Japan Earthquake: nuclear plants Fukushima part 2

[Charles Smalls]

More correctly they failed to show where the fuel is

Direct quote of the TEPCO July 27, 2017 Press release, page 8:
Unit 1- No massive fuel in the core area.[...] Most of melted fuel fell into the PVC and there is little fuel in the reactor core. (Source:
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170727_01-e.pdf)

That is word for word what I said. I made no comment on where the fuel currently is, only that results showed the RV, the reactor vessel was almost completely empty. I don't think you have grounds to criticize my statement.

On the follow up matter of fuel-concrete interactions and it's potential indications, I can't pretend to know. HOWEVER given the data and surveys TEPCO have taken and shared, I believe the hypothesis I put forward a couple of months ago has been reinforced by the latest results. If you see anything in the offical data to counter that viewpoint, be sure to comment.

You're looking for reasons to believe there was a "hot ejection". Maybe there was. Please don't stretch the evidence.

Belief is no part of my argument. All points I raise are cited and backed by current and relevant sources. 9 times out of 10 from TEPCO themselves. What I said on April 28th was that the Muon scan was very likely a pointless exercise. The data and the conditions in the sister units strongly suggested that the major amount of fuel had already melted out of the unit 3 reactor vessel and would not be seen on the muon scan. That has now proven true. As far as calling this fuel melt out a "hot ejection", that is not a term I coined. I am not familiar enough with industry terminology to make that statement. The point I raised with yourself and @Hiddencamper is that it doesn't matter whether it occurred under that definition. The outcome is largely the same. The gross amount of fuel has melted out of the bottom head and onto/into the basemat. The camera data and muon results prove that. As I stated, to my understanding unless you want to calculate potential concrete damage/penetration, the pressure of the vessel at the time of melt out is largely irrelevant.

Again if there is any data or relevant experience to counter this view be sure to raise it.

 

[turi]

To pick up 530 Sv/hr readings near the x-6 penetration at the top of the PVC didn't fit the likely situation of large scale downward relocation of fuel.

The 530 Sv/h estimation is the one they have reestimated to be 70 Gy/h.

 

[Charles Smalls]

The 530 Sv/h estimation is the one they have reestimated to be 70 Gy/h.

That is what I meant. The original 530Sv/h number was difficult to account for given the likely model of gross fuel meltout into the pedestal area with marginal splatter along the way. This revised 70 Sv/h fits much more into the expected scenario and brings the situation across the 3 units into greater alignment. There are probably many circumstances or chain of events that could have caused a 530 Sv/hr reading around that area but having it recalibrated to ~70Sv/hr means there is one less "mystery" to be solved and the situation inside the unit becomes more textbook and as expected.

 

[Astronuc]

The Unit 2 results also make sense given the assumed picture of the three reactors. The incredibly high contamination readings didn't fit with the expectation that the gross amount of fuel had melted down through the CRD room and into the basemat. To pick up 530 Sv/hr readings near the x-6 penetration at the top of the PVC didn't fit the likely situation of large scale downward relocation of fuel.

However, the X-6 penetration is not at the top of the Primary Containment Vessel (PCV), but rather it is at the elevation of the bottom of the CRD housing.
See page 6 of handouts_170727_03-e.pdf.

That is what I meant. The original 530Sv/h number was difficult to account for given the likely model of gross fuel meltout into the pedestal area with marginal splatter along the way. This revised 70 Sv/h fits much more into the expected scenario and brings the situation across the 3 units into greater alignment. There are probably many circumstances or chain of events that could have caused a 530 Sv/hr reading around that area but having it recalibrated to ~70Sv/hr means there is one less "mystery" to be solved and the situation inside the unit becomes more textbook and as expected.

There is nothing 'text book' or 'expected' about what is being discovered. The change from 530 Gy/h to 70 Gy/h is apparently due to a recalibration. On page 6 of handouts_170727_03-e.pdf, there is one reading of 650 Gy/h that was revised to 80 Gy/h, and another reading of 210 Gy/ revised to 70 Gy/h. These readings (outside the pedestal (about 5 to 7 m from the inner surface of the pedestal), not immediately under the RPV) are still higher than approx. 20 Gy/h (now revised to less than approx. 10 Gy/h) in the region above the platform and immediately beneath the RPV at the inner surface of the pedestal. I would like to see more readings, particularly underwater, and if they can do gamma spectrometry, I'd want some idea of what radionuclides are being detected.

Readings on the dose level made on 1/26, 1/30 and 2/9 were apparently deduced from the camera noise, i.e., the effects of radiation on the camera electronics (CCD). According to the text on page 6 of handouts_170727_03-e.pdf, the radiation measurements on 2/16 were made with integral dosimeters, which are ostensibly on the self-propelled survey robot.

 

[Charles Smalls]

However, the X-6 penetration is not at the top of the Primary Containment Vessel (PCV), but rather it is at the elevation of the bottom of the CRD housing.

I know, you can see it clearly in this TEPCO handout:

Location of the measurement readings here:

What I am saying is that the x-6 penetration and the area of the assumed 500-600Gy/h readings are relatively high up in the containment, around the middle of the 'lightbulb' curve area compared to the PVC floor proper below. This was difficult to account for given the other data. The picture across all three units now seems to be gross fuel exit from all three reactor vessels, with the fuel melting through the perforated bottom-heads and falling from the RVs down onto the basemat/ pedestal floor (red star). They all present visual and analytical signs of that being the case. Having ultra high ~500Gy/h readings in unit 2 so far above the PVC floor where the fuel is likely to have landed and spread didn't fit in with this hypothesis. If accurate it would require an alternate scenario where a large mass of fuel or other huge contamination source would have to be localised somewhere in that immediate area to account for it. With the new more accurate readings, that alternate theory is no as longer necessary. General fuel splatter and nearby pipework with contaminated contents can explain these readings quite well. There is now less mystery surrounding possible locations for the bulk of the heavily radioactive material in Unit 2. Due to the negative Muon results, probe dosimeter readings and camera footage, we can now presume the bulk of Unit 2s 150 tons of nuclear fuel to be somewhere in or under the building basemat rather than floating around somewhere above the PVC floor. Again, this is a much more likely scenario given not just the data inside the building but also the persistent high level groundwater contamination around the reactor buildings themselves. I believe we are getting good data and information from TEPCO and that the more likely condition of the reactors and buildings will be the one that ties all the data together into on coherent picture rather than a jumble of poorly understood information.

If there is any relevant data or information I have missed, be sure to present it.

 

[Astronuc]

What I am saying is that the x-6 penetration and the area of the assumed 500-600Gy/h readings are relatively high up in the containment,

Actually, one made the statement ". . . the x-6 penetration at the top of the PVC . . .". The top of the PCV is about 35 m or so above the location of the X-6 penetration, which is about 7 m or so above the basemat.

General fuel splatter and nearby pipework with contaminated contents can explain these readings quite well.

Actually, it doesn't. If fuel splatter caused that higher activity outside the pedestal, then we should see high activity at the inner surface of the pedestal rather than lower activity (the green dot in the image one posted). And we don't. And there is no nearby pipe work to explain those activities.

Having ultra high ~500Gy/h readings in unit 2 so far above the PVC floor where the fuel is likely to have landed and spread didn't fit in with this hypothesis. If accurate it would require an alternate scenario where a large mass of fuel or other huge contamination source would have to be localised somewhere in that immediate area to account for it.

Actually, it depends on what radionuclides are causing the radiation, and the most radioactive nuclides are fission products, and the fission products that could find their way outside of the pedestal area are gases (Kr, Xe) and volatiles (Cs, I, Br, and possibly Te). Now, Te is in a decay chain that goes like Sb->Te->I->Xe->Cs->Ba->La. There is a decay chain involving Kr: As->Se->Br->Kr->Rb->Sr->Y->Zr. One of the high yield fission product pairs involved Zr¹⁰⁰ and Te¹³⁴, and there are combinations of Sr, Xe isotopes, including Sr⁹⁷ and Xe¹³⁷. Xe¹³⁷ decays by beta decay to Cs¹³⁷.

Fission from Pu-239 shifts the nuclides pair up in mass and Z, and Pu fission produces a higher ratio of Xe to Kr, and so one can see that fission favors the presence of volatiles in the Xe-decay chain. Chemically, there are a number of compounds involving Cs, I and Te, which contribute to the mobility of these species, and which favors the mobility of Cs. Volatiles and Xe transport with steam, so if the RPV was breached, it's possible that any vapor escaping the RPV would transport Cs and its precursors into the area below the core and out through the opening in the pedestal wall. Cs and I are also soluble in the water, so they could be carried to wherever the water flows, or seep into unprotected concrete.

Regarding the activity levels measured, from the Unit 1, the debris on the basemat floor has activity of about 5-15 Gy/hr, or an average about 10 Gy/hr under water. (handouts_170727_02-e.pdf, pages 8,9,12,13). TEPCO posits that fuel debris and some deposit (corrosion) is sitting on the floor below the RPV.

If one refers to handouts_170727_01-e.pdf, page 9, TEPCO compares the expected fuel location with what the muon scans reveal.

U1: ·No massive fuel in the core area. (Lower area of the RPV is not measured.)
My comment: So TEPCO doesn't know how much might be left in the RPV.

U2: ·High density materials that is considered fuel debris were found at the lower area of the RPV.
·Part of fuel possibly exists in the reactor core area.
My comment: TEPCO's analysis so far does not indicate that the bulk of the fuel in U2 left the RPV, and it certainly does not support the fact that fuel ended up below the basemat. It could be the bulk of the fuel is still in the RPV, but I have no way to assess the muon scans. It would be good if they could scan a non-damaged, or intact, BWR/4 of similar size and composition.

U3: ·The evaluation at present shows possibility that some fuel debris remain inside the RPV, but massive and high density material has not been found. (Measurement and detail evaluation are continuing.)
My comment: It was not pointless to do the muon scan, as one asserted on April 28 and reiterated on this page. Based on the scan of U2, which is of similar design, there was some expectation that they might find fuel in the core and RPV. It appears there is less fuel than is believed to be in U2. They are still measuring and evaluating.

In order to use muon imaging, one has to shoot through the intended mass toward the sky, since muons originate in the upper atmosphere by virtue of interactions of cosmic rays with protons or nuclei in the atoms in the air molecules. Muons come from the decay of charged pions or Kaons, which come from the annihilation of anti-protons or resonance particles. The count rate is so low, they probably do the scans over several months (based on the dates of the scans on page 10 of handouts_170727_01-e.pdf).
Ref: http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/cosmic.html#c2

Aiming west toward the hill, or downward do the ground, there would be no muons traveling toward the detector. To scan at the basemat floor from the turbine building (east of containment), they'd have to be below the elevation of the torus. From the westside, they'd have to be in the containment building where the torus is located. That's probably flooded and/or exposed to high levels of radiation. I don't think anyone is going into containment any time soon.

 

[jim hardy]

I don't think you have grounds to criticize my statement.

The camera data and muon results prove that.

No, they only suggest it's possible.

Muon imaging also suggests it might be up in the steam separator region.
We know there's fuel in the SFP
compare density of regions circled in yellow.

I certainly don't assert that's where the fuel is
I do assert that muon imaging failed to show us where it is and only suggests where it is not.

 

[MadderDoc]

The Main steam lines are typically 260 inches above the top of active fuel. Give or take a foot or two based on vessel size.

Yeah, yeah. But the MSIV (main steam isolation valve), which I was talking about, close to which the leak was detected back in 2014, is not at that level. It is situated much lower, in the MSIV room on the 1st floor of the unit 3 reactor building, neighbouring the TIP room. That's where the four main steam lines exit through the PCV wall, to go to the turbine. The leak was found at the steam line that comes out through the X7-D penetration, apparently at the expansion joint between the (outer) MSIV valve on that line and the outer PCV wall.

 

[MadderDoc]

Schematically, indicating where a leak was found in the MSIV room of unit 3 in 2014. Red dot marks the spot. From what I have seen about the current investigation of the PCV of unit 3, it appears to be flooded currently to closely the level of that leak.

 

[MadderDoc]

Compared to the east wall of unit 4, very little debris was found remaining at or close to the foot of the east wall of unit 3, after the explosions on March 14-15 in 2011. I wonder by which mechanism or force it happened to be like that, and where did the debris go.

 

[jim hardy]

I wonder by which mechanism or force it happened to be like that, and where did the debris go.

You'd think somebody would have written his memoir by now.
There's this old report http://www-pub.iaea.org/MTCD/Publications/PDF/Pub1710-ReportByTheDG-Web.pdf

If you recall the videos

unit 3 ejected a black cloud straight up

my guess is debris got scattered far and wide
you can see some of it on top of turbine building and a sizeable hole in the roof

The Japan Self-Defense Force guys described chunks of concrete falling on their vehicles.

Schematically, indicating where a leak was found in the MSIV room of unit 3 in 2014.

Hmmm if upstream valve wasn't shut off tight and that expansion joint failed in the 'quake or one of the aftershocks, it'd be a hydrogen leak down low in the building...

I wonder by which mechanism or force it happened to be like that,

Prevailing thought is still AFAIK hydrogen explosion. Have you heard any different ?

 

[etudiant]

Hmmm if upstream valve wasn't shut off tight and that expansion joint failed in the 'quake or one of the aftershocks, it'd be a hydrogen leak down low in the building...Prevailing thought is still AFAIK hydrogen explosion. Have you heard any different ?

A hydrogen leak lower in the unit 3 structure might help explain the explosion of unit 4.
It is difficult to envision a leak from near the top of building 3 somehow infiltrating unit 4 selectively.
Hydrogen leaks ferociously well, but if there is a path it will escape upward and there is no connection afaik except through the vent stack, which is connected pretty low. Or am I missing something obvious?

 

[MadderDoc]

Obviously, there was a hydrogen explosion in the building, and it is in the nature of such an event to scatter debris far and wide. However, after the explosion, you'd expect to find increasingly more debris lying around, the closer you get to the wreck of the building. But not so, closing in on the foot of the east wall of unit 3. You think black cloud cleaned that up?

As regards the leak in the MSIV room Tepco says it is at the bellows (or expansion joint) on the southmost steam line, I do not know how they know. In the video one could only discern the stream of water (estimated by Tepco to about 1.5-4.5 m3/h) coming down from that general area, while the southmost steamline itself and its expansion joint is hidden from sight by the next steamline in line. Presuming the inner main steam valve on the leaking steamline was shut, as it should be (they fail shut) the steamline would still be blocked for exiting steam and hydrogen from the RPV, irrespective of a failing expansion joint on the line outside the wall of the PCV:

 

[Astronuc]

It is difficult to envision a leak from near the top of building 3 somehow infiltrating unit 4 selectively.
Hydrogen leaks ferociously well, but if there is a path it will escape upward and there is no connection afaik except through the vent stack,

As far as I know, TEPCO believe that the hydrogen traveled between Units 3 and 4 through the ducts that were connected to a shared vent stack. See lower left in photo Jim posted. The round (circular) ducts are about the elevation near top of primary containment or near base of the secondary containment. Unit 4 had a fire (deflagration) in the upper containment, while Unit 3 had an explosion.

Obviously the hydrogen had to escape the RPV, where the Zircaloy was reacting with the water coolant. Hydrogen then had to flow out of the RPV in the PCV, then into secondary containment.

After the earthquake, we don't know how much damage was done to joints and seals in the various systems, or cracks in the structures.

 

[MadderDoc]

At 14:31 on March 13, Tepco employees observed over 300mSv/h at the north side airlock on the first floor of unit 3, R/B while noting that "there was white gaseous substance inside", in other words, steam. that had come from within the primary containment was gushing into the first floor of the building. The measured pressures in the RPV and PCV of unit 3 had at that stage started tracking each other closely, consistent with a failed barrier between them.

The pressure of the presumably combined system came at times close to, but not much above the max design pressure for the PCV, until the next day March 14 when the explosion in the building occurred shortly before noon during a 'high' excursion of the pressure to slightly above the max design pressure for the PCV:. Throughout that period one can reasonably assume, that steam from within the RPV/PCV, as was observed in the afternoon on March 13, had been continually leaking out into the first floor of unit 3. The leak at the MSIV could've been a source, inasmuch , judging from the video, everything in the MSIV room shows sign of having been exposed to steam for an extended time.

 

[jim hardy]

However, after the explosion, you'd expect to find increasingly more debris lying around, the closer you get to the wreck of the building.

I'd expect heavy debris to get hurled by an explosion not so far as light stuff like wall and roof panels.
Indeed those plank looking pieces on top of turbine building are, if i recall correctly, pieces of trim-like panel .
The massive concrete columns on ocean side remained standing , those on West side toppled over but didn't go anyplace.

But not so, closing in on the foot of the east wall of unit 3. You think black cloud cleaned that up?

No, i think it threw the debris further Eastward probably a lot of it clear into the water. It moved some heavy stuff, recall what looked like an airconditioner hanging off East side and head bolt tensioner pushed out on North side.

What's your thoughts ? Do you think there was steam in it ? I don't remember any reports of water raining out of it.

I'll look for that Cryptome satellite picture taken just minutes after the explosion... Probably not quite enough resolution in it though.

 

[MadderDoc]

What's your thoughts ? Do you think there was steam in it ? I don't remember any reports of water raining out of it.

There was obviously steam in it, and quite a lot, or its buoyancy becomes inexplicable.
http://gyldengrisgaard.dk/fuku_docs/unit3cloud45/

 

[MadderDoc]

I don't remember any reports of water raining out of it.

I'll look for that Cryptome satellite picture taken just minutes after the explosion... Probably not quite enough resolution in it though.

No, it may have rained from it and you wouldn't be able to see it from that satellite photo:
http://gyldengrisgaard.dk/fuk/20110314_1104 satellite/index.html

The only indication that it might have rained out of it would be the vertical dark streak that develops under it and follows it, during the first dozen seconds of its lifetime, while it travels towards the ocean, then disperses. That could perhaps be interpreted as black rain.

 

[MadderDoc]

As far as I know, TEPCO believe that the hydrogen traveled between Units 3 and 4 through the ducts that were connected to a shared vent stack. See lower left in photo Jim posted.

The ducts Tepco believe carried the hydrogen from unit 3 to unit 4 are of too small a diameter, much smaller than the quite visible 'fat' ducts, in order to be visible in the photo Jim posted. These smaller diameter ducts are meant to carry the exhaust from the SGTS in units 3 and 4 respectively, and they merge close to the stack to a single duct that goes to the stack. The same ducts are used to carry the exhaust from vent operations, whether it be from the PCV, or from the suppression pool, to the stack. Tepco believe vents going out from unit 3 through the duct unintendedly didn't all go up the stack, but rather backflowed through the duct coming from unit 4 to end up there, entering the building through its SGTS system (the isolation valves in this system fail open) -- thus providing the fuel for the explosion.

 

[jim hardy]

Thanks Doc for that Glyndengaard link. I'd forgotten where that was.
Big black chunks falling out of the initial black cloud that got projected up then moves right with wind

Followed by white smoke or steam ?

Not to throw the conversation back six years, just refreshing my memory.

old jim

 

[MadderDoc]

Followed by white smoke or steam ?

Not to throw the conversation back six years, just refreshing my memory.

No, that would be pretty awful :-). I'd say, dust. I consider the white ground-near clouds to be secondary clouds of dust, originating from the debris of the shattered upper floor walls, which was initially spread by the explosion in a perpendicular direction to the walls, horizontally away from the building.

Due to the viewing angle, the main part of the white stuff you are looking at, in that frame, would be dust, formed from the cast out shattered panels of the upper west wall. The hydrogen explosion spread that material initially westwards 'across the road' towards the hillside. So now the lighter stuff, dust, is coming back in the opposite direction, towards the building, and further towards the ocean. The analogy would be like if you had been throwing a handful of dirt away from you, forcefully, against the wind...;-)

 

[MadderDoc]

Thus, looking at this famous photo, the cloud visible is in the main what lingers of these secondary clouds of dust. Also clearly visible, are the two distinct white clouds of steam rising from the area of the edge of the equipment pool, and from the area of the transfer gate to the SFP, respectively. Tne PCV was at that stage obviously leaking out steam in that direction, too.

 

[jim hardy]

Tne PCV was at that stage obviously leaking out steam in that direction, too.

Thanks - i had forgotten the steam was so distinct in that photo.

The land based camera was farther West looking East
so debris thrown toward the water was hidden by the cloud.

 

[MadderDoc]

Thanks - i had forgotten the steam was so distinct in that photo.

The land based camera was farther West looking East
so debris thrown toward the water was hidden by the cloud.

Yes. Farther south and west that is. The direction to the videocamera would be indicated by a line drawn from the center of the unit 3 building to the stack. In the video we are therefore incidentally looking in at the clouds movement with the wind, in an angle that is not too distant from 90 degrees, which is not too bad. Still, unfortunately, there is no other imagery of the event from any other direction.

 

[turi]

Something completely different: Until November they will test whether 3 wheel bicycles are useful for the workers on the premises. The trial will be restricted to roads with a speed limit of 20 km/h. A few roads have a 40 km/h speed limit where the bicycles won't be allowed.
http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_170803_03-j.pdf

 

[turi]

The first of 8 dome parts has been installed on unit 3 as part of the preparations for the fuel removal from the spent fuel pool.

 

[MadderDoc]

And now to something much lower down. This is a composite of two frames from the recent submersed investigation inside the piedestal in the PCV of unit 3. The camera is looking straight up, as revealed by reflections of its light from the water surface above (the reflections can be seen at about center in the upper frame).

I take it, that the several similar round 'foobars' (those with eight spots around the edge) which are in unobstructed view, pointing down towards the camera, are the bottom ends of control rod drive mechanisms (CRDMs) or what's left of them, hanging down from the RPV bottom head which is somewhere above the water surface, and that they are in unobstructed view, because their support frame has come off, and fallen away to somewhere down below.

There appear at first sight to be two sizes of these foobars -- however, if there is actually only one size meant to be present, they are just at different distances from the camera, meaning the large-looking foobars dive deeper into the water, in their current position, than the smaller-looking foobars. I imagine that could be because their lower supporting frame is gone missing, so they have somehow been free to move to a deeper position than the support frame would normally have allowed them to. The support frame, I imagine, is normally affixed to the RPV bottom head with vertical iron sticks, sort of like hanging under it. I don't know how much of this makes sense, I know very little of the CRDM and how it works, and have not been able to find much about it. Comments or directions are most welcome.

 

[MadderDoc]

The first of 8 dome parts has been installed on unit 3 as part of the preparations for the fuel removal from the spent fuel pool.

Well, there's an impressive reason, why Jim Hardy will need a lot of patience waiting to see the upper head area of the drywell. More humbly, I would like to see just the second shield plug layer, seeing the middle concrete slab in the top layer has broken and sunk ~30 cm _into_ the space the second concrete layer occupies. Assumedly, by Tepco without harming it. I'd like to see that. But that will take nearly as much patience. :-)

 

[jim hardy]

however, if there is actually only one size meant to be present, they are just at different distances from the camera, meaning the large-looking foobars dive deeper into the water, in their current position, than the smaller-looking foobars.

I would expect all CRDM's to be identical with bottoms at same elevation.

I couldn't tell from the pictures whether apparent distance was real or an artifact of "fisheye lens" .

But a melt through would depend on whether they had flooded the drywell to wet the vessel bottom. The old Hodge 'Severe Accident ... ' paper
describes that and is the reason he recommended cutting vents in the vessel skirt to provide for wetting.

Just stumbled across this Interesting 1988 paper that describes how they thought back then a melt through and pour would progress.

https://www.osti.gov/scitech/servlets/purl/6700980

Right there would be a sudden source of steam and pressure spike.

 

[MadderDoc]

a melt through would depend on whether they had flooded the drywell to wet the vessel bottom.

I can't imagine they ever managed to flood the drywell to that level. As I recall from reports, a pressure spike was recorded in the drywell and the wetwell, in timely connection with the sudden decompression of the RPV that occurred at ~9:00 on March 13. Some sources, including Tepco now says an increase in the drywell/wetwell pressure unintendedly produced conditions to activate the automatic decompression system, such as to _produce_ the sudden decompression of the RPV. The problem with explaining the decompression lies in its very steepness, from about 7 MPa to about 1 MPa in about 2-3 minutes. This has been difficult to model under the assumption that the RPV had _not_ suffered a large hole in connection with the pressure event.

 

[MadderDoc]

I would expect all CRDM's to be identical with bottoms at same elevation.
I couldn't tell from the pictures whether apparent distance was real or an artifact of "fisheye lens" .

They look level and identical in the photo from unit 5 which Tepco offers as reference. They look actually also a lot more densely and orderly packed. But seeing the support for the CRDMs in that section obviously has gone walkabout , some of the CRDMs could well have done so too.

 

[HowlerMonkey]

With the grate/mount missing, you would think they would all fall out but I guess it's possible that the ones we see remaining were bent over inside where the core belongs or otherwise distorted such that they can't fall out.

I believe they are getting a lot better at inspecting so we may see something soon enough.

 

[jim hardy]

This has been difficult to model under the assumption that the RPV had _not_ suffered a large hole in connection with the pressure event.

Instrument tubes should melt higher up in the core region where the heat is produced providing a lot of small leaks

CRDM tubes are larger of course. But i don't know whether they're open on outside of vessel like the TIP tubes are. .
Those aren't the only penetrations down there, though.

I'm no BWR guy so won't speculate further. It will unfold and surely will have some surprises for all of us.

 

[MadderDoc]

Instrument tubes should melt higher up in the core region where the heat is produced providing a lot of small leaks

CRDM tubes are larger of course. But i don't know whether they're open on outside of vessel like the TIP tubes are. .
Those aren't the only penetrations down there, though.
<..>.

Seeing the CRDM tubes are meant to guide a push or pull action on control rods, I imagine they would be kind of cylinders, with a piston inside. That would mean they are not in that sense open on the outside of the vessel. There would of course be other tubes for instrumentation or other stuff I do not know about. If I get what you mean, these are basically hollow tubes closed in the hot end. I'm not even an 'R guy :-)

Here is a photo of the premises (unit 5) for reference. I believe I can see a set of thinner shiny tubes coming down about there in the middle of it. (The somewhat thicker and more matte tubes, I see as the withdrawn bottom portion of the piston action in the CDRM guide tubes.)

 

[jim hardy]

Seeing the CRDM tubes are meant to guide a push or pull action on control rods, I imagine they would be kind of cylinders, with a piston inside. That would mean they are not in that sense open on the outside of the vessel.

That's my conception too. I've never been under a BWR, though.

BWR's have something called "TIP" for "Traversing Incore Probe" system .
https://www.nrc.gov/docs/ML1125/ML11258A339.pdf

I believe it is similar to our PWR "Flux Mapping System" which consists of tubes entering the vessel at bottom and extending up to top of fuel. They're of course closed at top . Other end is open so the moveable detectors can enter to measure neutron flux inside the core.. Much like the BWR TIP system.
We had fifty of those tubes in my PWR. Each is about 5/16 inside inch diameter i forget exactly.
If they all melted it would make fifty small leaks roughly equivalent to.. √ [(5/16)² X 50] = √4.88= 2.21 inch effective diameter .

That NRC training handout taken from BWR Technology manual describes 31 TIP tubes,
√ [(5/16)² X 31] = √3.02= 1.74 inch effective diameter .A BWR guy familiar with the system will be more knowledgeable .

old jim