Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Bandit127]

Nikkkom and NUCENG.

There is no doubt that your recent discourse is well informed. Your posts are based on knowledge and experience that most of us can only aspire to.

However, discourse it is. A thrust and parry of argument and opinion.

Yes, it forwards our knowedge and therefore it has some value. But the price we observers pay is high in trudging through each personal retort and each base derogatory term used.

In all, it does not fit the dignity of this board and I request that you take it elsewhere. Please.

 

[zapperzero]

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111128_01-e.pdf
Gas temperature in S/C of Unit 2

11/27 6:50, it was confirmed that it read approximately 84, increasing in a staircase pattern. On the other hand, it
was also confirmed that the temperature changes of the bottom of the Reactor Pressure Vessel and the
water in the pool of the Suppression Chamber were smaller than that of the inside of the Primary
Containment Vessel (Drywell) and that there was no significant change in the temperature.

 

[Astronuc]

New analysis of Fukushima core status
30 November 2011
A new analysis of the accident at Fukushima Daiichi indicates more extensive melting probably occurred at unit 1 than previously thought, although the predicted status of units 2 and 3 remains about the same.

The bulk of unit 1's nuclear fuel went through the bottom of the reactor vessel as well as about 70 centimentres of the drywell concrete below, according to the analysis released today. However, the corium did not breach the steel containment vessel 1.9 metres further down within the concrete, or the boundary of secondary containment some 7.6 metres further still.
. . . .

http://www.world-nuclear-news.org/RS_New_analysis_of_Fukushima_status_3011111.html

 

[clancy688]

http://www.world-nuclear-news.org/RS_New_analysis_of_Fukushima_status_3011111.html

Thanks for the link, Astronuc.

"The bulk"? For the bulk to disappear from the vessel, there have to be much larger leaks than previously thought of. At least I don't believe that dozens of tons of fuel- and control rods would be able to leak through a couple of holes only a few square centimeters wide. So there has to be a much larger hole.
Previous discussions regarding leaks discarded the theory of holes on that scale because of some temperature sensors at the bottom of the RPV, some of which still were "alive". Which's highly unlikely if the core came marching through. So how does this new analysis fit with those sensors still reporting?

 

[tsutsuji]

http://mainichi.jp/select/today/news/20111201k0000m040066000c.html According to a study made by the Institute of Applied Energy (Ministry of Economy and Industry) released on 30 November, between 85% and 90% of unit 1's fuel fell down into the primary containment vessel. According to their estimate, fuel cladding tubes were damaged 5 hours 31 minutes after emergency shutdown, and the RPV bottom was damaged 7 hours 21 minutes after emergency shutdown. At units 2 and 3 about 70% of the fuel fell down into PCV. In another study, Tepco says that "considerable amount" of fuel fell down into PCV at unit 1, without saying exactly how much, and that at units 2 and 3, "some of the fuel" fell down into PCV. In all three units, core-concrete reaction took place according to Tepco. At unit 1, a maximum 65 cm depth of concrete was eroded, which, in the worse case, leaves only a 37 cm thick layer of concrete between melted fuel and the PCV's steel bottom. As there is a 7.6 m thick layer of concrete below that, the ground has not been reached by the melted fuel. The contaminated water is believed to leak through, for example, interstices in pipes. A 12 cm depth of concrete was eroded at unit 2, and 20 cm at unit 3.

Tepco's handouts on this topic (Japanese) :

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_07-j.pdf Estimate of fuel damage

Fuel damage workshop :
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_09-j.pdf Condition of fuel
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_03-j.pdf Plant behaviour after accident
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_04-j.pdf Improvement and assessment of JAEA model based on real conditions
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_05-j.pdf List of approaches to grasp fuel condition
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_06-j.pdf MAAP analysis and core-concrete reaction study

http://www.yomiuri.co.jp/science/news/20111130-OYT1T01135.htm According to the Institute of Applied Energy study, up to 2 m of concrete was eroded at unit 1. It leaves open the possibility that the RPV, which is supported by concrete, is inclined. According to a JNES study, although unit 1's RPV is damaged, units 2 and 3's RPVs are not damaged.

http://www.asahi.com/national/update/1130/TKY201111300697.html Tepco announced the results of a study saying that most of unit 1's fuel has fallen into the PCV. The melted fuel has been stopped, but part of the concrete is eroded. As the melted fuel is still 37 cm above PCV bottom, there is no "China syndrome". At units 2 and 3 most of the fuel is still in the RPV. Tepco, which had previously admitted that some of the fuel was leaking from the RPV, is now recognizing that the fuel condition is worse than that. At Three Mile Island in 1979, the fuel had remained inside RPV.

http://www.jiji.com/jc/c?g=soc&k=2011113000590 According to Tepco, it is estimated that "considerable amount" of fuel at unit 1, "some fuel" at units 2 and 3 fell from RPV into PCV. Those results were released at the "Technical workshop on core damage estimates" held by the NISA. According to Tepco, nearly all of unit 1's fuel fell down into PCV. In case all of unit 1's fuel has fallen into PCV, the depth of eroded concrete is 65 cm.

 

[Astronuc]

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111122_03-e.pdf English version.

Thanks for the link, Astronuc.

"The bulk"? For the bulk to disappear from the vessel, there have to be much larger leaks than previously thought of. At least I don't believe that dozens of tons of fuel- and control rods would be able to leak through a couple of holes only a few square centimeters wide. So there has to be a much larger hole.
Previous discussions regarding leaks discarded the theory of holes on that scale because of some temperature sensors at the bottom of the RPV, some of which still were "alive". Which's highly unlikely if the core came marching through. So how does this new analysis fit with those sensors still reporting?

See the links posted by tsutsuji.

Related article in Nuclear Engineering International (Nov 28, based on Nov 22 handouts from Tepco, those cited by tsutsuji) - http://www.neimagazine.com/story.asp?storyCode=2061233


Bear in mind, it is an analysis, not the result of observing the RPV and core. The analysis would imply essentially complete loss of cooling of the core, i.e., nearly adiabatic conditions, and effectively no water in the bottom plenum, or rather no make up. The article reports this for unit 1. It appears that Units 2 and 3 had some loss of the core, possibly through vessel penetrations, e.g., control rod drive mechanisms.

Even without melting, if a lot of fuel reacted with the coolant, there could have been dissolution of the fuel which could have washed out. The effect would have been more or less the same, namely the loss of gaseous and volatile fission products to the containment and vent systems.


I'd like to read the report.

 

[tonio]

A question for the experienced nuclear engineers.

As far as I understood, during the meltdown most or all the fuel elements in the RPV's lost their zirconium cladding. Thus a mixture of UO2 and reaction products, both in the RPV’s and at the bottom of the secondary containment, was exposed to cooling water that probably dissolved most or all of the more soluble/volatile reaction products (a.o. Cesium?) and part of the less soluble reaction products. As a result, a significant part (most?) of the radioactivity entered the cooling water and spread with it though the reactor buildings, trough the cooling systems and into the soil and the sea.

Wouldn’t it be much better that, in case of a meltdown, the fuel would be allowed to exit the RPV and the leak onto a thick layer of some kind of metal with a moderate boiling point (lower than that of concrete and steel) which it then would melt and mix with? The surface of such a pool of metal/fuel mixture could subsequently be cooled and solidified with cooling water, minimizing the dissolution of radioactive reaction products in the passing cooling water.

 

[clancy688]

It's called Core Catcher.

 

[Rive]

...According to a study made by the Institute of Applied Energy (Ministry of Economy and Industry) released on 30 November, between 85% and 90% of unit 1's fuel fell down into the primary containment vessel.

I have a gut feeling that (this time) they are overestimated it. According to the temperature and water feed graphs of U1-U3 there is not so much difference between the units.

Don't shoot, it's just a gut feeling

 

[tsutsuji]

http://www3.nhk.or.jp/news/html/20111201/t10014325321000.html In the Tepco study the worse case for unit 2 is 57% of the fuel falling down into PCV, creating a 12 cm deep concrete erosion. For unit 3 the worse case is 63% of the fuel falling down into PCV, creating a 20 cm deep concrete erosion. Tepco explained that those results have no consequence on the achievement of the below 100°C "cold shutdown". But as in unit 1 there is only about 40 cm of water at the bottom of PCV, it is questionable if cooling is stable enough and if it can be juged that "cold shutdown" is achieved.

http://news.tbs.co.jp/20111130/newseye/tbs_newseye4890525.html The study results that were shown today are nothing more than estimates. Recently, reactor temperature did not rise as expected although the water injection rate had been reduced. The recovery work is expected to go on, as if groping one's way in the dark.

 

[LabratSR]

New Video Released By TEPCO - Radiation Measurement at Fukushima Daiichi

http://youtu.be/TAPsmgrGw30

 

[zapperzero]

This

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_06-j.pdf

shows a diagram of corium flow. It is oddly specific.

EDIT: part of a big release of documents by TEPCO. I am surprised no-one except tsutsuji (thanks, again) is discussing it here, but it appears they now believe that they have core on the floor in all three reactors.

I wonder, how long they have been working under this assumption? Maybe as long as us?

 

[Rive]

Speaking only for myself: I'm waiting for English translation.

I wonder, how long they have been working under this assumption? Maybe as long as us?

It's interesting that (AFAIK) they did not tried to start the Core Spray (assuming there is a CSS) in U1. Maybe because they were already sure that there is nothing left there what can be cooled only by the Core Spray?

Ps.: I wonder if the state of the molten core on/in the containment bottom concrete was considered as a criteria regarding the recommended water level in the reactor building basements?

 

[tsutsuji]

I translated the diagrams about unit 1 on pages 21, 22, and 27 of http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_06-j.pdf

 

[Cire]

I'd still like to know how you get measurements from the bottom of the RPV if the core melted the bottom of the RPV out. All the wiring, the thermocouple junctions, everything would be vaporized. All those signals would report being "open circuit"

The thermal radiation alone would burn up all the wiring within 20+ feet just from being in proximity of the fuel. Not to mention the magnetically attached thermocouples would have all dropped off and be hanging by their wiring.

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111202/0440_goninshiki.html Plant manager etc. were mistakenly believing that the isolation condenser was running. Tepco's internal investigation panel has completed its interim report. Concerning the fact that unit 1's isolation condenser was manually shutdown shortly after 6 PM on 11 March, the report says that the managers in the seismic-isolated building and in Tokyo were mistakenly believing that the isolation condenser was still running because a data saying that the water level was above fuel had been obtained from a water level gauge. As it is thought that the water level gauge, at that time, was not measuring the true value, it is possible that based on mistaken information, they held a mistaken awareness. Fuel started being uncovered 4 hours after the earthquake at around 7 PM at the earliest. According to the interim report, it is 8 hours after the earthquake, at around 11 PM that the managers began being aware of the possibility of fuel damage, after they grasped that radiation was rising.

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111202/index.html Tepco's internal investigation panel's interim report has been publicly released. It leaves many questions unanswered such as why huge quantities of radiation were released, and via which route they were released. The report does not depart much from what was previously said. The response to the accident is judged a number of times to have been "valid" or that "its direction was good". The cause of the accident is said to be "an unpredictable tsunami which entirely robbed safety functions". The report does not address the question of why Tepco did not immediately report to the government its own prediction, made 3 years earlier, of tsunamis higher than 10 m. Eight months after the accident, there is still an enigma concerning the operation of unit 1's cooling equipment, and concerning the route via which the radiation from unit 2, which is said to be the largest amount, was released.

http://www.tepco.co.jp/cc/press/11120203-j.html Tepco's internal investigation - interim report (Japanese)

 

[m82]

Are there any photos of the pressure vesel inside Fukushima reactor buildings (before accident)?
Also, how thick is the pressure vesel?

 

[Rive]

I translated the diagrams about unit 1 on pages 21, 22, and 27 of http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_06-j.pdf

Thanks.

I could translate some parts of the documents by copying the text to the googletrans: really interesting, thanks for them.
One question. AFAIK as the fuel melts, some parts of it forms some solid crust around the liquid corium. Do anybody knows anything about the heat produced by the liquid corium and the crust, relative to the decay heat produced by the whole fuel assembly?

The analysis assumes that the whole fuel amount slumped down to the bottom of the containment: but the liquid part will escape on the first leak of the RPV, and only the crust will be left behind to erode the RPV further...

 

[zapperzero]

Thanks.
The analysis assumes that the whole fuel amount slumped down to the bottom of the containment: but the liquid part will escape on the first leak of the RPV, and only the crust will be left behind to erode the RPV further...

If there is a crust, it would be metallic in composition, something like a steel foam with decay products mixed in. It would probably not be hot enough to do further damage, even if left uncooled

 

[Astronuc]

One question. AFAIK as the fuel melts, some parts of it forms some solid crust around the liquid corium. Do anybody knows anything about the heat produced by the liquid corium and the crust, relative to the decay heat produced by the whole fuel assembly?

Melting solids absorb heat (heat of fusion). The chemical reactions, e.g., oxidation of the Zircaloy would cause heat - probably on the order of decay heat.

The issue with corium, melted core, is that is increases the risk of breach of the pressure vessel, with an additional concern that if liquid metal falls into water, there will be a steam explosion.

 

[Rive]

Sorry, I wasn't clear enough. I have to think it again.

So, as the core debris melting through the lower plenum and falling to the bottom of the RPV, the temperature there will ramp up: the slowly decreasing decay heat (and possibly other chemical heat sources) against the cooling effect of the RPV wall and the melting.

When the RPV wall breached, the liquid (and most hot) part of the core debris will immediately leave the RPV. This would create only a/some smaller leaks instead of cutting down the whole RPV bottom.

When this happens, the heat generated within the RPV will be reduced to the heat generated by the remaining crust, while the cooling will be ~ the same as before.

As the average temperature of the containment rises, the cooling of the RPV will decrease, so maybe all this will be iterated some times.

But at the end there must be an equation between the cooling by the RPV (around the melting point of iron) and he heat generation of the stuff still inside. After this equilibrium reached, there will be no more liquid corium leaving the RPV.

If the decay heat of the corium, and the RPV 'natural' cooling can be estimated then maybe we can give a number for the amount of hot stuff still inside U1 RPV.

?

 

[Idaho_246]

Melting solids absorb heat (heat of fusion). The chemical reactions, e.g., oxidation of the Zircaloy would cause heat - probably on the order of decay heat.

The issue with corium, melted core, is that is increases the risk of breach of the pressure vessel, with an additional concern that if liquid metal falls into water, there will be a steam explosion.

Can we therefore assume that there was no water in the PCV when the corium dropped. When water was later introduced, the corium would be cooled from the top and sides but not from the bottom. Why then did it not penetrate down further into the PCV?

 

[I_P]

Can we therefore assume that there was no water in the PCV when the corium dropped. When water was later introduced, the corium would be cooled from the top and sides but not from the bottom. Why then did it not penetrate down further into the PCV?

This information from TEPCO is based on modeling/simulation, not observations. Until they can physically check all such determinations will be educated guesses using the external observations (like radiation levels, composition of the emitted particles, sensor readings, etc) as inputs. The answer to your question "Why did it not penetrate down further..." is that no one really knows at this point where the corium is, except that there have not been steam explosions so far...

Still, many months after the accident, we don't know many many things and the accident scenarios put forth are best guesses based on the available (and incomplete) evidence.

 

[etudiant]

It would be helpful to have some idea of the scale of a possible corium steam explosion.
The current decay heat is less than 5 megawatts per reactor, afaik and there is water being injected at a steady pace, so at least the top of the corium is in water. That should limit any explosion.
A steam explosion would require the underside of the corium to reach new moisture and flash it into steam in a confined space, which seems to be a recipe for a corium burble rather than a massive explosion, unless I missing something important.
Has this eventuality been modeled in any public documents?

 

[zapperzero]

Can we therefore assume that there was no water in the PCV when the corium dropped. When water was later introduced, the corium would be cooled from the top and sides but not from the bottom. Why then did it not penetrate down further into the PCV?

It's the other way around. This is a scenario we are discussing. The scenario, being something of a worst-case, ASSUMES that there was no water. We don't know that to be the case (although it may be, at least for Unit 1, where the IC was off).

 

[tsutsuji]

I have translated the abstract at the top of http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_09-j.pdf

Abstract

An estimate of Fukushima Daiichi units 1, 2 and 3's core status was publicly released on 23 May, and although cores in all three units were largely damaged and melted fuel had moved or fallen into the lower plenum, while not refuting the possibility that melted fuel had fallen ouside of the reactor pressure vessel, we estimated that most of it was being cooled in the vicinity of the lower plenum. The present public release consists of core status obtained by MAAP analysis and core status estimates based on core status evaluation that can be inferred from the behaviour of measured temperatures in every part of the plant.

Material for core status estimates were obtained by various operations, investigations, studies, and analysis performed since May. The estimates that were obtained are the following ones:

1) From the temperature behaviour of every part when injection rate is changed or when injection route is changed, it can be inferred regarding unit 1, as the RPV temperature decrease is large, that there are few fuel debris inside RPV, and regarding units 2 and 3, that fuel debris exist inside RPV.

2) As a result of calibration and of filling unit 1 and unit 2's water level gauges' reference tools with water, we can infer that a water level is not formed at the original fuel location inside the reactor, and that there is no fuel at the original location.

3) Conducting nuclear substance analysis of the gasses inside unit 1 and unit 2's primary containment vessels, we infer from cesium concentrations that fuel melted more at unit 1 than at unit 2.

4) Evaluating the heat balance of decay heat production and cooling, we estimate that the part of unit 1's decay heat that could not be cooled by the isolation condenser and by the high pressure core injection is about 3 times higher than that of units 2 and 3, and that this lead to early fuel and RPV damage.

5) From an estimate based on a RPV heat balance model, we infer that as of 10 October, at both units 2 and 3, the proportion of uncovered fuel is 3% or less, and that the fuel is largely submerged.

6) From an analysis concerning core-concrete reaction, we infer that even at unit 1, which is thought to contain the highest proportion of fallen fuel, the pedestal floor's erosion depth does not reach the PCV's inner wall.

Based on the above information and general analysis, it was possible to take further the estimations of core status we had performed in May. As a result, concerning unit 1, it can be thought that nearly all of the fuel that melted after the accident fell down into the lower plenum, and that there is almost no fuel left in its original reactor location. It can be thought that a great part of the debris that fell down into the lower plenum, fell down onto the primary containment vessel's pedestal. However, although the fuel debris create a core-concrete reaction, they were stopped due to the cooling performed by water injection and due to the decrease of decay heat, and we estimate that in the present situation, it remains inside the PCV where it is receives stable cooling. Concerning units 2 and 3, it can be thought that the fuel debris are for one part in the core's original territory, and for another part they have fallen into the lower plenum or onto the pedestal, and we estimate that in whichever place they might be, they are receiving stable cooling.

Nevertheless, far from observing directly into the reactor and into the primary containment vessel, this is a core status estimate based on various indirect informations and analysis, and in the future, using some methods, we want to grasp the situation through direct observation.

 

[Astronuc]

Earthquake not a factor in Fukushima accident
http://www.world-nuclear-news.org/RS_Earthquake_not_a_factor_in_Fukushima_accident_0212111.html
02 December 2011

The tsunami of 11 March was the 'direct cause' of the accident at the Fukushima Daiichi nuclear power plant, concluded an official investigation report. It dismissed the idea that earthquake damage was a major factor in the accident.

A safe emergency shutdown was achieved within seconds of the magnitude 9.0 earthquake, said the Fukushima Nuclear Accident Investigation Commission composed of experts independent of plant owner Tokyo Electric Power Company. Control rods were fully inserted within seconds and all 13 diesel generators started as per design when tremors disconnected the grid connection. Instrumentation was working correctly, as were cooling systems.

Shaking recorded at the site was around the maximum that the plant was designed to cope with and still maintain nuclear safety but walk-down checks by plant staff showed no indication of significant damage to coolant systems.

. . . .

I think this is premature.

Industry folks like myself are waiting to get a look inside before concluding what actually happened. Before that, we can only make some engineering/educated guesses/speculations based on external or indirect evidence. That means trying to piece together or make sense of the activity releases, the explosions, the visible damage, sounds reported by those onsite or by instrumentation, . . . .

IF the core melted, then what the instruments tell us about water levels, temperatures and pressures may not be accurate, or maybe some are, but others aren't. See the plots by Jorge Stolfi. One question then is - at what point during the event did the instrument readings become unreliable?

IF the core melted then there was essentially no cooling of the core, which essentially means no coolant in the core. The core may have melted - but at what temperature. Stainless steel melts at about 1450°C, Zircaloy-2 melts at 1800°C and the UO₂ (+TU+fission products) melts at 2800°C. On the other hand, rapid oxidation of Zircaloy occurs at lower temperatures, so that could have reacted with whatever coolant was present and produced H₂ + ZrO₂, which is the source of the hydrogen. The cladding for these units is barrier cladding, which means it has Zr-Fe liner which can oxidize pretty rapidly at high temperature. Once the Zircaloy cladding fails, the fuel (UO₂+fission products) is exposed, and at high temperature, the (M=U,Np,Pu,Am,Cm)O₂ oxidize to higher order oxides M₄O₉, M₃O₈, and MO₃, the latter of which is more soluble in water. The use of seawater, and the tsunamic flooding, complicated the scenarios regarding what happened with whatever contaminated coolant escaped. So some, or a lot, of fuel material and core could have simply chemically reacted and become an aqueous solution.

It's not yet clear at what temperature the melting occurred (IF any melting occurred) - anywhere between 1400°C and 2800°C, or perhaps slighly higher (that all depends on whether or not there was some level of heat transfer to the RPV and other structure via whatever fluid (aqueous solution, steam or gas) was present in the core).

One critical question: Was there coolant in the bottom of the RPV? That is where the control rod drive mechanisms/tubes reside. If the core melted, how did it manage to melt through the core support plate? If it did, then it had to displace any coolant present, while it melted the control rod drive mechanisms, as the then corium collected on the bottom of the RPV. Then it would have to had continue melting through the RPV (~ 5 inches or 127 mm), while melting the stainless steel guide tubes and the control rod drive mechanism (CRDMs).

Now the density of the corium is complex because it depends on the forms and proportions of melted material. Zircaloy-2 has a density of ~6500 kg/m³, stainless steel about ~8000 kg/m³, and UO2 about 10400 kg/m³ (water = 1000 kg/m³), so a molten mass can displace water.

Could the housing for the CRDMs have rupture during the earthquake? That's not clear, and TEPCO reported that the reactors scrammed. Could some other components or piping rupture during the earthquake, or during after shocks? Certainly any failure of the piping connected to the primary system would have made it difficult to get water to the core.

As for water under the RPV but in the PCV, that's not clear. If there was water present, but the rate at which a melted core dropped through the RPV was slow enough, there would not necessarily be a steam explosion.


Some useful data here (I know both authors) - http://www.ornl.gov/info/reports/1989/3445606042920.pdf

Assessment and management of ageing of major nuclear power plant components important to safety: Metal components of BWR containment systems
http://www-pub.iaea.org/MTCD/publications/PDF/te_1181_prn.pdf

Assessment and management of ageing of major nuclear power plant components important to safety: BWR pressure vessels.
http://www-pub.iaea.org/MTCD/publications/PDF/TE_1470_web.pdf

Integrity of Reactor Pressure Vessels in Nuclear Power Plants: Assessment of Irradiation
Embrittlement Effects in Reactor Pressure Vessel Steels
http://www-pub.iaea.org/MTCD/publications/PDF/Pub1382_web.pdf

http://nuclearsafety.info/ageing-management-and-long-term-operation/

The reactor pressure vessel (RPV) wall thicknesses for the BWR fleet were obtained from Reference 9, shown in Table 2-3. The maximum wall thickness is 7.125 inches (181 mm) and the minimum wall thickness is 4.47 inches (113.5 mm). The maximum vessel inner diameter is 254 inches (6.45 m) and the minimum vessel inner diameter is 185 inches (4.7 m).

The average wall thickness of the BWR fleet is 5.897 inches (150 mm). There is one vessel each at 4.47 inches (113.5 mm), 5.063 inches (128.6 mm) and 5.29 inches (134.4 mm). All other vessels are at 5.375 inches (137 mm) or thicker.

http://pbadupws.nrc.gov/docs/ML0906/ML090630402.pdf

9. BWR Vessel and Internal Project, BWRVIP-60-A, "Evaluation of Stress Corrosion Crack
Growth in Low Alloy Steel Vessel Materials in the BWR Environment," Technical Report
1008871, Electric Power Research Institute, Palo Alto, CA, June 2003.

BWRVIP-203NP: BWR Vessel and Internals Project
RPV Axial Weld Inspection Coverage Evaluation
EPRI 1016572 NP
Final Report, January 2009

 

[tsutsuji]

I attach a translation of the figures on pages 12 and 15 of http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_09-j.pdf

 

[clancy688]

(IF any melting occurred)

Excuse me for being unpolite, but what the hell is that supposed to mean? Granted, we don't have any clue what the situation inside the RPV really looks like.
But a few months ago TEPCO recalibrated the water level gauges of Unit 1 and discovered that, at that time, the water level was actually BELOW the bottom of the fuel. At the same time, temperature readings indicated that the fuel was sufficiently cooled, therefore well below its former position.
I'd like to read an explanation which describes how nearly all of the fuel could have relocated to the lower RPV plenum without melting.

There were times when we couldn't say for sure if there has "only" massive cladding damage or also melting occured. But those are long gone.
There is molten fuel in Fukushima. That's what I consider a fact.

 

[etudiant]

Not sure if melting is inherent in the core destruction process.
Presumably the fuel rod cladding could get oxidized and flake off before anything melts, with the fuel pellets falling down to collect on top of the core support plate.
That heap of fuel might then melt from the inside out and the melt would move as the TEPCO simulation suggests. Certainly it is hard to envision the fuel breaching the RPV without melting, although
I am fascinated by Astronucs comment that:

Once the Zircaloy cladding fails, the fuel (UO2+fission products) is exposed, and at high temperature, the (M=U,Np,Pu,Am,Cm)O2 oxidize to higher order oxides M4O9, M3O8, and MO3, the latter of which is more soluble in water. The use of seawater, and the tsunamic flooding, complicated the scenarios regarding what happened with whatever contaminated coolant escaped. So some, or a lot, of fuel material and core could have simply chemically reacted and become an aqueous solution.

That is a novel idea, that the fuel might in fact have leached out to some extent. However, that should be detected by the water treatment process and nothing has even hinted at that afaik.