Japan Earthquake: nuclear plants Fukushima part 2

[Rive]

You need information that does not yet exist to make the assumptions you are making.

However, the already existing information allows to make such assumptions on far more solid base than we used to make here way back.

At this point it is already the 'no gross damage on bottom head' line is, which is more in need of fitting informations and backing. The 'gross damage on bottom head' gained the support of Mr Occam.

 

[jim hardy]

Now i see,
this

is the area depicted in Charles Small's post , image oriented about 90 degrees CCW from the drawing

What a mess.

I wonder how i missed those last February ?

 

[jim hardy]

since the molten material will (most likely) find its way through (or: around) the control rod mechanisms sooner than it would eat up the bottom head wall.

So in case of complete meltdown what we should look for is not a missing bottom, but big holes around some control drives.
Also, it won't look like the whole grading there would be hollowed out, but like big molten/deformed/missing sections across the floor there.

Quite plausible.
One man's "big hole" is another man's "localized breach ".

 

[jim hardy]

Molten metal in water can produce very exciting phenomena, however the bulk of what you'd expect to come out from a RPV such as the Daiichi ones. would not be molten metal in its reduced form. Rather one should think of it as a lava-like substance, containing predominantly different oxides of uranium, zirconium, and iron.

Hydrides too ?

 

[Sotan]

(I'd just like to say that I for one love the discussions here and have nothing against trying to read as much as possible into the information made public by Tepco and others involved. That opinions differ sometimes, is only normal. But thank you all who contribute!)

Here's a video of April 2017 from NHK.


"The road to decommissioning 2017. The nuclear fuel debris. The "wall" that must be faced" would be a tentative translation of the title. It was aired after the investiga
tions in Unit 1 and 2 that showed the "sediments" and the missing grating, among others. NHK talked to experts and even enhanced some of the images and videos taken by Tepco.

It is 49 minutes long and, unfortunately, only in Japanese, but I found the animations shown throughout the video extremely interesting and suggestive. They might come in handy in the controversy regarding how the bottoms of the RPVs were pierced or damaged during the accident. Of course they are just models, and here and there NHK stresses that the true situation is not yet known - but I suppose they do convey the opinions of the various experts that were consulted, and perhaps even the opinion of Tepco, regarding the meltdown process as it proceeded in the 3 damaged units. See 0:56, 2:07, 5:55, 7:30 (the investigation in Unit 1 PCV), 9:55 (Unit 2 meltdown and investigation and the very high radioactivity readings), 12:28, 20:40 (the possible source of those high readings somewhere up in the PCV of unit 2)... and so on, I think it's worth clicking on the video progression bar every couple of minutes or so.

(I could translate what's being said if anyone is interested in certain points of the video but sadly I don't have the time for a full translation at the moment.)

 

[MadderDoc]

Hydrides too ?

No metal hydrides. They wouldn't be stable in that kind of environment.

 

[MadderDoc]

The latest videos from Tepco have now captured concrete underneath the reactors in this exact spalled state:

http://photo.tepco.co.jp/date/2017/201707-j/170721-01j.html

Notice the flaking and layering on the right hand surface. Typical of the high heat exposure you would expect from fuel contact. Now this is inside of Unit 3 but that goes towards my main point, a largely similar condition across the three units with signs of meltdown and RV penetration/ meltout/ ejection across all three units. In terms of Unit 1, the amount of damage to the basemat is very likely substantial if the amount of what looks like spalled concrete is an accurate indicator.

The frame is from the second video inspection inside the unit 3 pedestal. The whiteish eroded supposedly concrete surface seen to the right would be the inside pedestal wall. It is not clear at which height above the floor, however the inspection on that day was supposed to be about locating the grating platform under the CRDMs. Consistent with that, the metal structure about center of the frame could be the remaining wall mounts for the platform. If that is the case, the camera is looking at the pedestal wall somewhere well above the PCV floor. This does not seem to me a likely spot to see fuel/concrete interaction.

Now, seeing the grating 'wasn't there', possibly the camera took some detour, or a dive deeper to make these images, in which case there is, I believe, a circular metal structure along the pedestal wall rather close to the PCV floor. Still we are looking at a vertical concrete surface. Certainly, I have thought about where or if I might see signs of fuel/concrete interaction in the videos from unit 3, however I left it as indeterminate, under the impression, that much material appears to have landed in the bottom section af the pedestal, and on top of that some sedimentation has occured. IOW, I am not sure I would be able to see spalled concrete down there, if there were any, seeing it might have been covered by other material.

 

[Rive]

This does not seem to me a likely spot to see fuel/concrete interaction.

Spalling does not necessarily requires direct concrete-fuel interaction. It requires only heat.

At the time of RPV break the whole bottom of RPv were red (yellow?) hot I guess.
After the break there was a shower of yellow (white?) hot material under the RPV.

However, i cannot say that what we see there is spalling for sure: it more looks like broken down plaster.

 

[Charles Smalls]

Spalling does not necessarily requires direct concrete-fuel interaction. It requires only heat.

At the time of RPV break the whole bottom of RPv were red (yellow?) hot I guess.
After the break there was a shower of yellow (white?) hot material under the RPV.

However, i cannot say that what we see there is spalling for sure: it more looks like broken down plaster.

It's pretty much guaranteed. Concrete spalling occures anytime you have molten fuel ejection from an RPV into a dry PVC which we know happened in all three Units. You can read a lot about the process and what potential implications Tepco are facing in many of the published journal papers and conducted experiments on loss of coolant (LOCA) and station black out (SBO) accidents. They have been modelling and testing how they progress since the 1980s. MCCI Concrete-Fuel Interaction and Corium Coolability, BWR containment failure analysis during degraded-core accidents and Simulation of a BWR Lower Head RV in an Accident are all good reads. This paper is especially good because it's a straight forward essay and compares a lot of previous experiments with a whole section on spalling in nuclear accidents.

The image from the video is good because it's above the water line so 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.As for this new Tepco video... to get an idea of how the Tepco investigation team is puting together it's own data is fascinating. So many interesting images:

(00:59) straight away we see a model suggesting they presume all three PVC's were essentially dry at the time of fuel melt through. It looked that way for 1 and 2 but I thought as 3 is so flooded, it may have been so at the time of the accident but the flooding of the building seems to be something that happened later on. As far as concrete-fuel interactions, if there was no quenching at the time of core exit, it leaves the basemat more open to core damage.

(02:14) This is one of the most interesting images. As said before, RVs that have CRDs and other equipment that load from the bottom are inherently weaker by design. If you want to keep a liquid substance in a container, having holes in the bottom instantly makes that more difficult. This particular plumbing out let seems to be what Tepco suspect was the primary fuel escape route and shows how you can have gross fuel exit but still see a relatively intact CRD structure as we see. The disturbing thing from this and the other image above, is that they think the RV could have been breached even before the rest of the fuel had melted.

(02:17) This suspected outlet also goes on to explain very well the particular 'spray and splatter' pattern of the hot fuel exit and why we see the CRD room floor grating was melted away in the manner it was.

(13:42) There is the 1x1 meter central melt hole mentioned here the other day which confirms the overall scale of the damage.

(01:12) This graphic shows the molten fuel flowing straight out of the pedestal into the PVC with the splatter along the way. This is where the spalling would primarily occur.

(08:05) This seems to refer to the Unit 1 survey but should be pretty much the same across the three units with the molten fuel exiting the pedestal door into the PVC proper. That black mass is supposed to be the fuel but according to the handout report, the sample readings came back to low to be fuel which is why I am more confident of my original 'spalled concrete with fuel around or under this area' assumption.

(33:25) This is the other interesting section. Now that muon, robot investigations and site data all indicate gross fuel exit across the three units, the next question becomes where the fuel went from there and can that be used to explain the other situations on the site such as the persistent groundwater contamination on site. I had assumed that the burrowing action of the fuel into the basemat combined with explosion/earthquake damage had allowed the fuel to interact with the ingress of site groundwater to cause the contamination problem there. Basically with the fuel going under the PVC/pedestal floor. Tepco appear to be working with a different assumption.

(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.

(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. I do not know enough about the plant design to say if this is actually the natural progression for fuel escape or what amounts of fuel would have traveled into that area. I find it strange the molten fuel could travel through that plumbing but they think they can use the same pipework to get a decent concrete delivery.

(46:20) Apparently they think the torus leak is the main site causing the groundwater contamination and is still a major issue for the recovery effort.

Many many thanks Sotan, this was a gem of a find.

 

[Sotan]

Thanks Charles, I too found it interesting for the suggestive animations and graphic depictions. Let me point out, though, that I am not sure how much in that is Tepco. NHK made that video and they do say they consulted many experts and Tepco's results... but I am not sure all those are in fact the hypotheses Tepco is working with. (It may well be so, but...)
NHK has aired quite a few of those special programs. They had another great one about the contaminated water, one about the first hours of the accident... But I found them too late and there is the language barrier too. This one was pretty recent though.

 

[MadderDoc]

The image from the video is good because it's above the water line so 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.

The image from the video from the unit3 inner pedestal area is below the waterline. Perhaps that's also what you meant to write. As regards unit 1 I haven't looked enough into that to really comment, I would be interested in the latest data, if you can give me a pointer. I noticed you wrote, that we know that molten fuel ejection from an RPV into a dry PCV happened in all three units. I can't say I know that it didn't, but how do we know that it did? Looking at the imagery from unit 3, I see lumps of solidified material that appear to have solidified on contact with water, as well as solidified material that may have dropped, while still having been sufficiently liquid to flatten and flow out somewhat, when it was delivered to its present position. At the level of the CRDMs we see material that has solidified and become stuck up there, somehow managing not to flow to a deeper level. While the whole area appears to have suffered at severe beating, the final result does seem to me to have been produced by more than one destructive event, like if something melted down, did some damage, then solidified, then remelted. Everything indicates to me that the core substantially ended up inside the pedestal, but when, or in which stages this final result was produced is not really clear to me. The window of opportunity would be from no earlier than the morning on March 13th until a week or so after that.

 

[Charles Smalls]

Thanks Charles, I too found it interesting for the suggestive animations and graphic depictions. Let me point out, though, that I am not sure how much in that is Tepco. NHK made that video and they do say they consulted many experts and Tepco's results... but I am not sure all those are in fact the hypotheses Tepco is working with. (It may well be so, but...)
NHK has aired quite a few of those special programs. They had another great one about the contaminated water, one about the first hours of the accident... But I found them too late and there is the language barrier too. This one was pretty recent though.

Right. 90% of it seems as expected. The RPV faliure so soon into the accident before even complete fuel melt seems uncharacteristic. I agree with the rest of the depictions.

The image from the video from the unit3 inner pedestal area is below the waterline. Perhaps that's also what you meant to write.

No no I actually did think it was above the water line. I'm surprised it shows up so clearly under water or even that the water is as debris free as it is. But again, the clarity of the water in not the relevant part, it's the visibly layered breakup of the material that is important. The fact that the water wasn't cloudy or disturbed enough to obscure it is by the by.

You wrote, that we know that molten fuel ejection from an RPV into a dry PVC happened in all three units. I can't say I know that it didn't, but how do we know that it did?

You're right and I am wrong here, I can't make that claim, not for sure. My apologies to everyone.

 

[MadderDoc]

I'm surprised it shows up so clearly under water or even that the water is as debris free as it is. But again, the clarity of the water in not the relevant part, it's the visibly layered breakup of the material that is important. The fact that the water wasn't cloudy or disturbed enough to obscure it is by the by.

A lot of water has percolated down through the area, cloudy suspended matter has had a long time for going into solution or to find rest as a sediment. The camera's being under water also has the effect that fewer radiation artefacts are produced in the images.

 

[Gary7]

(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.

This is incorrect. They merely suspect there is a hole in the torus, which is prohibiting them from filling it up with water. They do not speculate that fuel caused the hole (at least, not in this video). The red mark is just a graphical representation of the hole, not of fuel.

 

[Rive]

The real spall site of interest is directly outside the Unit 1 pedestal doorway to the PVC.
...
(08:05) This seems to refer to the Unit 1 survey but should be pretty much the same across the three units with the molten fuel exiting the pedestal door into the PVC proper. That black mass is supposed to be the fuel but according to the handout report, the sample readings came back to low to be fuel which is why I am more confident of my original 'spalled concrete with fuel around or under this area' assumption.

Now, this part. That stuff outside the pedestal definitely looks like a secondary deposit, carried there through the pedestal opening by water. Also there is not any sign of heat source below.

Not enough information available at this point, but by the look of it I would not expect many fuel outside the pedestal (in case of U1). About the linked papers and experiments: interesting stuff, but as being an engineer who likes to think with his hands (and being good with it), I would not take them into my heart... Their relevance is quite limited in this matter.

So far I could not find worthy material for control-below type BWR meltdown RPV simulation or experiment. Maybe you can help me with this?

 

[Sotan]

I don't know how to quote anymore - but Gary7 is correct in post #1414 above (thank you Gary7!).

 

[turi]

This morning Tepco had the last valve for the frozen wall opened. Let's see whether closing those last 7 m (of 860 m) makes a difference.
http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_170822_02-j.pdf

 

[Charles Smalls]

They do not speculate that fuel caused the hole (at least, not in this video). The red mark is just a graphical representation of the hole, not of fuel.

I wonder who's idea was it to use the exact same hot orange blob colour to represent the fuel mass and the suspected hole in the Torus? That's just poor graphic design.
I said at the time I couldn't understand how a mass of fuel would make it into the torus so this clarification makes sense. What else would have caused this damage to the suppression pool while leaving the plumbing strong enough for concrete injection I still don't know.

Now, this part. That stuff outside the pedestal definitely looks like a secondary deposit, carried there through the pedestal opening by water. [...] by the look of it I would not expect many fuel outside the pedestal (in case of U1).

Molten fuel is already viscous. Why would you need a separate process to account for its transportation through the pedestal doorway? It is said there was approximately 125 metric tons of fuel and fuel related materials loaded into Unit 1 at the time of meltdown. With the muon scans and other visual/reading data indicating that this mass melted out of the reactor, the melted liquid metal/corium pool had to flow somewhere from there. Down into the basemat or out through the pedestal are pretty much the only viable options. I don't think transportation by water is necessary. As for not expecting fuel outside of the Unit 1 pedestal, I think this is pretty much exactly what happened and Tepco themselves depicted this in their March 27 handout:

Note the grey blob representing a molten melt fuel mass pooling out of the pedestal doorway into the PVC. With gravity and normal fluid behaviour, once the molten fuel falls from the reactor, flowing outside the pedestal into the PVC is pretty much the only logical next step. The fact that there is an openly communicable doorway from the pedestal to the PVC just makes it easier. One thing you see on page 4 of this handout and a later release with the eventual sample results, was that they actually sampled two areas in the PVC specifically with this in mind. One called D2 right outside the pedestal doorway where they expected the fuel mass to travel causing high radiological readings along the way, and another sample site called D0 on the opposite side of the PVC well away from the pedestal doorway. The idea being that a reading at D0 far from the likely fuel exit point could be sampled to provide a 'background' measurement and then compared to the D2 readings taken outside the doorway to see how radiologically 'hot' that area of the PVC was by comparison.

About the linked papers and experiments: interesting stuff, but as being an engineer who likes to think with his hands (and being good with it), I would not take them into my heart

This is just a personal view but I'm surprised to read that. I don't know if there is a technical or scientific field that benefits or depends on practical investigations and assessments more than engineering. If you want to know how much steel is needed to support a bridge or how a given structure will behave in high wind situations or whatever, you don't build it first and figure out what happens after, you model and test. University studies, thesis papers, research groups, material evaluations... everything is modeled and tested. It's no coincidence that many of the more relevant 'Severe Nuclear accident' studies and investigations were published during the 1980's and 90's. After the Chernobyl disaster, knowing exactly happens during a Nuclear power plant disaster became extremely important work our and was tested very heavily. What happens to the reactor pressure vessel when the fuel can't be cooled, what gases are produced if the fuel catches fire, how long does it take a given amount of molten fuel to burn through a given amount of steel or concrete, what withstand the longest, how viscous is the fuel-corium mix, what structures does it have, what happens when it meets water, what happens when you add Boric acid... Pretty much the entire civilian atomic power operation and plant construction industry is based on the result of these outcomes and studies. Los Alamos, Oak Ridge, the IAEA, many of these studies are carried out or commissioned by the actual operators and providers of the technology using the actual components. The entire industry is practically built and sustained on the outcomes of these studies. Wanting to understanding a nuclear accident but not putting much stock in these studies would be like trying to understand a murder without believing in forensics. Not to over do the point but except for the few unfortunate cases of actual disasters that have accidentally happened, the models and studies are all the data we have about nuclear plant mishaps. I will try to find the one you asked for though.

@Sotan, is there anything that you can share about 02:10 into the video where they show the hot fuel leaving the RV via the central drain pipe? Is that said to be Tepcos actual working assumption on the fuel exit or just from NHKs understanding of the events?

 

[Sotan]

At 02:00 a "specialist in metals" says "If there isn't considerable heat around there, it wouldn't get that way."
At 02:04 a "specialist in the structure of nuclear power plants" says "This does appear to be debris that scattered around and then hardened".
(I don't think they are Tepco people.)

Then while we see the animation with molten fuel piercing the RPV and falling over the grating the voice over says:
"The specialists pointed out the possibility that the nuclear fuel that melted proceeded to destroy the inside of the reactor and fell down over a large area."

Then at 02:23 as we see the animation with the CRD rails where the robot met very high radiation values the voice over continues:
"Moreover, the presence of substances, apart from the debris, that emit powerful radiation, has surfaced."

Finaly at 02:35 the specialists mentioned talking, one says "If that is the case, the operations will be very difficult.", the other agrees and adds "more and more difficulties are observed".So maybe we shouldn't take this very seriously, the majority of it is NHKs presentation of things as well as they could be derived from talks with experts and from public info from Tepco. It would be nice if we had more from Tepco. After all, we saw a few minutes of footage taken by each robot. They recorded hours.

 

[Rive]

Why would you need a separate process to account for its transportation through the pedestal doorway?

It's not 'needed' but already 'recognized'. That stuff there looks like a secondary deposit, and it come from the pedestal opening.
Also, since heat sources has recognizable signatures on such deposits, it can be said that by the time that deposit settled down there was no (significant) heat source below.

It is said there was approximately 125 metric tons of fuel and fuel related materials loaded into Unit 1 at the time of meltdown.

Thanks to the different experiments and modellings you mentioned, we have no idea what happens with such mixture when the metallic part of it escapes from a control-below type BWR bottom head.

everything is modeled and tested.

We are talking about this after an event which was out of expectations. Based on this fact, I would call this according to the ancient word 'hubris'.

Again, can you please find me a paper (among the everything) about the meltdown in control-below type BWR?
I could not find anything in this 'everything'.

 

[Charles Smalls]

Again, can you please find me a paper (among the everything) about the meltdown in control-below type BWR?

I could not find anything in this 'everything'.

You can find pretty much everything relevant to this by googling 'Mark 1 BWR Lower Head'. The actual progression of the meltdown was probably discussed earlier in the thread and is a bit outside the scope we're looking at here but off the top of my head I would suggest BWR LOWER PLENUM DEBRIS BED MODELS FOR MELCOR.
Classic paper done by the Oak Ridge Labs, it looks mostly at what happens when the fuel lands on the bottom shelf of the reactor and then hits the lower head itself in a severe LOCA accident. The BWR lower head damage and penetration are also evaluated. The best part is that the specific reactor types it refers to are 1966 General Electric Mark 1 bottom controlled BWRs which is pretty much exactly what Fukushima is.

Relevant excerpts:
The portion of the BWR reactor vessel below the elevation of the core plate is formed by a cylindrical section [joined with a] hemispherical section (the lower head). Much of the volume immediately beneath the core plate is occupied by the control rod guide tubes. Also passing through this volume are source range, intermediate range [and] power range detector assemblies.
There are more than 200 bottom head penetrations as necessary to accommodate the 185 control rod drive mechanism assembly penetrations, 55 instrument guide tube penetrations, and a 5.1 cm drain line penetration near the low point of the bottom head.

That drain line penetration seems to be the main point of fuel escape depicted in the NHK video at the 02:14 mark. Someone such as @Hiddencamper who may have first hand knowledge may be able to say more.

Cont:
Given the perforated status of the BWR bottom head, it is reasonable to expect that the initial pressure boundary failure after lower plenum debris bed dryout would occur through the vessel penetrations and not by rneltthrough of the 21 cm thick bottom head itself.

This is what they mean when they say bottom loading control rod reactors are known to be inherently weaker by design. The rest of the paper is a detailed engineering data on temperature failures of different structures and components.

For the actual penetration mechanism of the vessel, my search engine suggests a NRC paper from the 1990s called Light Water Reactor Lower Head Failure Analysis and another called BWR Reactor Bottom Head Failure Modes.

The second one is pretty straightforward and on page 6 discusses failure at the bottom head penetrations:

Temperatures at the inner surface of the reactor vessel wall would eventually become sufficiently high to cause failure of the welds that hold the instrument tubes in place. However, it is probable that a different mode of failure for the instrument tubes would occur first. This predicted initial failure of the in-core instrument housing guide tubes for
the source, intermediate, and power range detectors (55 penetrations in all) involves melting of the portions of these guide tubes within the central portion of the bottom head debris bed; then, when the downward relocation and freezing of molten metals has progressed to the point that molten metals are standing in the central portion of the bed, these metals could spill into the failed instrument tubes and pour through the vessel wall.

Many models and studies give this type of failure for a BWR reactor which is why I would like to learn more about the drain line ejection shown in the NHK video as it would account very well for the characteristic 'splatter' ejection pattern observed inside of the units

 

[MadderDoc]

the characteristic 'splatter' ejection pattern observed inside of the units

I may have missed that. Could you post a photo of this, please?

 

[Charles Smalls]

I may have missed that. Could you post a photo of this, please?

The characteristic fuel splatter distribution observed in the Unit 2 robot investigation. It's in my #1397 post and Jim Hardys #1402 post.

As I said before, the 02:02 - 02:22 mark of Sotans NHK video offers a very plausible explanation of how you can have both this distinct wide spread splatter and melt damage pattern and a fairly recognizable CRD rod and hydraulic fluid line system above at the same time.

 

[MadderDoc]

The characteristic fuel splatter distribution observed in the Unit 2 robot investigation

Thank you. We haven't observed anything like this in unit 1 or unit 3, have we?

 

[Charles Smalls]

Thank you. We haven't observed anything like this in unit 1 or unit 3, have we?

Similar signs have been observed in Unit 1 and Unit 3 with signs of a suspected fuel puddle exiting the pedestal in Unit 1 and strong indications of a molten hot fuel exit yet relatively intact CRD structure in Unit 3. The Unit 3 images were captured in the latest investigation using that 'sunfish' robot.
To be honest, as soon as you see a negative muon scan which 1 and 3 both have, a fuel exit event similar to Unit 2 is pretty much the only likely outcome. We know all three reactors are of a mostly identical design and underwent similar bouts of cooling loss, so barring the intervention of some unknown a pretty dramatic action, the prognosis is likely the same across the three. Why the Unit 3 PVC is flooded where 1 and 2 are not is pretty much the only key difference as far as melt out implications goes.

 

[jim hardy]

Given the perforated status of the BWR bottom head, it is reasonable to expect that the initial pressure boundary failure after lower plenum debris bed dryout would occur through the vessel penetrations and not by rneltthrough of the 21 cm thick bottom head itself.

Those are small holes. Plenty of steam and water will exit that way
but i remain unconvinced significant amounts of metal and crumbled ceramic fuel will follow.

And i don't recognize anything i could characterize as 'characteristic' in those photographs.

No metal hydrides. They wouldn't be stable in that kind of environment.

Understood.
Uranium hydride's dissociation pressure at 500°C is, to my very limited knowledge, just 8 atmospheres maybe 125 psi.
So I thought there might be some formed before the reactor depressurizes itself. But then, if it's hot enough to melt those stainless steel tubes it's way above 500°C . Okay i didnt think it through. Thanks for listening to me think out loud.

UO2 melting temperature 5189°F is about 1.5X that of zirconium, 3371F °F, and around twice that of typical stainless steels ~ 2600°F.
So the fuel will remain solid crumbly fragments long after the metal melts.

I'll be a month digesting this one about hydrides.
http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/043/27043289.pdf
https://fas.org/sgp/othergov/doe/lanl/pubs/00818031.pdf

old jim

 

[Charles Smalls]

Those are small holes. Plenty of steam and water will exit that way
but i remain unconvinced significant amounts of metal and crumbled ceramic fuel will follow.

You know I didn't write that right? You know that's a finding quoted verbatim from an Oak Ridge paper and is reconfirmed in multiple NRC, Los Alamos and IAEA investigations using actual reactor grade steel, actual reactor components and fuel materials right? Besides the papers I linked, there are many many more from the 1980s all the way up to 2017 that all say the same thing. If you don't cool a bottom perforated RV, the fuel will leak out of the equipment holes. That's just the way it is.

The funny thing is, with most of these tests being commissioned, funded and carried out by the actual plant operators and governing bodies, from an economical and technological viability standpoint, the one thing I assume they didn't want to find in their studies was that if a bottom controlled BWR melts down, it's relatively straightforward for the stuff you wanted to keep contained in the RV to make it's way out. The perforated nature of the lower head speeds the whole process up. For them to actually publish multiple papers and reports saying that the outcome is the opposite of what they would hope it would be really says a lot about how clear/undeniable the melt out via perforation process is. The other big problem with your view is that it can't account for the negative muon scans on 1 and 3 either. Also, what ceramic 'crumbles' are you referring to?

Either way, I think the IAEA,Oak Ridge, Los Alamos, NRC, and Tepco know how nuclear reactors work so I'm pretty confident in their assessments.

 

[MadderDoc]

Similar signs have been observed in Unit 1 and Unit 3 with signs of a suspected fuel puddle exiting the pedestal in Unit 1 and strong indications of a molten hot fuel exit yet relatively intact CRD structure in Unit 3.

Well, it was the 'observed' bit I was wondering about, as regards a characteristic fuel splatter pattern in all units. I wouldn't like to have overlooked some imagery of it :-). Now I wonder how the CRD structure in unit 3 is supposed to be relatively intact -- as compared to what?. From the imagery that has been released It appears to me to be far more damaged than the similar structure in unit 2 (the only other CRD structure I have seen actual imagery of). I can well imagine the one in unit 1 could be in pretty bad shape. It's just, I haven't seen it.

Personally I'm past looking for indications of a molten hot fuel exit in unit 3, I consider that to be established fact. Also I find it a reasonable assumption that the initial failure mode involved instrumentation penetrations. However, I do think there is evidence to suggest, that the RPV bottom head failure in the case of unit 3 progressed further than that, such as to involve also CRD housing penetrations.

 

[jim hardy]

Personally I'm past looking for indications of a molten hot fuel exit in unit 3, I consider that to be established fact.

Thirty gallons of it is enough to make plenty of mischief.

 

[MadderDoc]

Thirty gallons of it is enough to make plenty of mischief.

Yeah, wouldn't like to have thirty non-metric units of that stuff falling on my toes :-). I am sure someone has calculated the volume of matter that can be produced from the melting down of the internals of this type of RPV, also pretty confident that most of that volume is now external to it.

 

[Hiddencamper]

Hey everyone.

The BWR owners group emergency procedure committee is in session this week. Sandia national labs is also there and they are doing a lot of modeling based on Fukushima.

One interesting thing is the discussion that they believe in unit 3, we may have as much as 85% of the core in the bottom head still. One of the bwrog issues is that the fuel appears to not come out as a molten mass but likely causes creep rupture around a penetration and you get a chunkier debris coming out, and if injection starts up it can freeze in place.

Apparently severe accident codes assume pure lava like core after a vessel breach which causes drywell liner failure rapidly. This isn't true based on observations at Fukushima and codes will be updated as more testing is done to account for this.

Some other interesting things the bwrs are looking at: we found out at some of the units that when trying to reflood the fuel after it's well over 2200 degF, the water increases the zirconium water reaction. If the reflood isn't rapid enough (several thousand gpm, 2000-5000) then you end up driving the reaction and not quenching the fuel before causing significant damage to the core. So now the bwrs are modifying the emergency contingencies for situations where you do not have a high flow ECCS system for reflooding the core, and looking at depressurizing the core earlier in events so that the core never reaches that superheated state and a small 300 gpm pump can easily keep it cooled.

Sandia is doing tons of modeling using branching trees for Fukushima style accident scenarios to get more info.

 

[jim hardy]

Either way, I think the IAEA,Oak Ridge, Los Alamos, NRC, and Tepco know how nuclear reactors work so I'm pretty confident in their assessments.

I read them in 2011. They are estimates by educated people. To put them forth as established fact is an error of logic.

Over and out on the subject.

 

[MadderDoc]

UO2 melting temperature 5189°F is about 1.5X that of zirconium, 3371F °F, and around twice that of typical stainless steels ~ 2600°F.
So the fuel will remain solid crumbly fragments long after the metal melts.

Yes, that is true. The metal oxides present in the fuel or produced during the heat-up (e.g. ZrO2 mp 2,988 K (4,919 °F ;-) ) would remain in a solid state while the temperature has become high enough to eat away at the supporting steel structures, or other metal present, through melting. IOW melted fuel (liquid metal oxides) in a still intact RPV bottom head is an unlikely situation.

 

[MadderDoc]

One of the bwrog issues is that the fuel appears to not come out as a molten mass but likely causes creep rupture around a penetration and you get a chunkier debris coming out, and if injection starts up it can freeze in place.

The fuel might come out also, mixed into a molten mass. There is a video highlighting some of the actual lava formations found in the pedestal area of unit 3. Some of the material appears to have been fluid on delivery to its present position, while other lava formations appear to have been quite rather more chunky, or already crusted up.

 

[Charles Smalls]

Hey everyone.

The BWR owners group emergency procedure committee is in session this week. Sandia national labs is also there and they are doing a lot of modeling based on Fukushima.

One interesting thing is the discussion that they believe in unit 3, we may have as much as 85% of the core in the bottom head still. One of the bwrog issues is that the fuel appears to not come out as a molten mass but likely causes creep rupture around a penetration and you get a chunkier debris coming out, and if injection starts up it can freeze in place.

The muon results already came back showing no significant amount of fuel remaining in the unit 3 RV. In Tepcos report they say:
The evaluation at present shows possibility that some fuel debris remain inside the RPV, but massive and high density material has not been found.
85% remaining in the Unit 3 RV lower head seems counter to all observations so far especially when compared to the other units. Are you sure you don't mean Unit 2? That unit may have some fuel remains whereas Unit 3 appears practically empty:

Even without the observational data, doesn't this '85%' hypothesis still need a substantial cooling event or quenching action it can point to which occurring during that time to cool and arrest the fuel mid escape. Do they suggest that vessel depressurisation by melt penetration lowered the pressure enough to restart water injection? It would have to be substantial because as you say, once the fuel has reached this superheated state, injecting a small amount of water is just as likely to speed it up as slow it down. I don't see this huge injection event to account for your hypothesis in the timeline data. As I see it, once the barn door is open and the first horse has bolted, a substantial intervention effort or event is needed to account for what stopped the rest of the stampede so to speak. Any likely candidates?

Also, any thoughts on the NHK central drain line leak hypothesis? I have been told that after fuel enters the drain line it can penetrate in 9 seconds with a fairly substantial flow rate but I am unable to confirm if this is similar to the pipework referenced in the graphic.