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Good article and translationSotan said:Hello again everyone.
Approaching 10 years since the accident there seems to be an increased interest in the situation of Fukushima Daiichi.
I am posting below the link to an article, originally in Japanese, translated with google - I think google translation from Japanese to English has improved considerably in recent times (I wonder if you agree). The article gives some opinions of experts regarding the (im)possibility of removing the fuel debris.
https://translate.google.com/translate?sl=ja&tl=en&u=https://news.infoseek.co.jp/topics/toyokeizai_20210308_414695/?l-id%3DTopMainTopic_busi-econ_1
Concrete can melt, but not this way, and when it really melts, the rebars going with it.Sotan said:(Does concrete "melt" ? Does it melt in such a way that the reinforcing steel bars remain? Rather then melting perhaps it is some different sort of chemical reaction that erodes it?)
Concrete could melt: "The melting point of quartz sand is about 1,650 degrees Celsius while the melting point of cement is approximately 1,550 degrees Celsius."Sotan said:some of the surrounding concrete to "melt". (Does concrete "melt" ? Does it melt in such a way that the reinforcing steel bars remain? Rather then melting perhaps it is some different sort of chemical reaction that erodes it?)
I did see your references and I'm not disputing them, but overall I find this to be bit weird. Ordinary house fires with modern plastic and wood furniture etc items easily reach "few hundred degrees" and more at ceiling level.Rive said:The 'concrete' part loses integrity at few hundred degrees and dislocates from the (still solid at that temperature) rebars.
Usually that kind of fire won't heat up the whole concrete panel to that temperature. In such cases only surface spalling (few cm depth) is expected.artis said:Ordinary house fires with modern plastic and wood furniture etc items easily reach "few hundred degrees" and more at ceiling level.
I talked about it back in 2017, the process is called spalling. Great visual demonstration of it here:Sotan said:It is speculated that the high temperature molten fuel that fell to the bottom of the reactor caused some of the surrounding concrete to "melt".[...] Now they worry that some of the molten fuels might not be located in the limited area right under the reaction vessel, but also spread to a certain degree in the surrounding areas too.
Charles Smalls said:you can clearly make out the textbook morphology of heat induced damage where the material breaks up in a characteristic layered sheet-like manner. But it's just an example. The real spall site of interest is directly outside the Unit 1 pedestal doorway to the PVC. This is the presumed pathway any molten fuel would have flowed along as it left the RV and the latest data shows that the deposited material found there is too inactive to be the fuel itself. That indicates that the fuel did pass through or under that area.
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Charles Smalls said:![]()
(34:03) According to this graphic, they suspect hot molten fuel somehow made it's way into the suppression pool and burned through the bottom of the torus to contaminate and leak water from there.
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(34:09) They seem to be experimenting with different concrete consistencies and mixtures so they can get a mix to pipe into the suppression and plug the suspected leaks.
That's a puzzling statement given that the steel would melt at a lower temperature than concrete, i.e., if the concrete had melted, the steel should have melted before the concrete. Perhaps the concrete reacted chemically with seawater and whatever compounds were in the seawater.Sotan said:"Photos taken by a remote-controlled robotic device sent into the No. 1 reactor found that a large portion of the concrete base supporting the pressure vessel appears to have melted, leaving only a metal framework holding up the pressure vessel.
Rebars does not supposed to have (significant) structural strength without the support of concrete. High temperatures are just extra.Sotan said:Is the whole thing now really supported by those steel bars only? High temperatures would not have weakened that steel dangerously?
According to areas marked red in this TEPCO pdf, they seem to be estimating severe concrete damage to ~27% of the pedestal concentrated around the doorway. Plus the pedestal inner walls.Sotan said:How the concrete got cleaned so nicely away from the steel? Temperature did not exceed melting point of steel. How thick was that wall? Is the concrete gone everywhere, 360 degrees? Or just in portions? Is the whole thing now really supported by those steel bars only?
These values do not account for the sensible heat needed to raise the concrete components from the decomposition temperature to the melt temperature [...] Of particular significance is the high melting temperature of the limestone concrete, which is higher than the melting temperature of steel
The gases rising from the decomposed concrete( H20 and C02 ) cam react with the zirconium metal present, leading to a heat source that can overshadow the decay heat [of the molten fuel].
> [Our] spreading analysis ignores the possible presence of water in and out of the pedestal. Such a dry case is expected to pose a much higher threat to the liner than the wet case, due to the considerable potential for melt quench and cooldown via thermal interaction with the water pool.
It is highly likely that almost all of the fuel debris migrated to the pedestal, spread over the pedestal floor, caused Link Removed , and eroded the concrete.
The causes of hydrogen generation include the cooling water-Zr reaction and MCCI in the core, but it is highly likely that most of the hydrogen generated by the former has already been discharged from the reactor building. The following two routes are conceivable as hydrogen release routes.
- Hydrogen generated in the core → safety relief valve (SRV) → S/C → D/W → PCV top flange → reactor building
- Hydrogen generated in MCCI→D/W→PCV top flange→Reactor building
After the high-temperature fuel debris migrated to the pedestal, it is possible that sufficient cooling water was not injected into the pedestal for about 10 days. Therefore, the pedestal is filled with high-temperature steam (approximately 400°C according to the rough analysis using the STAR-CCM+ code [1] ), and the inner wall of the pedestal is heated by radiation (the rough analysis using the STAR-CCM+ code [1] ] estimated that the temperature was approximately 800°C), and some of the concrete and other structural materials may have been damaged at this stage.
It is highly likely that the transition from the lower plenum to the pedestal consisted mainly of molten metal or a relatively low melting point such as U-Zr-O. Such migration of core material to the pedestal occurred within a period of several hours, and it is possible that most of the oxide-based debris remained inside the RPV. At the lower end of the CRD near the application in the RPV, deposits thought to be debris were found, and it is possible that a small amount of debris was released through this part. The gratings of the platform directly below this position and the intermediate frame below it have fallen off, but the grating of the adjacent part of the intermediate frame (the grating of the platform directly above remains) has also fallen off. In addition, considering the condition of the deposits remaining on the grating that remained on the platform, it is presumed that the spilled debris was a highly viscous fluid with a temperature below the melting point of steel. Since the temperature of outflow debris was not so high and the amount of outflow was small, it is considered that MCCI hardly occurred.
Since the D/W pressure began to rise from around 0:00 on March 14, it is believed that the core material that had migrated to the pedestal at this point was evaporating the cooling water that had been in the pedestal. However, since the D/W pressure rise was slow, it is highly possible that the migration of core materials to the pedestal was extremely slow. After 0:00 on March 14, the D/W pressure gradually increased until around 7:00 on the same day. This pressure increase is thought to correspond to the steam generation at the pedestal, and it took about 7 hours. It is presumed that pedestal migration of debris has occurred on the scale.
Investigations inside the pedestal confirmed deposits of 2 to 3 m, but this volume cannot be explained by the fact that the core materials are densely packed, and there are considerable gaps inside the deposits. There is a possibility that If a significant MCCI had occurred, the debris would have reached a considerably high temperature, which is incompatible with having many gaps in the sediments. Based on this, it has been pointed out that MCCI may not occur very often in Unit 3. On the other hand, it is possible that remelting occurred inside the pedestal sediment, and in that case, the phase state is expected to become complicated due to component segregation, remelting, and resolidification. Again, the solidification rate is likely to affect the properties of fuel debris.
I think "hard liquid mixture" simple means a very viscous material, perhaps shy of melting, where the material is not quite liquid, but has little strength/stiffness. In other words, in such a state, there is essentially no yield strength and the material flows, rather than creep. That usually a state of matter the mechanical/structural engineers would avoid.Sotan said:P37: refers to Unit 3. The new findings are at the bottom of the page. – As the fuel debris accumulated at the bottom of the RPV, a new rise in temperature greatly affected the course of the accident; the fuel debris containing a lot of metal melted first, at 1000-1300°C, but then, the fuel debris containing a lot of oxide compounds reached a high viscosity state (like a “hard liquid mixture”? sorry can’t find a good word), at 2000-2300°C, which damaged the RPV bottom on a rather large area and, over a few hours, fell on the pedestal floor.
I'm not sure where Pb would be used, even as shielding, inside the RPV.Sotan said:https://www.marlinwire.com/blog/what-is-the-melting-point-of-stainless-steelPb, lead from shielding components, with a melting point of ~330°C.
Ref: https://www.marlinwire.com/blog/what-is-the-melting-point-of-stainless-steel
- Grade 304. 1400-1450°C (2552-2642°F)
- Grade 316. 1375-1400°C (2507-2552°F)
- Grade 430. 1425-1510°C (2597-2750°F)
- Grade 434. 1426-1510°C (2600-2750°F)
- Grade 420. 1450-1510°C (2642-2750°F)
- Grade 410. 1480-1530°C (2696-2786°F)
Sotan said:there appears to be a sufficiently large route for then to some day guide one of those underwater robots into the pedestal
I'm really surprised that the Japanese government is so determined to dump the Fukushima water immediately.Sotan said:* Article in Asahi (google translated) reporting the request from Nuclear Regulation Authority to TEPCO "to evaluate the impact of radioactive material scattering outside the site, including the possibility of a hole (that might occur) in the containment vessel due to subsidence of the pressure vessel (for example in case of an earthquake), and to consider countermeasures". NRA is not convinced of TEPCO's optimist conclusion that the pedestal of Unit 1 is still able to support the reactor vessel in case of a large earthquake.
* There are many articles (like here or here or here) about the upcoming release of tritiated water from Fukushima into the ocean. All the work needed (including an underground tunnel) for this purpose has been done and efforts are being made to convince the public that the release is safe. IAEA representatives are visiting the site these days and IAEA is planning to issue a final report next month.
I assume you're worried about cesium and strontium? Presumably dissolved in the water and exchanged with water in the concrete?Astronuc said:Soluble fission products may permeated the concrete, which make decommissioning challenging.