Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[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

 

[AntonL]

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.

Ok, michael200, I take your point on doubling up so let's forget it. Thanks for sort of agreeing with my boiloff calculation and let's asume there was no leak in pool otherwise Tepco could not have waited until the 20th to replenish water, because should there have been a significant leak fuel exposure would have resulted before then.

But where does the Hydrogen come from for the explosion, forum members here agree that the explosion centre was lower down in the building on the north side, which does make sense because of the observed damage.

For that scenario I can also give a very imaginative explanation. When unit 3 was vented, could unit 4 have pumped hydrogen steam into unit 3. Unit 3 and 4 share a common exhaust stack and there was no power for fans to work to aid the exhaust procedure, Furthemore, we do not know if there are any dampers in the system and if installed I would imagine they fail open when power is lost.

Lets assume unit 3 pumped unit 4 full of steam and hydrogen, this would then be into the suppression chamber, then H2 will leak into the primary containment chamber, it would not immediately escape to the roof, as there is a seal between the reactor vessel and PCV so that PCV is not flooded during fuel transfer. The Hydrogen could have leaked out through the access hatch which would not have been sealed due to the maintenance taking place, This access hatch is on the ground floor, which is the right level for a lower explosion centre. The only question why the long delay between units 3 and 4 exploding.

@TCups have you considered that the access hatch could also have leaked at unit 3, this could explain a second and third explosion. Explosion 1 roof area, due to shock wave and vibration of the building the access hatch leaks and PCV burps through access hatch, now more hydrogen in lower building resulting in explosion 2, same happens again explosion 3 and PCV has lost a lot of pressure and the process stops, looking at the devastation on lower northern side this could work, also watch the video to the north a low level dust cloud expands further and faster than to the south

 

[Dmytry]

Ok, michael200, I take your point on doubling up so let's forget it. Thanks for sort of agreeing with my boiloff calculation and let's asume there was no leak in pool otherwise Tepco could not have waited until the 20th to replenish water, because should there have been a significant leak fuel exposure would have resulted before then.

But where does the Hydrogen come from for the explosion, forum members here agree that the explosion centre was lower down in the building on the north side, which does make sense because of the observed damage.

For that scenario I can also give a very imaginative explanation. When unit 3 was vented, could unit 4 have pumped hydrogen steam into unit 3. Unit 3 and 4 share a common exhaust stack and there was no power for fans to work to aid the exhaust procedure, Furthemore, we do not know if there are any dampers in the system and if installed I would imagine they fail open when power is lost.

Lets assume unit 3 pumped unit 4 full of steam and hydrogen, this would then be into the suppression chamber, then H2 will leak into the primary containment chamber, it would not immediately escape to the roof, as there is a seal between the reactor vessel and PCV so that PCV is not flooded during fuel transfer. The Hydrogen could have leaked out through the access hatch which would not have been sealed due to the maintenance taking place, This access hatch is on the ground floor, which is the right level for a lower explosion centre. The only question why the long delay between units 3 and 4 exploding.

@TCups have you considered that the access hatch could also have leaked at unit 3, this could explain a second and third explosion. Explosion 1 roof area, due to shock wave and vibration of the building the access hatch leaks and PCV burps through access hatch, now more hydrogen in lower building resulting in explosion 2, same happens again explosion 3 and PCV has lost a lot of pressure and the process stops, looking at the devastation on lower northern side this could work, also watch the video to the north the dust cloud expands more than to the south

why would the valve between reactor 4's suppression pool and the common exhaust stack be open?

 

[WhoWee]

maybe even build a cofferdam around the whole damn complex like this:

http://youtu.be/2kxVKkXW5Fg

Submerge it inside a giant above ground swimming pool?

 

[MadderDoc]

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

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.

 

[Krikkosnack]

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.

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"

 

[Rive]

Lets assume unit 3 pumped unit 4 full of steam and hydrogen...

Hi all,

By the actual status of the release pipes outside the buildings the pipes are broken near unit 3, most likely because of the explosion of unit 3.

But unit 4 exploded later than unit 3. So there was no connection between the two units when unit 4 exploded.

Is it possible that some hydrogen buildup remained in U4 so long from such source? I doubt it.

Otherwise this connection was my favorite idea too.

 

[javadave]

Is this article correct in that TEPCO could cool the reactors within days if they flooded them with water? Feed and bleed could take months and result in much more widespread radiation release, right?

http://www.bloomberg.com/news/print/2011-04-14/tepco-said-to-estimate-three-months-needed-to-stabilize-fukushima-reactors.html

 

[AntonL]

why would the valve between reactor 4's suppression pool and the common exhaust stack be open?

For maintenance and they needed to purge the nitrogen before anyone could enter so it could be open for fresh air purposes.

 

[Borek]

Is this article correct in that TEPCO could cool the reactors within days if they flooded them with water?

I don't see physics behind the idea. Flooding with water won't help, as water will get heated and evaporate, so there will be a need to constantly add more. It won't be much different from what they do now.

And it won't be shorter, as the problem is not with the heat that is already there - heat is still produced and will be produced for months to come.

 

[javadave]

I don't see physics behind the idea. Flooding with water won't help, as water will get heated and evaporate, so there will be a need to constantly add more. It won't be much different from what they do now.

And it won't be shorter, as the problem is not with the heat that is already there - heat is still produced and will be produced for months to come.

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

 

[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)?