Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[TCups]

SOMETHING FOR CONSIDERATION: DID THE EXPLOSION AT UNIT 3 SUCK?

Suppose, just for grins that something like this happens . . .

1) SFP3 and SFP4 are boiling and fuel is beginning to be exposed. Hydrogen is building up above the service floor of both Bldg 3 and Bldg 4

2) a large blast (pick your origin, mine is the primary containment, through the fuel transfer gate), epicenter SFP3, southeast corner of Bldg 3 occurs

3) the initial strong shock wave not only blows the blank out of Bldg 3's southeast corner, but also damages the structural integrity of the north face and NE corner of Bldg 4 perhaps not readily apparent in the satellite images that seem to show Bldg 4 intact, but let's speculate -- cracks in the concrete at the northeast corner. Bldg 4 is no longer air tight at the northeast corner.

4) a large Vesuvial plume of steam shoots straight up from SFP3, making, yes, Ross Perot's "giant sucking sound" at ground zero, between Bldg 3 and 4.

5) the vertical blast sucks the air from Bldg 4, through the structural cracks and perhaps through the fuel loading access tunnel. There aren't many doors or windows in Bldg 4, after all.

6) the accumulating hydrogen gas originally inside the upper level of Bldg 4 is not ignited, but instead, is sucked downward, perhaps toward the northeast corner of Bldg 4, by the secondary negative pressure.

7) after the blast at Bldg 3 subsides, any residual hydrogen gas still in the lower levels of Bldg 4 again rises, forming pockets of trapped hydrogen gas along the ceilings of the various compartments in the building surrounding the primary containment. Someone in an earlier post noted that the internal doors closed during operation might be open during "cold" service of Reactor 4, with fuel off-loaded.

8) SFP4 continues to churn out hydrogen, eventually re-filling the upper portion of the building with more hydrogen gas.

9) at some point, something ignites the accumulated hydrogen gas in Bldg 4, probably something hot in SFP4, and then,

10) the resulting blast propagated throughout the entire building, blowing out the upper and lower portions of Bldg 4, and in particular, finishing the job started at the northeast corner and north face of Bldg 4 by the initial shock wave of the blast from SFP3.

How's that for a SWAG? Also . . .



@ M. B:

Sir: I confess that I am not smart enough to decipher all of the audiographic info you sent after your in depth analysis of the sounds from the explosion of Bldg 3, but perhaps you would summarize and share your thoughts in a follow up post. Very interesting stuff. Thanks.

 

[MadderDoc]

<..>
The thermal image originally posted was this one. I have recombined the window/level scale and annotated what I see as Bldg 4, Bldg 3, Tower, and Pipe (darker = colder = water in pipe?). The pipe just to the east of the tower is near ambient temp (empty). The only hot spot between Bldg 3 and Bldg 4 (circled) is relatively small, east of the pipe next to the tower, and only somewhere near 25ºC.

Agreed except the position of the hot spot, where I think the perspective may be cheating you. The thermal camera is looking down from the east at the hotspot, in a line that goes towards the foot of the tower, but under the pipe.

If the thermal image pre-dates the "cleaned up" image <..>

It doesn't, I should have made the date-marking clearer. The 'before cleanup' image is from March the 20th, the 'after' from the 24th, and the thermal image with the hotspot is from the 25th.

I see nothing on the thermal image even vaguely resembling a truck, blocking activity or otherwise.

Neither do I -- I see just about what I would expect if there were a fuel rod there. :-) Someone else said that it probably was just heat from the cleanup truck, and that would explain why we see no hotspot on the later images.

However, what I said is, the later thermal images indicates that a firetruck has taken station under the pipe, obscuring the view to the position of the hotspot. This _too_ would explain why we don't see it on the later images...

(A third explanation could be, someone removed it, but that's too simple :-)).

I have attached a photo making clear where I think the hotspot is.

 

[michael200]

Even if recirc pump seals fail there is a breakdown nudging to limit leakage to less than 60 gpm with reactor operating pressure of 1050 psi . Leak rate would be much less at lower rpv pressure.

 

[MiceAndMen]

In relation to the weird thermal images at unit 4, I found a few things that might help understand what's going on there. In short, TEPCO is doing much more than a simple refueling op, and that's why it has been offline for 4 months already. This http://www.nisa.meti.go.jp/english/files/en20110406-1-1.pdf" says (p 35) this about the fuel in unit 4's SFP:

All fuel assembly was removed from the core and located in the pool due to the core shroud replacement

I googled around a bit and found a few other items of interest regarding the core shroud replacement program at Fukushima Daiichi. My understanding is that the "core shroud" is a cylindrical wrapper that surrounds the fuel core inside the reactor pressure vessel (RPV).

This http://www.toshiba.co.jp/nuclearenergy/english/maintenance/replace/shroud01.htm" .

Another pdf describes the steps taken to control radiation exposure during the http://www.irpa.net/irpa10/cdrom/00584.pdf" . There's a schedule on page 1 that indicates the core shrouds for units 2, 3 and 5 were replaced in the 1997-2000 timeframe.

And 12 years ago at the 7th Int'l Conference on Nuclear Engineering in Tokyo, a http://www.jsme.or.jp/monograph/pes/1999/ICONE7/PAPERS/SPECIAL/FP7903.PDF" that details how unit 3 was the first reactor at Fukushima Daiichi to undergo core shroud replacement. Initially scheduled to last 300 days, the project actually took 422 days to complete.

So if the NISA Seismic report is correct, Unit 4 is currently in the middle of a 10-month core shroud replacement effort. I think it's extremely unlikely that any fuel was being placed into or taken out of the RPV at the time of the earthquake. I'm not sure if any of this has any impact whatsoever on the current observed thermal properties of the site, or if the work being done would contribute in any significant way to the etiology (a nice new word I learned here recently) of events at unit 4.

 

[shogun338]

We would like to make a correction to a part of the appendix "Radioactivity
Density of Seawater" of "Detection of radioactive materials from the
seawater near Fukushima Daiichi Nuclear Power Station (14th release)"
attachment "Radioactivity Density of Seawater around 15km offshore from
2F(Bq/cm3)". We apologize for any inconvenience and would like to correct
as follows:

<Correction part and content>

"Radioactivity Density around 15km offshore from 2F(Bq/cm3)"

With regard to the data of chronological graphs of the result of seawater
nuclide analysis around 15km off shore from Fukushima Daini Nuclear Power
Station, we mistakenly reported and put on the press release the data of
1st sampling and 2nd sampling on April 5th vice versa. Therefore, we
would like to make a correction.
We would also like to make similar corrections for the press releases,
which were released on and after April 7th.
We have confirmed that the data, which was attached as "The result of
the nuclide analysis of the seawater <offshore>" is correct.

<Attachment>
"Radioactivity Density of seawater around 15km offshore from 2F(Bq/cm3)"
(corrected version)

 

[shogun338]

On April 12th 2011, in order to examine the condition of the spent fuel
pool of Unit 4 of Fukushima Daiichi Nuclear Power Station, we collected
200ml of the water in the pool with the concrete pumping vehicle.
On April 13th 2011, we conducted a nuclide analysis of radioactive
materials with the water and as a result have detected some radioactive
materials as shown in the exhibit.

We are planning to evaluate the result in further detail.

 

[shogun338]

Freshwater injection is confirmed to continue for Units 1 to 3. The transfer of contaminated water from Unit 2 turbine building to the condenser was started (12 April) and suspended (13 April) to check for any leakage. Temperature at the Unit 1 outlet nozzle shows a decreasing trend continuously for several days now.

To minimize migration of contaminated water to the open sea, on the ocean-side of the Inlet Bar Screen of Unit 2, the two temporary steel plates (3 plates in total) were installed to stop water from leaking out of the inlet bay (around 08:30 until 10:00, 13 April). In addition, a silt fence to prevent the spread of the contaminated water was installed in front of the Screen of Units 3 and 4. (13:50 UTC, 13 April).

In Unit 1 fresh water is being continuously injected into the RPV through the feed-water line at an indicated flow rate of 6 m3/h using a temporary electric pump with off-site power. In Units 2 and 3 fresh water is being continuously injected through the fire extinguisher lines at an indicated rate of 7 m3/h using temporary electric pumps with off-site power.

Nitrogen gas is being injected into the Unit 1 containment vessel to reduce the possibility of hydrogen combustion within the containment vessel. The pressure in this containment vessel has stabilised. The pressure in the RPV is increasing as indicated on one channel of instrumentation. The other channel shows RPV pressure as stable. In Units 2 and 3 Reactor Pressure Vessel and Drywell pressures remain at atmospheric pressure.

RPV temperatures remain above cold shutdown conditions in all Units, (typically less than 95 °C). In Unit 1 the temperature at the feed water nozzle of the RPV is 206 °C and at the bottom of the RPV is 119 °C. In Unit 2 the temperature at the feed water nozzle of the RPV is 167 °C. In Unit 3 the temperature at the feed water nozzle of the RPV is 92 °C and at the bottom of the RPV is 119 °C.

On 12 April in Units 3 and 4, fresh water (35 T and 195 T respectively) was sprayed over the Spent Fuel Pools using a Concrete Pump Truck. A sample of the water in the spent fuel pool was collected for analysis.

There has been no change in status in Unit 5 http://www.iaea.org/newscenter/news/tsunamiupdate01.html

 

[cphoenix]

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.

(calculation deleted by cphoenix for readability)

• 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 ?

If I understand correctly, you are assuming that a dose rate that would accumulate 50 mSv per year is dangerous. But this is not the case.

A person could probably accumulate 1 mSv in 1 second without danger. That is a dose rate of over 30,000 Sv per year.

The people in Iitate may have to evacuate, because it may not be safe to live there for several years. But even given the higher dose rate right after the radiation release/deposition, they probably have not accumulated enough radiation yet to be significantly at risk of anything. (I think I read 20 mSv per person?)

Chris

 

[cphoenix]

@ M. B:

Sir: I confess that I am not smart enough to decipher all of the audiographic info you sent after your in depth analysis of the sounds from the explosion of Bldg 3, but perhaps you would summarize and share your thoughts in a follow up post. Very interesting stuff. Thanks.

When watching explosion videos... Don't forget the speed of sound. If there isn't a noticeable lag between the visual and the audio, then someone messed with the sound track.

While I'm at it, let me float a new hypothesis for building 4:

(Edit, Apr. 15: This is no longer plausible, assuming that http://www.asahi.com/english/TKY201104140125.html is correct, and the water sample rules out any disintegration of fuel rods.)

1) Fuel abruptly went critical near the bottom of the pool. (Even a mild aftershock might have shifted geometry.)

2) The weight and inertia of the water kept the pressure on until a fair amount of heat built up. (Fractions of a second.)

3) A large steam void formed, displacing water from around the criticality. (Superheated water under pressure would have expanded quite a bit once the pressure was reduced.)

4) The void collapsed, producing a water hammer that punched through the bottom of the pool. A fairly symmetrical collapse might create a pretty massive force in a pretty small area.

5) Water, steam, and very hot fuel were ejected into the interior of the building, causing a large steam explosion in a confined space.

6) The fuel was no longer critical. The building was destroyed with more force below-decks than in the steel-frame section. The building was not charred or sooted (as building three was).

7) Debris from the pool covered the hole; it would still leak, but slowly. (Come to think of it, do we know for sure that pool 4 still holds water? Or are they just steam-cooling the rods with a continuous flow of water?)

 

[Rive]

...So if the NISA Seismic report is correct, Unit 4 is currently in the middle of a 10-month core shroud replacement effort...

Tks-tsk-tsk.

Is it possible that the replacement process was in 'dry' phase in the time of the earthquake?

What if the pool gates were broken during the EQ and the pool water flooded the empty RPV - leave the pool with only a limited amount of water, barely covering the rods? That could explain the hydrogen buildup. And this flow can explain the weird thermal image too (the water injected to the pool is still flowing to the RPV and beyond).

 

[razzz]

...Plant operator Tokyo Electric Power Company (TEPCO) said it was also analysing footage taken by an unmanned helicopter of the tsunami-hit plant and its explosion-charred reactor buildings to assess the damage.

A probe of the reactor four containment pool for more than 1,000 fuel rods -- exposed to the sky after a hydrogen blast ripped away the roof -- suggested that some of the rods had been damaged, but that most were intact...
http://newsonjapan.com/html/newsdesk/article/88963.php

More than likely 1,000 fuel assemblies.

If 1,000 fuel rods (which would makeup an odd number of assemblies) then they are woefully short rods.

1,000 fuel assemblies, where 1 assemblies consists of 60 individual fuel rods in formation, then the pool has doubled for temporary parking of both in-service assemblies waiting to be reloaded after the core remodeling and end of life assemblies being bathed until cool enough for removal elsewhere.

Wiki notes Unit 4 should have over 1,300 assemblies in the pool and I've read anything from 135 tons to over 200 tons of fuel in the pool of Unit 4 depending on what unit measure and who is doing the reporting. No doubt a lot of weight to deal with during a 9.0 earthquake which is about 2 minutes of continuous intense movements.

 

[jensjakob]

http://translate.google.dk/translate?js=n&prev=_t&hl=da&ie=UTF-8&layout=2&eotf=1&sl=da&tl=en&u=http%3A%2F%2Fing.dk%2Fartikel%2F118380-tepco-vil-fjerne-braendselsstave-med-kaempekranerhttp://www.asahi.com/english/TKY201104130196.html

 

[rowmag]

HI folks, new poster here, in Japan.

There was an article in the morning paper (Asahi Shimbun) discussing the mystery of what happened to Reactor 4's building. As Astronuc and shogun338 quoted above, they have taken samples of water from the #4 SFP, and found isotope levels that indicate a limited amount of damage to the fuel rods, but that most of the rods must still be intact. There is not a high enough level of radioactive isotopes in the water to suggest the rods became uncovered and suffered zirconium cladding oxidation.

The mystery then, if this is true, is that there should not have been enough hydrogen generated to blow up the building. (Another mechanism mentioned for generating hydrogen, splitting of water by radiation, should not have generated enough to explain what happened.) In fact, it is suggested that the rod damage may be the result of debris falling into the pool after the explosion.

Looking back at some of the early news reports from a month ago, here is what was reported at the time:

On 3/14:
Water temperature in #4 SFP is measured to be 84 degrees.

On 3/15:
~6:00: An explosion sound was heard, at almost the same time as an explosion is heard coming from the Reactor 2 building. Damage to the 5th floor roof is observed.

9:38: Flames seen near recirculation pump (4th floor, NW area), by a worker making inspection rounds.
~11:00 No smoke seen, so fire is assumed to be out. (Actually, they report that "the fire was verified to have naturally extinguished itself." Later, as I recall from watching it live on TV at the time, they admit they didn't really look, just went by absence of smoke. So whether it really extinguished itself is left unknown.)

Radiation levels near Reactor 4 building are 100 mSv/h.

2 holes reported in side of building 4, each 8 meters on a side.
Picture taken that day shows roof clearly gone as well.

On 4/16:
5:45: Flames spotted again coming from Reactor 4 building
6:15: Flames verified to have gone out, by visual inspection from a high ground elevation.

It was noted at the time that the place the flames was observed was the recirculation pump, which has electrical equipment and lubricating oil. (Indicating speculation of an electrical fire and/or lubricant oil fire.)

Looking at the pictures from the 15th, they are from the north side, so the recirculation pump area may simply have been the only area that was visible from where people were standing at the time, and may not be a relevant clue.

Anyway, not sure what to make of it all, but thought I'd put some strands out there. Ignore if these have all been gone over. (I read through the thread, but admit to skimming in parts...)

 

[ceebs]

http://www3.nhk.or.jp/daily/english/15_22.html"

On Wednesday, TEPCO finished transferring some of the wastewater -- about 660 tons -- from the No.2 reactor tunnel to a condenser in a turbine building.

It says the water level in the tunnel dropped 8 centimeters after the transfer, but had returned to its previous level by Friday morning.

Now that's about 40 hours, according to their figures quoted higher up the page they TEPCO are pumping 7 tons per hour into reactor 2, which makes 280 tons of water, not even half the amount needed to replace the water level in the tunnel. in fact you'd need more water than the amount inserted into reactors 2 and 3 put together.

 

[MiceAndMen]

Tks-tsk-tsk.

Is it possible that the replacement process was in 'dry' phase in the time of the earthquake?

What if the pool gates were broken during the EQ and the pool water flooded the empty RPV - leave the pool with only a limited amount of water, barely covering the rods? That could explain the hydrogen buildup. And this flow can explain the weird thermal image too (the water injected to the pool is still flowing to the RPV and beyond).

I was just reporting what might have been going on at Unit 4 at the time of the EQ. Some people were questioning why it was taking so long for a refueling op, and NISA is telling us it was not in the midst of a normal refueling op.

As for failure scenarios that explain what the thermal images show... I dunno. My working theories about the sequence of events that led to the explosions at units 3 and 4 are in flux at the moment. Damage to the SFPs and their contents is certainly on the menu. Many here think the floodgates/channel/chute between the SFP and the area above the PCV constitutes a "weak spot" of some sort. I'm not convinced about that.

 

[rowmag]

Reporting from Futaba, Japan—
The radiation gauge beeped, signaling that isotopes were in the atmosphere.
[...]
Miles past a police checkpoint, we finally saw it. In Japanese and English, a large blue sign. Fukushima, the place where no one else in the world wanted to be.http://www.latimes.com/news/nationworld/world/la-fg-japan-fukushima-20110415,0,271475.story

What a pointless article...

 

[hch71]

yet another long time follower first time poster...

WNN - Most fuel in Fukushima 4 pool undamaged
http://www.world-nuclear-news.org/RS-Most_fuel_in_Fukushima_4_pool_undamaged-1404117.html
14 April 2011

under the assumption that there might be a leak at the bottom of the SFP#4, how clear are the conclusions from this sample measurment? i presume the probe is taken from the top layer where water is constantly added?

actually, is it known how much water was added to SFP#4? this should allow to estimate whether there is some unaccounted loss by now - we know the approximate evaporation rate and we know that there is water in the pool.

final question: as far as i could gather fuel assemblies are a rather porous structure. so is it at all possible that a crack in the bottom of the SFP gets mostly plugged by fuel assemblies? someone suggested here that there might have been a crack in the bottom of the SFP that got clogged with fuel assemblies, which could possibly explain an explosion in the lower part of the building.

 

[Dmytry]

'most fuel' really means nothing. Most fuel in that SFP is old. But there is entire reactor core worth of fresh fuel and entire reactor core worth of slightly old fuel. The fresh spent and 'unspent' could of totally burnt down (more than entire reactor worth of fuel), yet most fuel would be undamaged. Looks like they are just trying to spin anything in the positive light.

 

[tsutsuji]

Assessment of the 7 April aftershock at the Onagawa nuclear plant :

a seismometer at the building housing the No. 1 reactor registered a quake acceleration of 476.3 gal vertically, against the 451 gal assumed for the facility.
http://search.japantimes.co.jp/cgi-bin/nn20110415n1.html

 

[clancy688]

I guess that will be the third or fourth time I'm asking this...

In hope that I'll get an answer this time:

http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/plot-un1-full.png

In this plot (provided by Stolfi, thx) you can see that the pressure in the core of Unit 1 is rising and rising. Furthermore, a couple of days ago the drywell radiation sensor topped 100 Sv/h and then went AWOL.

I have no idea of reactor physics, so I'm asking you: What could that imply? What's the meaning of this data? Recriticality? Especially with these heightened I-131 measurements in the trenches of Unit 1 and 2? Or nothing?

 

[timeasterday]

I guess that will be the third or fourth time I'm asking this...

In hope that I'll get an answer this time:

http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/plot-un1-full.png

In this plot (provided by Stolfi, thx) you can see that the pressure in the core of Unit 1 is rising and rising. Furthermore, a couple of days ago the drywell radiation sensor topped 100 Sv/h and then went AWOL.

I have no idea of reactor physics, so I'm asking you: What could that imply? What's the meaning of this data? Recriticality? Especially with these heightened I-131 measurements in the trenches of Unit 1 and 2? Or nothing?

I've been wondering about that too. Lately they indicate that the nitrogen injection is causing the pressure increase. I don't remember exactly when the nitrogen injection started but I seem to remember the pressure rising well before that.

EDIT: from the 4/7 IAEA report: "Instrumentation "B" for Reactor Pressure indicates that the pressure in the RPV is increasing and instrumentation "A" indicates that it has stabilized. NISA has indicated that some instruments in the reactor vessel may not be working properly. "

 

[TCups]

RE: BUILDING 4, CASK TRANSFER POOL & FUEL ROD ASSEMBLY LOCATION(S), AND CURRENT THERMAL IMAGES

A second diagram found here, source: PowerPoint Presentation - "Anatomy of a Tragedy: Fukushima Dai-lchi March 2011", downloaded from UCS website, here:

http://www.ucsusa.org/nuclear_power/nuclear_power_risk/safety/disaster-at-fukushima-anatomy.html

. . . this diagram:
http://i306.photobucket.com/albums/nn270/tcups/Picture3-1.png

and the previous schematic:
http://i306.photobucket.com/albums/nn270/tcups/Picture63.png

would both seem to confirm the location of a separate, gated "Cask Transfer Pool" (CTP) in addition to the larger Spent Fuel Pool (SFP) in the Mark 1 BWR design. Also, evidence suggests a cask and its contents were at some stage of the process of transfer at the time of the earthquake and tsunami which followed. While it is impossible to confirm that fuel assemblies in transfer may have been in the CTP, the possibility cannot be dismissed.

Additional diagrams of the relationship of the SFP, Reactor Vessel (RPV) and Equipment Pool (apparently AKA "Dryer/Separator Pit") are sourced here:
http://bigdustup.blogspot.com/2011/03/fukushima-daiichi-reactor-number-3-and.html

http://i306.photobucket.com/albums/nn270/tcups/tumblr_libecrgLyW1qbnrqd.jpg

http://i306.photobucket.com/albums/nn270/tcups/tumblr_libf94F96b1qbnrqd.jpg

And Reuters photo shows that the visible external damage to the south face of Bldg 4 is both above and to the west of the location of the SFP

http://i306.photobucket.com/albums/nn270/tcups/inreuterscom.jpg

http://i306.photobucket.com/albums/nn270/tcups/reactor4-water.jpg

It is also known and documented and posted here, some weeks back that the pneumatic seals for the transfer gates were electric-powered and that failure of the seals results in significant leakage from the SFP to the upper primary containment of the RPV, which could explain the presence of warm water in the region of the open primary containment of Unit 4 on current thermal images:

http://i306.photobucket.com/albums/nn270/tcups/Picture1-5.png

The apparent difference in temperature (warmer in the primary containment) could be explained, at least in part, by 1) active addition of fresh, cool water to the surface of SFP4, and 2) superimposition of the fuel handling machine (FHM) and portions of the larger overhead crane over SFP4 in the aerial thermal photography. Also, note that the observed position of the FHM and overhead crane after the accident would have been consistent with the process of ongoing cask transfer operations immediately before the accident.

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

I believe it is certainly possible that an "event" may have occurred in the CTP4, that an explosion originating in CTP4 may be a better fit for the external damage, at least on the south face of Bldg 4, and that damage to or loss of seal integrity of the gate(s) of SFP4 between SFP4 and the CTP or the primary containment, or both could explain any ongoing leakage from SFP4 as well as accumulation of warm water in the primary containment.

PS: What is a "Fuel Preparation Machine" and who is in the "Doghouse" (see two diagrams above)

http://i306.photobucket.com/albums/nn270/tcups/tumblr_libf94F96b1qbnrqd.jpg

http://i306.photobucket.com/albums/nn270/tcups/Picture3-1.png

 

[biffvernon]

http://www.youtube.com/watch?v=gWzPU5fgThA&feature=youtu.be

Video of fire engine almost getting its water to the right place.

 

[AtomicWombat]

in Tokaimura Criticality Accident
http://www.world-nuclear.org/info/inf37.html
The criticality continued intermittently for about 20 hours. It appears that as the solution boiled vigorously, voids formed and criticality ceased, but as it cooled and voids disappeared, the reaction resumed. The reaction was stopped when cooling water surrounding the precipitation tank was drained away, since this water provided a neutron reflector. Boric acid solution (neutron absorber) was finally was added to the tank to ensure that the contents remained subcritical. These operations exposed 27 workers to some radioactivity. The next task was to install shielding to protect people outside the building from gamma radiation from the fission products in the tank. Neutron radiation had ceased.

"mumble...mumble"

Now imagine this with many meters of water column on top of it, preventing quick expansion of bubbles. Then imagine this in a huge volume, where rapid formation of bubbles on one side compresses bubbles on another side.
Other very interesting reading (article from 2007):
http://search.japantimes.co.jp/cgi-bin/nn20070323a3.html

Something that may be required to sustain a criticality is the presence of water. Assuming the control rods are damaged or have melted away, water moderates thermal neutrons and keeps fission occurring.

If the water boils away then the moderating effect stops, the fission stops, but the fuel rods, which are no longer cooled, start to heat. Water is added back to stop the rods overheating, but this then restarts the nuclear reaction. It is possible this sort of vicious circle may have occurred.

I'm curious whether borated water is able to completely offset the moderating effect of water.

 

[AtomicWombat]

Something big, hot - and mobile, that's for sure. The other thermal images shows no hotspots at the same point.

But: there are earlier aerial photos (I could not find the date, but here is a link: http://cryptome.org/eyeball/daiichi-npp/pict6.jpg), showing a broken pipe of the venting system and some debris near the hotspot: and on later images the same pipe is near the reactor building and the area is clean from debris.

So: I think the thermal image shows the truck which cleaned up the area.

Or bulldozer. Remember they had to bulldoze over fuel elements (debris?) between units 3 & 4 to allow firetrucks access. I suspect they briefly set up one of those concrete pump type things near that site to top up SFP 3. The heat in that pool is so low that it would only need topping up occasionally.

 

[Jorge Stolfi]

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

Opps! Sorry. that was a typo --- should have been 1030 Sv/h. Thanks! Fixed now.

 

[AtomicWombat]

Because nothing says everything is under control like the government doing a runner

http://www.novinite.com/view_news.php?id=127294"

Not sure what to say...

 

[WhoWee]

http://www.youtube.com/watch?v=gWzPU5fgThA&feature=youtu.be

Video of fire engine almost getting its water to the right place.

At approx 1:38 - what was flying overhead - repeated also?

 

[Rive]

RE: BUILDING 4, CASK TRANSFER POOL & FUEL ROD ASSEMBLY LOCATION(S), AND CURRENT THERMAL IMAGES

AFAIK fuel casks are used only when the fuel is ready for dry storage / has no dangerous heat output which can damage the fuel or the assembly. That's why the SFPs exists.

 

[AtomicWombat]

Did Stotin's configuration really get supercritical? As I understand, it would be hard to tell the difference between (k slighty below 1) and (k slightly above 1) for a very short time. In both cases the rate of fission would be very high. The difference is that in the second case the radiation would have increased exponentially if the assembly had not been undone; whereas in the first case the reaction would remain at a high but constant level indefinitely.

There's no question Stolin's configuration was supercritical. It could hardly be mathematically anything else:
http://en.wikipedia.org/wiki/Demon_core"
It seems that criticality incidents can have quite different time courses. With nuclear weapons criticality accidents appear prompt and self-limiting - the burst of energy restores sub-criticality. With lesser enriched material, it appears criticality can sometimes be sustained without those nearby noticing. Astronuc can probably give more reliable information.
http://en.wikipedia.org/wiki/Criticality_accident"

What I was thinking is the fllowing. I am assuming a dense rack design like that in the Czech re-acking paper, with walls of boral (boron carbide powder clad in aluminum) sandwiched in steel around each assembly.

0. Pool cooling pumps stop.
1. Pool water boils off, and the assemblies become partly dry.
2. The dry parts of the assemblies slowly get hotter by decay heat (k still <<1).
3 When the temperature reaches 660C, the aluminum in the boral melts.
4. The boron carbide powder shifts inside the steel walls, creating "neutron holes".
5. As neutron absorption decreases, the k factor starts to increase.
6. Larger k means increased heat production that means more boral melting and larger k.
8. As k approaches 1 the temperature of the fuel slugs shoots up to >>1800C.
9. A few seconds later, fuel tubes and steel jackets melt.
10. The molten mass falls onto the remaining water causing a steam explosion.
11. The explosion blows away the overheated fuel and stops the chain reaction.
Note that this scenario does not require k>1, but only k large enough for fission to cause fast heating of (some part of) the fuel, from ~700C to over 1800C.

Does it make sense?

It seems to me that if the geometry of the fuel cells is maintained, even without the boron, criticality would require return of the water as a moderator. I may be wrong.

 

[Jorge Stolfi]

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.

Er, just because something is against the rules, it does not mean that it wasn't done.

They may have assumed that very old and/or fresh fuel did not need 20ft water. After all, while the fuel is in transit between reactor and SFP, I believe it is carried above the top of the spent fuel in the SFP, and therefore under less than 12 feet of water. Is this correct?

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).

I understand that re-racking had already been done at Fukushima Daiichi. If the new packing is like that described in the re-racking articles previously posted, then the capacity cannot be increased without going to a second layer.

Someone quoted a capacity for 1444 fuel assemblies for the SFP in each unit. Is that before or after re-racking? The same sources said that 1535 were currently stored in #4. How can we square these numbers?

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.

But the neutron absorbers used in the SFP (unlike those used in the reactor core) will melt or decompose before the temperature reaches 670C, which seems to be well below the threshold for significant zirconium+steam=H2.

 

[Jorge Stolfi]

It seems to me that if the geometry of the fuel cells is maintained, even without the boron, criticality would require return of the water as a moderator. I may be wrong.

Well, I assume that the boron baffles are there because they are needed.

Interleaving fuel and moderator seems to be the most efficient design, but if I understand correctly it is not necessary. Suppose the fuel gets compacted into a solid mass surrounded by water, away from absorbers. Most of the neutrons that escape the mass will scatter around the surrounding water and eventually find their way back to the mass. In other words the water should behave like a diffusing "mirror", just as a cloud cover eventually "reflects" most of the incident sunlight back to space.

If the fuel is in layer at the bottom of the pool, with water on one side only,
then almost 50% of the neutrons that are produced in it should be scattered back to it.

 

[Dmytry]

I've been wondering about that too. Lately they indicate that the nitrogen injection is causing the pressure increase. I don't remember exactly when the nitrogen injection started but I seem to remember the pressure rising well before that.

EDIT: from the 4/7 IAEA report: "Instrumentation "B" for Reactor Pressure indicates that the pressure in the RPV is increasing and instrumentation "A" indicates that it has stabilized. NISA has indicated that some instruments in the reactor vessel may not be working properly. "

it would imply that they cannot relieve the pressure (they would want to keep pressure low as the vessel may have been damaged). The re-criticality, I would not expect it to run nicely, i'd expect some oscillations.

Void coefficient is bad, folks. Positive void coefficient (as in RBMK) is obviously bad, negative void coefficient (as in BWR) is less obviously bad. Sudden power increase on one side collapses bubbles on other side (given the thermal lag between rods and water), resulting in larger power excursion that can collapse bubbles elsewhere even better, etc.

Then the BWR core. Ever thought about that: the fuel has to react on the top, where there's a lot of voids? Where the water density is a lot lower.
BWR control rods are inserted upwards, so that the top which has voids and less moderation also has less neutron absorption. The RBMK (Chernobyl's reactor) control rods were graphite tipped on the bottom so that the fuel on the bottom (where there's less voids) could react. Those are the kind of very dangerous design decisions that you have to do to increase fuel burnup when you have coolant that is also the moderator boiling inside the reactor, and when the void coefficient is not zero.
Entire BWR idea is on par with RBMK in terms of cutting the safety to make things cheaper, increase fuel burnup, etc. Boiling water and nonzero void coefficient = no good. In BWR, the fuel obviously has to be able to react on top where there's a lot of voids. A lot of excess reactivity is required. PWR is a much more reasonable design, but also more expensive.

When it comes to safety, the burden of proof is not on me that something scary can happen, but on designer that it can't happen.

Safety design (dangerous until proven safe) is an opposite of legal process (safe until proven dangerous), but everyone tends to debate in the 'safe until proven dangerous' way.

 

[AtomicWombat]

Tks-tsk-tsk.

Is it possible that the replacement process was in 'dry' phase in the time of the earthquake?

What if the pool gates were broken during the EQ and the pool water flooded the empty RPV - leave the pool with only a limited amount of water, barely covering the rods? That could explain the hydrogen buildup. And this flow can explain the weird thermal image too (the water injected to the pool is still flowing to the RPV and beyond).

Which reminds me. ON the UCS site they mention a failure mode whereby the seals around the gates on the SFP fail when there is a power loss. If the primary containment and rector were "dry" due to the shroud replacment the SFP water would leak there when the power failed.
http://allthingsnuclear.org/post/3964225685/possible-source-of-leaks-at-spent-fuel-pools-at"

 

[AtomicWombat]

I guess that will be the third or fourth time I'm asking this...

In hope that I'll get an answer this time:

http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/plot-un1-full.png

In this plot (provided by Stolfi, thx) you can see that the pressure in the core of Unit 1 is rising and rising. Furthermore, a couple of days ago the drywell radiation sensor topped 100 Sv/h and then went AWOL.

I have no idea of reactor physics, so I'm asking you: What could that imply? What's the meaning of this data? Recriticality? Especially with these heightened I-131 measurements in the trenches of Unit 1 and 2? Or nothing?

The radiation measurment rose after the start of N2 injection. N2 injection will do 2 things 1) stir up the contents of wherever it's being injected; and 2) create void space.

Void space in the water will reduce shielding from the radiation sources. Stirring could spread the radiation around.

All this is speculation. I don't even know whether the N2 is injected into the RPV or dry-well.

 

[Joe Neubarth]

There's no question Stolin's configuration was supercritical. It could hardly be mathematically anything else:
http://en.wikipedia.org/wiki/Demon_core"
It seems that criticality incidents can have quite different time courses. With nuclear weapons criticality accidents appear prompt and self-limiting - the burst of energy restores sub-criticality. With lesser enriched material, it appears criticality can sometimes be sustained without those nearby noticing. Astronuc can probably give more reliable information.
http://en.wikipedia.org/wiki/Criticality_accident"

It seems to me that if the geometry of the fuel cells is maintained, even without the boron, criticality would require return of the water as a moderator. I may be wrong.

You are quite right. Thank heavens when they frantically tried to refill the cement reservoirs alongside the Reactor, most of the fuel assemblies were gone. Basically, it was a self correcting transient of power.

 

[AtomicWombat]

Well, I assume that the boron baffles are there because they are needed.

The water is there for colloing and as a radiation shield. The boron baffles are there to avoid criticality in water.

Interleaving fuel and moderator seems to be the most efficient design, but if I understand correctly it is not necessary. Suppose the fuel gets compacted into a solid mass surrounded by water, away from absorbers. Most of the neutrons that escape the mass will scatter around the surrounding water and eventually find their way back to the mass. In other words the water should behave like a diffusing "mirror", just as a cloud cover eventually "reflects" most of the incident sunlight back to space.

If the fuel is in layer at the bottom of the pool, with water on one side only,
then almost 50% of the neutrons that are produced in it should be scattered back to it.

I agree. If the fuel ended up in a solid mass surrounded by water I suspect it would be hard to avoid at least transcient criticality events.

I understood from your original proposal that the criticality was occurring above the water layer where the boron was lost. But if the water was lost here as well I don't see how criticality could occur.

 

[TCups]

AFAIK fuel casks are used only when the fuel is ready for dry storage / has no dangerous heat output which can damage the fuel or the assembly. That's why the SFPs exists.

Rive - sure, understand. There was an excellent post earlier about dry cask fuel storage. But I was not sure that is the same thing a a fuel transfer cask. It may be. But I do know at the Fukushima Daiichi facility, a 7th spent fuel storage pool for the older, colder fuel exists and that it contains a substantial portion of the 40 year old spent fuel on site. As it it still stored in a pool, under water, it logically follows that there is some mechanism in place to transfer rods from one pool to another, and that in principle, though older and colder, those might still be hot enough to require immersion cooling, except for the short-term transfer process. This, however is an educated guess. Also, there must be a process for transferring into the SFP and subsequently into the reactor core, new fuel rods and fuel rod assemblies

I had not considered the existence of a fuel preparation machine in the SFP4 or how it might be used, but perhaps its size and placement in the diagram . . .

http://i306.photobucket.com/albums/nn270/tcups/FuelPrepMachine.jpg

. . . suggest one. Again, this is an educated guess on my part. Is the specific purpose of the fuel preparation machine to take new (ie, not highly radioactive individual fuel rods) and insert them into fuel rod assemblies while under water in the SFP, and conversely perhaps, to allow removal of individual fuel rods, perhaps damaged fuel rods, from a fuel rod assembly and thus repair the fuel rod assembly?

If so, then it would appear that the FHM would need to be all the way to the far edge of the SFP to access the fuel preparation machine, and that there may even be guide channels to allow new fuel rods to be lowered into the individual fuel rod assemblies from the service floor level at the south end of the SFP.

That is a lot to infer from a very basic diagram, but given the amount and quality of information forthcoming from TEPCO, inferential analysis of the photographs and diagrams is, unfortunately, most of what is available.

 

[Krikkosnack]

I'm trying to analyze more in detail this video to get some information about what flew in the sky ... mainly on its mass. Maybe someone can help me too.
http://energheia.bambooz.info/index.php?option=com_content&view=article&id=170%3Areactor-3-explosion-enhanced-view&catid=60%3Avideo&Itemid=85&lang=it

Some notes:
Take a look on the right side of the reactor 3 and follow the hard material falling down near the tower

1) at the time 0:16 there is an explosion of flammable material that expands almost sideways
2) immediately something shoot oneway directed upward the material as a recoil of the weapon (very strange, like a cannonball!)
3) about the time 0:21 comes to its maximum height
4) at the time 0:32 comes around on the ground near the tower under the law of gravitation http://en.wikipedia.org/wiki/Equations_for_a_falling_body

 

[biffvernon]

At approx 1:38 - what was flying overhead - repeated also?

I wondered that but then thought it was a bit of muck on the vehicle window; it reappears again a little later on.

 

[ceebs]

I wondered that but then thought it was a bit of muck on the vehicle window; it reappears again a little later on.

I was thinking Insect myself

 

[dinorobo]

There's new footage of inside reactor 4's building in this news report. You can see them taking a sample from the spent fuel storage pool:

http://news.tbs.co.jp/20110415/newseye/tbs_newseye4701752.html

I don't know if you can watch it outside of Japan though

 

[Rive]

I don't even know whether the N2 is injected into the RPV or dry-well.

To the drywell. One of my forum-mates made a calculation based on the data provided here: http://www3.nhk.or.jp/daily/english/07_37.html

The company says that after injecting 413 cubic meters of nitrogen gas until 5 PM on Thursday, the pressure reading inside the vessel was 1.76, up 0.2 from before the injection started.

For the volume of the drywell he got 3634.5m³: the 'factory data' of an MK1 containment is 3700m³ (don't ask for calculation details, it wasn't my work).

- As far as I know the drywell of unit 1 were injected with considerable amount of seawater.
- the JAIF reports refers to water injection to drywell of unit 1 as ' to be confirmed'.

Where is the water? Or is the calculation wrong?

...educated guess...

Mine was an kind of educated guess too.

About http://www.freepatentsonline.com/y2010/0232564.html" .

Side effect of googling that machine was this: http://www.aws.org/w/a/wj/2003/09/048/index.html

 

[Rive]

http://www.youtube.com/watch?v=gWzPU5fgThA&feature=youtu.be

Video of fire engine almost getting its water to the right place.

I've tried to 'deshake' that video with VirtualDub and Deshake filter but I have not enough bandwidth to upload the result...

 

[tsutsuji]

A new batch of pictures (some of them taken on April 14, 2011) is available at http://cryptome.org/eyeball/daiichi-npp8/daiichi-photos8.htm

 

[biggerten]

Because nothing says everything is under control like the government doing a runner

http://www.novinite.com/view_news.php?id=127294"

ITAR-TASS is not quite the AP. And furthermore, I'd be surprised if the US Government doesn't have plans in place to relocate DC if trouble arises, I'd bet most developed countries do.

 

[Borek]

I'm trying to analyze more in detail this video to get some information about what flew in the sky ... mainly on its mass. Maybe someone can help me too.

You can't calculate the mass from known trajectory. Every object having the same initial speed will behave identically (well, ignoring air resistance, but in the case of high density objects and not too high speeds that's quite good approximation).

 

[tsutsuji]

ITAR-TASS is not quite the AP. And furthermore, I be surprised if the US Government doesn't have plans in place to relocate DC if trouble arises, I'd bet most developed countries do.

The plans for relocating the capital of Japan is an old story. The timeline at http://www.mlit.go.jp/kokudokeikaku/iten/English/background/index.html starts in the 1950s, but in old times, emperors changed the location of their capital quite often : http://en.wikipedia.org/wiki/Capital_of_Japan

It is obvious that the occurrence of a major earthquake or any other large-scale disaster in the Tokyo Metropolitan area, where political and economic activities and population are concentrated, would dramatically impact not only Japan’s economy but also the global economy, as well as destroy lifeline services essential to people’s lives, disrupt the transportation network such as Shinkansen (bullet trains) and airports, and cause deterioration in public security. In preparation for such circumstances, it is becoming increasingly important to relocate the Diet and other organizations—the core functions of our country—outside the Tokyo Metropolitan area as a measure of risk management.

December 22, 2004
Inter-party Conference Committee of Both Houses on the Relocation of the Diet and Other Organizations
http://www.mlit.go.jp/kokudokeikaku/iten/English/reports/report_1222_2004.html

 

[clancy688]

The radiation measurment rose after the start of N2 injection. N2 injection will do 2 things 1) stir up the contents of wherever it's being injected; and 2) create void space.

Okay, thx.

There's new footage of inside reactor 4's building in this news report. You can see them taking a sample from the spent fuel storage pool:

http://news.tbs.co.jp/20110415/newse...ye4701752.html

I don't know if you can watch it outside of Japan though

Interesting video. Doesn't look as if there's much damage inside of Unit 4... oO

 

[triumph61]

The same Video but better . Sorry for my English