Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[TCups]

The limited resolution of the available photos do not make it feasible to 'find' any individual fuel rods.

Perhaps you mean looking for fuel rod _assemblies_.

If I have it right, assemblies would in a BWR -- complete with cladding -- be rather large square sticks, about 4.5 meters long and about 0.14 meter wide -- and that's within the resolution of at least some of these photos. I am not an expert, but I would expect these assembles sticks to be found with a brownish color (unless oxidized, when I think they might form a whitish oxide crust)

For size comparisons some of the roof material scattered everywhere comes handy.

The roof 'strips' shown here are from unit 4, and are about 7 m long and about 0.7 m wide. Roof strips from unit 1 and 3 are about the same length, but only half as wide. So good hunting.

Scattered fuel rod assemblies out of the spent fuel pool would heat rapidly, probably to temperatures around 200º-300ºC These would be the hottest things on any of the thermal images updated every other day. Instead, the hottest things on thermal imagery are in the 20º-60ºC range, as I recall, and correspond to either the contents of the various SFP's or areas of venting steam, smoke. I think the thermal imagery effectively excludes the possibility of any fuel rod assemblies laying around on the ground.

Also, note that the on going thermal imagery shows persistent heat in the SFPs of Bldg 3 & 4. The only logical explanation of a source for that heat signature at this time is that both SFPs retain all or most of their fuel rods, now immersed in water.
As before, Unit 4 is an enigma. We can probably exclude the possibility of any significant "blow out" of the bottom of the SFP4, given that it seems to be holding water. It is possible that something in the accessory "cask transfer" pool may have exploded, possibly venting to the lower building and to the outside south wall, however. The visible external damage at the level of the SFP4 is to the west of the main SFP.

But even this hypothesis is doubtful as, 1) it doesn't necessarily put hydrogen gas in the lower containment, and 2) only partially matches the external damage to the building.

It is conceivable that the force of the blast and shock wave originating from Bldg 3 may have caused internal structural damage to Bldg 4 not visible on the available exterior views of Bldg 4 immediately following the explosion, but again, this alone would not explain all of the apparent visible damage.

There is insufficient information regarding any other volatile or explosive substances in the lower portions of Bldg 4 (ie, oil for pumps, etc.) to allow any evidence-based speculation on that possible mechanism.

As for some duct or vent connecting the lower floors of Bldg 3 & 4 being the origin of hydrogen in lower Bldg 4, one is still left with 1) how did the hydrogen get to the lower Bldg 4 to vent, and 2) if there were a connection, then why didn't the explosion(s) at 3 also result in near simultaneous explosion at Bldg 4?

This returns as always to the only known initial source for the potential energy released in the explosion of Bldg 4 -- fuel rod assemblies

There are only 3 likely possibilities for the location(s) of fuel rod assemblies in Bldg 4, IMO:
1) SFP4 or,
2) adjacent cask transfer pool, or
3) a transportation cask containing fuel rods somewhere in the path from the access tunnel to the cask transfer pool

An unlikely 4th might be:
4) fuel rod assemblies reloaded into the core.
I am not sure that thermal images support the 4th possibility, however. And they are not as far as I know pumping water into Unit 4 primary containment or RPV.

 

[Astronuc]

Thanks. I was wondering what was meant by flooding the reactors with water. I thought that's essentially what they were already doing.

That is the intent. However, if there are leaks in the piping systems connected to the reactor pressure vessel (RPV), then the water level achieved may not cover the core, which is the objective. In such an event, they would be expected to flood the drywell in order to ensure that the water level is maintained in the RPV. However, if there are leaks in containment, then they may not be able to flood containment to the elevation required to cover the core. Then there is the matter of the evaporation of the water, which must be made up.

According to the available data, the water level measurements indicate that the core is not completely covered. But then, it could be that the instruments have been damaged (?).

 

[Astronuc]

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

Radionuclide analysis of water from the used fuel pool of Fukushima Daiichi unit 4 suggests that some of the 1535 fuel assemblies stored there may have been damaged, but the majority are intact.

 

[javadave]

That is the intent. However, if there are leaks in the piping systems connected to the reactor pressure vessel (RPV), then the water level achieved may not cover the core, which is the objective. In such an event, they would be expected to flood the drywell in order to ensure that the water level is maintained in the RPV. However, if there are leaks in containment, then they may not be able to flood containment to the elevation required to cover the core. Then there is the matter of the evaporation of the water, which must be made up.

According to the available data, the water level measurements indicate that the core is not completely covered. But then, it could be that the instruments have been damaged (?).

Thanks for the clarification!

 

[MadderDoc]

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

Indeed it does, and I've thought of that too.

The zoomout (see attachment) precludes that we are just looking at bits of a passing cloud. Then comes the illusion that the 'secondary' smoke fan could originate from further away, to the northwest there seemingly coming out of the forest on the hillside, but that _is_ just an illusion, the smoke-like tint there is not smoke, it is grey stuff on the ground thrown there by the explosion a few minutes earlier.

So, it certainly looks as if there is smoke coming from unit 4. I'll offer an alternative explanation in that the steam explosion at unit 3 threw out several large and steaming, hot objects. One of them may have landed in the yard between unit 3 and 4, and the smoke still coming from that could be what gives an illusion of smoke coming from unit 4.

 

[TCups]

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

Which, perhaps surprisingly, is consistent with the imagery, IMO.

BTW, the detailed technical drawing of the reactor building referenced just above doesn't fit the images of Unit's 3, 4. The Elevator is in the wrong location (west side, north end of the structure instead of the west side, south end of the structure.

I just love to color by number. The green overlay is the elevator shaft and access tunnel for the fuel casks. Shouldn't it be adjacent to the SFP, not the equipment pool?

Am I missing something -- is there another elevator shown and the shaft for the fuel cask transfer is not shown?

Ah Ha! Yes, The elevator shown is not the fuel cask shaft. My mistake. Hmmmmm. . .

 

[MJRacer]

It might be helpful to review this document: (http://cryptome.org/0003/daiichi-assess.pdf)

Selected quotes:

Unit 1:

reactor water level is unknown.
The volume of sea water injected to cool the core has left enough salt to fill the lower plenum to the core plate.
Recirculation pump seals have likely failed.
The fuel in this pool is all over 12 years old and very little heat input (<0.1 MW)
There is likely no water level inside the core barrel. Natural circulation believed impeded by core damage. It is difficult to determine how much cooling is getting to the fuel. Vessel temperature readings are likely metal temperature which lags actual conditions.
Spray water on steam plumes and planned containment vents for scrubbing effect.

Unit 2:

Same as 1 above, except for:
No comment on age of fuel in pool

the amount of salt build-up appears to be less than U-l
Based on the reports of RPV level at one half core height, the reactor vessel water level is believed to be even with the level of the recirculation pump seals, implying the seals have failed.
Low level release path: fuel damaged, reactor coolant system potentially breached at recirculation pump seals, primary containment damaged resulting in low level release.
There may be some scrubbing of the release if the release path is through the torus and water level is maintained in the torus.

Unit 3:

Similar to 2 above, except for:

fuel may have been ejected from the pool
Unit 3 turbine building basement has flooded. Samples of water indicate some RCS fluid is present...the likely source is the fire water spray onto the reactor building.

Unit 4:

Given the amount of decay heat in the fuel in the pool, it is likely that in the days immediately following the accident, the fuel was partially uncovered. The lack of cooling resulted in zirc water reaction and a release of hydrogen. The hydrogen exploded and damaged secondary containment. The zirc water reaction could have continued, resulting in a major source term release.
Fuel particulates may have been ejected from the pool

Engineering comment: Apparent failure of recirc pump seals implies inability to cover core no matter how much water pumped into RPV. For the experts: is this an unanticipated failure point? would adding another valve in the suction line be a possible solution?

 

[Astronuc]

This may help to understand the issues of hydrogen in a LWR plant.
Light Water Reactor Hydrogen Manual Use save target as, since the pdf is 14 MB. It also might be a slow server.

 

[|Fred]

yep elevator is human service, as you can see remains on the roof tops of unit 1.
I'm not convince the location is the same (N/W) for unit 4, I think I could be S/W.

Also I'm not convince that there is a temporary cast pool, sofar I'm voting for cast operation handled in the main SFP.

I believe the is an un accounted for crane structure above the utility pool adjacent to the reactor "opening" on unit 4

what is the truck size whit object at the bottom south of the west wall..

there is a fair amount of roof structure debrit on the south part of unit 4 where do they come from, the south part of the roof does not seems missing that many parts.

 

[MadderDoc]

<..> I think the thermal imagery effectively excludes the possibility of any fuel rod assemblies laying around on the ground.

The pixel-resolution of the thermal imagery I've seen spares about 0.2 m/pixel on the ground. With dispersion and such the real resolution must be worse, I'd reckon no better than 0.4 m/pixel and I think I am being generous.

Anyhow, what is that thing we see glowing -- all alone by itself :-) -- between unit 3 and 4, on the imagery from March 25th (attached thermal image and scale)?

 

[TCups]

yep elevator is human service, as you can see remains on the roof tops of unit 1.
I'm not convince the location is the same (N/W) for unit 4, I think I could be S/W.

Also I'm not convince that there is a temporary cast pool, sofar I'm voting for cast operation handled in the main SFP.

I believe the is an un accounted for crane structure above the utility pool adjacent to the reactor "opening" on unit 4

what is the truck size whit object at the bottom south of the west wall..

there is a fair amount of roof structure debrit on the south part of unit 4 where do they come from, the south part of the roof does not seems missing that many parts.

|Fred -
I believe it is probably some sort of insulation material. I also seem to recall a major HVAC ductwork structure overhead at the SFP end of the service floor on one of the priors.

Something just doesn't add up. If the fuel in SFP4 was only partially uncovered and most was not damaged, it just doesn't seem to match the degree of destruction seen in Bldg 4.

Here is another HVAC SWAG to think about . . .

Rather than the blast at Bldg 3 pushing anything through the connecting ductwork back into Unit 4, is it possible (perhaps even likely) that the blast at Bldg 3 would send significant pressure pulse of hot gas into the vent system, then up and out the vertical stack, in addition, of course, to the rest of the blast damage? That being the case, then, could a Venturi effect from the vertical stack shared by Units 3, 4 have created a sudden, transient negative pressure in the lower portions of Bldg 4 through the connecting vent system?

If so, then the a large volume of air is sucked out of the lower floors of Bldg 4, and a large volume of hydrogen gas that had accumulated above might be sucked downward into the building -- like smoke being pulled back into the bowl from one big puff off a giant briar pipe.

Pressure equalizes, hydrogen again rises, but it is now possible for it to accumulate in pockets along the ceilings of the lower floors. The partially exposed fuel in SFP4 continues to release more hydrogen, replenishing the fraction of hydrogen lost from the upper building. When, eventually, the hydrogen + oxygen mix explodes, then both the upper and lower floors are involved in the resulting blast.

(Talk about conspiracy theories . . .)

Could the pipe have held together long enough for that to happen? . . .

 

[Giordano]

The pixel-resolution of the thermal imagery I've seen spares about 0.2 m/pixel on the ground. With dispersion and such the real resolution must be worse, I'd reckon no better than 0.4 m/pixel and I think I am being generous.

Anyhow, what is that thing we see glowing -- all alone by itself :-) -- between unit 3 and 4, on the imagery from March 25th (attached thermal image and scale)?

Concrete truck?

 

[Dmytry]

|Fred -
I believe it is probably some sort of insulation material. I also seem to recall a major HVAC ductwork structure overhead at the SFP end of the service floor on one of the priors.

Something just doesn't add up. If the fuel in SFP4 was only partially uncovered and most was not damaged, it just doesn't seem to match the degree of destruction seen in Bldg 4.

Here is another HVAC SWAG to think about . . .

Rather than the blast at Bldg 3 pushing anything through the connecting ductwork back into Unit 4, is it possible (perhaps even likely) that the blast at Bldg 3 would send significant pressure pulse of hot gas into the vent system, then up and out the vertical stack, in addition, of course, to the rest of the blast damage? That being the case, then, could a Venturi effect from the vertical stack shared by Units 3, 4 have created a sudden, transient negative pressure in the lower portions of Bldg 4 through the connecting vent system?

If so, then the a large volume of air is sucked out of the lower floors of Bldg 4, and a large volume of hydrogen gas that had accumulated above might be sucked downward into the building -- like smoke being pulled back into the bowl from one big puff off a giant briar pipe.

Pressure equalizes, hydrogen again rises, but it is now possible for it to accumulate in pockets along the ceilings of the lower floors. The partially exposed fuel in SFP4 continues to release more hydrogen, replenishing the fraction of hydrogen lost from the upper building. When, eventually, the hydrogen + oxygen mix explodes, then both the upper and lower floors are involved in the resulting blast.

(Talk about conspiracy theories . . .)

Could the pipe have held together long enough for that to happen? . . .

I don't think that could happen. Firstly you need a specific geometry for venturi effect, secondarily, even if the pipe is at absolute vacuum, I do not believe it would suck in enough air. The air speed at the leak into vacuum (a hole in a space station) is ~= speed of sound. Just look at cross-section of pipe versus cross-section of entire upper floor to determine air inflow rate here. You're better off speculating that the wind blew through some hole and blew hydrogen down, except that doesn't make sense either because a hole would rather let hydrogen out.

 

[Dmytry]

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

 

[shogun338]

Storage tanks, pumps and monitoring equipment from Savannah River Site will be shipped from South Carolina to Japan to help in the battle to stabilize the damaged nuclear reactors at Fukushima.

Special
R-56 diagram: A high-tech, self-contained piece of equipment known as a “Radioactive Liquid Transport Assembly,” or LR-56, is being sent to Japan to help government officials who are monitoring contamination levels in the vicinity of the Fukushima nuclear power plant.
"This is what we can send them at this point to support their activities," said Jim Giusti, a U.S. Energy Department spokesman.
Equipment is also being gathered from other federal nuclear sites around the county, he said. Its transfer is being expedited as a "government to government diplomatic exchange" that will convey the gear directly to the Japanese government.
The primary items from SRS, identified by its management contractor, Savannah River Nuclear Solutions, include a "radioactive liquid transport assembly," a high-tech, self-contained, trailer housing a 1,000 gallon tank, pumps and a monitoring system. http://chronicle.augusta.com/latest-news/2011-04-14/srs-equipment-headed-japan?v=1302791614

 

[MadderDoc]

<..>
I believe the is an un accounted for crane structure above the utility pool adjacent to the reactor "opening" on unit 4

what is the truck size whit object at the bottom south of the west wall..

there is a fair amount of roof structure debrit on the south part of unit 4 where do they come from, the south part of the roof does not seems missing that many parts.

I also see a big green machine at the north end, over the utility pool.

I have no idea what the truck size object is. From looking at imagery from before the explosion I get the impression that it was there then, too. (see attached)

As to the debris at the south side, I believe it is a fair amount of the roof covering, on top of the concrete pillars and panels from the holes in the wall above (certainly mixed with assorted foobar, but quantitatively a small proportion)

 

[TCups]

I don't think that could happen. Firstly you need a specific geometry for venturi effect, secondarily, even if the pipe is at absolute vacuum, I do not believe it would suck in enough air. The air speed at the leak into vacuum (a hole in a space station) is ~= speed of sound. Just look at cross-section of pipe versus cross-section of entire upper floor to determine air inflow rate here. You're better off speculating that the wind blew through some hole and blew hydrogen down, except that doesn't make sense either because a hole would rather let hydrogen out.

OK, how about just the straightforward mechanism of a large blast with a vertical plume at Bldg 3 sucking the ground air upward and creating a large, but transient negative pressure region at "ground zero"? I am grasping here for some mechanism to explain a transient, negative pressure gradient in the lower portions of Bldg 4 . . .

 

[TCups]

Concrete truck?

No, the resolution is actually pretty good, but there is "blooming" of the heat signatures. The hot object is probably about 25ºC and looks to be about where debris have been cleared and a new "plate" of some sort is now on the ground, near the center of this photo.

In the thermal imagery, you can see the pipe, the tower, and actually, a fair amount of ground detail in the blue scale.

 

[MadderDoc]

Concrete truck?

I don't think so. The thermal image with this clear hotspot in the yard between unit 3 and 4 was taken somewhere between 6.34 am and 07.19 am on March 25th. It was a cold morning, as you can well see from the image, only about 2 deg. C. The location is right under some huge pipes, not exactly where one would think to place a concrete truck. Anyhow, Tepco states clearly that the concrete pump was not used at that time for doing anything to unit 3.

'Today's work for cooling the spent fuel pools
- At approximately 5:35 am, we started injecting seawater into the fuel
spent pool of Unit 3, using Fuel Pool Cooling and Filtering(clean up)
system (FPC) and finished at 4:05 pm.
- At around 2:35 pm, spraying to unit 4 by concrete pump track and
finished at around 5:30 pm.
- We are considering further spraying at other units and others subject
to the conditions of spent fuel pools.' (Tepco press release on the morning of March 25th)

 

[Most Curious]

That is the intent. However, if there are leaks in the piping systems connected to the reactor pressure vessel (RPV), then the water level achieved may not cover the core, which is the objective. In such an event, they would be expected to flood the drywell in order to ensure that the water level is maintained in the RPV. However, if there are leaks in containment, then they may not be able to flood containment to the elevation required to cover the core. Then there is the matter of the evaporation of the water, which must be made up.

According to the available data, the water level measurements indicate that the core is not completely covered. But then, it could be that the instruments have been damaged (?).

Could it also be that core no longer exists at that height? IOW, has 50+% of the core relocated lower in the RPV? (All three units.)

 

[Dmytry]

OK, how about just the straightforward mechanism of a large blast with a vertical plume at Bldg 3 sucking the ground air upward and creating a large, but transient negative pressure region at "ground zero"? I am grasping here for some mechanism to explain a transient, negative pressure gradient in the lower portions of Bldg 4 . . .

of bldg4 ... but not of bldg 2? Also, the explosion of #4 happened at different time.
I think at this point we sadly have to include some conspiracy/coverup in the explanation. But this also means we have no real data to make explanation from.

 

[Rive]

Concrete truck?

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.

 

[ceebs]

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

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

As powerful earthquakes continue to jolt Japan and radiation levels near Tokyo are rising, the Asian country's authorities are considering moving the capital to another city.

The most probable location for a new capital are Osaka and Nagoya, according to ITAR-TASS. Both cities are located near international airports.

The main conditions the new capital has to provide are a population over 50 000 and a sufficient capacity to accommodate the parliament, the government, the Emperor's residency and the foreign diplomatic missions.

 

[TCups]

The pixel-resolution of the thermal imagery I've seen spares about 0.2 m/pixel on the ground. With dispersion and such the real resolution must be worse, I'd reckon no better than 0.4 m/pixel and I think I am being generous.

Anyhow, what is that thing we see glowing -- all alone by itself :-) -- between unit 3 and 4, on the imagery from March 25th (attached thermal image and scale)?

? Are we talking about the same thermal image?

https://www.physicsforums.com/attachment.php?attachmentid=34365&d=1302808699

If so, then I don't see anything even near the size of a concrete truck between Bldg 3 and 4.

 

[TCups]

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.

Are you referring to this thermal image attached to the original post asking about the object between Bldg 3 and 4?

https://www.physicsforums.com/attachment.php?attachmentid=34365&d=1302808699

If so, something is way off. Nothing remotely resembling a truck is in this thermal image.

 

[Giordano]

Increasing ratio I-131/Cs-137 in "Sub-drainage of Unit 2" indicates criticality occurred somewhere in Unit 2?

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110408e6.pdf
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110414e18.pdf

 

[PietKuip]

Increasing ratio I-131/Cs-137 in "Sub-drainage of Unit 2" indicates criticality occurred somewhere in Unit 2?

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110408e6.pdf
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110414e18.pdf

And (Tepco has learned to make graphs!):
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110414e19.pdf

Increasing iodine in unit 2, decreasing (as it should) in the other drains.

 

[clancy688]

Increasing iodine in unit 2, decreasing (as it should) in the other drains.

As far as I can see, it's increasing in unit 1 as well... what's with the constantly rising core pressure and missing drywell radiation in Unit 1 which can be observed in Jorge's plot?

 

[Dmytry]

criticality - i do expect criticality. Someone posted report where they said the fuel above water line turned into small dust on the bottom of PV. It's 3 reactors with significant fraction of fuel on the bottom of pv, what are the chances that none will go critical? The dust fuel can get stirred into water and go critical in that form, if it is not critical as a lump. They better be borating the water, but as they don't have cooling loop, they'd run out of boron-10 fairly quick. Unless they cool it by evaporation, in which case they'd be venting into atmosphere a lot of stuff.

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

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

you got to be kidding me. That better be a joke.

 

[MadderDoc]

Concrete truck?

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.

I do believe the area had been cleared for passage already by March 24th. See 'before and after 'cleanup'' attached. The position of the hotspot is at the far foot of the exhaust tower.

On later thermal images, it appears that the pool has been regularly douched from a firetruck about at the time of thermography, and the truck blocks the view to the hotspot.

 

[Jorge Stolfi]

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.

I must be using the wrong terms. Presumably your k is the expected number of first-generation children of a single neutron introduced in the mass. If k > 1, as you say, the number of fissions increases exponentially in time, and we get a standard nuclear explosion.

On the other hand, if k < 1, then a spontaneous neutron created within the mass will generate T = k + k^2 + k^3 + ... = k/(1 - k) fissions in total, before all neutrons get lost. When k << 1, T is approximately k and therefore small too. However, as k aproaches 1 the number of fissions T created by one spontaneous neutron gets arbitrarily large. For k = 0.9, each initial neutron generates 9 fission events, on the average; for k = 0.99, it is 99 fissions. The chain reaction for one initial neutron indeed will die off, but a steady supply of N spontaneous neutrons per second will produce N*T fissions per second.

Of course this ignores the effects of energy, direction, and location within the mass. But the point is that fission can generate an arbitrarily large amount of heat power even if k < 1. Indeed, in a woking reactor the goal is to keep the net k below but very close to 1. Isn't that so?

(Wikipedia says that in a reactor one has "delayed criticality", i.e. k > 1 but secondary fissions are "delayed" so that the process becomes stable. This does not seem right: if the net k is greater than 1, delaying the children fissions will still yield an exponential growth with a smaller but still positive rate, growing ever faster without limit. Isn't that so?)

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.

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.

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.

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?

 

[Dmytry]

I think it can get to well over 1800 celsius with good ol chemistry alone - zirconium in steam exothermic reaction, then zirconium-uranium dioxide reaction, and of course the simple zirconium-air reaction. The fission would be bad though. Also, it probably wouldn't fission without water, unless aluminium works as moderator. However, when the fuel falls down between the rods - who knows. They'd better have proof it couldn't happen.

 

[Jorge Stolfi]

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.

The "most pristine" source I have is the faxes that are included in the twice-daily NISA press releases. Those faxes have been reporting the CAMS readings in Sv/h in exponential format; the only conversion I did was to fixed format, x100. Do you recall the source of those "raw readings"?

 

[TCups]

I do believe the area had been cleared for passage already by March 24th. See 'before and after 'cleanup'' attached. The position of the hotspot is at the far foot of the exhaust tower.

On later thermal images, it appears that the pool has been regularly douched from a firetruck about at the time of thermography, and the truck blocks the view to the hotspot.

OK, understand, but - - -
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. If the thermal image pre-dates the "cleaned up" image, then the hot spot is about where a large square of metal plate or plywood has been laid. I see nothing on the thermal image even vaguely resembling a truck, blocking activity or otherwise.

I have rotated your before/after clean up image to match the orientation of the thermal image, though the angle is different, and I have indicated the approximate location of the small hot spot between Bldg 3 and 4.

 

[shogun338]

I do believe the area had been cleared for passage already by March 24th. See 'before and after 'cleanup'' attached. The position of the hotspot is at the far foot of the exhaust tower.

On later thermal images, it appears that the pool has been regularly douched from a firetruck about at the time of thermography, and the truck blocks the view to the hotspot.

They have been using remote controlled excavators to clear away debris . There burying some of the hotter stuff and putting a lot of debris in small containers taking it to a temporary storage point just to the west side of the reactor buildings near a storage shed . (approx. 2.5mSv/h around containers) that have been filled .