Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[NUCENG]

I completely agree with Nikkom perspective. It is unbelievable how close the analogy is to implantable medical devices too. Where the shuttle accident reports were evaluated in detail and lessons learned. Sadly the reality always was that before the money was invested, every lesson was used correctly and the burden of proof was on the engineers to prove it was safe, but after the product was launched commercially, then the burden of proof was shifted to "prove to me it is unsafe". Most recalls in the implantable medical device industry have been made a lot worse by this behavior. NPPs are in the post commercial launch situation and thus the mentality is prove it is unsafe before I do something. That is not the mentality of the patient that has the device or the public that will suffer the consequences of failure.

I do not understand how any progress can be made without some risk. What do you want? It would be wonderful if new technologies like medical implants, or space exploration, or nuclear power, or anything else you care to name could be born without any potential for errors or flaws, or unconceived risks, but can you name one development that has? The logical consequence of what you just wrote is that we can never do anything new. Doctors should tell people they are going to die because we never could risk reactions to vaccines or medications, or surgery or any of the thousands of medical developments that came before implants.

The Black Plague did not end urbanization. The Titanic did not end shipbuilding. The Hindenberg did not stop commercial aviation. The Tacoma Narrows did not end bridge building. Louis Washkansky's death did not stop medical science from trying to correct heart disease. The Challenger has not ended Space exploration.

The burden on engineers (and doctors) is to find solutions to problems. It is impossible to prove that anything is "safe." In solving problems they have to balance benefits and risks. Society has to agree with that assessment of benefit and risks or the engineers work will never be built. The job doesn't stop there. A technology must be proved over time, and adapted, updated, and improved.

Your analogy is inappropriate, your logic is absent, and your conclusion is dead wrong. The standard you are trying to demand would halt all progress, including development of solar and wind power generation. (Electricity has risks all by itself, whatever the source.)

 

[zapperzero]

The Black Plague did not end urbanization.

Gentlemen, methinks it's time to take this to the political thread?

 

[NUCENG]

Gentlemen, methinks it's time to take this to the political thread?

I agree.

 

[Gary7]

Another layman's question: Assuming meltdown of a portion of the fuel (or most/all of the fuel) in the reactors of Fukushima Daiichi #1, #2, and #3, what condition could we expect the corium to be in after 2+ months of cooling. Is there a possibility that the various fuel masses have completely solidified by now, or would the center still be liquid/molten?

Clarification: Let me say that I understand the heat comes from decay of the fission products, and that even if water is continually poured onto it, it will continue to generate heat for years. I suppose what I am asking is whether or not the addition of steel, concrete, zirconium, etc... will enable the corium to solidify (or to form a crust that is sufficiently deep that it ceases to pose a problem with regard to moving around the underside of what is left of the reactors).

 

[Caniche]

60 ton lumps enriched to 5%,

 

[Bioengineer01]

I think there's been some interest, just not many comments. I, for one, haven't yet found the time to do much more than a quick skim-through.

EDIT: the idea of printing out e-mails, then collating them in a scanned PDF? Who came up with that one? There's zero excuse here - while I can understand why rules and regulations would have treeware master copies, this is just stupid, verging on willfully incompetent.

If I sound frustrated, it's 'cause I am. I have good text processing, indexing and search tools at my disposal - and none of them work!

In the past (10 years ago +) I have used very successfully OCR programs that can read pdf files and output a pdf file with a built-in searchable text layer. Extremely useful when building a database with pdf files.

 

[etudiant]

Another layman's question: Assuming meltdown of a portion of the fuel (or most/all of the fuel) in the reactors of Fukushima Daiichi #1, #2, and #3, what condition could we expect the corium to be in after 2+ months of cooling. Is there a possibility that the various fuel masses have completely solidified by now, or would the center still be liquid/molten?

Clarification: Let me say that I understand the heat comes from decay of the fission products, and that even if water is continually poured onto it, it will continue to generate heat for years. I suppose what I am asking is whether or not the addition of steel, concrete, zirconium, etc... will enable the corium to solidify (or to form a crust that is sufficiently deep that it ceases to pose a problem with regard to moving around the underside of what is left of the reactors).

If we assume that all of the core is neatly collected into one lump, we have a blob that needs to dissipate somewhere around 5 megawatts continuously to stay in a steady state. Water has a latent heat of vaporization of about 2000 joules/gm, so we need to vaporize about 2.5 kg of water every second, or about 9 tons/hr. Do note that this is boil off, although it may re condense in the water flooding the plant.
That is about the rate at which the reactors are getting fed, so we have a measure of stability.

However, that says nothing about the state of the fuel, except that if it were still a superhot melt the vapors given off from the reactors would show some of the chemical dissociation products. However, TEPCO has been only modestly informative about the reactor airborne emissions and I've no competence to assess that which they have released. Because no one expert has raised any alarm, I assume the fuel is largely immobile, while still in dire need of continuous cooling.

 

[robinson]

Wouldn't any melted spent fuel also be an issue? Seems like at least one fuel pool is steaming away as well.

 

[etudiant]

Wouldn't any melted spent fuel also be an issue? Seems like at least one fuel pool is steaming away as well.

The spent fuel does need cooling as well, but normally a cooling circuit keeps the refrigeration going.
Afaik, that has been restored at the SFPs for reactors 1-3, but is still not there for reactor 4, because the explosion blew out part of the cooling pipes. They have been able to access an essential valverecently and expect to be able to restore cooling to SFP 4 as well. TEPCO hopes to wrap up that work this month.

 

[joewein]

If we assume that all of the core is neatly collected into one lump, we have a blob that needs to dissipate somewhere around 5 megawatts continuously to stay in a steady state. Water has a latent heat of vaporization of about 2000 joules/gm, so we need to vaporize about 2.5 kg of water every second, or about 9 tons/hr. Do note that this is boil off, although it may re condense in the water flooding the plant.
That is about the rate at which the reactors are getting fed, so we have a measure of stability.

On June 1 the remain decay heat output in unit 1 was 3.7 MW while in units 2 and 3 it was 6.3 MW, which is 0.26% of thermal output at shutdown. Over the next 8 months that will go down to 0.21% of thermal output at shutdown, so it's essentially steady now (http://mitnse.com/2011/03/16/what-is-decay-heat/" ), since most of the iodine-131 and other shortlived isotopes are largely gone.

Water injection in units 1 and 2 is currently running at 3.5 t/h, while unit 3 is receiving 9 t/h, even though unit 2 and 3 should have the same decay heat output. If all the heat was being dissipated by boiling they would consume the same amount of water. Unit 3 is receiving more water because measured temperatures at the lower end of the RPV were higher.

If temperatures in unit 2 are lower despite only receiving 1/3 of the amount of water, perhaps most of the fuel there has already left the RPV and has splashed onto the containment concrete floor. Alternatively, the shape of the corium may obstruct water flow in the RPV of unit 3, so water boils on top of it but doesn't flow past it.

Units 2 and 3 should be largely identical, except that unit 3 was more recently refueled and therefore the average burn-up rate of its fuel should be lower, hence there should be marginally *less* decay heat output in unit 3 than unit 2.

60 ton lumps enriched to 5%,

Unit 1 had 68 t fuel enriched to 3.4% while unit 2 and 3 had 94 t enriched to 3.6%.

 

[joewein]

Wouldn't any melted spent fuel also be an issue? Seems like at least one fuel pool is steaming away as well.

The fact the pool is steaming shows that it has water in it, which is good. If they got the heat exchanger hooked up, it would be even better of course.

The spent fuel pool with the most active fuel and cause for the biggest worries was unit 4, but radioactivity levels there are much lower than in the reactor building (R/B) basement water:

Unit 4 SFP: 123 Bq/cm3 (Cs-134 + Cs-137)

Unit 1 R/B: 250,000 Bq/cm3 (Cs-134 + Cs-137)
Unit 2 R/B: 6,100,000 Bq/cm3 (Cs-134 + Cs-137)
Unit 3 R/B: 3,100,000 Bq/cm3 (Cs-134 + Cs-137)

This is why TEPCO doesn't believe in major fuel damage in the #4 SFP.

Its radioactivity levels are also orders of magnitudes lower than in the #1 through #3 SFPs.

Unit 1 SFP: 26,000 Bq/cm3 (Cs-134 + Cs-137)
Unit 2 SFP: 191,000 Bq/cm3 (Cs-134 + Cs-137)
Unit 3 SFP: 290,000 Bq/cm3 (Cs-134 + Cs-137)

Number three has the worst numbers, perhaps because it had that big blast. It is the one with lots of concrete and steel on top of the fuel assemblies seen in http://www.youtube.com/watch?v=KugIrnThul0" (you don't actually see the fuel, except for a single handle sticking out).

 

[tsutsuji]

Under the timeline, nuclear fuel will be removed from the pools for spent fuel at the No. 1 to 4 reactors starting in fiscal 2014. Officials hope to finish removing such fuel from the first reactor by fiscal 2016.
http://mdn.mainichi.jp/mdnnews/news/20110711p2a00m0na003000c.html

http://www.tv-asahi.co.jp/ann/news/web/html/210710029.html Picture of the repaired hose fitting at the water treatment facility.

http://www.nikkei.com/news/category...39797E3E2E2E2;at=DGXZZO0195165008122009000000 Tepco is going to repair the damages caused by the tsunami in the walls of the water inlet by planting steel tubes in the water. This requires opening the fence, and allowing some radioactive materials from the inlet to flow into the harbor. 14 m³/h of nitrogen will be injected into unit 3 starting 12 July. The robot found 50 mSv/h near the heat exchanger on the second floor of unit 2, which hampers the work that has to be done there. 6 people among the 9 initially feared have been confirmed to have exceeded 250 mSv.

 

[etudiant]

On June 1 the remain decay heat output in unit 1 was 3.7 MW while in units 2 and 3 it was 6.3 MW, which is 0.26% of thermal output at shutdown. Over the next 8 months that will go down to 0.21% of thermal output at shutdown, so it's essentially steady now (http://mitnse.com/2011/03/16/what-is-decay-heat/" ), since most of the iodine-131 and other shortlived isotopes are largely gone.

Water injection in units 1 and 2 is currently running at 3.5 t/h, while unit 3 is receiving 9 t/h, even though unit 2 and 3 should have the same decay heat output. If all the heat was being dissipated by boiling they would consume the same amount of water. Unit 3 is receiving more water because measured temperatures at the lower end of the RPV were higher.

If temperatures in unit 2 are lower despite only receiving 1/3 of the amount of water, perhaps most of the fuel there has already left the RPV and has splashed onto the containment concrete floor. Alternatively, the shape of the corium may obstruct water flow in the RPV of unit 3, so water boils on top of it but doesn't flow past it.

Units 2 and 3 should be largely identical, except that unit 3 was more recently refueled and therefore the average burn-up rate of its fuel should be lower, hence there should be marginally *less* decay heat output in unit 3 than unit 2.



Unit 1 had 68 t fuel enriched to 3.4% while unit 2 and 3 had 94 t enriched to 3.6%.

It is interesting that the temperatures are being stabilized with these low levels of water injection. The decay energy created must go somewhere, so if it is not taken up by the injected water, it must be heating up the water in the plant. Given that there are about 100,000 tons of water in the plant and that we are apparently dissipating about two thirds of the energy ( about 10 megawatts) into the existing pool. The specific heat of water is about 4 joules/gram, so the conversion would be roughly:
10x10**6j/sec / 4 j/gm =2.5 x10**6 gm/s heated 1 degree. There are 10**11 grams (10**5x10**6) of water in the plant, so the hourly heating of the plant pool should be 10**11 gm / 9x10**9 gm/hr (3.6x10**3s/hr x2.5x10**6gm/sec), which rounds out to one degree every 10 hours.
This is clearly too high, as the plant would be swimming in boiling water after a few weeks, which it is not. So where has the energy gone?

 

[robinson]

I'm just going by what I see happening in the rare video. #3 seems to have a boiling fuel pond.

 

[hellbet]

So where has the energy gone?

Tepco only cares of the "inside" part of the melted fuel that is the main part of so-called "corium" has probably escaped out the containment. How would you possibly care of something that is out of range? Frankly, I'm afraid we're now facing at some possible "melt-out" event taking place after all sort of barely admitted "melt-something".

 

[joewein]

I'm just going by what I see happening in the rare video. #3 seems to have a boiling fuel pond.

No it doesn't. That video is not exactly new.

According to status updates, an "alternative cooling system" for the spent fuel pool of unit 3 has been in place since July 1:

<Cooling by the alternative cooling system for the Spent Fuel Pool>
July 1 11:00 ～ July 8 08:20, July 8 14:24 ～
(Source: http://www.nisa.meti.go.jp/english/press/2011/07/en20110711-1-2.pdf" )

Furthermore they are working on one for unit 4 too:

The alternative cooling system for the Spent Fuel Pool of Unit 3 was
temporarily suspended due to the installation works of the alternative
cooling system for the Spent Fuel Pool of Unit 4. (from 08:20 till 14:24 July 8)
(Source: http://www.nisa.meti.go.jp/english/press/2011/07/en20110710-1-1.pdf"

 

[joewein]

Tepco only cares of the "inside" part of the melted fuel that is the main part of so-called "corium" has probably escaped out the containment.

What makes you think the melted fuel has left the containment? Where do you think it is now located and why do you think so?

Of course a lot of the more volatile substances in the core have left the containment, as one can see from the radiation levels in the basement of the reactor building and turbine hall, but the less soluble substances are likely to be either inside the RPV or on the containment floor.

If all of it had left the RPV, the RPV bottom temperatures would not respond to the water flow because there would be no heat source to interact with the water flow, even more so if all of it had not only left the RPV but also the containment.

 

[tsutsuji]

No it doesn't. That video is not exactly new.

According to status updates, an "alternative cooling system" for the spent fuel pool of unit 3 has been in place since July 1:Furthermore they are working on one for unit 4 too:

Concerning unit 4 they said that they could restore a vital pipe called "FPC-68" for that purpose : http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110709_02-e.pdf. Details explaining why they had to stop unit 3 SFP cooling equipment to allow starting the construction of the cover structure at unit 4 are provided in http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110708_01-e.pdf .

The spent fuel does need cooling as well, but normally a cooling circuit keeps the refrigeration going.
Afaik, that has been restored at the SFPs for reactors 1-3, but is still not there for reactor 4, because the explosion blew out part of the cooling pipes.

As Joewein says above, the cooling has been restored for the SFP at unit 3. What is making you think it has been restored for unit 1 as well ?

On http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/11071106_table_summary-e.pdf the SFP temperature at unit 1 is still marked today with "instrument failure" which makes me doubt much equipment has been restored there.

Concerning unit 2, the SFP circulating cooling system has been running since 31 May, according to http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110711_05-e.pdf page 3. It is also marked as "May 31 17:21 Started full-fledged operation of the alternative cooling system for the Spent Fuel Pool" on http://www.nisa.meti.go.jp/english/press/2011/07/en20110711-1-2.pdf page 5.

 

[etudiant]

Concerning unit 4 they said that they could restore a vital pipe called "FPC-68" for that purpose : http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110709_02-e.pdf. Details explaining why they had to stop unit 3 SFP cooling equipment to allow starting the construction of the cover structure at unit 4 are provided in http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110708_01-e.pdf .



As Joewein says above, the cooling has been restored for the SFP at unit 3. What is making you think it has been restored for unit 1 and unit 2 as well ?

On http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/11071106_table_summary-e.pdf the SFP temperature at unit 1 is still marked today with "instrument failure" which makes me doubt much equipment has been restored there. The news I summarized above about unit 2 says that the radiation hampers the work, so I also doubt a lot of work has been performed at unit 2. See also http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110711_04-e.pdf concerning the radiation measurements in unit 2 reactor building.

The JAIF status report summary here:

http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1310358115P.pdf

states that the SFP cooling function for reactors 2 and 3 have been restored and that SFP for reactor 1 is now receiving water from the SFP coolant clean up line ( not sure what that represents).
I was mistaken to believe that the reactor 1 SFP was already getting circulation cooling.

 

[tsutsuji]

The JAIF status report summary here:

http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1310358115P.pdf

states that the SFP cooling function for reactors 2 and 3 have been restored and that SFP for reactor 1 is now receiving water from the SFP coolant clean up line ( not sure what that represents).
I was mistaken to believe that the reactor 1 SFP was already getting circulation cooling.

And I was mistaken concerning unit 2. I edited my previous post concerning that unit.

If I try to gather information concerning unit 1 SFP :

NISA press conference 2011.06.24 11:10 http://www.ustream.tv/recorded/15577343 : it is expected that the original primary circuit at the unit 1 SFP cooling system can be reused instead of creating a new one as was previously thought.

http://www.nisa.meti.go.jp/english/press/2011/07/en20110711-1-2.pdf page 3 : "<Fresh water injection to SFP via FPC (using the temporary motor-driven pump) > May 29 11:10～15:35, June 5 10:16～10:48, July 5 15:10～17:30" (compare with unit 2 SFP on page 6, saying the last water injection was performed on May 30)

http://www.meti.go.jp/press/2011/07/20110711006/20110711006-1.pdf page 8 : the amount injected into SFP on July 5 was about 75 tons.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110711_05-e.pdf page 3 : "status of cooling" marked with "no plan on 7/11". ( http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1310358115P.pdf translates　http://www.jaif.or.jp/ja/news/2011/110711status_of_countermeasures-j-188.pdf's 検討中 as "planned", but a more accurate translation would be "under study" ; conversely, unit 4's 作業中 should translate as "under work" rather than "Construction to be prepared").

from the SFP coolant clean up line ( not sure what that represents).

They mean that they no longer pour water using a concrete pump truck, using instead the original pipes of the Fuel Pool Cooling and Purification System, sometimes shortened as "FPC". Apparently the pump (according to http://www.ustream.tv/recorded/15577343 ) and the pipes in the primary system are OK, but the secondary system (the pipes and pump(s) bringing sea water for cooling) has collapsed. I am not sure about the status of the heat exchanger.

By the way, concerning unit 4, there is a big translation mistake. 20A/B弁閉操作完了　on http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110709_02-j.pdf means "finished the closure operation of valve 20A/B", not "opened 20 A/B valve" as translated in http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110709_02-e.pdf

 

[Bandit127]

What a surprise (or more precisely - Quelle suprise!)

Removal of reactor fuel won't start until 2021
http://www.yomiuri.co.jp/dy/national/T110711004878.htm"

I hope this forum (and this thread) are still open when they finally get into the reactors and work out what happened. But that day is a long way away yet.

 

[etudiant]

What a surprise (or more precisely - Quelle suprise!)
I hope this forum (and this thread) are still open when they finally get into the reactors and work out what happened. But that day is a long way away yet.

If anything, that schedule seems very ambitious. Seen that it took a decade for the TMI reactor fuel to be removed, it would be surprising if this much larger and more complicated accident took a similar amount of time. Afaik, there is no effective way to decontaminate concrete, simply because the radioactive material will have infiltrated deep beyond the surface, helped by the natural aging of the concrete as well as the fissures created by the earthquake and the explosions. So just approaching the lower levels of the reactors to start that part of the cleanup will be a major challenge. Imho, the site will take decades to remediate.
More to the point, once the emissions from the site have been capped, further cleanup work will be very expensive, not only in financial but even more so in human terms, with zero economic reward. Japan is saddled with a massive dead weight loss project because of this disaster.
These costs may be so large that industry is forced to shift to smaller reactors that fail somewhat more gracefully, because no country can afford these consequences.

 

[Cire]

If all of it had left the RPV, the RPV bottom temperatures would not respond to the water flow because there would be no heat source to interact with the water flow, even more so if all of it had not only left the RPV but also the containment.

I've argued this several times. You don't have RPV bottom temperatures if corium melted out the bottom of the RPV. You don't have temperatures at all if the bottom of the RPV ever reached or exceeded 1000C. The thermocouples and their leads would have melted off long before the RPV failed.

 

[etudiant]

I've argued this several times. You don't have RPV bottom temperatures if corium melted out the bottom of the RPV. You don't have temperatures at all if the bottom of the RPV ever reached or exceeded 1000C. The thermocouples and their leads would have melted off long before the RPV failed.

True, but if that were the case, there would not be readings that fluctuate with the volume of water injected.

 

[NUCENG]

I've argued this several times. You don't have RPV bottom temperatures if corium melted out the bottom of the RPV. You don't have temperatures at all if the bottom of the RPV ever reached or exceeded 1000C. The thermocouples and their leads would have melted off long before the RPV failed.

You may be assuming a complete failure of the vessel bottom which may not have happened. The instrument and CRDM penetrations of the vessel are more likely to have failed first allowing corium to exit the vessel. The thermocouples in US plants are qualified to at least 2300 degrees F per USNRC Regulatory Guide 1.97.

 

[joewein]

I've argued this several times. You don't have RPV bottom temperatures if corium melted out the bottom of the RPV. You don't have temperatures at all if the bottom of the RPV ever reached or exceeded 1000C. The thermocouples and their leads would have melted off long before the RPV failed.

Perhaps you are thinking that if the corium melted its way out of the RPV, all of the bottom would have been heated to 1500 deg C (melting point of steel) to create an exit path. I don't think that's necessarily the case. In a BWR the control rods are inserted through the bottom of the RPV. If the core melts inside the RPV, corium could flow into the control rod channels and escape from below there without having to take out the entire bottom of the RPV. Therefore in my opinion corium outside the RPV is entirely consistent with the thermocouples still functioning.

The fact that temperatures at the core bottom went up in unit 3 when they cut back on cooling water flow to stem the flooding of the basements and trenches, and dropped back again when they stepped it up again suggests that those thermocouple readings are not totally phantom readings.

Furthermore, dry well and RPV temperature readings (4 sensors in total) broadly went up and down together during those periods, which gives me some confidence that RPV readings are not totally bogus like the core water level readings turned out to be. See http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/out/plot-un3-t-T-full.png" .

What I don't have a clear picture of is how many MW of heat could have been conducted away by the 160 mm (BWR3) / 138 mm (BWR4) of steel of the RPV and of the (unknown) m2 of concrete floor inside the containment bulb.

The containment would eventually be pierced by a melted core if less heat is conducted away from the surface of the corium than is generated as decay heat inside, leading to a rise of temperature beyond the melting point of concrete. How realistic is that at this stage? We would need to understand the heat flow, which depends on conduction and convection.

If the RPVs are pierced at the bottom from a meltdown then water injected through the feed water pipe at the top of the RPV should leak out through the bottom, unless it all boils off inside the RPV from decay heat of any portion of the corium still left inside.

If cooling water leaks out of the RPV it may create a shallow pool inside the containment, at the bottom of which the corium will cause continuous boiling. Steam should be rising and perhaps recondensating inside the containment steel wall that carries heat away into the building. In that case the containment floor is not the only surface area removing heat from the corium.

If steam condensates on the containment walls and trickles down into the corium pool again, it could boil multiple times before eventually leaking out through damaged seals. That would explain why the apparently sufficient amount of cooling water in units 1 and 2 is less than the theoretical amount boiled away per hour by the predicted decay heat output.

That makes me wonder if the 5 cm gap between the steel and concrete portion of the containment could somehow be used for air-cooling the containment.

 

[Joe Neubarth]

Concerning unit 4 they said that they could restore a vital pipe called "FPC-68" for that purpose : http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110709_02-e.pdf. Details explaining why they had to stop unit 3 SFP cooling equipment to allow starting the construction of the cover structure at unit 4 are provided in http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110708_01-e.pdf .



As Joewein says above, the cooling has been restored for the SFP at unit 3. What is making you think it has been restored for unit 1 as well ?

On http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/11071106_table_summary-e.pdf the SFP temperature at unit 1 is still marked today with "instrument failure" which makes me doubt much equipment has been restored there.

Concerning unit 2, the SFP circulating cooling system has been running since 31 May, according to http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110711_05-e.pdf page 3. It is also marked as "May 31 17:21 Started full-fledged operation of the alternative cooling system for the Spent Fuel Pool" on http://www.nisa.meti.go.jp/english/press/2011/07/en20110711-1-2.pdf page 5.

Considering the degree of site contamination, my hat if off to the Japanese. They have accomplished a lot.

 

[tsutsuji]

http://www.asahi.com/national/jiji/JJT201107120052.html At 8:40 AM on 12 July, a leak was observed on a surveillance camera, and the water treatment facility was manually stopped 10 minutes later. The leak location is in the Areva system, in the close proximity of the leak that occurred and was repaired on 10 July. The desalination facility is still running.　Repairs are shown on http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110712_02-j.pdf (top picture is before repair, bottom picture is after repair).

http://www.nikkei.com/news/headline...1949EE3E0E291838DE3E0E2E5E0E2E3E3E2E2E2E2E2E2 The leaked volume was 10 l. The leak location was exactly the same as two days ago. The facility was started again at 4:58 PM after changing the fitting for a stainless steel one. It is thought that the zinc-coated cast iron one installed two days ago was corroded by the ferric sulfate flowing in the hose.

http://www.asahi.com/national/update/0711/TKY201107110485.html For the purpose of breakwater reinforcement work repairing the tsunami damages at the south of the water inlet, the silt fence will be opened 36 times for two hours each time in the upcoming 3 months to let the steel sheet pile driving boat to come and go. This rises the fear that some of the 1.2 terabecquerels in the water inlet could flow into the sea. The silt fence opening time being limited, Tepco says the consequence on the periphery is limited. Radiation measurements in the sea will be intensified. Local governments have been notified.

http://news.tbs.co.jp/newseye/tbs_newseye4773907.html the nitrogen hose will be connected on unit 3 on 12 July afternoon. The nitrogen injection will start at unit 3 this week (nitrogen injection is already being performed at units 1 & 2).

http://www.yomiuri.co.jp/science/news/20110712-OYT1T00502.htm 　Minister Goshi Hosono said the details on a new middle to long term study team whose purpose is to study the decommissioning of Fukushima Daiichi will be announced on 19 July. They will have to find a solution so that the final disposal is located elsewhere than in Fukushima prefecture.

http://www.ustream.tv/recorded/15943265 NISA press conference, 12 July : The original heat exchanger at unit 1 SFP can be reused. Tepco hopes the SFP cooling system can be started at the end of July or within the first decade of August.

Operation of the diesel generators was carried out as follows due to the
preparatory construction of Yonomori line for duplication of line (July 11);
D/G 5A started (03:03), connected to the grid (03:19) and stopped
(09:07).
D/G 5B started (03:37) and connected to the grid (03:44).
D/G 6A started (04:17) and connected to the grid (04:21).
D/G 6B started (04:31) and connected to the grid (04:36).
・The power supply from Yonomori line was suspended due to the preparatory
construction for Yonomori line for duplication of line. (05:01, July 11)
http://www.nisa.meti.go.jp/english/press/2011/07/en20110712-1-1.pdf

http://www.meti.go.jp/press/2011/07/20110712005/20110712005-1.pdf page 23 : white smoke observed at unit 4 on 12 July 6:30 AM.

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110712_01-j.pdf page 2 : water injection into unit 4 reactor well and dryer storage pool started at 11:22 AM on 12 July. It had to stop at 12:03 because of a leak in the connection of the injection line.

 

[Cire]

You may be assuming a complete failure of the vessel bottom which may not have happened. The instrument and CRDM penetrations of the vessel are more likely to have failed first allowing corium to exit the vessel. The thermocouples in US plants are qualified to at least 2300 degrees F per USNRC Regulatory Guide 1.97.

According to GE they are actually a Type T Thermocouples with a 350C upper limit. They are magnetically attached which is another big problem. Does anyone know of a magnet that still works when heated over 1000C? Only the thermocouples on the RPV head are attached mechanically.

[PLAIN]http://img89.imageshack.us/img89/7498/thermo.jpg

 

[NUCENG]

According to GE they are actually a Type T Thermocouples with a 350C upper limit. They are magnetically attached which is another big problem. Does anyone know of a magnet that still works when heated over 1000C? Only the thermocouples on the RPV head are attached mechanically.

[PLAIN]http://img89.imageshack.us/img89/7498/thermo.jpg[/QUOTE]

My bad, thermocouples were dropped for RG 1.97 per
http://pbadupws.nrc.gov/docs/ML1109/ML11098A049.pdf

You are correct about type T range. Thanks for the correction.

 

[Barneysee]

On June 1 the remain decay heat output in unit 1 was 3.7 MW while in units 2 and 3 it was 6.3 MW, which is 0.26% of thermal output at shutdown. Over the next 8 months that will go down to 0.21% of thermal output at shutdown, so it's essentially steady now (http://mitnse.com/2011/03/16/what-is-decay-heat/" ), since most of the iodine-131 and other shortlived isotopes are largely gone.

Joe, the data you use to estimate the current heat output assumes that there was no leakage of nuclides as has happened at Fukushima.

The actual current heat output should be considerably less than those figures.
From your source:

"This data is not produced from measured data on the actual reactors at Fukushima, but from using a well established model that is routinely used to estimate decay heat from shutdown reactors.

#9044 Jorge Stolfi

once the fuel is completely molten, the radioactive elements that remain in the liquid corium will produce 30% of the decay heat power that would be produced by the intact fuel;
the other 70% of the decay heat power is due to more volatile elements that will end up elsewhere.

 

[joewein]

Thanks for pointing that out. I had forgotten about that...

So assuming the cores completely melted in all three units, the current heat output of the cores should be more like 1.2 MW in unit 1 and 2.1 MW in units 2 and 3.

The balance of 2.5 MW in unit 1 and 4.2 MW in units 2 and 3 should mostly be in the leaked water, on RPV and containment walls and around pipes, wherever evaporated volatile isotopes could condensate or get leached out.

Does anybody remember how many curies of Cs-134 and Cs-137 were leaked into the atmosphere according to NISA estimates?

 

[MiceAndMen]

According to GE they are actually a Type T Thermocouples with a 350C upper limit. They are magnetically attached which is another big problem. Does anyone know of a magnet that still works when heated over 1000C? Only the thermocouples on the RPV head are attached mechanically.

What document is that from?

There is a lot that is unknown about thermal properties during a core melt event.

http://www.tec-sim.de/images/stories/lecturenotes-late-in-vessel-phenomena.pdf

On page 16 of that document:

Though in the TMI-2 accident no external cooling was available, the vessel wall did not experience any noteworthy thermal attack in contradiction to what all simulations predict to date. The thermal attack on the vessel wall was limited to a hot spot in which the internal vessel wall reached temperatures of ~1100°C in a region of approximately 0.5 meter width, which was rapidly cooled after approximately 1/2 hour.

There is the distinct possibility that molten material relocated out of the RPV without necessarily requiring the entire bottom head to fail. There's also the possibility that such a magnetically attached thermocouple became "no longer attached" to the vessel at some point.

 

[Barneysee]

So assuming the cores completely melted in all three units, the current heat output of the cores should be more like 1.2 MW in unit 1 and 2.1 MW in units 2 and 3.

Considering the tens of thousands of gallons of water which have washed through the containment, those numbers would be an unlikely to achieve upper limit.

But with the good comes the bad - more from Jorge:

On the other hand the corium will contain many long-lived isotopes....

While the contribution of an element to the heat production rate is inversely proportional to its half-life (among other things), its potential for health damage is largely independent of it, at least for lifetimes up to a decade or two. So, while the corium keeps 30% of the decay heat production, it may include a larger fraction of the total health damage potentia of the original fuel.

Thus the decay from this point will be at a slower rate due to the predominance of long-lived isotopes.

In effect, we are now at the same point a plant which had a normal shutdown would have been - several decades later.

 

[htf]

They are magnetically attached which is another big problem. Does anyone know of a magnet that still works when heated over 1000C?

Cobalt would be a Candidate (T_c ~ 1400K). This is the material with the highest Curie temperature I know of.