Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Jorge Stolfi]

The Alvarez et al document about re-racking claims that

"The standard spacing for new dense-pack racks today is 23 cm—barely above the 21.4 cm spacing in reactor cores. This “dense-packed” fuel is kept sub-critical by enclosing each fuel assembly in a metal box whose walls contain neutron-absorbing boron [...] the cross-section of the portion of a densepack box that is not obstructed by fuel rods would be about 0.032 m2

Assuming that this "standard dense packing" was used at Fukushima Daiichi, we get 1/0.23^2 = 18.9 assemblies per square meter. Then 1535 assemblies in a single layer would use 81.2 square meters. What are the dimensions of the #4 pool?

In the absence of any cooling, a freshly-discharged core generating decay heat at a rate of 100 kWt/tU would heat up adiabatically within an hour to about 600C, where the zircaloy cladding would be expected to rupture under the internal pressure from helium and fission product gases, and then to about 900C where the cladding would begin to burn in air. [...] Steam cooling could be effective as long as the water level covers more than about the bottom quarter of the spent fuel. [...] When the water [level falls to near] the bottom of the fuel assembly, it appears doubtful that [steam flow through the assemblies] could keep the peak temperature below 1200C for fuel less than a hundred years post discharge.

However, the claim that steam cooling would be effective with 75% of the fuel uncovered seems to assume that the whole assembly is at the same temperature, i.e. that heat generated in the top part is efficiently transported down the assembly to the water-covered part. Is my reading correct? Is that claim valid?

 

[Astronuc]

And, while spontaneous fission is an insignificant source of heat, even a momentary sub-critical chain reaction could cause a steam explosion, could it not?

I recall that famous accident where someone was trying to demonstrate a sub-critical chain reaction with two chunks of plutonium...

A subcritical (k < 1) system decreases in power to some low level which is that left by spontaneous fissions or other neutron sources. A subcritical system would not cause a steam explosion.

Only if a system went supercritical and achieve a certain power density very rapidly, would there be a possibility of a steam explosion, and likely the system would have to be prompt critical with a significant amount of positive reactivity (i.e., k >> 1.006), which is not the case at Fukushima.

The famous bare critical sphere demonstration, e.g., the one in which Louis Slotin died, was a supercritical assembly with nearly pure fissile material. Such material is not used in power reactors.

 

[Dotini]

Such material is not used in power reactors.

You don't suppose they were above fooling around with forbidden materials?

Respectfully submitted, seeking correction
Dotini

 

[StrangeBeauty]

Would someone like to comment on these images
http://www.houseoffoust.com/fukushima/fukushima.html

Seriously though, the images the above link do not match my (perhaps faulty) recollection of most recent mapping the service floor layout to the buildings done by our esteemed colleagues here. It places the reactor in a different place in the building (not next to the east wall, but in the mid-south part of the building), and the SFPs slightly west of the middle of the south wall in building #4. Can someone please check my memory as this seems like new/revised info to me. Thanks.

 

[jmelson]

from the link above...
Beta particles are just electrons from the nucleus, the term "beta particle" being an historical term used in the early description of radioactivity. The high energy electrons have greater range of penetration than alpha particles, but still much less than gamma rays. The radiation hazard from betas is greatest if they are ingested.

so is it possible to say that beta particle derived from plutonium and uranium are indistinguish from normal "electricity in the air"? and therefore unmeasurable?

No, they are certainly measurable, and the high energy distinguishes them from static electricity.
They have energies of hundreds of KeV to several MeV, and that allows them to penetrate at least some layers of the skin. When these electrons are stopped, the energy from their momentum is given off as gamma rays. So, in fact, if the Betas have something like a thin metal foil to stop them, then any sensitive (to low energies) gamma detector will detect their presence.

Jon

 

[razzz]

Would someone like to comment on these images
http://www.houseoffoust.com/fukushima/fukushima.html

Nice work georgiworld, that took some time and effort. Can't add much. Looks like entombment looks impossible due to debris having to be removed first. Everything contaminated and not approachable doesn't help.

'Pit' looks like it took a direct hit to the lid from falling object.

Always wondered how the working deck/platform was constructed. Need a drawing to see how it is assembled and what supports it. Is it anchored down or 'float' independently?

Many weak points when your vessel/capsule is glowing red and venting, ready for takeoff. Flange and pass-through(s) just for starters besides overheating leading to a stress crack qualify as weak points.

 

[razzz]

http://af.reuters.com/article/energyOilNews/idAFTFD00673420110413

http://www.welt.de/vermischtes/weltgeschehen/article12901818/Schickt-Tepco-Obdachlose-ins-AKW.html

 

[StrangeBeauty]

Would someone like to comment on these images
http://www.houseoffoust.com/fukushima/fukushima.html

Adding onto my post above -- slogged through many past pages and found several posts from Fred that were good in trying to overlay the reactor floor layout on the existing destroyed buildings -- see this one for example:

https://www.physicsforums.com/showpost.php?p=3235798&postcount=3302

The images above disagree with the placement of the SPF here. I found compelling the combination of the photo of the #4 reactor room (is this really an actual picture of that exact room as is claimed?) then _flipped_ (which is odd), which then matches up with the other images showing the yellow, removed containment cap. Second opinions?

 

[TCups]

Would someone like to comment on these images
http://www.houseoffoust.com/fukushima/fukushima.html

g-

I will risk a comment. The mark-ups in many cases don't match the pre-earthquake pictures or the building schematics.

If this were indeed is a picture of the inside of Unit 4, Fukushima Diiachi, then there is an important clue to what has happened at Unit 4:

http://www.houseoffoust.com/fukushima/reactor4_insideplain.jpg

The mark up of the SFP4 stops short. The mark up should include an accessory pool in the corner of the SFP used for cask transfer. The elevator shafts have to be in the southwest corners to mate with the external access tunnels at the south end of the west walls of Units 3 and 4.

At Fukushima Diiachi, the reactor core, RPV and primary containment are located in the center line of the building on the east-west axis but are located to the east of the center line of the north-south axis, I believe. I suspect "Unit 4" here refers to Fukushima Diana not Fukushima Diiachi.

The floor plan layout that seems to match both the schematic drawings and the actual photography is much closer to this:

http://i306.photobucket.com/albums/nn270/tcups/Picture63.png

http://i306.photobucket.com/albums/nn270/tcups/TopFloor-Floorplan.jpg

http://www.houseoffoust.com/fukushima/reactorcutaway.jpg

That leaves the bulk of the floor storage space to the west side of the access floor, which is where the yellow drywell cap can be seen. I had assumed the pressure vessel's cap would be radioactive enough that after its removal, it would be transferred and placed in the equipment pool to the north of the reactors primary containment plug.

There are pictures that seem to support this in several of my earlier posts.

It has not been conclusively confirmed, at least to me, but I believe that the cask pool for the Bldg 3, 4 design was not incorporated into the corner of a larger SFP as in your marked up photo, but rather was a separate, smaller pool, gated and located between the fuel cask elevator shaft and the SFP of Units 3, 4. If so, then the cask pool may have great significance in the apparent pattern of explosion damage at Unit 4.See also:

http://i306.photobucket.com/albums/nn270/tcups/aerial-floorplan.gif

http://i306.photobucket.com/albums/nn270/tcups/FHM.jpg
Note the absence of an accessory cask pool in the corner of the SFP, and also the concordant location of the yellow drywell cap. Compare to the inside view of Fukushima Diana here, which does have the cask pool in the corner of the SFP

http://i306.photobucket.com/albums/nn270/tcups/Picture38.png

And compare to what I believe to be the layout of the Fukushima Diiachi core and pools seen in this shot. The red arrow indicates the path of transfer of fuel rods through the transfer chute into the SFP, the cutaway detail and green arrow show the access through a much larger gate to the equipment pool and, I believe, the RPV cap in the equipment pool.

http://i306.photobucket.com/albums/nn270/tcups/Picture2-4.png

PS: See also this recent video

http://www.youtube.com/watch?v=_L5cc8yNvDA&feature=youtu.be

 

[AtomicWombat]

The real question is: How the hell would it reach detonation limit, 18%, without getting ignited first? You can explain it when it's leaking from the reactor, having cooled down sufficiently, but there's zirconium burning in steam (and air) there, at more than 1000 degrees Celsius or so, I'd guess hot pieces of oxide flying around, etc.
From 4% to 18% it is deflagration, not detonation.
See this:
http://www.iaea.org/ns/tutorials/regcontrol/appendix/app9344.htm
and check wikipedia etc:
http://en.wikipedia.org/wiki/Hydrogen_safety

I'm really kind of confused. I just can't believe in hydrogen detonation in reactor 4. Does not compute. I don't believe in nuke steam explosion in SPF either.

Very good point. I'd forgotten about the difference between flammability and explosive limits.

Hydrogen is lighter than air so it will tend to accumulate near the ceiling of the reactor buildings and could build beyond the explosive limit provided no source of ignition was present.

Whether it would burn in air as soon as it is produced above the SFP would presumably depend on the H2 production rate and the air supply - given the temperature is likely high enough.

This video demonstration - where hydrogen is produced in a steam-zirconium reaction - shows that the hydrogen is not necessarily immediately burnt in air.
http://vimeo.com/22209827"

 

[AtomicWombat]

http://search.japantimes.co.jp/cgi-bin/nn20110413x1.html"

"The radiation level 6 meters above the spent-fuel storage pool at the crippled Fukushima No. 1 nuclear plant was measured at 84 millisieverts per hour Tuesday. Normally, it's 0.1 microsievert.

Tepco used a robot to take a water sample from the pool Tuesday to analyze the radioactive materials in it, which can tell them in greater detail what is happening to the spent fuel rods."

If they can measure radiation 6 metres above SFP 4 and sample it with robots, why can't they take a decent high resolution photo!

I feel more like a mushroom every day.

 

[hbjon]

I bet you didn't know that the garbageake mushroom has elevated levels of vitamin D when exposed to brief amounts of UV radiation. Much like the workers on site in Fukushima. They will probably see their vitamin D levels increase as well. Vitamin D is a beneficial nutrient the human body cannot store for long periods of time. Go easy Borek, go easy.

 

[densha_otoko]

Thanks for the updates. BTW, according to some Japanese documents I read a few days ago (cannot find now) the 4/08 R1 Drywell radiation reading was actually 187 Sv/h, not EXACTLY 100 as reported in some places. A Japanese source said 100 was the max limit reportable for some reason, but a conversion of the raw readings data released by Tepco translated to 187 Sv/h.

It bugs me that they delayed reporting #1 data right after the nitrogen injections started and then stopped reporting Drywell radiation after it started to go wild this week. And in your chart today we see a Torus radiation spike in #1. #1 is not looking stable indeed.

Also trying to find #4 fuel pool data trends. I think #4 pool has MOX fuel rods as well. Kyodo news says #4 has 204 unused/new fuel rods in addition to the 1,331 rods stored there. (http://english.kyodonews.jp/news/2011/04/85295.html)

Does the presence of unspent fuel rods make the situation more volatile or dangerous?

Updated my plots of #Fukushima reactor variables to NISA release 91 (13/apr 13:00) : http://bit.ly/gAuxse

(Note that release 89 merely repeated the data of release 88 for units #1 and #2, which were measured at 06:00 on apr/12.)

 

[Reno Deano]

You run the effluent out of the reactors into the water processing facility/ship then pump it back to the plant. Depending on temperatures you run it through appropriate radiator/coolers before it gets to the ship.

Have experience with one of those systems (cobalt 60 removal) they would soon be such a high radiation hazard (due to fuel particle buildup on the filtration system) that it would need to be towed out to sea to reduced area radiation levels. Also, the inplant radwaste system is not designed (shielded) for fuel particle removal.

Dean

 

[Reno Deano]

I notice from the NEI reports that Unit 1 is having water continously injected! Why not recirculated? Where is the water going? Seems to indicate that the RPV and primary containment are breached, and they cannot keep the core covered. If hydrogen is being generated in such a large scale to need a Nitrogen blanket, then the core must be uncovered or cycling through covered and uncovered. Your thoughts folks.

Dean

 

[PietKuip]

Does the presence of unspent fuel rods make the situation more volatile or dangerous?

Unspent fuel produces less decay heat, but it is more likely to go critical.

 

[Dmytry]

Unspent fuel produces less decay heat, but it is more likely to go critical.

what the hell is unspent fuel doing in the pool? It doesn't need to be cooled, and in fact there's garbageton of precautions in transportation to prevent it from going critical if truck falls off bridge into a river.

 

[Samy24]

Updated my plots of #Fukushima reactor variables to NISA release 91 (13/apr 13:00) : http://bit.ly/gAuxse

(Note that release 89 merely repeated the data of release 88 for units #1 and #2, which were measured at 06:00 on apr/12.)

There is a jump in the Csupc readings (1930) of unint 1 in your plot regardng release 91. Where did you get this data?

 

[razzz]

I notice from the NEI reports that Unit 1 is having water continously injected! Why not recirculated? Where is the water going? Seems to indicate that the RPV and primary containment are breached, and they cannot keep the core covered. If hydrogen is being generated in such a large scale to need a Nitrogen blanket, then the core must be uncovered or cycling through covered and uncovered. Your thoughts folks.

Dean

One would have to conclude at the least that the plumbing is broken, crack or missing to all three cores whether or not the cores remain inside some kind of containment due to the fact all the cooling liquid going in comes out contaminated. So, like unit 4, the fuel rods are all exposed to the environment without watering. A description like 'containment' would be a misnomer as far as I'm concerned.

 

[bidou]

Hello everyone.
First I want to thank you all. I follow this discussion since 10 days and its really interesting.
Big Kudos!

I have some questions about the Corium.
We know that the Fuel is in pretty bad shape.
Some says Corium may be present.
Others are more confident and says that Corium IS present.
(I've read that after 10 hours without cooling everything is potentialy melting).
So what do you think? Are Corium ponds present in Fukushima?

Second question.
IF Corium is present. Can it go critical?
Corium is an indistinct molten mass of a little bit everything.
Is it possible that it goes critical? Or hardly?
IF Corium is present and it can go critical.
What are the risks? Does this "only" throw radiation everywhere or can it heat so rapidly to explode?

Yes I know there is many IF and many questions :/
Thank you in advance.

 

[georgiworld]

Nice work georgiworld, that took some time and effort. Can't add much. Looks like entombment looks impossible due to debris having to be removed first. Everything contaminated and not approachable doesn't help.

I must clarify that these images are not my work. I noticed new images and a different interpretation of old ones and wanted feedback from the experts on this forum. The website is quite comprehensive and deserves attention. There are probably errors in the analysis, which I hope the members here can point out.

I have thanked the author for her work and in no way want to take credit for the fine job done.

 

[georgiworld]

I wonder if someone can clear something up for me. From the beginning I thought Reactor 4 posed the most danger since the fuel in it, both spent and unspent, were outside of containment. Since then most attention has focused on the three other reactors and the possible breaching of the RPV with a concomitant oozing of corium .

There have now been reports of fires at number 4, which has refocused attention on it and which concerns me greatly.

As I understand it, the decay heat that is emanating from the spent fuel has created the need for a great deal of water to be added in order to remain cool. While this effort appears to have worked since there remains water in the pool, the amount of heat generated maybe too much to deal with and the excess heat has found another outlet.

Now what I'd like to know is could this heat be melting the unspent fuel rods? Is that what is on fire?

Are we looking at an open-air reactor with widespread dissemination of radiation?

Can anyone reassure me that this is not the case.

 

[Arm_Chair_QB]

Hello everyone.
First I want to thank you all. I follow this discussion since 10 days and its really interesting.
Big Kudos!

I have some questions about the Corium.
We know that the Fuel is in pretty bad shape.
Some says Corium may be present.
Others are more confident and says that Corium IS present.
(I've read that after 10 hours without cooling everything is potentialy melting).
So what do you think? Are Corium ponds present in Fukushima?

Second question.
IF Corium is present. Can it go critical?
Corium is an indistinct molten mass of a little bit everything.
Is it possible that it goes critical? Or hardly?
IF Corium is present and it can go critical.
What are the risks? Does this "only" throw radiation everywhere or can it heat so rapidly to explode?

Yes I know there is many IF and many questions :/
Thank you in advance.

There is the saying: never say never. Certainly this recent tragedy is once again a testament to this philosophy and a stark reminder to all engineers and scientists.

Given that disclaimer, these civilian nuclear reactors are designed with contingency in place to ensure they (for the most parts) do not go critical when they aggregate into a lumped mass corium at the bottom of a reactor containment and beyond. This is because firstly the nuclear fuel is of low purity in terms of fissile elements. Secondly, neutrons generated directly from fission of say uranium have too high energy - this results in a low probability of causing other fission events of other uranium atoms thereby eventually freezing out a chain reaction.

These reactors are purposely designed to have neutron moderators placed between fuel rods to slow down and reduce the energy of these neutrons from fission, so that they can cause fission and sustain a chain reaction. In other words, when fuel rods are melted down into a corium, there is presumably no more neutron moderators between the fuel rods and fuel elements (that is, the water), thereby from that point on you are primarily concerned with high energy neutrons generated from fission which have low probability of sustaining a chain reaction while thermal neutrons (lower energy) that can sustain a chain reaction are reduced in a hypothetical corium configuration.

This is the best case scenario of course. If it so happens that the civil structure of the plant itself unknowingly had neutron reflecting material nearby, it may be able to sustain a reaction to a degree. Theres also the possibility that the corium achieves localized critical mass configurations within the corium that can sustain a reaction (though not likely as the fuel is purposly designed to have low purity and is assumed to be homogenous in composition). All of this is purely hypothetical of course as are any other plausible scenarios people can draw up.

That is also not the end of the story of course, as life is never that perfect: it is not just 100% fuel configuration with control rods in place or 100% corium lump mass. If it is partially melted with a corium and some spent fuel assembies, presumably with neutron moderating water inbetween, this can again lead to criticality, especially since control rods that would have otherwise prevented criticality may be ineffective to block neutrons between the corium and the fuel assemblies though they will be far away in distance and the reaction rate should not be very high.

 

[elektrownik]

New thermal images, very bad, so hot: http://www.mod.go.jp/j/approach/defense/saigai/tohokuoki/kanren/230414.pdf

 

[bidou]

Thank you for the answer.

 

[Maxion]

New thermal images, very bad, so hot: http://www.mod.go.jp/j/approach/defense/saigai/tohokuoki/kanren/230414.pdf

Please not the scale of the temperature. Nothing is very hot.

 

[PietKuip]

Please not the scale of the temperature. Nothing is very hot.

Look at the right column: some temperatures are rising quite a bit.

Also, some hot spots are too small on the detector to resolve the real temperature.
The are measuring from 3000 ft high, and do not dare to take a closer look.

 

[jpquantin]

A research team from Kyoto, Hiroshima, Kokugakuin, Nihon universities presents its "Interim Report on Radiation Survey in Iitate Village area conducted on March 28th and 29th" : http://www.rri.kyoto-u.ac.jp/NSRG/seminar/No110/Iitate-interim-report110404.pdf. It includes a gamma ray radionuclide analysis of soil samples.

Thanks, tsutsuji ! This pointed to the level triggering actions in case of radiation exposure. I later found http://www.jnes.go.jp/bousaipage/english/an-3-12.htm" :

• For an estimated exposure between 10 and 50 milliSieverts (per year I guess), people should shelter in door
• Above estimated annual dose of 50 milliSieverts, people should shelter in concrete building or evacuate

Well with the provided report, I was able to do a calculation of the hour dose on today that would trigger either the first or second action.

WARNING : This calculation is based on readings on graphs in the report, so there could be some errors. It is also an extrapolation of measurements in Iitate.

Projected total dose after 84 days would be 96 miiliSieverts in Magata, 62 milliSieverts after 42 days (graph page 6). After 21 days, I considered the dose linear in time. That would lead to a total of 323.47 milliS in Magata in one full year (96 up to day 84, then prorata, approx 0.80 per day)

To reach 50 milliS per year, this is a factor of 6.47. Reading at Magata on March 31st was 24 microSieverts per hour. Assuming the rate fell by decay from a 10/13 factor (reading on graph page 5 - not very accurate) till now, reading at Magata today should be around 18.46 microSieverts per hour.

Therefore, applying the 6.47 factor, the hourly limits today should be :

• Above 2.85 microSieverts per hour : shelter in concrete or evacuate
• Above 0.57 microSieverts per hour : shelter indoor

Now if all this is accurate, look at thehttp://www.mext.go.jp/component/english/__icsFiles/afieldfile/2011/04/14/1304852_041319.pdf" , and determine which area should have received action 1 or 2. Fukushima inhabitants would have been told to stay indoor.

Any comments ?

 

[Dmytry]

Except that is BS because it is doses for the corpses who don't breathe etc. In short, for the people who'd really remain indoors as ordered and die of starvation or lack of water. It's really wrong to calculate this stuff to 2 digits accuracy. The radiation levels (see current chernobyl levels thread) vary a LOT between locations within same city block (middle of road, drain, roadside, may differ by order of magnitude). Bottom line is - they should of evacuated a lot of people they didn't evacuate, and they did evacuate a lot of people they did not have to evacuate.
edit: also, where the hell is US radiation measurement plane that US offered a few weeks ago? WTF guys. Is there some reluctance of government to use such plane and admit to the people that they're nowhere as prepared as other countries? Are they opting not to do such measurements and pretend they do everything they could?

 

[Krikkosnack]

Worst case scenario Fukushima fuel pool with plutonium catches fire

http://energheia.bambooz.info/index.php?option=com_content&view=article&id=168%3Aworst-case-scenario-fukushima-fuel-pool&catid=60%3Avideo&Itemid=85&lang=it

http://vimeo.com/22352930

 

[AntonL]

Can someone please be so kind and repost the link to the large cross-sectional plan of the nuclear reactor that was posted some time ago, not the GE handout - I tried very hard and cannot find it

 

[bytepirate]

Can someone please be so kind and repost the link to the large cross-sectional plan of the nuclear reactor that was posted some time ago, not the GE handout - I tried very hard and cannot find it

this one? http://fukushimafaq.wikispaces.com/Reactor+Blueprints

 

[cola]

#4 was undamaged after #3 exploded: http://www.digitalglobe.com/downloads/featured_images/japan_earthquaketsu_fukushima_daiichi_march14_2011_dg.jpg

there also seems to be no truck at no. 4 at that date, while later on there was one.

 

[AntonL]

this one? http://fukushimafaq.wikispaces.com/Reactor+Blueprints

yes - you are a star - thanks

 

[Krikkosnack]

Fuel Rods of the reactor 4... are really those?
http://www.houseoffoust.com/fukushima/rods.html

 

[curious11]

Much more likely to be beams from structures on the harbourfront that were obliterated by the tsunami.

 

[clancy688]

#4 was undamaged after #3 exploded: http://www.digitalglobe.com/downloads/featured_images/japan_earthquaketsu_fukushima_daiichi_march14_2011_dg.jpg

Doesn't it look like as if there's white smoke coming from the eastern (sea) side of unit 4?

 

[Astronuc]

Fuel Rods of the reactor 4... are really those?
http://www.houseoffoust.com/fukushima/rods.html

Most of those pictures do not point to fuel rods, which are slightly less than 0.5 inch in diameter. The top images show a 9x9 fuel assembly.

 

[biffvernon]

Doesn't it look like as if there's white smoke coming from the eastern (sea) side of unit 4?

That smoke is continuous with the smoke towards bottom right of picture towards the trees. I guess just an eddy a few moments earlier from the #3 plume.

 

[SayaX]

Hi all. I've been reading quietly, but I wanted to contribute here since I've been keeping up with the disaster through the Japanese media.

This was posted shortly ago, I don't know if it's good news or not. But curious what people think of it. The translations are my own and while it's not perfect (I'm not a professional or a native speaker), the news is about an hour old.

http://www.yomiuri.co.jp/science/news/20110414-OYT1T00938.htm?from=main4
溶融燃料「粒子状、冷えて蓄積」１～３号機分析
Melted/fused fuel, "has grown cool and accumulated in a grain like state", analysis of Unit 1-3

注水冷却が続けられている東京電力福島第一原子力発電所１～３号機について、日本原子力学会の原子力安全調査専門委員会は１４日、原子炉などの現状を分析した結果をまとめた。
About the pouring water refrigeration situation at TEPCO Fukushima Daiichi Nuclear Power Plant Units #1~3, members of the Japan Atomic Mechanics Society(?)'s Nuclear Power Safety Investigation Advisory Committee has compiled the analysis of the nuclear reactor's present condition.

　３基は核燃料の一部溶融が指摘されているが、専門委は「溶融した燃料は細かい粒子状になり、圧力容器の下部にたまって冷えている」との見解を示した。
The 3 groups have pointed out that while there is indication that partial melting has occurred, the professional committee is of the opinion that "the melted fuel has become fine grain, and has collected at the bottom of the pressure vessel and has grown cold".

　専門委では、東電や経済産業省原子力安全・保安院などが公表したデータをもとに、原子炉の状態を分析した。
The professional committee, using the data made available by TEPCO, METI, NISA etc, have made an analysis of the nuclear reactor's current state.

　それによると、圧力容器内の燃料棒は、３号機では冷却水で冠水しているが、１、２号機は一部が露出している。１～３号機の燃料棒はいずれも損傷し、一部が溶け落ちている。溶融した核燃料は、冷却水と接触して数ミリ以下の細かい粒子に崩れ、燃料棒の支持板や圧力容器下部に冷えて積もっていると推定している。これは、圧力容器下部の水温が低いこととも合致している。沢田隆・原子力学会副会長は「外部に出た汚染水にも、粒子状の溶融燃料が混じっていると思われる」と説明した。
According to this, regarding the fuel rods inside the pressure vessel container, in number 3 they have been submerged with coolant, however 1 and 2 has been partially exposed. In addition, 1~3 fuel is damaged, partially melted and then falling. They assume the melted fuel is then coming into contact with the coolant and crumbling to particles/grain no bigger than several millimeters, and collecting at the bottom of the pressure vessel or fuel rod support board/planks. It has been agreed that the water temperature at the bottom of the pressure vessel is cold. The Atomic Mechanics Society President explained that in the contaminated water going to the outside, particle sized melted fuel is thought to be mixed in with it.

（2011年4月14日21時02分 読売新聞）
April 14th, 2011 21:02 Yomiuri Shimbun

Source: http://www.yomiuri.co.jp/science/news/20110414-OYT1T00938.htm?from=main4

 

[rmattila]

Sr-89 and Sr-90 activities are being reported (http://www.enewspf.com/latest-news/...-nuclear-accident-13-april-2011-1430-utc.html):

MEXT reported on measurements of strontium-89 (half-life: 50.5 days) and strontium-90(half-life: 28.8 years) in three samples taken in one village in the Fukushima prefecture on 16 March. The activities in soil for Sr-89 ranged from 13 and 260 Bq/kg and for Sr-90 between 3.3 and 32 Bq/kg. Sr-90 was also distributed globally during nuclear weapons' testing in the atmosphere, typical global levels of Sr-90 in surface soils are in the order of one to a few becquerel per kg. Strontium was also measured in plant samples in four others villages, with values ranging from 12 to 61 Bq/kg for Sr-89 and 1.8 to 5.9 Bq/kg for Sr-90.

As a first impression, the reported activity ratio of Sr-89 to Sr-90 (of the order of 10:1) seems rather low for reactor fuel with the burnups one would assume for core average at 3 - 4 months into the cycle. I would have assumed the ratio to be somewhat higher - perhaps 15 : 1 or so, but then again: from what I've been told, Sr activities are pretty difficult to measure accurately.

Would someone happen to have burnup calculation results regarding Sr89/90 ratio for 8x8 or 9x9 BWR fuel as a function of exposure? It might be worthwhile to compare the ratios obtained by using different lattice codes & CS libraries, and different assembly designs (not knowing what they actually used at Fukushima).

 

[Astronuc]

I wonder if someone can clear something up for me. From the beginning I thought Reactor 4 posed the most danger since the fuel in it, both spent and unspent, were outside of containment. Since then most attention has focused on the three other reactors and the possible breaching of the RPV with a concomitant oozing of corium .

There have now been reports of fires at number 4, which has refocused attention on it and which concerns me greatly.

As I understand it, the decay heat that is emanating from the spent fuel has created the need for a great deal of water to be added in order to remain cool. While this effort appears to have worked since there remains water in the pool, the amount of heat generated maybe too much to deal with and the excess heat has found another outlet.

Now what I'd like to know is could this heat be melting the unspent fuel rods? Is that what is on fire?

Are we looking at an open-air reactor with widespread dissemination of radiation?

Can anyone reassure me that this is not the case.

The decay heat was stronger in the cores of Units 1, 2 and 3, because they had recently shutdown (May 11). They then lost cooling on May 12-13. Unit 4 had been shutdown for about 100 days (Nov 30, 2010 - Mar 11, 2011), so the decay heat was significantly diminished. However, there is still heat on the order of 0.2% of operating power, or about 3.7 MW. That is still a significant amount of heat.

The SFP lost cooling so there was not removal of heat by the normal closed loop, but instead without cooling the SFP water increased in temperature to the point were it would start to evaporate at a higher rate. In addition, without good circulation, the relatively stagnant pool water has very low heat transfer coefficient, so the water next to the fuel rods could boil. It is also possible that the SFP may have cracked somewhere because of the seismic loading (but this is not known or confirmed).

It is surmised that the cladding of the spent fuel in SFP #4 oxidized to the point of rupture, thus releasing fission gases (radioisotopes of Xe, Kr), and some volativles I, Cs. Fuel particles would be released only if the cladding split open, or experienced circumferential fractures - which would be possible if there was excessive hydriding of the cladding. The hydrogen is generated from the reaction of Zr + 2 H₂O => ZrO₂ + 2 H₂, which is basically a corrosion reaction. Normally it occurs at a very slow rate of several microns per year. The corrosion rate increases exponentially with temperature of the metal-oxide interface. Normal operating temperatures are on the order of 300C, but it is possible that higher temperatures are achieved if the cooling is inadequate. What temperatures the fuel actually experienced has yet to be determined, and it requires a forensic analysis, particularly an evaluation of the damage, oxidation and cladding microstructure in connection with compuational analysis with appropriate CFD tools.

 

[michael200]

Fuel in pool: 1331 spent fuel assemblies and 204 brand new assembles and some of them simply gone critical.
I already can hear Gunderson reporting that unit 4 is an open air reactor.

However let's analyse SPF 4
Capacity = 1425 m³ http://www.nisa.meti.go.jp/english/files/en20110406-1-1.pdf"

Decay Heat:
2000kW for spent fuel from unloaded reactor last operation 29 Nov 2010 (can be calculated)
+ 400kW long term stored spent fuel (estimated on the high side)
2400kW total

Assuming SPF4 was at 30^oC
To raise 1425m³ by 70^oC using 2.4MW it would take 48 hours
so SPF 4 has started to boil somewhere between March 13 afternoon to March 14 morning.

The pool is about 11 metres deep,
so to boil away 1 metre or 1425/11= 129.5m³ of water using a 2.4MW heater 33.8 hours
I cannot find the detail drawing that was posted here before to confirm the depth so I took depth of SPF 7

From earthquake (3pm 11/3) to unit 4 explosion (6am 15/3) if I can calculate correctly are 87 hours,
thus about (87-48)/33.8 = 1.2 metres of water would have boiled away at time of explosion.
that is the 4 metre fuel rods have 5.8m water covering them -
so where does the Hydrogen come from??

Helicopter crews on 17 March reported spotting water in SPF4 thus they concentrated on
dropping water on unit 3 - proof that there is ample water in the pool.

Spraying water into unit 4 was only started on March 20 at 9:40 thus
at time of explosion 6am 15/3 water level -1.2m
10am 20/3 114 hours later a further level drop of 3.4 metres due to boiling
thus when water spraying started on 20March the level was down 4.6 mtres
excluding leakage or spillage, this leaves the 4 metre fuel rods 2 metres under water.
Tepco set their priorities correctly and started spraying water at the right time.

http://www.nirs.org/reactorwatch/accidents/6-1_powerpoint.pdf" that the storage capacity of all fuel pools at Fukushima as 8310 fuel assemblies,
that is 1444 fuel assemblies can be stored at each units 2 to 5, based on SFP volume.

However, SPF4 had 1331+204 = 1535 fuel assemblies stored which is more than stated capacity! http://www.nisa.meti.go.jp/english/files/en20110406-1-1.pdf"

Therefore we can speculate that Tepco double layered at least two or three spent fuel racks,
this would explain the early exposure of fuel to air, hydrogen generation, fuel damage etc.
and we can speculate that two or three racks worth of spent fuel may be destroyed.

Debunk that.
Have I solved the hydrogen, spent fuel pool water mystery?

Now, should my calculation and speculation be proven true, Tepco needs to do a lot of explanation.

Let's please stop this discussion about "double layers" of fuel in the SFP. The depth of the fuel pool is about 40 ft and the height of a fuel assembly in the SFP racks is about 14 ft. Plant technical specifications require a minimum water level of about 20 ft above irradiated fuel in the SFP. This technical specification requirement could never be met if two fuel assembilies were stacked on top of each other. The very idea of such a thing would be impractical.
When a utility increases the capacity of the SFP, they do it by replacing the exisiting used fuel storage racks in the SFP with racks that allow the fuel to be placed closer together (higher density). The geometry is analyzed and possible change to the neutron absorber panels in the fuel racks are changed to preclude criticality of the assemblies in the SFP.

Back to loss of inventory in the U4 SFP: AntonL's boiloff calculations look pretty good. There probably was also several feet (maybe 3-4 ft total) of SFP inventory loss due to pool sloshing during the earthquake. However, the inventory boiloff plus the inventory loss from sloshing wouldn't explain uncovery of the irradiated fuel in the U4 SFP. It is possible that the earthquake caused a leak in the spent fuel pool (perhaps in the gate plugs that are removed when performing refueling operations between the reactor and the SFP). Such a leak, if large, would explain the additional inventory loss. However, the limited photos of the damage to the U4 reactor indicate that there was a VERY hot fire with explosions in the lower levels of the U4 reactor building. How H2 generation and ignition at the elevation of the U4 refuel floor caused this lower building destruction is beyond me.

 

[Astronuc]

Hi all. I've been reading quietly, but I wanted to contribute here since I've been keeping up with the disaster through the Japanese media.

This was posted shortly ago, I don't know if it's good news or not. But curious what people think of it. The translations are my own and while it's not perfect (I'm not a professional or a native speaker), the news is about an hour old.

I don't believe the fuel necessarily melted. It certainly did have contact with the coolant, then seawater. Various chemical reactions are possible well below melting temperatures. It is certainly likely that the Zr-alloy cladding was several corroded (oxidized), which is the source of hydrogen. The oxidation could have proceeded to cladding rupture and perforation (breach), and possibly fracture or cracking. If cracking occured, particularly axial splits, then the fuel has intimate contact with the coolant, and it is possible that the ceramic fuel oxidized, possibly to the point where particles of fuel dropped out and settled to the bottom of the core, or perhaps in the regions below the core.

If there has been water in the bottom half/third of the core, then melting is not necessarily an outcome (it might if temperatures were sufficiently high). On the other hand, dissolution of the fuel (to some extent) would certainly occur if there was large scale failure of the cladding.

 

[Dmytry]

Wild theory: could SFP4 have gone critical, after damage to the fuel rods spewing the fuel pellets that fell down? Then, a powerful excursion deep under water (perhaps with positive 'damage coefficient' so to say, when steam generation in one place makes criticality worse elsewhere) *shook* the spent fuel pool, damaging the walls as by man-made quake? The water around would of acted as sort of tamper, preventing quick formation of steam bubbles and even redistributing pressures between bubbles, potentially leading to instability. To think about it, it could have experienced a power excursion, followed by formation of voids and lifting of water, then the voids could collapse as the water was falling down.

(I presume cladding entirely destroyed unless shown otherwise, as the reaction of zirconium with steam is exothermic, and with air, even more so, and as is the reaction of zirconium with uranium dioxide. Simply put, it is fire - or if you wish, exothermic reactions that speeds up at higher temperatures. To call it oxidation is true but makes it into a sort of under-statement)

 

[georgiworld]

Thank you Astronuk, as always your technical assessment is spot on.

My question however is not about what happens to the cladding of the spent fuel but what effect the heat will have on the unspent fuel which was stored in SPF#4. Now while the decay heat was greater in the reactors that were operational at the time of the incident, it was contained in the RPV.

My fear is that the unspent fuel in the SFP of #4 will go critical. Is that possible?

 

[Astronuc]

Thank you Astronuk, as always your technical assessment is spot on.

My question however is not about what happens to the cladding of the spent fuel but what effect the heat will have on the unspent fuel which was stored in SPF#4. Now while the decay heat was greater in the reactors that were operational at the time of the incident, it was contained in the RPF.

My fear is that the unspent fuel in the SFP of #4 will go critical. Is that possible?

The spent fuel pool racks are design to prevent criticality. There is neutron absorber material present. IF somehow that neutron absorber material was lost, then there would be a chance of criticality. The configuration of the pool is important. It's not clear to me where the fresh fuel was located with respect to the reinsert and discharge fuel.

Even if the SFP went critical, this does not necessarily lead to an explosion. An explosion requires a rapid release of energy, and that doesn't happen if a system is critical (k = 1) or slightly supercritical (k > 1, but < ~1.006).

The fresh fuel would have no fission products, and no decay heat. Except for criticality, it would be pretty benign. Even if fresh fuel went critical, there would be no fission product accumulation, unless it was critical for many days or weeks, but the fission product inventory would not be significant. The problems at Unit 4 were only days after losing cooling.

 

[Dmytry]

Thank you Astronuk, as always your technical assessment is spot on.

My question however is not about what happens to the cladding of the spent fuel but what effect the heat will have on the unspent fuel which was stored in SPF#4. Now while the decay heat was greater in the reactors that were operational at the time of the incident, it was contained in the RPV.

My fear is that the unspent fuel in the SFP of #4 will go critical. Is that possible?

I think it gone critical before, after geometry change due to the fire. The geometry has to be specifically designed as to avoid criticality. The pool was re-racked to higher capacity.

Possible, definitely it is possible, did it happen, we can't know for sure (lack of good data), will it happen, we can't know either.

Things to know about criticality:

1. The criticality in the fuel of that reactor type is not possible without moderator to slow down neutrons (water is moderator). The enrichment must be greater than 6% to allow criticality without moderator.
2. thus, there, fast criticality (without moderator, on fast neutrons) is impossible (unless some process separates plutonium and uranium, seems unlikely).
3. The temperature increase of the fuel leads to Doppler broadening of the absorption of neutrons by U238, decreasing the reactivity. This limits the temperature.
4. Thus, the criticality does not imply nuclear explosion

You can look at the list of criticality accidents here:
http://www.johnstonsarchive.net/nuclear/radcrit.html
to know what to expect.
The criticality is not hell on Earth event. In all the mess, it could even be going on somewhere unnoticed. It's not necessary a kaboom, even though it might be a kaboom.

 

[OnlyOneTruth]

Thanks for the link to blueprint for reactor building 1: http://fukushimafaq.wikispaces.com/Reactor+Blueprints

The reactor buildings of nos 2,3,4 are different: bigger in their horizontal dimensions and placed differently in respect to the corresponding turbine buildings.

Has anyone seen a blueprint for the no4 reactor building? How does the internal layout look like? Can one assume that 2,3,4 share a common internal layout?

Another thing: An annotation to picture http://i306.photobucket.com/albums/nn270/tcups/r735227_5964756.jpg of https://www.physicsforums.com/showpost.php?p=3196018&postcount=463 mentions thermal damage (from fire?) to the northside upper part of No.4. Has this been confirmed? To me that would explain that aspects of no4 damage that are bound inwards.

BTW Thanks to all contributing, have been reading here for the last week. And sorry if my question is noobish. I'm just an amateur trying to understand what's going on.

 

[Earthmover]

Anyone have any thoughts on collapsing the fukushima power station into the pacific with this techinque:

http://youtu.be/S1f6vbiuUt0
http://youtu.be/l5jfaXSFfGQ

Just the thought of these reactors and all that stored waste spewing radiation into the atmosphere for who knows how long...well its just damn scary
Instead of trying to bring the pacific to the reactors put the reactors in the pacific.
At that point the release of radiation into the atmosphere will cease and men could clean it up using deepsea salvage operations.
maybe even build a cofferdam around the whole damn complex like this:

http://youtu.be/2kxVKkXW5Fg