Japan Earthquake: nuclear plants Fukushima part 2

[Azby]

Here's the English-language short report of the July 19th Unit 3 submersible survey:
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170719_01-e.pdf

 

[Sotan]

Quick report again (only in Japanese for now) about the Unit 3 PCV investigation of July 21st:
http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_170721_09-j.pdf
Photos: http://photo.tepco.co.jp/date/2017/201707-j/170721-01j.html

They sent the robot inside the pedestal and took some images.
They identified "what appears like once molten mass(es), now solidified", as well as "fallen objects". (Separately on NHK TV I saw a Tepco representative stating that these solidified icicle-like masses have a high probability of being "molten fuel and various parts", which mixed and later solidified.)
Same as in the first day, they saw damaged structures and fallen support fittings of CRD-housings.
During today's operations too they couldn't see/confirm the grating platform where it should have been.
Tomorrow they will try to go down into the basement of the pedestal. Due to many obstacles and fallen objects they think they might not be able to retrieve the robot.

 

[Hiddencamper]

Just so everyone remembers, unit 3's Automatic Depressurization System activated and the unit was allegedly depressurized when the core melt occurred. So it will likely look different than units 1/2

 

[Azby]

Once again, English version of the brief report has been posted here: http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170721_01-e.pdf

 

[Astronuc]

They sent the robot inside the pedestal and took some images.
They identified "what appears like once molten mass(es), now solidified", as well as "fallen objects". (Separately on NHK TV I saw a Tepco representative stating that these solidified icicle-like masses have a high probability of being "molten fuel and various parts", which mixed and later solidified.)
Same as in the first day, they saw damaged structures and fallen support fittings of CRD-housings.

It has certainly made the news.

http://abcnews.go.com/Technology/wireStory/melted-fuel-time-fukushima-plant-48769520

It is difficult to know at what one is looking. The CRD housings are clear in some pictures (looking up from underneath), but in others it's not clear if one is looking up or down. Certainly there is a lot of corrosion and corrosion products. If there were chunks of molten fuel below the pressure vessel, then I would expect somewhere there should be a relatively large hole in the pressure vessel.

Once again, English version of the brief report has been posted here: http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170721_01-e.pdf

In one image described as the CRD housings under Unit 5 (CRD housing at Unit 5 and CRD housing supporting clamp), there seems to be a lot of rust on some of some of the components.

 

[Charles Smalls]

Far more interesting than expected.

First and foremost; the water level. The water level in Unit 3 is remarkably high. The probe results from the other 2 units show water levels much, much lower.
At 1:40 in the video we see the probe swimming close to submerged control rod drives.
[COLOR=#black]. [/COLOR]

Knowing that the CRD bottom ends are located at the top of the pedestal and that the bottom of the pedestal is openly connected to the outer pvc area, that means the entire unit 3 Primary Containment 'light bulb', must be flooded to an amazing level (Similar to the light blue area below but also including the outer area).

Considering that the unit building is above the water table and sea level, they must be pumping water into keep it flooded.

The second interesting thing is the structure of the deposited fuel melt. Anybody experienced in lava or melt flows would be able to say whether these globular formations were formed in air or in water, i.e. did the fuel melt burn through the reactor onto a dry floor where it could burrow further or was it quenched as it entered an already flooded containment.

Lastly, muon results. It will be interesting to see when they are released, how they match up with what appears to be visible fuel deposits in the Unit 3 pedestal area. That could give some information as far as the lack of results from the scans on units 1 and 2 i.e. is the fuel in units 1 and 2 higher or lower etc.

Unit 3 being so heavily damaged but still holding that much water is definitely the most surprising thing to see though.

 

[Hiddencamper]

Far more interesting than expected.

First and foremost; the water level. The water level in Unit 3 is remarkably high. The probe results from the other 2 units show water levels much, much lower.
At 1:40 in the video we see the probe swimming close to submerged control rod drives.
[COLOR=#black]. [/COLOR]
View attachment 207633

Knowing that the CRD bottom ends are located at the top of the pedestal and that the bottom of the pedestal is openly connected to the outer pvc area, that means the entire unit 3 Primary Containment 'light bulb', must be flooded to an amazing level (Similar to the light blue area below but also including the outer area).View attachment 207634

Considering that the unit building is above the water table and sea level, they must be pumping water into keep it flooded.

The second interesting thing is the structure of the deposited fuel melt. Anybody experienced in lava or melt flows would be able to say whether these globular formations were formed in air or in water, i.e. did the fuel melt burn through the reactor onto a dry floor where it could burrow further or was it quenched as it entered an already flooded containment.

Lastly, muon results. It will be interesting to see when they are released, how they match up with what appears to be visible fuel deposits in the Unit 3 pedestal area. That could give some information as far as the lack of results from the scans on units 1 and 2 i.e. is the fuel in units 1 and 2 higher or lower etc.

Unit 3 being so heavily damaged but still holding that much water is definitely the most surprising thing to see though.

As I said earlier remember unit 3 was depressurized so it did not have a hot debris ejection. Hot debris ejections are extremely likely to cause containment damage under the conditions units 1/2/3 were operating in.

The severe accident guidelines at the time direct operators to flood containment following a core breach to establish "Minimum Debris Submergance Level". This is to stop or prevent a core concrete interaction. So they did just that. Unit 3's containment appears to be in much better condition than 1/2. There is speculation that the fact that unit 3 didn't have a containment failure led to hydrogen gas buildup in the upper head of the drywell and the head "burping" as a way to release gas pressure which caused the different hydrogen explosion than we saw at unit 1. There is history of BWR drywell a burping through the drywell head seals during containment pressure testing under some more extreme conditions so this isn't unheard of.

 

[jim hardy]

There is history of BWR drywell a burping through the drywell head seals during containment pressure testing under some more extreme conditions so this isn't unheard of.

Closure bolts yield ?

 

[Charles Smalls]

unit 3 was depressurized so it did not have a hot debris ejection.

Latest results and images seem to indicate Unit 3 did in fact sustain a substantial hot debris ejection.

Going by the definition of:

"an explosion through the bottom of the reactor [where nuclear] material scatters all across the basemat of the containment liner below the bottom of the reactor. If a core catcher was installed when the plant was built, the material will spread across the core catcher, and will not be able to eat through the concrete at the bottom of the containment. If there wasnt, it could potentially undergo a reaction with the concrete for a while, where it can eat through the containment basemat. The whole time this is occurring, if it is not cooled and water is not injected to the containment, then the containment will start to heat up, overpressurize, crack, and leak radioactive material directly outside." - HiddencamperTepco engineers and the new images seem to be suggesting this is exactly what happened. We know there was no "core catcher", which is why I raise the point about whether the pedestal area was flooded at the time the core ejected. Depending on how deep into the concrete the fuel ate and how damaged the basemat was by this and the earthquake explosions is very relevant. If the basemat is no longer water tight and the building is flooded to a height of 30 feet above sea level or so, there must be a lot of water flowing past the fuel into the environment. That would explain the groundwater contamination issues at the site.

Again, whether this new found apparent fuel will match up with the muon scans and what that says about the negative results on 1 and 2 could have a lot to say about fuel locations and depths over there too. Very interesting time and I'm glad TEPCO is sharing so much information

 

[jim hardy]

I raise the point about whether the pedestal area was flooded at the time the core ejected.

I see at least some CRD mechanisms still in place
which infers not gross vessel failure
so i doubt there was any significant 'ejection'though a smaller drip is sure plausible
which into water would cause a steam explosion likely scattering the fuel.

When they find the actual fuel its physical arrangement will give more insight to what went on that morning.

 

[Hiddencamper]

Latest results and images seem to indicate Unit 3 did in fact sustain a substantial hot debris ejection.

Going by the definition of:

"an explosion through the bottom of the reactor [where nuclear] material scatters all across the basemat of the containment liner below the bottom of the reactor. If a core catcher was installed when the plant was built, the material will spread across the core catcher, and will not be able to eat through the concrete at the bottom of the containment. If there wasnt, it could potentially undergo a reaction with the concrete for a while, where it can eat through the containment basemat. The whole time this is occurring, if it is not cooled and water is not injected to the containment, then the containment will start to heat up, overpressurize, crack, and leak radioactive material directly outside." - HiddencamperTepco engineers and the new images seem to be suggesting this is exactly what happened. We know there was no "core catcher", which is why I raise the point about whether the pedestal area was flooded at the time the core ejected. Depending on how deep into the concrete the fuel ate and how damaged the basemat was by this and the earthquake explosions is very relevant. If the basemat is no longer water tight and the building is flooded to a height of 30 feet above sea level or so, there must be a lot of water flowing past the fuel into the environment. That would explain the groundwater contamination issues at the site.

Again, whether this new found apparent fuel will match up with the muon scans and what that says about the negative results on 1 and 2 could have a lot to say about fuel locations and depths over there too. Very interesting time and I'm glad TEPCO is sharing so much information

I'm out of town right now, but when I get home I'll link a TEPCO report that shows the Automatic Depressurization System actuated and depressurized the reactor preventing a hot debris ejection.

A hot debris ejection is when the vessel is still pressurized. If you have a hot debris ejection while the suppression pool exceeds the heat capacity temperature limit and the pressure suppression pressure, then containment failure is virtually guaranteed. The fact that unit 3 is retaining this much water demonstrates that it is far less damaged than units 1/2 which can't even keep the pedestal region flooded.

There are pressure trends which were recorded on unit 3 where you can clearly see the ADS actuation. The logic was made up because primary containment and suppression chamber pressure were so high that it dummies the low pressure ECCS running signal which is a permissive for ADS to actuate.

 

[Hiddencamper]

Latest results and images seem to indicate Unit 3 did in fact sustain a substantial hot debris ejection.

Going by the definition of:

"an explosion through the bottom of the reactor [where nuclear] material scatters all across the basemat of the containment liner below the bottom of the reactor. If a core catcher was installed when the plant was built, the material will spread across the core catcher, and will not be able to eat through the concrete at the bottom of the containment. If there wasnt, it could potentially undergo a reaction with the concrete for a while, where it can eat through the containment basemat. The whole time this is occurring, if it is not cooled and water is not injected to the containment, then the containment will start to heat up, overpressurize, crack, and leak radioactive material directly outside." - HiddencamperTepco engineers and the new images seem to be suggesting this is exactly what happened. We know there was no "core catcher", which is why I raise the point about whether the pedestal area was flooded at the time the core ejected. Depending on how deep into the concrete the fuel ate and how damaged the basemat was by this and the earthquake explosions is very relevant. If the basemat is no longer water tight and the building is flooded to a height of 30 feet above sea level or so, there must be a lot of water flowing past the fuel into the environment. That would explain the groundwater contamination issues at the site.

Again, whether this new found apparent fuel will match up with the muon scans and what that says about the negative results on 1 and 2 could have a lot to say about fuel locations and depths over there too. Very interesting time and I'm glad TEPCO is sharing so much information

This link discusses the rapid pressure drop in unit 4. They pretty much disprove everything but the ADS which gives the trace they saw (and lines up with what I've seen in simulator scenarios)

http://www.tepco.co.jp/en/press/corp-com/release/betu14_e/images/140806e0122.pdf

 

[Charles Smalls]

Is the rv pressure status at the time of the core ejection relevant? If the new images are accurate then it still shows that the core ejected from the RV and splattered around the CRD room, pedestal and outer PVC area. Whether it was complete and immediate ejection (i.e. under pressure) or a depressurised slow and partial release would only be relevant to calculate potential basemat penetration. Seeing as the explosions and earthquake put the integrity of the basemat at the time of the meltdown into the unknown I don't think it matters.

The fact that unit 3 is retaining this much water demonstrates that it is far less damaged than units 1/2 which can't even keep the pedestal region flooded.

Well this is the really interesting part. The pedestal is open to the outer pvc at the bottom here:

So the water level inside the pedestal must be the same outside in the pvc. Is the outer PVC designed to be water tight to that level? If so then maybe they just add relatively small amounts of water to keep the building flooded. If not then they must be pumping in massive amounts but need to in order to provide some sheilding around the fuel in a relatively problematic location.

What does that mean for unit 1 and 2? That they don't need the same amount of forced flooding because their cores are lower down in the basemats closer too or actually screened by the water table?

The persistent groundwater contamination is coming from somewhere. It's just interesting to figure out whether unit 3 is the best of the bunch or the worst.

 

[Hiddencamper]

Is the rv pressure status at the time of the core ejection relevant? If the new images are accurate then it still shows that the core ejected from the RV and splattered around the CRD room, pedestal and outer PVC area. Whether it was complete and immediate ejection (i.e. under pressure) or a depressurised slow and partial release would only be relevant to calculate potential basemat penetration. Seeing as the explosions and earthquake put the integrity of the basemat at the time of the meltdown into the unknown I don't think it matters.
Well this is the really interesting part. The pedestal is open to the outer pvc at the bottom here:

View attachment 207677

So the water level inside the pedestal must be the same outside in the pvc. Is the outer PVC designed to be water tight to that level? If so then maybe they just add relatively small amounts of water to keep the building flooded. If not then they must be adding massive amounts but need to in order to provide some sheilding around the fuel in a relatively problematic location.

What does that mean for unit 1 and 2? That they don't need the same amount of forced flooding because their cores are lower down in the basemats closer too or actually screened by the water table?

The persistent groundwater contamination is coming from somewhere. It's just interesting to figure out whether unit 3 is the best of the bunch or the worst.

The SRVs discharge steam in a relatively controlled fashion into the suppression chamber. While a 1000+ psig bottom head rupture will catastrophically expand in the containment system while its already beyond its pressure limit. The severe accident mitigation basis specifically discuss the importance of depressurizing the vessel to prevent a hot debris ejection while the containment is above the pressure suppression pressure because you will exceed the design limit of the containment. The SAGs also tell you to immediately depressurized the vessel once you recognize that adequate core cooling is lost because of this.

There is a huge difference between depressurizing to the pool and rupturing the vessel at pressure.

As for the water level, this is how bwrs are designed. After a core melt breaches the reactor you were supposed to flood the suppression pool until water backfills the under pedestal region and submerged the fuel (called the Minimum Debris Submergence Level or MDSL). This changed post Fukushima for most bwrs... but that's irrelevant. The goal of a post vessel failure, whether it's due to core melt or LOCA, is to flood containment to backfill the vessel and resubmerge the fuel. The containment is designed to be essentially leaktight. After backfilling, you pull the reactor head off and dig the debris out from the top using the water as shielding.

Now we know unit 1/2 aren't leaktight anymore due to the damage they had. Neither one is capable of submerging the undervessel grating. However unit 3 looks like it's still nearly leaktight so this may be an option.

 

[turi]

What does that mean for unit 1 and 2? That they don't need the same amount of forced flooding because their cores are lower down in the basemats closer too or actually screened by the water table?

The persistent groundwater contamination is coming from somewhere. It's just interesting to figure out whether unit 3 is the best of the bunch or the worst.

According to this document (July 13, 2017) http://www.tepco.co.jp/en/press/corp-com/release/betu17_e/images/170718e0101.pdf the amount of water injected to each reactor is comparable (70, 67 and 70 m³/day for units 1, 2 and 3 respectively).

EDIT: And as of July 20 is 72 m³/day for each unit.

 

[jim hardy]

images are accurate then it still shows that the core ejected from the RV and splattered around the CRD room, pedestal and outer PVC area.

Where in the images do you see that?

 

[Astronuc]

Various news organizations are publishing pictures purportedly being 'likely molten fuel'. Unfortunately, there is no scale or reference, so it is impossible to judge the size of what is considered to be molten fuel, or a mixture of molten steel and fuel. The orange/brown is likely corrosion products from steel. The dark or black material could be molten fuel.

http://www.japantimes.co.jp/news/20...fuel-bottom-reactor-3-fukushima/#.WXVM8HlK2po

https://www.bloomberg.com/news/arti...ted-fukushima-fuel?utm_medium=bd&utm_campaign

I would hope that the robot can grab some samples for analysis of the composition.

 

[jim hardy]

Thanks Astro.

 

[Sotan]

There's a new quick report (in Japanese) regarding the third day of investigation in Unit 3 (22 July)
http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_170722_05-j.pdf
Pages 1-2 show new photos from inside the pedestal.
Page 3 lists short conclusions: The investigation of 22 July aimed and succeeded to obtain information regarding the lower part of the pedestal and surroundings. Once molten and then solidified masses were observed, as well as fallen objects such as pieces of grating, and sediments/deposits. The analysis will continue.
The photos can also be seen here:
http://photo.tepco.co.jp/date/2017/201707-j/170722-01j.html

 

[jim hardy]

Thanks Sotan !

Has anyone looked in that 294 megabyte zipped folder?

3号機 PCV内部調査動画（22日調査結果速報）(2:23)

動画を再生／ダウンロード（294MB）

 

[Sotan]

Hey Jim. I hadn't even noticed that! I couldn't have played it at work anyway, but now I am home and I just looked at it and it's awesome, much more suggestive than the still photos. The swimming robot did a great job, I just wished I understood more or what I see. Reminded me a little of Titanic - and also of those scenes in which a paleoanthologist says "see here how well this bone is preserved" and all I see is a rock. The large pipe corroded (melted) so bad that it is missing a few patches (01:37) through which we see a smaller pipe inside? The blue-greenish stuff that we've seen in another unit too (00:48). The mix of materials at 01:20. The amount of light, floating sediment that is raised by the robot's propellers.

By the way the clickable folder name in the link given by Jim is something like link1 / link2, link1 is "play the movie" and link 2 is "download the file".

Edit: Then I found the video images of 21st too. Totally as impressive! Take a look (I only know this way to access them : http://www.tepco.co.jp/tepconews/library/archive-j.html?video_uuid=f1ak69jq&catid=69619)
Especially intriguing moments: 00:12 and 00:53, that can't be steel corrosion, too conspicuous and non-uniform? they rather look like blobs of material splattered over structures. What's that at 00:19, a little current of water - under water? Or bubbles? And at 00:44, a large irregular mass near the still nicely rounded pedestal wall.

 

[Charles Smalls]

Where in the images do you see that?

My apologies for not posting links to the suspected molten fuel deposit videos, I assumed they were commonly known as mainstream media have been covering it quite a lot over the last few days. As I said in my earlier post, the notion that this is ejected fuel isn't based on seeing the video images, TEPCO spokesman Takahiro Kimoto is on record since last week indicating they suspect the images are of melt out fuel. "You can see something melted and solidified fallen from the inside of the reactor" he said. (Images and the quote source are here: www.cnn.com/2017/07/24/asia/fukushima-robot-nuclear-fuel-detected/index.html)

Unit 2 inspection already showed very strong indications of gross melt out there so I don't think it should be any surprise for unit 3.

As you say, as far as the ratio of fuel that exited the RV, I don't think the size of the hole in the CRD roof is relevant. Molten fuel being a liquid, once container penetration is achieved, the entire contents are able to exit the RV just as well through a small bore hole as a large one. Bottom loading control rods are a known inherent weak-point in this particular reactor design after all. News sources are calling the deposit sizes 'large' but whether that is from casual arm chair eyeballing or from actual TEPCO officials looking at known equipment and CRD remains captured in inspection videos and using them for scaling is unknown. If we get enough images to tile together as was done for Unit 2, we should have a clearer picture.

@Hiddencamper , Thank you very much for such an informative reply. I had no idea that the PVC was inherently designed to be water tight that way. This explains so much about events during the accident with the helicopters dumping water on the reactor buildings and what they were trying to achieve. Very interesting.

As far as this part:

There is a huge difference between depressurizing to the pool and rupturing the vessel at pressure.

I understand that it is better to have RV depressurisation than pressurised RV rupture, but if the depressurised reactor then goes on to suffer a melt through anyway, what difference does it ultimately make? I am correct is assuming that it only matters as far as trying to calculate how much the concrete under the reactor will be damaged/attacked and how long you have to re-establish cooling before radioactive materials leak directly outside?

 

[jim hardy]

Especially intriguing moments: 00:12 and 00:53, that can't be steel corrosion, too conspicuous and non-uniform? they rather look like blobs of material splattered over structures.

Thanks Sotan

my Windows is dyng of update poisoning, Microsoft's preferred means of demise. It downloaded the file but refuses to play it. Perhaps i'll try the other machine tonight.

Splattered ? As in perhaps thrown against the walls by expanding steam ? That might explain a lot.

I would be very interested to see upper head area of the drywell (or is PCV correct term? The big light bulb..) Do you recall any photos ? . .

 

[jim hardy]

Got it to play
THANKS, Sotan and Charles ..

I see what you mean. Around 1:21 and 1:50 could pass for slag.

but speaking for myself I have to resist the natural tendency to speculate whatever is most exciting . So at this point i wouldn't assert that it's less benign than melted plastic cable insulation. .

Maybe someone who's been under a BWR will recognize the mechanical parts.

Astro nailed it - got to get a sample of the stuff.. Activation products in it will tell a lot.

 

[Rive]

I think it is at 1.18. That's something dense and solid hanging down (!) from CRD support.

 

[MadderDoc]

<..>
Now we know unit 1/2 aren't leaktight anymore due to the damage they had. Neither one is capable of submerging the undervessel grating. However unit 3 looks like it's still nearly leaktight so this may be an option.

Now, they are pumping in 70m3/day, so that would presumably be the volume that is leaking daily, at the present level of flooding. As I recall it, they were pumping in close to double that rate in early 2014, then cut it down to about 100m3/day later during that year. At about that time a significant leak from the PCV was detected at the level of the MSIV penetration (X-7) (~ at O.P. 12.000, which would be on the first floor of the reactor building). Apparently that is also the level to which the vessel is currently flooded. Due to the leak at MSIV penetration, they may not be able to flood the containment to a higher level, lest it spills over there.

 

[Hiddencamper]

Thanks Sotan

my Windows is dyng of update poisoning, Microsoft's preferred means of demise. It downloaded the file but refuses to play it. Perhaps i'll try the other machine tonight.

Splattered ? As in perhaps thrown against the walls by expanding steam ? That might explain a lot.

I would be very interested to see upper head area of the drywell (or is PCV correct term? The big light bulb..) Do you recall any photos ? . .

The outside of the upper head of the drywell requires you to drain out the spent fuel pool and upper cavity and remove the shield plugs.

To see the inside of the drywell head you need to go to the upper elevation of the drywell and open some bellows hatches. Not something a robot can accomplish.

 

[Hiddencamper]

My apologies for not posting links to the suspected molten fuel deposit videos, I assumed they were commonly known as mainstream media have been covering it quite a lot over the last few days. As I said in my earlier post, the notion that this is ejected fuel isn't based on seeing the video images, TEPCO spokesman Takahiro Kimoto is on record since last week indicating they suspect the images are of melt out fuel. "You can see something melted and solidified fallen from the inside of the reactor" he said. (Images and the quote source are here: www.cnn.com/2017/07/24/asia/fukushima-robot-nuclear-fuel-detected/index.html)

Unit 2 inspection already showed very strong indications of gross melt out there so I don't think it should be any surprise for unit 3.

As you say, as far as the ratio of fuel that exited the RV, I don't think the size of the hole in the CRD roof is relevant. Molten fuel being a liquid, once container penetration is achieved, the entire contents are able to exit the RV just as well through a small bore hole as a large one. Bottom loading control rods are a known inherent weak-point in this particular reactor design after all. News sources are calling the deposit sizes 'large' but whether that is from casual arm chair eyeballing or from actual TEPCO officials looking at known equipment and CRD remains captured in inspection videos and using them for scaling is unknown. If we get enough images to tile together as was done for Unit 2, we should have a clearer picture.

@Hiddencamper , Thank you very much for such an informative reply. I had no idea that the PVC was inherently designed to be water tight that way. This explains so much about events during the accident with the helicopters dumping water on the reactor buildings and what they were trying to achieve. Very interesting.

As far as this part:I understand that it is better to have RV depressurisation than pressurised RV rupture, but if the depressurised reactor then goes on to suffer a melt through anyway, what difference does it ultimately make? I am correct is assuming that it only matters as far as trying to calculate how much the concrete under the reactor will be damaged/attacked and how long you have to re-establish cooling before radioactive materials leak directly outside?

Dumping water from helicopters was only for the spent fuel pool. Not the containment. No real reason to do that.

If you have a pressurized melt while the containment is already beyond its limits the steam explosion can catastrophically damage the containment. We are talking about a small breach in the bottom head region, where the sudden and rapid expansion of steam out of the breach causes a catastrophic failure. The drywell is designed to handle this only when it is within certain initial conditions, and if you already exceed the primary containment pressure limit (PCPL) and the suppression pool exceeds the heat capacity temperature limit and suppression pressure, then drywell failure is very likely during a pressurized vessel breach.

When you initiate ADS, the steam from the reactor is discharged into the suppression pool. Even if the pool is already at saturation temperature, this is a controlled discharge of 25-40% rated steam flow, and as the containment system pressurizes slowly the suppression pool helps to act as a buffer to prevent an instantaneous failure of the drywell.

During a LOCA in a BWR, even if initial conditions are all met, drywell pressure momentarily exceeds the design pressure before the suppression pool can do its job. If you already exceed the pool and drywell limits and a hot debris ejection occurs, the pressure spike will likely fail the containment.

Remember that SRVs discharge to the suppression pool, while a vessel rupture discharges to the drywell and only eventually makes its way to the suppression pool after vent clearing.

Additionally in the case of unit 3, the ADS blowdown caused the containment vent rupture disc to finally rupture, allowing a controlled vent of decay heat to protect the containment system.

 

[Hiddencamper]

Now, they are pumping in 70m3/day, so that would presumably be the volume that is leaking daily, at the present level of flooding. As I recall it, they were pumping in close to double that rate in early 2014, then cut it down to about 100m3/day later during that year. At about that time a significant leak from the PCV was detected at the level of the MSIV penetration (X-7) (~ at O.P. 12.000, which would be on the first floor of the reactor building). Apparently that is also the level to which the vessel is currently flooded. Due to the leak at MSIV penetration, they may not be able to flood the containment to a higher level, lest it spills over there.

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.

 

[turi]

Results for Unit 3 muon measurements are in: No massive material has been found in RPV, meaning that the fuel has mostly be gone from the RPV. Page 8 shows a nice overview of the expected fuel locations and water levels in all 3 units:
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170727_01-e.pdf

 

[turi]

New analysis of the unit 1 investigations: For one location they eliminated the possibility that there is fuel debris, for two other they are not sure because deposits above possible melted fuel might provide shielding. http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170727_02-e.pdf

 

[turi]

Dose rates at unit 2 were reestimated. The first estimates from the image noise level were too high:
"[...] when making preparations to conduct the internal exploration of the PCV in a low radiation level environment, the threshold values were lowered to 50 in order to check operation of the equipment, but were never returned to 70 before conducting the PCV internal exploration".
So estimates at the CRD went from 530 Gy/h to 70 Gy/h. The latter value is also under the assumption "that Cesium 137 is the single major radiation source in the Primary Containment Vessel".
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170727_03-e.pdf
Edit: Corrected whitespaces.

 

[Charles Smalls]

All three unit results pretty much as expected:

The latest Unit 1 update report is one of the more interesting. The Unit 1 muon scans already indicated the RV was mostly empty and the most likely direction for the liquid fuel to travel was out through the pedestal opening into the PVC proper. The fact that a mass of deposited material was picked up by the probe cameras in this area suggested fuel remains. Now the new readings from this material suggests that the radioactivity levels are actually too low for it to be actual fuel.

To quote the July 27, 2017 report:
"Existence of fuel debris cannot be examined in the case of thick deposits due to their shield. [They consider that] there is no fuel debris, or that thick deposits and structures have profound shield effect."

I suggested before that hot fuel-concrete interactions can result in a process known as spalling where the high heat breaks up and redeposits the spalled concrete material in a1 process. (I think there's a video attached in one of my May or June posts.) But due to the results of this new relatively weak reading, I think the likelihood has been increased that this sediment material coating the surfaces in Unit 1 is actually spalled or deposited concrete of the basemat with the actual fuel melt further down in the building footings.

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.

Unit 3 scans show the reactor vessel was empty as the fuel appears to have melted out through the bottom head i.e. hot ejection. This means that the muon scan and it's target area was misdirected as I suspected. If they had been more pessimistic/realistic and used the results of the first two scans, they could have adapted the plan here to observe the lower portion of the pedestal area to gain valuable information to match up with the new probe results and provide more insight to the situation.

Just to go back to @Hiddencamper , this is what I meant by hot debris ejection being a relative term. Data may show that preemptive steps like ADS steam purging took place, but either way, a gross fuel exit and spread seems to have occurred and as we know large explosions took place inside the unit building so the end result is largely the same.

Very interesting to see the growing picture of the situation across the units and glad Tepco is sharing as much as they are. Many thanks to Sotan, Turi and others for providing the information and translations.

 

[HowlerMonkey]

Post worst case opinion, get refuted...repeat.

 

[jim hardy]

The Unit 1 muon scans already indicated the RV was mostly empty

More correctly they failed to show where is the fuel.

a gross fuel exit and spread seems to have occurred and as we know large explosions took place inside the unit building so the end result is largely the same.

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

I have my own ideas too.

TEPCO seems to be proceeding with meticulous caution and attention to detail. Patience is the best policy for us "Sidewalk Superintendents" ..