Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[razzz]

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 35504

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

I'd ask for your money back on that manual whereas doesn't specially describe the vault or its procedures even though it's called out in the drawing with dimension. Mentioned in section 12 of the .pdf as a part of (FT) Fuel Transfer System (starting @ pg. 132 and noted on the last pg. 153). BTW, using Foxit Reader for .pdf.

 

[Jorge Stolfi]

... Stolfis Plots were showing constant water levels and rising pressure for Unit 1. ...

I have fallen behing with my plots due to other work. But now I wonder whether it is worth keeping them updated.

For one thing, TEPCO has been releasing a lot more readings and more often, including plots; and other people have been doing it too.

More seriously, the latest news about #1 mean that *both* its water level readings, for the past two months, were garbage --- not inaccurate, not biased, not noisy, but just total garbage. Since the readings for #2 and #3 have similar values and behavior, it is very likely that they are garbage too. (Indeed the black smoke event for #3 sems to coincide with some major event in the RPV emperatures and other variables --- a core breach perhaps?)

Now what trust can we put in the other measurements? I have this mental image of a room somewhere in the reactor building, half-full with water, with a big chunk of fallen concrete in the middle and all the pressure and water level gauge pipes disappearing under it. Or of the "RPV bottom" termocouple inside the drywell, hanging in mid-air by its wires, gently swaying under a drizzle of warm borated water from a ruptured pipe pipe above.

Anyway, I will think about it over the next weekend, when I have more time. Sorry... and all the best.

 

[Borek]

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

Just to clarify - water is not superheated just because its temperature is above 100 deg C. It is superheated when it is heated above its boiling point - but deep in the pool pressure is higher than on the surface, so boiling point is higher as well - which means water is not superheated. It becomes superheated when pressure falls.

I superhate the paragraph above for the abuse of the word superheated.

 

[Jorge Stolfi]

If I udenrstand the rawings, the bottom of the drywell is covered by a thick layer of concrete. So the corium would hit concrete right after leaving the RPV, before it gets to the drywell wall.

 

[NUCENG]

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.

US NRC representatives, yes, I saw a leaked report that was based on initial feedback reports to NRC by the first team they sent to Japan. I only questioned whether the US military was involved in that decision or recommendation. Millitary NBC warfare specialists are trained to protect the troops in the field, not the general public. Things like evacuation areas and shelter-in-place recommendations and such considerations are something NRC works with in detail. I believe the military response would have been to provide monitoring and support and the civilians would have been making the recommendations and decisions. Since Japan didn't implement the 50 mile recommendation I conclude the final decision was made by Japan.

You are right about containment pressure testing. In the US Pressureized leakage testing is performed up to a test pressure that is specified in Technical Specifications - the license for the plant. This is a design basis test that will provide protection in a design basis accident. Please remember, that the failure to either protect the site from tsunami or to provide flood protection for the emergency diesel generators and switchgear took this event beyond the design basis.

Emergency Operating Procedures at US nuclear plants would initiate containment venting before exceeding the pressure and temperature limits of the containment. Apparently Japanese procedures allowed the pressure to get to double the design pressure, and decision delays allowed it to get even higher. By the time the pressure topped out leakage from the drywell cap was assured.In Unit 1 the containment still appears to be holding partial pressure. Units 2 and 3 appear to have no pressure difference to atmosphere and may have complete containment failure. In Unit two a failure in the torus probably occurred leading to an explosion below ground level. Containment failure in unit 3 led to a masssive explosion like the entire building was filled with hydrogen.

If drywell penetrations for cables exceeded 500 deg they would have failed. Other potential sites for damage include the transition from the bulb to the cylinder of the drywell and at the vent line bellows between the drywell and torus, But none of these failures would have happened if the event hadn't gone past the design basis.

There was a post earlier that said that the faillure to design for beyond design basis events was a problem. Apart from the logical impossibility, I actually think that makes a twisted kind of sense.

If the TEPCO design protected the site with a seawall big enough to handle the tsunami or if they had protected the safety systems from flooding it would have been within the design basis. If they couldn't do that they could have designed for redundant equipment to replace the functions that would be lost. But by doing any of these things they would convert a beyond DBA into a DBE.

 

[TCups]

Just to clarify - water is not superheated just because its temperature is above 100 deg C. It is superheated when it is heated above its boiling point - but deep in the pool pressure is higher than on the surface, so boiling point is higher as well - which means water is not superheated. It becomes superheated when pressure falls.

I superhate the paragraph above for the abuse of the word superheated.

. . . as when the deeper water, under higher pressure is violently agitated, and moved to the surface of the pool, correct?

 

[razzz]

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

I think you described Unit 1's explosion perfectly, at least the water flashing to steam part. See beginning at 32 seconds http://www.youtube.com/watch?v=OFt1OHU1u9s"

 

[AntonL]

I have fallen behing with my plots due to other work. But now I wonder whether it is worth keeping them updated.
...
Anyway, I will think about it over the next weekend, when I have more time. Sorry... and all the best.

Jorge - your plots have served there purpose, I admired your diligent work transferring the data, I presume manually, and thank you for it.

Now that the data is available in tabular forms in pdf files, like http://www.tepco.co.jp/nu/fukushima-np/f1/images/011_1F1_05130600.pdf it is easy to transfer this to MS-Excel using http://www.anypdftools.com/pdf-converter.html even though the Tepco files are locked for copying. The trial version allows 5 pages of conversion which is more than enough.

Once in Excel format it is easy enough to generate a plot of the area of interest and using all the new parameters for what ever they are worth.

 

[Concrn&Curius]

Any reaction welcome:

Unit 4 SFP. Is this a picture of melted fuel assemblies?

1. Take a look at the attached image of the Caltech presentation mentioned in a post a few days ago. In the graph, you can see that the two most intense radiological releases followed events at the unit 4 SFP. (I believe as measured onsite, but the presentation is not abundantly clear)

2. Take a look at the attached image from the unit 4 SFP.

3. If you were TEPCO, which assemblies would you check first? The ones that were most recently taken out = hottest = most likely to melt?

4. Would you know where these were located in the pool (yes) and would likely check them first?

5. Recall the Gundersen demonstration of the zircaloy heating, and subsequent change in the characteristics of the metal

6. I’m not sure if the “handles” of the fuel assembly (the part lifted by crane) are also zircaloy?

7. You can clearly tell in the picture that some of the assembly handles are considerably less reflective and dull. It would make sense if this was due to light obstruction, but the light seems to be uniformly cast throughout the rest of the image, and equally reflective in other parts of the pool.

*Note – I have been following since the beginning. Apart from the news aspect, read this forum has been an excellent lesson in critical reasoning. I am only trying to understand what has happened because I find it fascinating this event has pushed us collectively to the edge of our knowledge. I only post on this because I have thought about it critically, and cannot arrive upon a good answer.

 

[AntonL]

Just to clarify - water is not superheated just because its temperature is above 100 deg C. It is superheated when it is heated above its boiling point - but deep in the pool pressure is higher than on the surface, so boiling point is higher as well - which means water is not superheated. It becomes superheated when pressure falls.

(inserted by AntonL, for the not eagle eyed removed size and colour)
I superhate the paragraph above for the abuse of the word superheated.

I now split the paragraph into two as the first sentence is introduction to everything that follows

 

[PietKuip]

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)

There was also fresh fuel in the same #3 pool. Which to me seems a risky thing to do. A nuclear explosion cannot be excluded.

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?

This is all we know:
"When the No. 3 reactor housing exploded on the morning of March 14, levels inside the bunker jumped as much as 12-fold, [Kazuma Yokota] said, checking dates and times in a pocket diary. "
http://www.bloomberg.com/news/2011-04-25/japan-s-terrifying-day-saw-unprecedented-exposed-fuel-rods.html

 

[Jorge Stolfi]

The basic premise of a spent fuel pit is you build it so criticality is impossible. You do that by two different means:

I understand that that the original arrangement was safe because of its low density, but after re-racking with close-packed assemblies they had to rely on absorbers. It seems they use them even in the casks, out of concern for possible criticality --- even though the casks have no moderating water and hold much less fuel.

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.

Others have mentioned boral (same boron carbide powder as boraflex but in an aluminum matrix instead of silicone plastic). IIRC TEPCO uses boral in the casks. On the other hand it is the first time I hear of borated steel in the SFP. I know it is used in reactor control rods; but also in SFPs?

Zircalloy starts reacting significantly with water at 800C, right?Aluminum melts at <700C and silicone should decompose into gas + silica ash well before that. So if #4's explosion was due to H2, it must have come from the SFP, and therefore some of the neuron absorbers must have collapsed. (Would that be visible in the movie? The heads of the assemblies are a feet or two away from the fuel, so they would remain relatively cool, no?)

The last obstacle is the lack of moderator. A couple of days ago I proposed that, once the fuel becomes partly dry, the water level inside the racks could begin to oscillate due to delayed feedback between rising level and increased steam production. That could be a mechanism for first getting rid of the boral and then returning water to the scene, independent of external spraying.

Note also that keff = 0.95 does not allow an exponenially increasing fission chain reaction, but still it means that the natural level of spontaneous fission is amplified by 1/(1 - 0.95) = 20 times. How significant is that?

 

[swl]

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スキマーサージタンクレベル (mm)

FCP Skimmer Surge Tank Level (mm)

 

[TCups]

REGARDING THE EXPLOSIONS OF BOTH UNITS 1 AND 3 - DID STEAM FLASH OCCUR?

I think you described Unit 1's explosion perfectly, at least the water flashing to steam part. See beginning at 32 seconds http://www.youtube.com/watch?v=OFt1OHU1u9s"

Hey, I think razzz is right. There was a steam flash at Unit 1, too, just a lot smaller volume of water flashed to steam. Excellent observation, IMO, razzz. Thank you.

Q: Why the difference between the relatively small steam flash at Unit 1 and the massive steam flash at Unit 3?

A: The difference may have been:
1) the explosion in the drywell at Unit 1 and the amount of rapid turbulence venting from the fuel transfer chute or concrete plug "burp" or both caused in SFP3 vs only an overhead hydrogen explosion in Unit 1
2) the total amount of thermal energy stored in SFP3 > SPF1 before the explosion.

Obviously these aren't mutually exclusive, and it seems to me, both were probably the case.

@anton:
Brilliant analysis on the thermodynamics of the steam flash at Unit 3, Anton! I am glad there is someone here at PF with the brains to help back up at least some of my wild speculation and off-the-wall theories. Thank you, sir.

 

[razzz]

REGARDING THE EXPLOSIONS OF BOTH UNITS 1 AND 3 - DID STEAM FLASH OCCUR?



Hey, I think razzz is right. There was a steam flash at Unit 1, too, just a lot smaller volume of water flashed to steam. Excellent observation, IMO, razzz. Thank you.

Q: Why the difference between the relatively small steam flash at Unit 1 and the massive steam flash at Unit 3?

A: The difference may have been:
1) the explosion in the drywell at Unit 1 and the amount of rapid turbulence venting from the fuel transfer chute or concrete plug "burp" or both caused in SFP3 vs only an overhead hydrogen explosion in Unit 1
2) the total amount of thermal energy stored in SFP3 > SPF1 before the explosion.

Obviously these aren't mutually exclusive, and it seems to me, both were probably the case.

@Anton:
Brilliant analysis on the thermodynamics of the steam flash at Unit 3, Anton! I am glad there is someone here at PF with the brains to help back up at least some of my wild speculation and off-the-wall theories. Thank you, sir.

I'm just guessing but the wild card in Unit 3 is the MOX fuel.

 

[MadderDoc]

Explosion Unit 3 – Putting it together.
<..>somewhere near the surface the what is left over from the >100^oC hot water would convert to steams and boiling would only be apparent near the surface of the water. <..>

Stop there. Just because a volume of water is above 100^oC, doesn't mean that this water can all convert to steam. The liquid to vapour transition for water is strongly endothermic (energy demand 2257 MJ/ton).

If you have 1 ton of water at, say, 110^oC at atmospheric pressure, this unstable system has energy only to convert itself to a mixture of 980 kg liquid water and 20 kg water vapour at 100^oC. If you extend this to a sfp of 1400 m³, about 30 tons of water vapour could be produced.

Problem is, as I see it, that this amount of vapour is insufficient to explain the size of the mushrooming cloud and the apparent size of its lifting capability.

 

[TCups]

REGARDING THE HIGH SPIKE OF MEASURED RADIATION ASSOCIATED WITH UNIT 3'S EXPLOSION

This is all we know:
"When the No. 3 reactor housing exploded on the morning of March 14, levels inside the bunker jumped as much as 12-fold, [Kazuma Yokota] said, checking dates and times in a pocket diary. "
http://www.bloomberg.com/news/2011-04-25/japan-s-terrifying-day-saw-unprecedented-exposed-fuel-rods.html

Could there be another plausible explanation? For example:

Could the spike in measured radiation levels be explained by the explosive release of volatile iodine and cesium (or other highly radioactive isotopes) from the explosion and venting of the contents of the drywell of Unit 3, lofted by the associated steam flash? I believe there were smaller spikes in measured radiation during controlled venting. It would seem to make sense that explosive venting would be a much larger spike.

Could some of the contents of SFP3 also have been damaged by an explosion and steam flash and also lofted and scattered, perhaps also contributing to the spike?

 

[artax]

Having previously not understood the mechanism for instantaneous conversion of water to steam and the possible resulting high energy explosion, I now do see that it's possible. The PRINCIPLE section of this page:- http://en.wikipedia.org/wiki/Boiler_explosion. explains.
However I think that for the explosion of unit three to be due to a steam explosion, it would mean there had been a catastrophic failure of the pressure vessel to produce so much steam instantly.
I think this has been ruled out due to the radioactivity and isotopes found.

Do we have an approximate figure for the power of that explosion...based on the dimensions of the rising cloud...or the fact that it was heard 30 miles away?

 

[TCups]

Stop there. Just because a volume of water is above 100^oC, doesn't mean that this water can all convert to steam. The liquid to vapour transition for water is strongly endothermic (energy demand 2257 MJ/ton).

If you have 1 ton of water at, say, 110^oC at atmospheric pressure, this unstable system has energy only to convert itself to a mixture of 980 kg liquid water and 20 kg water vapour at 100^oC. If you extend this to a sfp of 1400 m³, about 30 tons of water vapour could be produced.

Problem is, as I see it, that this amount of vapour is insufficient to explain the size of the mushrooming cloud and the apparent size of its lifting capability.

Yes, clearly a large amount of energy is required for phase change from water at 100ºC to steam at 100ºC (the old chemist seems to remember the heat of transition is 880Kcal/mole), but water will boil to water vapor at room temp in a vacuum (ie, from a rapid drop in pressure). Agitating heated water at the bottom of the pool, bringing it to the top of the pool, would have occurred (ie, a rapid decrease in the pressure). Consider also not vaporization but atomization of water droplets resulting from a drywell explosion, then combined with mixing the atomized water droplets with a secondary hydrogen explosion in the upper building.

Regrets that I cannot provide the calculations or perhaps even a very detailed description, but it seems that there may have been several factors contributing to the rapid production of steam. Also, I don't know that flashing of tons of water to steam were involved, though again, I fall short of PF standards in not being able to provide the calculations needed to support a reasonable estimate of the volume of water which may have flashed to steam in the explosion at Unit 3.

 

[Rive]

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.

I'm still skeptic about the SFP. The first (hydrogen) explosion most likely 'cleared' the roof and most of the walls (like on U4). But the vertical column still moved a lot of (new) debris/dust. Where is the source of that dust/debris? The SFP has no such severe structural damages, it cannot supply so much material.

So we are searching for a barrel and a hole too. The NW corner has both of them. IMO that part is more logical source than the SFP.

Ps.: I cannot discard the possibility of criticality accident in SFP#3, but the energy release of such accident has a hard upper limit - there are no structural damages in the SFP itself.

 

[MadderDoc]

I think you described Unit 1's explosion perfectly, at least the water flashing to steam part. See beginning at 32 seconds http://www.youtube.com/watch?v=OFt1OHU1u9s"

I think the steam cloud whose condensed top we can see rising swiftly to dissipate over unit 1 is just from the hot humid air produced by the hydrogen explosion.

This air mass is rising because it is hot and humid, i.e has a lower density than atmospheric air. It is swiftly dissipating because the amount of water from the hydrogen explosion was not huge, and it is white/colorless, because contained in it was after all only air and steam.

Unit 3 that is another story.

 

[SteveElbows]

Any reaction welcome:

2. Take a look at the attached image from the unit 4 SFP.

I explained what is labelled on that picture as being damaged grid & damaged vertical row in the past. This is not actually damage to the fuel assembly storage racks, it is actually handles/lugs that are part of the racks themselves (as opposed the the fuel handles), handles that protrude above the rest of the rack. And so when viewed from that angle at that low resolution, it can appear like a distortion to the grid.

If you watch the 2nd, longer unit 4 fuel pool video then you can see these more clearly, along with other features.

edit - I attach an image from the 2nd video that shows these handles clearly.

 

[AntonL]

Stop there. Just because a volume of water is above 100^oC, doesn't mean that this water can all convert to steam. The liquid to vapour transition for water is strongly endothermic (energy demand 2257 MJ/ton).

If you have 1 ton of water at, say, 110^oC at atmospheric pressure, this unstable system has energy only to convert itself to a mixture of 980 kg liquid water and 20 kg water vapour at 100^oC. If you extend this to a sfp of 1400 m³, about 30 tons of water vapour could be produced.

Problem is, as I see it, that this amount of vapour is insufficient to explain the size of the mushrooming cloud and the apparent size of its lifting capability.

Do not think the the upward moving column of dust as a shotgun propulsion but rather as an atmospheric thermal of sucking up the dust. when the steam condenses back to water droplets the stored heat is released into the air causing more heating and updraughts. (same process as in cumulus clouds that have up drafts of high enouh velocity for tennis sized hail stones)

 

[jlduh]

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) .

Nisa is saying no need to inject as much water in N°1 if fuel has fallen to the very bottom of RPV...

http://www3.nhk.or.jp/daily/english/13_26.html

NISA: no need to flood No.1 reactor
An official of Japan's nuclear safety agency has suggested that a nuclear fuel meltdown at one of the damaged Fukushima reactors means that filling the reactor's container with water may be meaningless.

Hidehiko Nishiyama told reporters on Friday that melted rods at the bottom of the No. 1 reactor are being cooled by a small amount of water.

Whata are your opinions about that? Can some calculations be made (i imagine that they consider that most of the heat could be removed by steam production if there is no real flow except the leaks (which might be important though...).

But... we see no steam coming out, so it's only leak flow that would cool the melted core?

I'm getting lost.

 

[MadderDoc]

Yes, clearly a large amount of energy is required for phase change from water at 100ºC to steam at 100ºC (the old chemist seems to remember the heat of transition is 880Kcal/mole), but water will boil to water vapor at room temp in a vacuum (ie, from a rapid drop in pressure). Agitating heated water at the bottom of the pool, bringing it to the top of the pool, would have occurred (ie, a rapid decrease in the pressure).

However true that is there is still no free lunch as regards the heat of transition.
It must be served, or you will not have any vapour no matter how much you tune down the pressure.

Consider also not vaporization but atomization of water droplets resulting from a drywell explosion, then combined with mixing the atomized water droplets with a secondary hydrogen explosion in the upper building.

I am not sure what you are saying there. The problem I see is one of size of cloud and one of lifting capability. Water in tiny droplets do not take up much space, and do not contribute to lift.

Regrets that I cannot provide the calculations or perhaps even a very detailed description, but it seems that there may have been several factors contributing to the rapid production of steam. Also, I don't know that flashing of tons of water to steam were involved<..>

OK, so you don't know that flashing of tons of water to steam was involved. However you do know, I believe, that 'something' propelled a diversity of heavy objects 100s of meters into the air. If a huge amount of water vapour rising didn't do it, what did?

 

[SteveElbows]

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.

I feel that you are ruling out weather phenomenon too definitively here. Failure to take into account a range of weather phenomenon has caused repeated wild speculation over the last week, based on live feed images. There is now a danger that if something really does change, we may miss it because we are so used to these false stories caused by live feed images.

As you can see, people are mostly not too surprised to hear of stuff coming from the north end of the building. Perhaps it would be helpful for you to review a video that was shot early on, and shows an area where this stuff is likely to be coming from:

http://www.youtube.com/user/modchannel

Check out the above at around 3:08, 3:23 and 3:48 for a view of what was escaping from the north side in the past.

 

[AntonL]

Nisa is saying no need to inject as much water in N°1 if fuel has fallen to the very bottom of RPV...

http://www3.nhk.or.jp/daily/english/13_26.html
Whata are your opinions about that? Can some calculations be made (i imagine that they consider that most of the heat could be removed by steam production if there is no real flow except the leaks (which might be important though...).

But... we see no steam coming out, so it's only leak flow that would cool the melted core?

I'm getting lost.

"Feed and Bleed" or "Chernobyl by Drip Feed" whatever you want to call it continues by the NISA comment, closed loop cooling is the only way forward.

@NISA and @Tepco if you run out of creative thinking please enlist as many specialists from around the world, to brain storm, analyse and come up with viable solution. At the moment your direction remind me of silk paper blowing in the wind.

 

[MadderDoc]

Do not think the the upward moving column of dust as a shotgun propulsion but rather as an atmospheric thermal of sucking up the dust. when the steam condenses back to water droplets the stored heat is released into the air causing more heating and updraughts. (same process as in cumulus clouds that have up drafts of high enouh velocity for tennis sized hail stones)

Right, exactly. A cumulus cloud needs no barrel, it has direction 'up' due to gravity and it has considerable lifting properties. So why invoke a need for a barrel in the case of the unit 3 cloud?

 

[clancy688]

Q: Why the difference between the relatively small steam flash at Unit 1 and the massive steam flash at Unit 3?

A: The difference may have been:
1) the explosion in the drywell at Unit 1 and the amount of rapid turbulence venting from the fuel transfer chute or concrete plug "burp" or both caused in SFP3 vs only an overhead hydrogen explosion in Unit 1
2) the total amount of thermal energy stored in SFP3 > SPF1 before the explosion.

I don't know if that has probably something to do with the different explosions in Unit 1, 2 and 3, but I just recently discovered that Unit 1 had a totally different emergency cooling system compared to Unit 2 and 3.
And because of that, Unit 1 probably had far less water and steam at time of its explosion than Unit 2 and 3 at time of theirs.

Both images are taken from this Caltech presentation: http://www.galcit.caltech.edu/~jeshep/fukushima/ShepherdFukushima30April2011.pdf


According to these images, it's normal under emergency cooling conditions that the Drywell in Units 2 and 3 is filled with hot steam.

 

[smartie18]

Who knows what really happened there actually. I heard so many conspiracy theries about the Japan thing that I am almost starting to believe some of them. It's weird that the Japanese would let something like that happen.