Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[NUCENG]

Overpressure in the drywell would first be vented to the torus, which is one reason why I can't accept (yet) that the cap would unseat and release gas before other primary containment failure. Proceeding further, even if the cap did unseat and release gas, as soon as enough was released the internal pressure would drop below the threshold needed and the seal would be re-established. Not sure about the time for that to happen, though, and how much gas would need to be released to reduce the pressure far enough.

What if the weakest part of the cavity above the cap was the path to the equipment pool? There would be no hydrostatic pressure at all on the other side of that barrier if the pool was empty.

Will respond more after digesting the rest (and dinner!).

Yes the drywell pressure vents to the torus. Steam is condensed in the suppression pool to help reduce the pressure increase. However, the torus is heating up and some of the released material are non-condensibles like hydrogen gas. As time goes on both drywell and torus pressure increase. The unit 1 containment pressure was more that twice the design pressure, That was more than enough to fail the drywell cap seal ring and stretch the holddown bolts. It is likely this process was repeated at unit 2 and unit 3.

 

[jim hardy]

"So it's prohibited to discuss the criticality of SFP #3 in this forum? "

i'm not a reactor physicist but did take a course in it a little over forty years ago.
Here's a link to a short course :
http://www.if.uidaho.edu/~gunner/ME443-543/LectureNotes/ReactorPhysics.pdf

it's pretty decent explanation of the basics.

i'll refer to one just page in my attempted explanation below.

I too looked into pool criticality and mentor is right - you need to know basically how many atoms per cubic centimeter you have of fuel, water and poisons to predict criticality with accuracy. We don't have that.

There are people who earn their living doing that calculation and they could give you a convincing answer yea or nay if they had the info Mentor mentioned.. I will tell you why i did not pursue it any further.

The basic premise of a spent fuel pit is you build it so criticality is impossible. You do that by two different means:
1. Put so much space between fuel assemblies that a flying neutron stands a really good chance of missing a fuel atom on its way out of the neighborhood. If the chance of a neutron missing a fuel atom is more than 0.55 (page 12) then criticality is impossible. It doesn't take much distance.

2. Put poison in the pool so that a flying neutron stands a decent chance of hitting a poison atom instead of a fuel atom. That adds to the probability it'll miss a fuel atom. You can use closer geometry if you have poison.

Some places do both. In PWR's there's loads of boron in the pool water, and they've taken to adding boron to the construction materials for the racks to further assure safety in case the pool somehow gets diluted with unborated water.

BWR's don't use borated water like PWR's do so if they use poison it's either those Boraflex plastic inserts or boron bearing steel racks.

I emailed Arnie Gundersen over a month ago and asked him - since he said his company made the Fukushima racks how about telling us something about them - did he use boraflex or boron steel or simple geometry to assure legal requirement on subcriticality (Keff < 0.95 which is the requirement stateside) .
He never answered but just went on a mild rant in next video about prompt criticality which to me sounds like he's BS'ing. I now think he has alarmist leanings but that's just my opinion.

so my OPINION as an old guy who has drunk beer with real reactor engineers is this:
If you took all the stuff in the 3 spent fuel pool and ran it through your blender it would not go critical.
There should be old control rods and probably Boraflex in there too. The steel racks themselves are a mild poison and a great one if they're boron steel.
And there's not a lot of fuel in 3 either, the Japanese are ahead of USA on that note. To get the pool critical you'd have to get the boron out and still have water. And the seawater they added has the property chlorine(NaCl) is another mild poison.

That sort of cross checks with impression i took away from that underwater video. The rubble looked to me blown into pool from higher up and laying on top of fuel. Criticality should have emptied the pool and steam cleaned it.

That's my opinion. I hope a genuine reactor engineer chimes in for you."""if tepco would not have considered the possibility of criticality, why would they have added boron to the cooling water?""
If i were in their shoes I'd do it just to make sure.
If they have Boraflex plastic in the racks, when fuel got uncovered did it melt out the Boraflex? Borating would prevent criticality when you cover fuel with water again.

 

[TCups]

ROOF DAMAGE FLOOR PLAN OVERLAY, UNIT 3

Find attached my assessment of the state of the roof structure of unit 3, after the explosion.

Based on visual inspection of photos, each field in a 16x24 matrix covering the entire roof structure was assessed to one of five categories of damage, see legend. The method used gives the assessment a resolution of about 1.5 meter.

Fragments found on the roof of unit 3 were assessed, as were fragments of the roof structure locatable to the south and the east side of the building. Due to poor photo coverage and their inter-mixture with other debris fragments which ended to the north of the building could not be inspected. Those parts of the structure from the N end and from the SE corner which could not be inspected were assessed based on plausibility, judging from visually inspected close-by or bordering fields.

This, of course is MadderDoc's work combined with MiceAndMen's recent post attachment showing a pretty accurate floor plan. Interesting.

 

[rowmag]

Something else, could someone tell me what is the title of this parameter (Unit 1)

https://spreadsheets.google.com/spr...ZDbX39YK-iFb0Iw&hl=ja&authkey=CP6ewJkO#gid=41

FPC skimmer surge tank level (mm)

 

[etudiant]

ROOF DAMAGE FLOOR PLAN OVERLAY, UNIT 3



This, of course is MadderDoc's work combined with MiceAndMen's recent post attachment showing a pretty accurate floor plan. Interesting.

Thank you very much for this. It is by far the best visual summary yet of the current situation.

It would help if someone would reverse the image. The descriptions show as mirror writing currently.

 

[TCups]

Thank you very much for this. It is by far the best visual summary yet of the current situation.

It would help if someone would reverse the image. The descriptions show as mirror writing currently.

Anecdote:

Many years ago, when I was in training, there were some "Grand Old Men" of the profession of Radiology who read only X-Ray films on a light box -- long before the days of CT, Ultrasound, MRI and other cross-sectional images. One of them in particular was a mentor of mine, now long since dead. I won't mention his name, but I will say that he could, with the flick of a wrist, like dealing cards, toss 14" x 17" films up under the clips of a bank of a dozen viewboxes like a professional poker player deals the cards. Of course, some of the films would invariably end up upside down or backwards. But if you ever pointed that out to him, his reply was: "Doctor, turn them over in your mind." The guy could read more off an upside down chest x-ray than a lot of my present day colleagues can decipher from a high-resolution CT. RIP, "Big Dave".

Here 'tiz:
Reference credit to: MadderDoc - Color Overlay, MiceAndMen - recent floor plan post

 

[mscharisma]

"So it's prohibited to discuss the criticality of SFP #3 in this forum? "

i'm not a reactor physicist but did take a course in it a little over forty years ago.
Here's a link to a short course :
http://www.if.uidaho.edu/~gunner/ME443-543/LectureNotes/ReactorPhysics.pdf

it's pretty decent explanation of the basics.

i'll refer to one just page in my attempted explanation below.

I too looked into pool criticality and mentor is right - you need to know basically how many atoms per cubic centimeter you have of fuel, water and poisons to predict criticality with accuracy. We don't have that.

There are people who earn their living doing that calculation and they could give you a convincing answer yea or nay if they had the info Mentor mentioned.. I will tell you why i did not pursue it any further.

The basic premise of a spent fuel pit is you build it so criticality is impossible. You do that by two different means:
1. Put so much space between fuel assemblies that a flying neutron stands a really good chance of missing a fuel atom on its way out of the neighborhood. If the chance of a neutron missing a fuel atom is more than 0.55 (page 12) then criticality is impossible. It doesn't take much distance.

2. Put poison in the pool so that a flying neutron stands a decent chance of hitting a poison atom instead of a fuel atom. That adds to the probability it'll miss a fuel atom. You can use closer geometry if you have poison.

Some places do both. In PWR's there's loads of boron in the pool water, and they've taken to adding boron to the construction materials for the racks to further assure safety in case the pool somehow gets diluted with unborated water.

BWR's don't use borated water like PWR's do so if they use poison it's either those Boraflex plastic inserts or boron bearing steel racks.

I emailed Arnie Gundersen over a month ago and asked him - since he said his company made the Fukushima racks how about telling us something about them - did he use boraflex or boron steel or simple geometry to assure legal requirement on subcriticality (Keff < 0.95 which is the requirement stateside) .
He never answered but just went on a mild rant in next video about prompt criticality which to me sounds like he's BS'ing. I now think he has alarmist leanings but that's just my opinion.

so my OPINION as an old guy who has drunk beer with real reactor engineers is this:
If you took all the stuff in the 3 spent fuel pool and ran it through your blender it would not go critical.
There should be old control rods and probably Boraflex in there too. The steel racks themselves are a mild poison and a great one if they're boron steel.
And there's not a lot of fuel in 3 either, the Japanese are ahead of USA on that note. To get the pool critical you'd have to get the boron out and still have water. And the seawater they added has the property chlorine(NaCl) is another mild poison.

That sort of cross checks with impression i took away from that underwater video. The rubble looked to me blown into pool from higher up and laying on top of fuel. Criticality should have emptied the pool and steam cleaned it.

That's my opinion. I hope a genuine reactor engineer chimes in for you."""if tepco would not have considered the possibility of criticality, why would they have added boron to the cooling water?""
If i were in their shoes I'd do it just to make sure.
If they have Boraflex plastic in the racks, when fuel got uncovered did it melt out the Boraflex? Borating would prevent criticality when you cover fuel with water again.

I have a couple of questions about that but should mention first (again) that I'm completely "nontechnical," even if the wording of the questions may reveal that right away anyhow.

I read somewhere recently (probably something I found a link to in this forum) that stacking the fuel very tightly to get more fuel in the pool to evade the problem of final storage as long as possible may seriously impede the cooling capability via water. Would the dense stacking also negate the here desired and described effect of the distance for a neutron being too great to meet a fuel atom? Would the fact that the too closely stacked fuel racks' cooling via water is impeded also mean the added poisons are not present in sufficient quantity/distribution so that the desired effect of the neutron hitting a poison atom rather than a fuel atom is also negated or minimized?

If my thinking is completely off on this, I'd be ok if someone just said so without spending/wasting time on a lengthy explanation. But if it's not totally off, any explanation would be appreciated. Thanks.

 

[rowmag]

Why would there be a need to relocate Tokyo?
I see nothing remotely threatening that city.

Posted a reply here:
https://www.physicsforums.com/showpost.php?p=3299228&postcount=165

 

[jmelson]

What I am wondering is what effect a burst of Gamma radiation would have on a stockpile of new and spent fuel rods ? Any whizzkids know the projected results? surely it must have been modeled given 7 reactors and sfp's in close proximity? I mean risk assessment and all that.

Gamma rays won't do much to Uranium or spent fuel. The only thing you need to worry about is neutrons. Gamma radiation, if intense enough, is a health hazard, but the fuel couldn't care.

Jon

 

[razzz]

Why would there be a need to relocate Tokyo?
I see nothing remotely threatening that city.

Tokyo is seldom downwind from Fukushima.

Pay attention to typhoon season and associated storms this year.

 

[jmelson]

a spokesman for the power giant said when a faulty gauge had been repaired, it showed water levels in the pressure vessel 5m (16ft) below the level needed to cover fuel rods.
"All the fuel is unprotected at this point and the water levels are below that," said Junichi Matsumoto.

Well, since there was a hydrogen explosion, damage to the fuel elements is pretty well assumed. So, they replaced the differential pressure transducer, but they really have no idea of the condition of the sensing tubes in the RPV. I gather there is one tube that opens below the fuel, and one that runs up to above the fuel. Now, I really don't understand how this thing works. It seems that the upper tube needs to be kept free of standing water to get the right reading. If the upper tube if allowed to fill with water, then you only measure the height of water in the TUBE, not the RPV.
So, I don't know how they make that work. But, it seems they'd either need to heat the upper tube or drain it. And, of course, the tubes may be damaged by explosion, overpressure, overheating, collapse of support structures in the RPV, earthquake, etc.

So, just by replacing the diff. pressure sensor, I'm not sure that makes the readings of water level any more reliable than they were before.

Anybody have comments?

Jon

 

[razzz]

NUCENG, I didn't forget about you but as I read things of interest I accept them as common knowledge (just new to me) esp. news article, then later find out they are scrubbed or discounted or ridiculed for various reasons. So without links I tell you that the US did 'suggest' Japan move the no-go zone to 50km, shut down and inspect any NPP in the vicinity of the great quake, stop using saltwater as soon as possible and after a NRC report to Congress some congress critter made a statement that Unit 3 had a crack which he quickly retracted. I'm sure the US military and GE had lots of input for NRC's reporting.

By the by, the air testing procedures of vessels before restart don't include heating to hellish temperatures where seals would burn out, flanges would warp, and bolts/studs would acquire a new memory.

 

[MiceAndMen]

ROOF DAMAGE FLOOR PLAN OVERLAY, UNIT 3


This, of course is MadderDoc's work combined with MiceAndMen's recent post attachment showing a pretty accurate floor plan. Interesting.

It sure is. MadderDoc's colorized grid picture some pages back was what prompted me to post that floorplan which I only found just last night. Nice work overlaying the two. (and a great story re. Big Dave in your post after that )

 

[MiceAndMen]

Edit: My "Theory of Theories" post regarding the theories posted in this thread and their relation to the Physics Forums rules against posting "personal theories" has been moved to my blog page here. Probably a better place for it.

https://www.physicsforums.com/blog.php?u=323135

 

[razzz]

It's a sad state of affairs when you have to rebuild events and timelines from bootleg videos, pics and references that are more precise than what is officially available.

 

[razzz]

I see the latest layout diagram shows a 'New Fuel Storage Vault' I wonder if those racks are excitable?

 

[rmattila]

I gather there is one tube that opens below the fuel, and one that runs up to above the fuel. Now, I really don't understand how this thing works. It seems that the upper tube needs to be kept free of standing water to get the right reading. If the upper tube if allowed to fill with water, then you only measure the height of water in the TUBE, not the RPV.

No, the upper tube (=the reference tube) must be completely filled with water. Then the pressure difference in the upper and lower tubes is proportional to the difference in the water level of the tank (=pressure seen by the lower tube) and the axial location of the upper tube connection.

So in order to be reliable, both tubes must be filled with water similar to that in the tank to be measured, and the possible difference in the temperature of the tubes and the tank must be compensated for.

 

[tyroman]

I believe DW head seal leakage MAY have been part of the hydrogen leak path, although I doubt it was the ONLY one. In the case of Unit #3, it appears to still be leaking.


There is another link I am searching for that had a list of the known / estimated failure pressures of various components of the DW & SC. If someone finds it, please post it!

I posted this document on 4/30/2011. Probably not the study you are searching for, but it is relevant to the discussion of drywell head leakage [as well as other potential leakage paths]...

https://www.physicsforums.com/attachment.php?attachmentid=35050&d=1304204379

See page 5 of 7 where it says;

"For the BWR Mark I -- a leak area of 35 sq in was estimated at -- 117 psig --. Most of this leak area (approximately 95%) is attributed to the drywell head which is predicted to unseat at a pressure of 27 psig."

.

 

[MiceAndMen]

No, the upper tube (=the reference tube) must be completely filled with water. Then the pressure difference in the upper and lower tubes is proportional to the difference in the water level of the tank (=pressure seen by the lower tube) and the axial location of the upper tube connection.

So in order to be reliable, both tubes must be filled with water similar to that in the tank to be measured, and the possible difference in the temperature of the tubes and the tank must be compensated for.

Is the task of calibraion all done at a central place, such as the picture shows? I'm wondering (go ahead and laugh) if it would be possible for a specialized robot to do that job, knowing that the other reactor buildings may not be safe for people to enter any time soon. Is that station on the ground floor?

 

[MadderDoc]

Currently catching up on posts, only on page 428 so far, but I found something that may be of interest...

New Refueling Floor Diagram
<..>
http://min.us/mvnWv8N

The general shape and location of the pools (with North to the left), the drywell head laydown area, the elevator, the main equipment hatch, and the stairwells in the NW and NE corners all seem to be in pretty good agreement with how I envision the "typical" Fukushima floorplan to be. It seems like a good additional point of reference.

Yes, and thank you for posting this. Of particular interest to me is that it shows a diversity of shield plugs different from what I'd envisioned. Specifically it shows the top plug to be sliced not in two semicircular sections as I had imagined it to be, but actually three, a middle section spanning across the full diameter, and at the sides two tangential slices, together forming the full circle.

It also appears the top plug is double layered, each layer with three such sections, and the total top plug construction would then seem to made out of six sections. I do hope someone will correct me if I am wrong, also if we have reason to believe that Fukushima top plugs are differently constructed.

My interest in the shape of these plugs stems from the existence of several hard to explain concrete slabs in the mess in the ruins of the NW corner of unit 3. One large slab in particular did strike my eye at first glance, and I still have no good explanation of its existence. However I had completely rejected that this slab could possibly be a shield plug, since I 'knew' that a shield plug is semicircular, while this slab on closer inspection appeared to be narrower than a full semicircle, and rather more shaped like the tangential slices I see now in this diagram.

 

[~kujala~]

Mr. Kaieda wants to make a point, I guess we have to believe him and it's now official - so Kyodo News didn't make a mistake after all...

Economy, Trade and Industry Minister Banri Kaieda said it is a fact that the water injected into the No.1 reactor leaked away because of a hole or holes created by the meltdown.

http://www3.nhk.or.jp/daily/english/13_16.html

I guess it would be really nice if this fact would later turn out to be a non-fact. But if that would indeed happen then also averything else TEPCO/government states as a fact could be a non-fact too. So I hope this fact stays as a fact although it's a sad affair.

 

[sp2]

http://www.youtube.com/watch?feature=player_embedded&v=FptmoVcgpqg"

2, 3 and 4 puffing away like locomotives...

Anyone want to comment on why #3 appears to be smoking from the north end of the building? It's hard to tell with the wind blowing but at this time of day the prevailing winds change from offshore to onshore and they swirl back and forth. If you watch for a few minutes you can catch them at a null point.

I was out all evening, but noticed the same thing earlier (around 5 p.m., EST).

All three reactors (#1 was out of frame) were putting out LOTS of steam (more than I think I've seen in a while), and 3, specifically was spewing out a pretty focused, fairly intense stream straight to the north.
That also looked a little different than what I've seen lately.

 

[sp2]

Could be that the wind picked up. Lots of wind sounds on the live feed now.

That was definitely not a function of wind.

Wind was clearly blowing to the SE at that point.

Steam (presumably) was spewing straight out of #3, sideways, toward the north, and then getting blown by the wind out to sea, towards the SE.

 

[PietKuip]

I too looked into pool criticality and mentor is right - you need to know basically how many atoms per cubic centimeter you have of fuel, water and poisons to predict criticality with accuracy. We don't have that.

There are people who earn their living doing that calculation and they could give you a convincing answer yea or nay if they had the info Mentor mentioned.. I will tell you why i did not pursue it any further.

The basic premise of a spent fuel pit is you build it so criticality is impossible. You do that by two different means:
1. Put so much space between fuel assemblies that a flying neutron stands a really good chance of missing a fuel atom on its way out of the neighborhood. If the chance of a neutron missing a fuel atom is more than 0.55 (page 12) then criticality is impossible. It doesn't take much distance.

2. Put poison in the pool so that a flying neutron stands a decent chance of hitting a poison atom instead of a fuel atom. That adds to the probability it'll miss a fuel atom. You can use closer geometry if you have poison.
[...]
so my OPINION as an old guy who has drunk beer with real reactor engineers is this:
If you took all the stuff in the 3 spent fuel pool and ran it through your blender it would not go critical.
There should be old control rods and probably Boraflex in there too. The steel racks themselves are a mild poison and a great one if they're boron steel.
And there's not a lot of fuel in 3 either, the Japanese are ahead of USA on that note.

To get the pool critical you'd have to get the boron out and still have water. And the seawater they added has the property chlorine(NaCl) is another mild poison.

I agree with this, but there was a big explosion, accompanied by a radiation spike inside a bunker. So it is legitimate to speculate how this might have come about.

It would be difficult to make spent fuel go critical, and prompt criticality (k>1.006) in spent fuel should be impossible. But Tepco kept a charge of fresh fuel in the same pool. Maybe the only stuff that kept the fresh fuel from going critical were neutron absorbers like boraflex or boral. That kind of stuff cannot stand high temperatures. And the aluminum of boral would desintegrate in hot alkaline water. So after the spent fuel had heated the pool, the fresh fuel might have caused an explosion.

I am a physicist, and one may quote me, click on my username for details about me. But I have no experience whatsoever of nuclear reactors. I do not know anything about packing of fuel in fuel pools. I have discovered many things about the nuclear industry in this thread that I find highly surprising or even horrifying. Keeping fuel in storage in the attic of reactor buildings is one of those surprises.

PS: It would be highly desirable to get some seismic data about that explosion. Of course there is a tremendous background of seismic activity in Japan, but an expert should be able to find it in the data. It is unfortunate that many nearby records have gaps because of the power outage at the time of this event.

 

[MadderDoc]

It was reported re unit 1, that a faulty gauge had been repaired a few days ago, and after repair it showed water levels to be significantly lower than previously thought. It was reported that the new readings indicated water levels in the pressure vessel to be 5 m (16ft) below the level needed to cover fuel rods.

However the most recent plant parameters released (measurement time May 13 5 am) still reports the water level in the pressure vessel to be 'only' 1.7 m below the design top of the fuel?

 

[MiceAndMen]

I see the latest layout diagram shows a 'New Fuel Storage Vault' I wonder if those racks are excitable?

The word "vault" doesn't appear at all in the text of the source document, so there's no insight about that question contained therein. I can't seem to find the website I downloaded it from originally, but I've attached 3 pages from it including the title page and the 2 diagrams I recently posted (including the entire full-size dual-unit floorplan).

View attachment GE BWR4 Tech Manual.pdf

The entire 153 page document is a 7.6 MB pdf and I uploaded it here in case anyone is interested. The refueling floor diagram is near the very end on page 146.

http://min.us/mvnWf9P

 

[rowmag]

It was reported re unit 1, that a faulty gauge had been repaired a few days ago, and after repair it showed water levels to be significantly lower than previously thought. It was reported that the new readings indicated water levels in the pressure vessel to be 5 m (16ft) below the level needed to cover fuel rods.

However the most recent plant parameters released (measurement time May 13 5 am) still reports the water level in the pressure vessel to be 'only' 1.7 m below the design top of the fuel?

The B channel, which has not been recalibrated, is still reporting -1700 mm.
The A channel, which was recalibrated, is reporting "Downscale," or below the measurable range.

 

[Samy24]

It was reported re unit 1, that a faulty gauge had been repaired a few days ago, and after repair it showed water levels to be significantly lower than previously thought. It was reported that the new readings indicated water levels in the pressure vessel to be 5 m (16ft) below the level needed to cover fuel rods.

However the most recent plant parameters released (measurement time May 13 5 am) still reports the water level in the pressure vessel to be 'only' 1.7 m below the design top of the fuel?

TEPCO has only "repaired" the reactor water level sensor A. Sensor B is the old one. A is reported as "Off scale" (water level lower than 5 m below the "former" top of fuel) .

 

[AntonL]

Explosion Unit 3 – Putting it together.

For the Hydrogen explosion two sources can be considered
1. Hydrogen leaked during venting
2. Hydrogen escaping from Dry Well cap as per TCups

Whichever is correct or a combination of both, does not necessarily account for the spectacular vertical column, something else must have happened, and we need a barrel pointing to the sky, this barrel is the spent fuel pool. Again we two possibilities

a. Nuclear explosion ala Gunderson
b. Steam explosion by flash boiling

I think we can discard Gunderson nuclear explosion, that to me is the wildest speculation for a fuel pool that was fairly sparsely packed. It contained 548 spent fuel assemblies with an estimated heat load of 200kW (compare this to SPF 4 with 1535 fuel assemblies and a heat load 2000 to 2400kW)

Now we need to investigate if flash boiling is at all possible. Between loss of cooling at 15:42 on 11 March to the explosion at 6:20 on 15 March, 86.6 hours have passed. The water holding capacity is of SFP-3 is 1425m³

Calculating the temperature rise of the water using 200kW and 1425m³ yields 104.50^oC

To this we got to add the initial temperature, 20 to 25 degrees C and subtract any radiated losses and cooling from surface by evaporation, thus the water was on the brink of boiling or started to boil.

The next question to ask can the water be super heated, and the answer is yes.

We must remember that the pool is some 11 metres deep. Allowing a 1 meter below floor water level the top of the fuel rods are 6 metres below water surface ( pressure 1.6 atmospheres) and the boiling point of water at that depth is 113^oC

Thus >100^oC hot water would rise from the fuel assemblies to the top, mix with the just boiled water (100^oC) or cooler water from the surface that is descending heating that up, and somewhere near the surface the what is left over from the >100^oC hot water would convert to steam and boiling would only be apparent near the surface of the water . One can also envisage that the pool has not started to boil at all due to the heat loss to atmosphere during the heating phase but the water being super heated as there a no seeding points in the upper pool where boiling can initiate.

The Hydrogen explosion and its associated shock waves initiate the flash boil, this intern physically expels water from the pool, lowering the pressure lower down, feeding the flash boil process. Now we have a lot of steam rising through the destroyed roof and as the steam condenses back to water, heating the air and thereby fuelling the explosive thermal that rises some 500 metres into the air.

In conclusion, as documented by the explosion video, we have a hydrogen explosion which is radiating in all directions dictated by building geometry and explosion dynamics. This is followed by flash boiling of the spent fuel pool, creating a powerful thermal lifting the roof and sucking debris high in the sky. I my opinion, we are not looking at explosive shotgun event.

 

[rowmag]

I agree with this, but there was a big explosion, accompanied by a radiation spike inside a bunker. So it is legitimate to speculate how this might have come about.

Do we know how big this spike was in absolute terms, or was it just 12 times the normal background level in the seismic refuge building?