Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[mscharisma]

http://www.spiegel.de/wissenschaft/technik/0,1518,751818,00.html

Thank you. And it's actually funny that the article is from March 18 and very well makes my point: the scale changes.

Being on the brink then meant that cooling was imperative since "otherwise, a meltdown of the core could occur or, respectively, continue in the reactors."
"Die Kühlung der Reaktoren und der Abklingbecken der Reaktoren 1 bis 4 ist entscheidend. In den Reaktoren könnte sonst eine Kernschmelze einsetzen beziehungsweise weiterlaufen."
For Unit 1, we're past that and on a new brink, I suppose.

 

[default.user]

Salad ideas:

The corium is very quickly a firm skin.
At this time, borated water is not able to do its work.
The gescmolzenen pellets in the corium are critical and can tear the skin.
Borated water penetrates and prevents the critical situation.
Neutrons are absorbed. The skin will close again and the result is again criticality. The skin tears open again and borated water penetrates. Neutrons are absorbed and prevented the criticality.
The theater goes on until all the fission products are exhausted. The corium remains hot until the fission products are exhausted.
So far so good.
As long as not burst the vessels, one can talk from a funny cat and mouse game.

 

[GJBRKS]

Chubu Electric Power Co. said Sunday that cooling system trouble delayed the 'cold shutdown' of the No. 5 reactor at its Hamaoka power plant in Shizuoka Prefecture for about two hours earlier in the day, while ruling out any external release of radioactive substances.

http://english.kyodonews.jp/news/2011/05/91196.html

Has it reached cold shutdown yet ?

(EDIT : yes it has ... http://search.japantimes.co.jp/cgi-bin/nn20110516a5.html )

The Nagoya-based firm said the problem in the reactor's cooling system was found Saturday evening after a gauge indicated that around 400 tons of seawater had flowed into the condenser at around 4:30 p.m., most likely because of a piping problem.

The water also found its way into the reactor, making it necessary to desalinate it, the company said.

That's kinda what you see ... One report speaks of 2 hours trouble on a Sunday , the other says the problem had been found Saturday starting at 4:30 p.m. ...Tsk Tsk

 

[default.user]

I forgot to mention:

While the skin breaks and water comes with the liquid part of the corium in connection arises suddenly steam. This steam can be derived as barrier-free. Otherwise there is a steam explosion. In addition, the highly radioactive steam.

 

[mscharisma]

You highlight vividly how the spin is corrupting the message. Then Japan's government wonders about the emergence of 'rumors' in other countries.
Afaik, the likelihood of another major blast is much smaller now, although reactor 3 could still experience a steam explosion if/when the core remains believed to be still in the reactor pressure vessel melt through and fall into the now flooded dry well. The only question is how much more damage is likely to come, hopefully relatively little, as the bulk of the volatile radioactives have already been boiled off.

Beyond that small comfort, the situation will remain as is for the rest of the year and perhaps much longer,
a shattered site littered with intensely radioactive debris, flooded with a lake of water 100x300x3 meters deep, so radioactive that it can only be approached briefly. That will make cleaning up the spent fuel pools a very long term proposition, so airborne emissions are also likely to continue for a long while.
If the readings from sewage plants near Tokyo and beyond are correct, the accident has already contaminated the larger part of Honshu to the point that the government will have to raise the allowable radiation contamination standards. Seafood and seaweed products are likely at risk for a very long time.

The spin and hence the correct/best interpretation of the scientific data (by you guys, of course, not me) is what's so important.

It's maybe just natural that a progression from bad to even worse and yet worse is difficult to accurately put into perspective, but at the same time I'm concerned about deliberate minimization by the plant operator, government, and mass media and the consequences for the affected population. For those of us Europeans and Russians who are old enough to remember the Chernobyl aftermath, very similar to the Fukushima situation, the info for the public started with "don't worry about it," went to "well, it's bad but not dangerous," to "yeah, it's worse than thought, but not deadly." (obviously paraphrased)

If the risk from Units 1-3, at a minimum, is the continued contamination of ground- and seawater through radioactive water leakage, I hope every layperson makes a decent effort to understand what's going on and what the consequences for him/her may be.

Many thanks to you knowledgeable folks and experts who put so much effort into collecting and interpreting the scientific data as it becomes available. Even if we laypersons may not comprehend the highly technical stuff, we at least don't have to rely on the spin in press releases and mass media reports because you instead provide us with -at a minimum- educated guesses what's going on and we can then guess what it all means for each individual in his/her respective geographic location.

 

[etudiant]

http://english.kyodonews.jp/news/2011/05/91196.html

Has it reached cold shutdown yet ?

(EDIT : yes it has ... http://search.japantimes.co.jp/cgi-bin/nn20110516a5.html )

Wow! That is a serious screw up.

To have sea water enter the reactor means that they really gummed up the shutdown operations.
Under normal circumstances, heads would roll. Now, it's just a problem in a plant that is dead anyways.
Still does not speak well for the competence of the operation.

 

[KJones]

This site is complaining that Chubu delayed 20 hours before making an announcement about the seawater: http://criticality.org/2011/05/hamaoka-shutdown-underway-problems/

Based on Chubu Electric’s 20 hour delayed announcement on the morning of May 15th, they confirmed that seawater has been detected in the water circulation system.

Chubu Electric’s excuse for the 20 hour delay

Quoting NHK news this is Chubu Electric’s response when asked about the 20 hour delay in releasing information to the public about the cooling water contamination problems,

“There has not been environmental releases of radiation, because Chubu Electric is not legally subjected to reporting the situation to the government, we did not make any immediate announcement when it occured.” Chubu Electric, May 15th press annoucement.

 

[Uagrepus]

There are only a few images from the period after 3 blew but before 4 went up.

I don't think the resolution is high enough to be 100% sure, but it looks to me like there was already debris fallen onto the pipe in the place where it is later shown to be broken. And I think its always been a pretty likely bet that it was falling walls of reactor 3 that caused the damage.

Yes, in your pictures it already looks like the broken pipe *after* Unit 4 explosion:

 

[etudiant]

The spin and hence the correct/best interpretation of the scientific data (by you guys, of course, not me) is what's so important.

It's maybe just natural that a progression from bad to even worse and yet worse is difficult to accurately put into perspective, but at the same time I'm concerned about deliberate minimization by the plant operator, government, and mass media and the consequences for the affected population. For those of us Europeans and Russians who are old enough to remember, very similar to the Fukushima situation, the info for the public started with "don't worry about it," went to "well, it's bad but not dangerous," to "yeah, it's worse than thought, but not deadly." (obviously paraphrased)

If the risk from Units 1-3, at a minimum, is the continued contamination of ground- and seawater through radioactive water leakage, I hope every layperson makes a decent effort to understand what's going on and what the consequences for him/her may be.

Many thanks to you knowledgeable folks and experts who put so much effort into collecting and interpreting the scientific data as it becomes available. Even if we laypersons may not comprehend the highly technical stuff, we at least don't have to rely on the spin in press releases and mass media reports because you instead provide us with -at a minimum- educated guesses what's going on and we can then guess what it all means for each individual in his/her respective geographic locations.

There is no doubt that the leadership , both corporate as well as national, is treading a fine line, trying to minimize concern without doing real violence to the truth. So any public statement should be read in the context that it aims to reassure first, even before it informs.
The problem this creates, at least imo, is that it does not adequately take into account the extra vulnerabilities of children, who play in the dirt a lot and who are more susceptible to contamination damage.
Of course, the damage done to everybody, children included, from a panic or mass evacuation would be even worse, so the same logic applies, reassure and tell the unavoidable minimum. Not an easy balancing act.

 

[AntonL]

Sea water in Hamaoka cooling system

If sea water is detected in the cooling loop of the reactor, leads me to believe that there is a problem in the heat exchanger of the cooling system. In the closed loop you have reactor water which exchanges its heat to the sea water. If there is a crack in the heat exchanger sea water can be sucked into the reactor circuit, either due to Bernoulli effect at high flow rates or the system was at under pressure as steam condensed to water at the cooling stage and fresh water was not replenished fast enough.

 

[GJBRKS]

Sea water in Hamaoka cooling system

If sea water is detected in the cooling loop of the reactor, leads me to believe that there is a problem in the heat exchanger of the cooling system. In the closed loop you have reactor water which exchanges its heat to the sea water. If there is a crack in the heat exchanger sea water can be sucked into the reactor circuit, either due to Bernoulli effect at high flow rates or the system was at under pressure as steam condensed to water at the cooling stage and fresh water was not replenished fast enough.

Would such a system have 3 cooling loops , another heat exchange at the reactor ?

If not then I guess contamination of reactor water into the environment can not be ruled out.

(I'm not saying it would be radioactive , just that it would have breached containment)

 

[SteveElbows]

Wow! That is a serious screw up.

To have sea water enter the reactor means that they really gummed up the shutdown operations.
Under normal circumstances, heads would roll. Now, it's just a problem in a plant that is dead anyways.
Still does not speak well for the competence of the operation.

I don't think they had stated that the Hamaoka closure was to be permanent. I think it was supposed to remain shut whilst they came up with new measures against things like tsunami. Of course its possible that it would never reopen if such measures could not be completed or the politics remained very difficult, but that's not how they wanted to present the story at this time.

 

[turi]

A PDF about unit 1 shows two floor plans. I don't know if they show anything not known here yet:
http://www.meti.go.jp/press/2011/05/20110515001/20110515001-5.pdf
(I believe it's about the staircase in the north-west where they have seen water accumulating).

 

[Caniche]

Tepco have been reading this forum for ideas, I postulated this a month ago


and again later

That was just too easy,do another one cos I can't quite believe Tepco are that dumb. Then again on reflection ,no need.

 

[jlduh]

That's odd.

3/11 - 14:46: Tohoku earthquake
3/11 - 15:41: Tsunami impact
3/11 - 16:36: Battery failure in Unit 1
3/11 - 17:07: Isolation Condenser active in Unit 1
3/12 - 05:30: Unit 1 primary containment pressure 820 kPa
3/12 - 06:46: +16 hours
3/12 - 10:17: Venting of Unit 1
3/12 - 11:20: Fuel rods 90 cm exposed in Unit 1
3/12 - 15:36: Explosion of Unit 1

The meltdown occurred before venting and before the fuel rods got exposed?

If they say now truth, the only explanation is see is that... they were lying before! Well, we can understand it, why panicking people recognizing the reactor just melted in the first 16 hours?

Is their new assessment not far from the old original document assessing what would happen in a timely manner in case of severe accident in a BWR mark I reactor?



By the way, have these studes already been posted here?

1) VERY INTERESTING STUFF IN IT, details and schematics including failure modes

THE IMPACT OF BWR MK I PRIMARY CONTAINMENT FAILURE
DYNAMICS ON SECONDARY CONTAINMENT INTEGRITY


http://www.osti.gov/bridge/servlets/purl/5835351-nR29Hq/5835351.pdf



2) IDENTIFICATION AND ASSESSMENT OF BWR IN-VESSEL
SEVERE ACCIDENT MITIGATION STRATEGIES

http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/24/072/24072657.pdf

 

[jlduh]

That was when all the industry experts told us in TV that such a thing was impossible.

Yep, i remember well also...

It will be very interesting to re-read this thread from scratch after some time.

 

[bytepirate]

Nice work, Anton!
(BTW: I remember there were some counterarguments against your theory. Now that TEPCO has adopted your theory is there any more validity in these counterarguments? TEPCO engineers must know their plants so their evaluations about possible theories have a certain level of assertiveness, which the outsiders lack.)

i still don't buy that. there were more than 24 hours between the explosions of #3 and #4...
and we have this information ('tepco says...') only second hand. the same quality of information as we had before for a contradictory explanation (explosion in #4 blowed away a gate and reflooded the SFP).
let's wait and see ;-)

something else:
as i am not able to see the 'live feed', can anyone who can see it confirm, that everything looks normal?
the webcam shows something, that might be smoke: http://pointscope01.jp/data/f1np/f1np1/pic/20110516060032.jpg (its the same pic, that the webcam shows right now)

 

[mscharisma]

There is no doubt that the leadership , both corporate as well as national, is treading a fine line, trying to minimize concern without doing real violence to the truth. So any public statement should be read in the context that it aims to reassure first, even before it informs.
The problem this creates, at least imo, is that it does not adequately take into account the extra vulnerabilities of children, who play in the dirt a lot and who are more susceptible to contamination damage.
Of course, the damage done to everybody, children included, from a panic or mass evacuation would be even worse, so the same logic applies, reassure and tell the unavoidable minimum. Not an easy balancing act.

No doubt, the responsibility is great and the considerations multifold, and I sure wouldn't want to trade places with anyone in Japan's government or Tepco management. However, personally, I feel more reassured by information, even if bad or difficult to comprehend, than by reassurance first and gradual information later. Agree with your assessment on the impact on children, btw.
But I don't want to get this thread off track. Just wanted to get a "reality check" from more knowledgeable people on my layperson's impression that the overall trend of things is going towards worse rather than better and express my gratitude to you guys who tirelessly try to make sense out of the mess of data and/or lack thereof.

 

[SteveElbows]

If they say now truth, the only explanation is see is that... they were lying before!

I would not accuse TEPCO of always telling the truth, but there are more explanations for this than mere lying.

They learned something when humans were able to enter reactor 1 building, just as they learned something when they increased the water rate but levels of water did not rise. This allows them to be a bit more certain about what happened.

Certainly they did not seem to like the word meltdown being used in the early days, but they didnt do very much to discourage the theory, only to discourage ideas that the core may have left the rpv, so depending on what definition of meltdown people were using, it is not necessarily wrong for TEPCO to dislike the word. I don't think their position has changed completely. More detail on the timescale, and not hiding behind %age of fuel damage assumptions anymore, but in other ways similar to the past. They say the core is still in the reactor vessel, same as before. And they have long said that they think water is cooling the fuel even though water levels are below what would be top of the fuel. Maybe in the past they wanted to give impression that fuel was now half melted, so half its original height, with water still high enough to cover it. But now they have water measurements that are much lower, so they adjust the assumed height of the fuel downwards so that water still covers it.

 

[~kujala~]

The full theory for #4 includes also the explanation for the fires. Now TEPCO thinks that the explosion may have caused oil to catch fire:

By assessing damage to the building, TEPCO says the blast was likely due to a hydrogen explosion, and that oil may have triggered fires after the blast.

http://www3.nhk.or.jp/daily/english/16_02.html

It was not so long when somebody said here:

Is it possible that the explosion could have immediately heated a small part of the oil over fire point and also ignited it? The result of this would be a couple of fires here and there that would be extinguished by itself? The explosion itself could be explained by AntonL's theory (radiolysis) or something else.

 

[mscharisma]

as i am not able to see the 'live feed', can anyone who can see it confirm, that everything looks normal?
the webcam shows something, that might be smoke: http://pointscope01.jp/data/f1np/f1np1/pic/20110516060032.jpg (its the same pic, that the webcam shows right now)

What the live web cam shows at the moment is not much of anything across the immediate shore line. It looks like they're completely fogged in and the buildings cannot be seen at all. Maybe the fog bank depicted off-shore in your still photo has moved ashore.

 

[SteveElbows]

something else:
as i am not able to see the 'live feed', can anyone who can see it confirm, that everything looks normal?
the webcam shows something, that might be smoke: http://pointscope01.jp/data/f1np/f1np1/pic/20110516060032.jpg (its the same pic, that the webcam shows right now)

The weather has been getting in the way of clear images on the feed so far this morning in Japan, but it did clear up enough earlier (about half an hour ago) that I was able to take a clear look for some minutes. Steam from reactor buildings 2 & 4 was showing up very well, but as discussed before on this thread we cannot read much into this, due to the impact weather can have on the visibility of this stuff. It was also rising straight upwards at some moments, and was visible fairly high in the sky, which would also make it show up better on TEPCOs webcam pictures.

 

[jlduh]

According to Tepco, hyrogen produced in the overheating of the reactor core at Unit 3 flowed through a gas treatment line and entered Unit No. 4 due to a breakdown of valves. Hydrogen leaked from ducts in the second, third and fourth floors of the reactor building at Unit 4 and ignited a massive explosion.

http://online.wsj.com/article/SB10001424052748703509104576325110776621604.html

Very nice domino effect (one more) if this proves true.

Would be interested to see what was the rated probability of this event in the safety assessments... I already mentionned the fact that some of these safety studies are lacking seriously on this domino effect side (and "thinking the unthinkable", as they said recently In France for the reassessment of french nuclear plants to be done ASAP)

It seems god plays with dices and wins much more than expected by experts in this desaster.

 

[Borek]

as i am not able to see the 'live feed', can anyone who can see it confirm, that everything looks normal?

I see lots of fog. In fact there is so much fog, I can't see NPP buildings, just tower tops. But that's not much different from what I have seen several times this time of the day before.

 

[Caniche]

Just can't remember the Hindenberg mushroom cloud and that was a big hydrogen sucker.May be modern hydrogen is different?

 

[Caniche]

I see lots of fog. In fact there is so much fog,

Pea souper in so many ways

 

[bytepirate]

... but as discussed before on this thread we cannot read much into this, due to the impact weather can have on the visibility of this stuff...

i know that. i am the guy, that lives next to some cooling towers.
only fog on the webcam right now, but the picture before was 'supicious'. let's assume everything is ok. we will know soon, if its not.

 

[TCups]

OK, so you claim to have seen the ignition, I should have suspected someone would :-) But at least I can ask you to reconsider. It is not easy, I know. As the old sage said, 'the subtlest act is to set another before you'. But, I am presenting you with new evidence, so that would be a good reason to reconsider.
I have looked at those images and the preceding discussion. The video used to produce these frames are, excuse me, crappy. No wonder Jorge protested! I would too, if I had found this site earlier. But, fortunately much better video sources are available, and for your convenience frames from such a video is at gyldengrisgaard.dk/fuku_expl3/

The source video for those frames, unlike the one you have been looking at, comes in HD, and it comes prezoomed to the interesting parts. Consequently you can see the same as you can in the poor video, but also -- sans comparison -- more.

Here is a side by side mount of the first frame showing a flash of fire from the two sources, sticking with your numbering, this is frame number 2:

And here is a side by side mount of the frame immediately _preceding_ the first frame with a flash of fire, frame number 1:

From both sources it is clear that an explosive event is ongoing in frame 2.
From the better source it is clear that an explosive event is ongoing in frame 1, too.

Now we can say, these were two separate events. In frame 2 we could then be seeing the ignition of event 2, and this is undoubtedly outside the building. In frame 1 we see the effects of another event, but we cannot say where that one ignited.

Alternatively we can say that the two frames show the same explosive event at two different points in time, separated by 0.03 seconds. In frame 1 the flash of fire from the event is not yet visible behind the building. In frame 2 it has grown to become now visible over the building. Neither of the two frames are able to show us the ignition.

I am sure it is well known what Ockham would have to say about this. Ockhams principle is more of a philosophical nature, than a scientific one, admittedly. Still, it seems to be a sound principle also in science, that we consider the simpler explanation first, and only adds further assumptions as needed.
Yes OK, that's, er, quite simple, -- at least it could've been more complex.

However, a simpler explanation exists: A single explosion with a flash of fire coming from youguesswhere shooting out through the eastern wall, the flash of fire becoming visible to us 0.03 seconds later.

@MadderDoc

Sir: Surely you know that the history of this thread shows that I and many others have been considering and re-considering evidence from day 1. Your video evidence looks different from my video evidence, hence I must reconsider, and will. But, surely you do not think my comment was made without consideration to begin with, as having taken the time to do the frame by frame analysis of the video I had was done, and was done to the best of my abilities.

Further, even on reconsideration, the point is a subtle one and does not alter my contention that the explosion at Building 3 originated first from the primary containment, vented over and through the upper portions of SFP3, exited the southeast corner of Bldg 3, and was followed by a more generalized explosion of Bldg 3 and a vertical column of steam, the origin of which was vaporization of at least part of the water in SFP. The later carried aloft large amounts of heavy debris and a large amount of radioactivity which could come from the volatile contents of the primary containment or the damaged contents of SFP3 or both.

If presented with new evidence that substantively refutes that, I shall reconsider that, too.

T-Cups and Antoni must be auditioning for Fox or one of the other news groups. Using limited knowledge, convoluted science, and conjucture. BTW, radiologically clearing a place and entering a cleared place are to diametrically opposites. Evidently you have not worked in contaminated areas or had to cleared them. Pictures do not always tell the whole story...that is how Fox news and others get attention. Have you got enough yet?

@AntonL

Anton: I haven't heard back from Fox News yet. You?

NewsFlash: It is amazing how far the photographic evidence, combined with limited knowledge, convoluted science, and conjecture can sometimes take one in pursuit of the truth.

PS: MadderDoc

A critical comparison of the side-by-sides you provided shows that, yes, there are some differences in resolution, but the far larger differences between the two images are in the color saturation and hues. What, if any significance may be attributed to that?

Do you think the higher resolution image shows conclusive evidence of explosion of the entire upper floor of the building or a differential expansion of the southeast portion of the roof and walls? Do you believe that the images you added conclusively refute the contention that "fireball" conflagration of the gas cloud occurred mostly or entirely after the gas was ejected? In short, I get the message you think my interpretation is in error, but I am not getting a clear picture of your differing interpretation and the significance of the difference in your interpretation vs mine.

 

[etudiant]

Just can't remember the Hindenberg mushroom cloud and that was a big hydrogen sucker.May be modern hydrogen is different?

You are surely correct, a pure hydrogen explosion or combustion is a pretty low key event, in fact the flame is invisible, at least afaik.
So the orange fireball from reactor 3 strongly indicates substantial additional combustible/explosive material was involved. Normally flames are luminous because of white or red hot particles, generally of carbon, that are carried in the heated flow. Where these came from in this case is not yet clear.

 

[DSamsom]

Tepco does not trust the other water levels either. Will try to check on reactor 2 and 3.

TEPCO says the gauges at the No.2 and 3 reactors might not be showing the actual water levels, and that the worst case is that the rods have melted down.

http://www3.nhk.or.jp/daily/english/16_04.html

 

[clancy688]

1) VERY INTERESTING STUFF IN IT, details and schematics including failure modes

THE IMPACT OF BWR MK I PRIMARY CONTAINMENT FAILURE
DYNAMICS ON SECONDARY CONTAINMENT INTEGRITY


http://www.osti.gov/bridge/servlets/purl/5835351-nR29Hq/5835351.pdf

Indeed, very interesting. Thank you very much for those documents. The first one is of special interest. I think it could explain what happened to Unit 2. Maybe there are hints as to what happened to Unit 3, but I didn't found any.

A second potential mechanism for MK I primary containment failure
in an unmitigated severe accident is drywell liner (shell) ablation due
to direct attack by molten corium. The ability of molten metals to
erode steel structures is well documented.4 While significant uncertainties
surround the behavior of core/concrete reactions and corium
spreading in a MK I containment configuration,2 preliminary analyses
indicate failure of the MK I drywell liner is quite likely if core
debris does contact the inner liner surface5.
Should the liner fail near the drywell floor elevation, the most
probable sites for blowdown entry into the secondary containment are the
reactor building basement torus room and the second floor of the reactor
building (Exhibit 2).

March 14th, 13:25 - RCIC fails in Unit 2
March 15th, 06:00 - Explosion in Unit 4
March 15th, 06:10 - Pressure drop in torus of Unit 2
March 15th, 06:20 - Explosive sound near torus of Unit 2
March 15th, 06:51 - Radiation dose at site boundary around main entrance exceeds limit value (11,9 mSv/h are measured some time later at the main gate)
March 15th, 12:00 - Large release starts and continues into Wednesday (I'm writing this down here because I'm trying to think out of the line. Unit 4 burned 4 hours earlier. I think nearly everybody thought that the radiation originated in Unit 4. But now TEPCO's giving us an explanation that the explosions and fires weren't connected to SFP #4 - so could it've come from Unit 2? An indication of a big containment and RPV breach?)

It was reported on March 14th / 15th that the fuel rods were completely uncovered, though I don't know for how long. If we look back how fast the core meltdown occurred in Unit 1, it's possible that there was also significant meltdown in Unit 2. So here comes my theory:
Some Corium pierced the RPV and got into contact with the quoted liner. Then it failed and there was a blowdown to the torus room.

I'm not sure if that means that the torus itself is damaged. If there's a blowdown from the primary containment to the torus room (where the torus is located), one could easily mistaken that as a torus damage. Especially, if you're unable to check out the situation because of high radiation readings.

 

[Borek]

You are surely correct, a pure hydrogen explosion or combustion is a pretty low key event, in fact the flame is invisible, at least afaik.
So the orange fireball from reactor 3 strongly indicates substantial additional combustible/explosive material was involved.

I have been thinking about it. Yes, orange flame doesn't look like hydrogen, more like something organic, carbon rich - oil, gas, paint thinner, something like that. Yes, hydrogen flame is almost invisible. However, I have no idea what will happen if hydrogen is mixed with cement dust (or any other inflammable dust). If the dust is fine enough I can imagine it gets hot enough to start glowing orange/red, like soot does.

It can be completely off, that's why I refrained from mentioning the idea till now, but - in a way - you called for that.

 

[TCups]

You are surely correct, a pure hydrogen explosion or combustion is a pretty low key event, in fact the flame is invisible, at least afaik.
So the orange fireball from reactor 3 strongly indicates substantial additional combustible/explosive material was involved. Normally flames are luminous because of white or red hot particles, generally of carbon, that are carried in the heated flow. Where these came from in this case is not yet clear.

Uhhhh . . . If an explosion of superheated gas blasts out a large hole in the wall and roof of a concrete building, might it be reasonable to assume a quantity of dust and particulate would result from the mechanical damage, and that that dust and particulate, in the presence of heat and air (oxygen), might form a fireball? Somehow the picture of a sawdust cannon comes to mind.



or lycopodium powder:


But even so, I am again somehow left with the feeling we are debating, by analogy, the size and trajectory of the bullet that was used to assassinate President Lincoln, not the result.

 

[bytepirate]

I have been thinking about it. Yes, orange flame doesn't look like hydrogen, more like something organic, carbon rich - oil, gas, paint thinner, something like that. Yes, hydrogen flame is almost invisible. However, I have no idea what will happen if hydrogen is mixed with cement dust (or any other inflammable dust). If the dust is fine enough I can imagine it gets hot enough to start glowing orange/red, like soot does.

It can be completely off, that's why I refrained from mentioning the idea till now, but - in a way - you called for that.

this looks orange:
this one as well:
one more: http://www1.teachertube.com/viewVideo.php?album_id=&title=Combustion_of_Hydrogen&video_id=77053&vpkey=

i don't know anything about chemistry, but i claim: hydrogen explosions on video look orange ;-)

 

[MiceAndMen]

I forgot to mention:

While the skin breaks and water comes with the liquid part of the corium in connection arises suddenly steam. This steam can be derived as barrier-free. Otherwise there is a steam explosion. In addition, the highly radioactive steam.

The study and analysis of a core meltdown is an inexact science, at best, because until March there had only been one BWR or PWR core meltdown in history. These 2 PDFs make it abundantly clear just how complex the analysis is:

http://www.tec-sim.de/images/stories/severe-accident-phenomenology.pdf
http://www.tec-sim.de/images/stories/lecturenotes-late-in-vessel-phenomena.pdf

One quote from the second presentation:

Though in the TMI-2 accident no external cooling was available, the vessel wall did not experience any noteworthy thermal attack in contradiction to what all simulations predict to date.

Interesting. Apparently, by all rights, the TMI-2 corium should have melted through the pressure vessel. It did not, and nobody really knows why.