows by 33% for units #1, 2, & 3 RPVs - from 4.5 tons per hour down to 3 tons per hour – over the first three months of this year. Unit #1 coolant flow was lowered in January, unit #2 in February, and unit #3 in March. The resulting temperature increases due to the reduced flows were used to guesstimate the current corium locations.
For unit #1, the way temperatures measured at the bottom of the RPV rose as the coolant flow was reduced, indicates that some corium is lodged in the control rod drive mechanisms beneath the RPV. But, the company makes no attempt to say why they feel the corium is entangled in the CRDMs.
With unit #2, the RPV bottom head temperature also increased in tandem with the reduction in coolant flow, at a rate almost imperceptibly greater than with unit #1. Because of this, Tepco assumes that the majority of the unit’s corium may be massed in the RPV’s bottom head.
Tepco also says that the temperature trends tend to confirm the findings of the muon tomography images for units #1 & 2.
For unit #3, Tepco says the water in the Primary Containment Vessel is at a higher temperature than the RPV’s bottom head. Thus, the company assumes most of the corium melted through the bottom head and now lies under the water atop the PCV floor.
Unfortunately, the Tepco assumption with unit #1 makes no sense and the conclusion postulated for unit #3 defies confirmation by the data posted by the company!
If any of the three units that experienced severe fuel damage in March 2011 had a melt-through of the bottom head, it was unit #1. Let’s say that it did. The corium exiting the 8-inch-thick bottom head would be somewhere in the vicinity of 2,000oC, which would be more than enough to cause liquification of the CRDMs beneath. Tepco says some of the melted-through corium became lodged in the CRDMs because of the way temperature changed when coolant flow was reduced. But, when we compare the data posted by Tepco relative to units #1 and #2, we find the changes are almost identical. (4) If “some” of the unit #1 corium is entangled in the CRDMs, then the temperature drop for the unit #1 bottom head should be markedly less than with unit #2; but it is not!
Thus, Tepco’s temperature-based conclusion for unit #1 is in error. Since the unit #1 and unit #2 temperature changes are almost the same, the degree of assumed melt-through should be the same for both units. But, Tepco ruthlessly holds to its opinion that only unit #1 experienced catastrophic melt-through. If unit #2 didn’t experience catastrophic melt-through, then neither did unit #1! Muon tomography shows the unit #2 did not melt-through, so the temperature indicates that a complete melt-through for unit #1 did not occur.
Almost adding insult to injury, the robot investigation inside unit #1 PCV that began March 18,2017, revealed that the water in the bottom of the PCV is remarkably clear. On the other hand, it was discovered in February that the water below the CRDM maintenance carousel for unit #2 PCV appears murky! There seems to be a consensus agreement that corium will cause water to become murky, but water underneath the unit assumed to have melted-through – unit #1 – has a much higher degree of clarity than unit #2. There is no speculation on this conundrum… and your friendly Fukushima reporter has no idea, as well.
Next, there’s the problem with Tepco’s unit #3 conclusion – there is literally nothing in the Tepco data postings for March, 2017, to support it! There are two data points listed for “PCV temperature” – monitors TE-16-002 and TE-16-004. There is no listing of the actual readings for either of them. The atmosphere temperature readings in the PCV seem in line with the PCV temperatures for the unit; the PCV is at a slightly higher level than the air in the PCV, which is the same for all three units. How Tepco concluded that the water in the PCV is at a higher temperature than its RPV bottom head is a complete mystery! The actual data suggests otherwise. Thus, we must place Tepco’s conclusion with unit #3 in question!
Most importantly, while poring over the thermal data postings of unit #1, 2, & 3, I noticed something I had overlooked until now. Only unit #1 PCV has Xenon-135 in its internal atmosphere. (5) Xe-135 is produced inside a full power reactor core to the tune of ~1014 (100 trillion) Becquerels per cubic centimeter. The vast majority comes from the almost immediate decay of Iodine-135. Another major source of the isotope is fissioning of Curium-242 (Cm-242), which is bred from the Uranium and Plutonium in the core by neutron capture. But, all of this activity dies off to nearly nothing less than a week after fission stops due to Cm-242’s 9.2 hour half-life (t1/2).
However, Cm-245 is also bred in the core during power operation, albeit at a much lower rate than with Cm-242. Cm-245 has a t1/2 of 8,500 years. It’s still in the corium, in appreciable abundance. It has a probability of fission, and there are enough neutrons being generated in corium to cause a sub-critical production of Xe-135 by Cm-245 fissioning. In addition, Cm-244 is also bred and has a relatively significant spontaneous fission probability, adding to the current Xe-135 production in the corium.
Xenon is an inert gas, thus the Xe-135 will necessarily migrate through corium, and rapidly percolate through water. This is how it becomes a part of the internal atmosphere of a PCV, if its RPV has melted-through. Unit #1 PCV’s atmosphere has registered a trace of Xe-135 ever since Tepco began posting its data in 2012. It is scant – in the order of 1x10-3 (.001) Bq/cm3… but it is there. This can only mean that at least some of the corium produced inside unit #1 found its way out of the RPV and likely resides somewhere within the pedestal below. (Not lodged in the CRDMs) On the other hand, no Xe-135 has been detected in the atmosphere of either #2 or #3 PCV!
This provides quite compelling evidence that there was no melt-through of any degree with either unit #2 or #3 in March, 2011! Thus, my assertions of January, 2012, of no catastrophic melt-through of unit #1, and no partial melt-throughs for either unit #2 or unit #3, still hold. In fact, the Xe-135 evidence (above) makes it a virtual slam-dunk for my conclusions relative to units #2 & #3!
(Thanks to colleague Lars Jorgensen for reminding me about Cm-244's spontaneous fissioning.)
1 - New Data Obtained on Debris Locations Based on Temperature Changes Using Reduced Injections of Water; Japan Atomic Industrial Forum, June 20, 2017. http://www.jaif.or.jp/en/new-data-obtained-on-debris-locations-based-on-temperature-changes-using-reduced-injections-of-water/
2 - Locating Fuel Debris inside the Unit 2 Reactor Using a Muon Measurement Technology at Fukushima Daiichi Nuclear Power Station;https://www4.tepco.co.jp/en/nu/fukushima-np/handouts/2016/images/handouts_160728_01-e.pdf
3 - Reactor imaging technology for fuel debris detection by cosmic ray muon measurement: status report in Unit-1; https://www4.tepco.co.jp/en/nu/fukushima-np/handouts/2015/images/handouts_150319_01-e.pdf
4 – Parameters related to the plants at Fukushima Daiichi Nuclear Power Station, Archive pages for January-April, 2017; Tokyo Electric Power Company. http://www.tepco.co.jp/en/nu/fukushima-np/f1/pla/index-e.html
5 - Parameters related to the plants at Fukushima Daiichi Nuclear Power Station, Archive page for June 26, 2017; Tokyo Electric Power Company. http://www.tepco.co.jp/en/nu/fukushima-np/f1/pla/2017/images/17062605_table_summary-e.pdf