Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[jim hardy]

Thanks Nuceng

In PWRs with large dry containments the pressure instumentation transmitters or sensors are usually inside the containment building but outside the shield wall. so they may be measuring gauge pressure to the containment.

Exactly how mine was built (Westinghouse 3 loop). Sensors were gage.
I do not recall whether the subcooled margin monitor used gage or absolute sensor. I do recall raising that question in design review.

[QUOTEIn BWRs the instrument transmitters or sensors are all located outside the PCV and can be identified as gauge or d/p by the fact that the d/p sensors have two lines and the gage instruments have one which means the difference is to secondary containment pressure. ][/QUOTE]

I did not know that. I assumed they'd be inside drywell.
Sum total of my BWR experience is a very brief (like an hour) tour of Duane Arnold plant thirty+ years ago.

If i learn something every day , and can turn things around so (rate of absorb) > (rate of forget), i may know something someday.

Thanks !



old jim

 

[jim hardy]

I think there could be a main valve preload spring to determine the differential (see attached, from the GE Manual).

Great drawing.

Bottom of piston can be assumed at upstream pressure
and top of piston at either upstream or downstream pressure depending whether pilot is in left or right position.

So:
Free Body Diagram when pilot is in right (open) position:
Force up = (area of piston bottom) X Pupstream + (area of seat) X Pdownstream
Force down = (area of piston top) X Pdownstream + (area of seat) X Pupstream + spring preload

when those two are equal valve can open.

From your drawing (presumably unscaled) - on my screen;
diameter of seat = 0.569 inch
diameter of piston = 1.025
diameter of shaft = 0.350

Let's see if we can "normalize" as nuclear egineers love to do:

diameters relative to diameter of seat:
diameter of seat = 0.569 inch / 0.569 = 1
diameter of piston = 1.025 / 0.569 = 1.801
diameter of shaft = 0.350 / 0.569 = 0.615

and areas relative to area of seat
area of seat =( 0.569 inch / 0.569)^2 = 1,
area of piston = (1.025 / 0.569)^2 = 1.801^2 = 3.243
area of shaft = (0.350 / 0.569)^2 = 0.615^2 = 0.378
area of piston bottom = 3.243 - 0.378 = 2.865

oksy it's all ratio'ed to seat area so we won't need ∏r^2 for a while

now back to free body diagram

Force up = Pupstream X (area of piston bottom) + Pdownstream X area of seat
Force down = Pdownstream X (area of piston top) + Pupstream X (area of seat) + spring preload

By assuming Pdownstream = zero(gage) we can make it a lot simpler

Force up = Pupstream X (area of piston bottom )
and
Force down = Pupstream X (area of seat) + spring preload

Equating those two
Pupstream X (area of piston bottom ) = Pupstream X (area of seat) + spring preload
Pupstream X (area of piston bottom - area of seat ) = spring preload

Valve can open when
Pupstream = spring preload / (area of piston bottom - area of seat )
Pupstream = spring preload / ((2.865 - 1)(area of seat))

Pupstream = spring preload /(1.865 X area of seat)

If it takes 50 psi to open valve
50 X 1.865 = springload/ area of seat
93.25 X area of seat = spring preload

So a seat area of say 4∏ sq inches (wild-a** guess at 4 inch disc) requires but a 1171 pound spring preload.
That still sounds high to me
but compare that spring to the one required for a direct acting valve like the Crosby

4∏ X 1100 psi = 13,283 pounds and that's a really stout spring. Pilot design allows 10X reduction in spring .

Oops i neglected weight of piston in above. Surely it's less than spring preload in case valve is installed upside-down.

Sorry for the digression - it do this for my own sanity checks. If it has entertained you it was worthwhile.
Corrections or suggestions welcome. I feel like there's an arithmetic mistake in it
but got to go now.
Would you guess that spring might be about size of an automobile suspension coil spring?

later
old jim

 

[MadderDoc]

<..> I feel like there's an arithmetic mistake in it
but got to go now.
Would you guess that spring might be about size of an automobile suspension coil spring?

I thought they were smaller , and it is by far not the first time I have underestimated the size of things in a nuclear plant! That was enjoyable, also to see how you tackled the problem with that unifying approach. I could find nothing wrong with the arithmetic. :-)

 

[MadderDoc]

Unit 2 March 14th - March 15th

The behaviour of pressures of the RPV and PCV in Unit 2 after it was depressurised at about 18:00 on March 14th
is grossly inconsistent with the hypothesis that the RPV was henceforth held depressurised by way of the relief valve.
The hypothesis should be rejected.

This hypothesis would predict that the RPV pressure was held at a level at about 0.35 MPa above that of the wetwell,
and the RPV pressure was by far not held at such level.

After the initial depressurizing followed a short period with the expected behaviour, but then RPV pressure was seen rising and falling
in three large peaks that went way outside what an actuated SRV would allow.

Concurrent with the second and largest of these RPV pressure excursions,
the drywell pressure (as well as the drywell CAMS reading, not shown in the figure) rose dramatically.

After the third and last RPV pressure excursion, the RPV and the drywell pressure equilibrated at level more than 0.5 MPa above that of the wetwell. This state was upheld until the morning of March 15th, when, concurrent with the onset of steaming from the reactor building, the pressures of the RPV and the drywell decreased, while the wetwell pressure went downscale.

Seeing the hypothesis that there was pressure relief through a SRV is not viable, and the RPV pressure did nonetheless not increase to hit the roof (automatic safety valve opens at 7 Mpa), the RPV must have found or made itself other channels for pressure relief during this period. IOW, most plausibly, the RPV was damaged.

 

[MadderDoc]

Unit 3 March 13th - March 15th

In unit 3, during the period after it was depressurised in the morning of March 13th,
a prominent feature of the RPV and PCV pressures
is that their variations henceforth appear to have been very closely correlated.
However while drywell and wetwell also appear to have been close to pressure equilibrated,
the RPV pressure tracked their variations only from a lower pressure level.

The RPV's being at a lower pressure than the PCV is difficult to explain, except by assuming that the RPV pressure readings are in error, in the sense: affected by a systematic error producing too low readings. With the added assumption that the barrier between the atmospheres of the RPV and the PCV had degraded, it could be explained how all three compartments would then effectively have been pressure equilibrated.

A hypothesis that the SRV was held open over the period fails, for a surprising reason: there is no indication that the RPV after its initial depressurising ever again attained a pressure of 0.345 MPa above the pressure of the wetwell, and therefore the main valve of the SRV would have come shut shortly after the initial depressurizing, and would have remained closed, whether or not the actuation of the valve was maintained.

Relief from the system had then to be by other means. There are in fact known S/C vents at about 42 and 46-47 hours after the earthquake, concurrent to peaks in the graph. Further opening and closing of S/C vents have been assumed for the models such as to fit the variations in pressure that followed -- except for the abrupt relief at 68 hours, the occurrence of which is so far unexplained by the models.

 

[jim hardy]

Did those show up in jstolfi's (remarkable) plots ?

This one shows rpv tracking drywell with the offset you noted:
http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/out/pcor-PCA-PD-un3-full.png

i don't know what he used for time zero.


Maybe nuceng knows where that "A" reactor pressure sensor's tap is located.
I have been suspect of its readings since 3/21/11 when it went impossibly high then cured itself.
That shows on another of jstolfi's wonderful plots:
http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/out/plot-pres-un3-t-I-full.png
it's too wide for page or i'd post it

and
http://www.nytimes.com/2011/03/21/world/asia/21japan.html?pagewanted=all

The Tokyo Electric Power Company, which runs the plant, appeared to have experienced a serious setback as officials said that pressure buildup at the ravaged No. 3 reactor would require the venting of more radioactive gases.

But at a news conference a few hours later, officials from the power company said that the pressure had stabilized and that they had decided they did not need to release the gases immediately, which would have heightened worries about wider contamination among the population. They said they were unsure what had caused the pressure to rise, highlighting the uncertainty engineers must still grapple with at Fukushima.

just one of those little nagging questions. This obsessiveness is part of my aspergers i think !

old jim

 

[MadderDoc]

Did those show up in jstolfi's (remarkable) plots ?

This one shows rpv tracking drywell with the offset you noted:
http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/out/pcor-PCA-PD-un3-full.png
i don't know what he used for time zero.

Jorge uses midnight between March 10 and March 11 as time zero, and includes a much longer time series than the one I have been focusing on. As regards the abrupt relief at 68 hours (Jorge 82 hours) that has been left unexplained by the model, it may be a hint that 68 hours is also the time of the explosion of Unit 3.

Maybe nuceng knows where that "A" reactor pressure sensor's tap is located.
I have been suspect of its readings since 3/21/11 when it went impossibly high then cured itself.
That shows on another of jstolfi's wonderful plots:
http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/out/plot-pres-un3-t-I-full.png
it's too wide for page or i'd post it

and
http://www.nytimes.com/2011/03/21/world/asia/21japan.html?pagewanted=all

I think Tepco would have been referring to the gradual build up of pressure that was seen at that time. That build up of pressure levelled out at not too critical levels, and Tepco canceled intervention, it seems, and all the while pressure started to drop back again. It was on the decreasing flank of that pressure variation that one of the RPV sensors returned very high readings (~10 MPa) for an hour or so, cause unknown. However, that was well after Tepco's consideration to vent, and also after the cancellation announcement, so probably unrelated to those matters. There could be reason to think something untoward happened (on top of all the untoward that had already happened), in connection with that apparent transient -- in data it appears as being the last throes of the reactor as a pressure containing system.

just one of those little nagging questions. This obsessiveness is part of my aspergers i think !

old jim

Lol. Just keep in mind, paraphrasing the proverb: "Those who restrain obsession, do so because theirs is weak enough to be restrained" :-)

 

[jim hardy]

The RPV's being at a lower pressure than the PCV is difficult to explain, except by assuming that the RPV pressure readings are in error, in the sense: affected by a systematic error producing too low readings.

in measuring low pressures the relative elevations of measurement points becomes significant. The offset you noticed , ~90kpa(?) equates to ~30 feet of water.

But it takes someone who knows the physical plumbing runs to know where the measuring taps are located. And whether there's a long vertical run of pipe in the line between sensor and mesauring point. Usually pressure sensors are calibrated to include head due to full sense lines and if that fill is lost (eg boils away when containment is hot and pressure is low) , the reported reading will be in error by amount of fluid lost.

Again these are just little details to be uncovered . Your observations are right on.
I did a lot of troubleshooting in my day . As the increasingly fine details emerge these little questions all resolve and a true picture emerges. But it sure morphs a lot along the way.

Nice work !

It was on the decreasing flank of that pressure variation that one of the RPV sensors returned very high readings (~10 MPa) for an hour or so, cause unknown.

~10mpa is ~1400 psi. As i say I'm just waiting for that detail to unravel . I believe it was a measurement error and I've not heard it explained.
But you and Elbows are much better versed than i am. If you run across a "why" for that one, please post.

old jim

PS - Thanks for indulging my OCD.

 

[MadderDoc]

in measuring low pressures the relative elevations of measurement points becomes significant. The offset you noticed , ~90kpa(?) equates to ~30 feet of water.

But it takes someone who knows the physical plumbing runs to know where the measuring taps are located. And whether there's a long vertical run of pipe in the line between sensor and mesauring point. Usually pressure sensors are calibrated to include head due to full sense lines and if that fill is lost (eg boils away when containment is hot and pressure is low) , the reported reading will be in error by amount of fluid lost.

The instrument would then read out too high values, if I understand the setup. Then of course, it could be the measured PCV pressure readings in unit 3 which are erroneously too high during this period, rather than the RPV's pressure readings too low.? At least I cannot now exclude that possibility, that may come later. As you say, it would not be unwelcome at all having some input from a BWR guy.

 

[jim hardy]

The instrument would then read out too high values, if I understand the setup. Then of course, it could be the measured PCV pressure readings in unit 3 which are erroneously too high during this period, rather than the RPV's pressure readings too low.?

well - if the pressure tap is above the transmitter
the transmitter would be calibrated to report less pressure than it sees.
That's because the condensed water in the vertical line adds to the pressure as you traverse down it. Kirchoff's pressure law ?


So if the pressure tap were near top of vessel and the sensor lower than that,
when sensing line dried out,
reported pressure would be low by the height of fluid lost.

but i don't know physical arrangement in a BWR. Mine i knew pretty well.

old jim

 

[MadderDoc]

What, if anything, could have been a _technical_ rationale of Tepco of letting firefighters risk their lives to splash water to the top of Unit 3 in the days following March 16th 2011?

 

[zapperzero]

What, if anything, could have been a _technical_ rationale of Tepco of letting firefighters risk their lives to splash water to the top of Unit 3 in the days following March 16th 2011?

The real and present danger of pool dry-out of course. We've been over this before, if from a different angle - the firefighting helicopters found multi-Sievert/h fields above unit 3, suggesting either massive gas/particulate releases (unlikely, for a variety of reasons) or shine from a mostly empty pool.

Of course, I don't see TEPCO ready to admit that yes, they almost lost Tokyo.

 

[zapperzero]

Then of course, it could be the measured PCV pressure readings in unit 3 which are erroneously too high during this period, rather than the RPV's pressure readings too low.

Common sense would suggest that whichever is closest to atmospheric is true, fwiw.

 

[MadderDoc]

The real and present danger of pool dry-out of course. We've been over this before, if from a different angle - the firefighting helicopters found multi-Sievert/h fields above unit 3, suggesting either massive gas/particulate releases (unlikely, for a variety of reasons) or shine from a mostly empty pool.

Those high radiation fields were measured, I believe, on March 16th, on which day there was massive steam release from the building -- Tepco evacuated their own people temporarily out of fear that the PCV was leaking, and videos were taken from helicopters showing steam sources to be consistent with that fear. Otoh, there does not seem to have been any cause on that day to think the pool was even boiling.

Therefore I do not find it readily credible that Tepco the next day should have thought the danger of dry-out of the pool of unit 3 was real and present to a degree of imminence that the brave people of the civil forces should think they had cause to be willing to risk their lives in desperate measures to splash just some water into it.

 

[MadderDoc]

Common sense would suggest that whichever is closest to atmospheric is true, fwiw.

Please explain.

 

[zapperzero]

Those high radiation fields were measured, I believe, on March 16th, on which day there was massive steam release from the building -- Tepco evacuated their own people temporarily out of fear that the PCV was leaking, and videos were taken from helicopters showing steam sources to be consistent with that fear. Otoh, there does not seem to have been any cause on that day to think the pool was even boiling.

Therefore I do not find it readily credible that Tepco the next day should have thought the danger of dry-out of the pool of unit 3 was real and present to a degree of imminence that the brave people of the civil forces should think they had cause to be willing to risk their lives in desperate measures to splash just some water into it.

They weren't expecting that water to make it into the PCV, no? So what else is left? A fire (for which there is no evidence whatsoever) or a pool that is _believed_ to be emptying fast.

 

[MadderDoc]

They weren't expecting that water to make it into the PCV, no? So what else is left? A fire (for which there is no evidence whatsoever) or a pool that is _believed_ to be emptying fast.

But you can't just assume that they did have a technical rationale relating to conditions of the plant. It is well known that people may act out of a variety of motivations. For what we know there was as little evidence that the pool was emptying fast, as there was of a fire.

 

[zapperzero]

Please explain.

I'd expect that one vent path or another eventually opened and remained open, so I'd intuitively go for the lower number, when asked to estimate which of the three readings of our steaming teapot is correct.

Moreover, the three sensor readings track each other pretty nicely - except at the initial two peaks, where the RPV pressure is shown as highest (which makes sense, as it could probably still hold pressure at those times). What seems more likely? That the RPV pressure is over-, then under-reported by the same instrument, or that the other two are consistently over-reported?

 

[zapperzero]

But you can't just assume that they did have a technical rationale relating to conditions of the plant.

Why not? Everything else, including the aborted decision to abandon the site, was based on a technical rationale.

It is well known that people may act out of a variety of motivations. [/ quote]

I don't really buy the "they did this to be seen doing something" idea anymore. Something WAS being done, quite arduously and visibly in fact. Trucks and pumps were moving in, power lines were being repaired etc etc.

For what we know there was as little evidence that the pool was emptying fast, as there was of a fire.

No. There was a simple calculation as to how long it takes for the pool to boil after the pump stops. There was also an elevated and increasing, but rather uniform radiation field around Unit 3 (on the ground), which at least to some must have said"gamma shine".

 

[MadderDoc]

Why not? Everything else, including the aborted decision to abandon the site, was based on a technical rationale.

:-) That is some statement but even if assumed it doesn't follow that this one would have to be.

I don't really buy the "they did this to be seen doing something" idea anymore. Something WAS being done, quite arduously and visibly in fact. Trucks and pumps were moving in, power lines were being repaired etc etc.

Now I didn't suggest Tepco did anything to be seen doing something. Perhaps they did, as I said people are known to act from a diversity of motivations, including this one.

There was a simple calculation as to how long it takes for the pool to boil after the pump stops.

Yes, and no doubt Tepco did the calculation, only the result must've shown that the pool ought to be days from boiling and many days from fuel exposure. You also never heard a Tepco official say the Unit 3 pool was drying out fast. Nor was it expressed at the next level of the foodchain, NISA, who rather thought at the time that the Unit 4 pool was the one in need of urgent attention.

If Tepco really thought the situation of the unit 3 pool was so desperate, then why did they clear the ground at unit 4 for a concrete pump asap, while it took them days to not even make a passageway at unit 3 such that the rescue teams could approach the pool?

There was also an elevated and increasing, but rather uniform radiation field around Unit 3 (on the ground), which at least to some must have said"gamma shine".

To be sure the dose rate was high around unit 3 after the explosion. That just puts some seriousness to the question, what was Tepco's technical rationale for letting people go in there to splash water onto the building.

 

[MadderDoc]

I'd expect that one vent path or another eventually opened and remained open, so I'd intuitively go for the lower number, when asked to estimate which of the three readings of our steaming teapot is correct.

Moreover, the three sensor readings track each other pretty nicely - except at the initial two peaks, where the RPV pressure is shown as highest (which makes sense, as it could probably still hold pressure at those times). What seems more likely? That the RPV pressure is over-, then under-reported by the same instrument, or that the other two are consistently over-reported?

Thanks for expounding your thoughts. There were actually two RPV sensors in good agreement with each other, making the two parties even :-), perhaps we could assume the average.

 

[jim hardy]

Yes, and no doubt Tepco did the calculation, only the result must've shown that the pool ought to be days from boiling and many days from fuel exposure.

i think there's somewhere a tabulation of how much water was added to respective pools.

I had assumed unit 3 pool leaked , because of degree of damage to building from explosion and later videos showing what looked like rebar bent into pool.

My memory is notorious for dyslexia-like cross-connects so i won't venture a guess at how much water was added to unit 3 pool compared to others.
Were i organized i could find those numbers.

Low pool level of course means too little water to provide full shielding above fuel in pool.


This post not much of a contribution i know .
But i was among those thinking at the time #3 pool level was low and gamma backscatter was reason for high fields on ground. Lots of makeup water compared to others would support that.

 

[MadderDoc]

i think there's somewhere a tabulation of how much water was added to respective pools.

I have a note of a tabulation from Tepco, saying that as of the morning of April 1, 90 tons had been applied to SFP1, 4802 tons to SFP3, and 1278 tons to SFP4. There was no figure for SFP2 in that source according to my note. The figure for Unit 3 would have included helicopter dumps and much building splashing.

In the equidistant periods 2-22 April, and 1-21 April for SFP3 and SFP4, respectively, the concrete pump was used for injection at both units, and there was spent 963 minutes pumping to SFP3, and 2301 minutes to SFP4, which can be at least an indicator for the relative demand between those two units. (Edit: I see I've made another note, estimating the pumping rate of the concrete pump to be about 50t/h)
I am quite sure I very recently saw a more recent accounting for the injections to the pools with graphs and all. I'll see if I can dig it up.
Edit: I found that document i was thinking of , but it includes only accounting for unit 4, sorry. It indicates that the volumes referred to above were not sufficient to maintain the level of that pool.
It has also this tabulation of the decay heat n MW, of the pools as of March 11th 2011, and October 17th.
Unit 1 SFP 0.18 0.14
Unit 2 SFP 0.62 0.43
Unit 3 SFP 0.54 0.39
Unit 4 SFP 2.26 1.15
Common pool 1.13 1.12

I had assumed unit 3 pool leaked , because of degree of damage to building from explosion and later videos showing what looked like rebar bent into pool.

Perhaps it did/does, but my point is that Tepco at the time of all that display of bravery at unit 3 do not seem to have had any observation to indicate that the pool was leaking, boiling, nor drying out. Tepco also never said anything to that effect, nor did NISA. The rest of the world were excused to think that such desperate measures as shown in the telly would imply a critical situation, but Tepco never said there was a critical situation with the spent fuel pool of unit 3. All they said was the rather obvious, that if the spent fuel pools were to be not served with water, they might eventually dry out and bad things could happen.

I was among those thinking at the time #3 pool level was low and gamma backscatter was reason for high fields on ground. Lots of makeup water compared to others would support that.

It is a natural thought when someone is seen desperately spraying water to a pool, that it must be because he thinks the level in the pool is critically low. And I think that also comes close to what the fire-fighters thought they were doing, while they were bombarding the steam plume they could see over the building with water from their spray cannons.

But my question is, what did Tepco think these men were doing. Tepco, unlike the fire-fighters, cannot be assumed to have not realized from the helicopter observations and videos on March the 16th, that there was water in the pool and it was not boiling.

 

[gnasch]

I remember reading shortly after the helicopter water drops that japanese officials put this into the context of "calming the americans", presumably upon the impression of Mr. Jaczko going ballistic over the pools and recommending an 80 km evacuation radius. No source remembered though.

 

[zapperzero]

I have a note of a tabulation from Tepco, saying that as of the morning of April 1, 90 tons had been applied to SFP1, 4802 tons to SFP3, and 1278 tons to SFP4. There was no figure for SFP2 in that source according to my note. The figure for Unit 3 would have included helicopter dumps and much building splashing.

This would support the idea that they thought SFP3 was drying fast, no?

Tepco, unlike the fire-fighters, cannot be assumed to have not realized from the helicopter observations and videos on March the 16th, that there was water in the pool and it was not boiling.

The helicopters "saw" a 7.5 Sv/h field at 90 meters above the unit and a smoking or steaming pool filled with rubble. These are not encouraging observations.

 

[SteveElbows]

The interim report is suitable vague when it comes to the initial observations of reactor 3 pool. It talks of how observations showed that unit 4 pool looked ok, and it says they were supposed to observe reactor 3 pool as well, but makes no mention of what those observations showed.

Page 272 onwards:

http://icanps.go.jp/eng/120224Honbun04Eng.pdf

There certainly is at least one document that goes into full detail about the time & volume of spraying to the pools, but I cannot lay my hands on it right now.

Personally if I were them I would have started these operations as soon as possible, so as to leave more wiggle room and reduce the chances that other events could get in the way of water spraying by the time it was absolutely necessary.

I know that many months ago some attention was paid to why video of one of the firefighter missions showed someone spraying the wrong side of the building. I don't have a wonderful explanation for this, although I wouldn't read too much into it, especially as the ground between units 3 & 4 was not very hospitable at the time.

 

[MadderDoc]

I remember reading shortly after the helicopter water drops that japanese officials put this into the context of "calming the americans", presumably upon the impression of Mr. Jaczko going ballistic over the pools and recommending an 80 km evacuation radius. No source remembered though.

Mr Jaczko's technical rationale was that the SFP4 had run dry by March 16th, and this he based on the information he had coming in through a chain of liaison in which he thought at the other end was a direct link to Tepco. Mr Jaczko would seem to have had an impeccable technical rationale - however based on flawed intelligence it was. Tepco on March 16th appears to have evaluated -- based on observations from helicopter -- that the water level of the SFP4 was but 2-3 meter below normal, leaving ample water above the top of the fuel.

 

[zapperzero]

Personally if I were them I would have started these operations as soon as possible, so as to leave more wiggle room and reduce the chances that other events could get in the way of water spraying by the time it was absolutely necessary.

That's common sense, business-as-usual thinking. In crisis mode, with limited resources and very limited information on which to base predictions, the emphasis shifts to rapid, decisive reaction to emerging events. Firefighter logic.

 

[MadderDoc]

This would support the idea that they thought SFP3 was drying fast, no?

Yes, one could say that, but be wary of supportive evidence for your ideas, cherish rather evidence that is suitable to demolish their opposites.

The helicopters "saw" a 7.5 Sv/h field at 90 meters above the unit and a smoking or steaming pool filled with rubble. These are not encouraging observations.

According to the WHO, the measured dose rate from the helicopters involved in the water dump operations was 4.13 milliSv/h at 1000ft (300m), and 87.7 milliSv/h at 300ft (90m). As regards what could be seen from helicopters, we can get some indication from the published video sequences that were shot on March 16th. They appear to show a water surface in the pool, and steam plumes originating from areas close to the pool gates on either side of the reactor well. That was probably also what a human observer would have seen from the helicopter, only more clearly.

 

[tsutsuji]

There certainly is at least one document that goes into full detail about the time & volume of spraying to the pools, but I cannot lay my hands on it right now.

https://www.physicsforums.com/showpost.php?p=3813126&postcount=12587