Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[rmattila]

Has anyone seriously considered creating small reservoirs next to our many piles of stored spent rods in the event that the power in the United States fails, and we need to cool them by a gravity-fed system until generators to power pumps are working?

The pool itself is supposed to work as such a reservoir, with typically enough water to cope at least a week or so before new water needs to be added.

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111214/0400_osensui.html Another leak of 30 litres of water, that remained indoors and was stopped by closing a valve, was found at the same evaporative concentration equipment on 12 December. The NISA is issuing a "severe reprimand" to Tepco. It is the first time the NISA issues such a reprimand since the one that concerned the identity checks at the site entrance in August.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111213_03-e.pdf Leakage from Sampling Line of Evaporative Concentration System 3 C

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111203/0735_reikyoku.html (3 December) Tepco's internal investigation report says that the suitability of the mechanism which makes the IC's valves automatically close when battery power is lost must be investigated. The mechanism is intended to prevent radioactive substances to be released outside of the plant in case of emergency, but in this case, it prevented the emergency cooling function from being performed.

http://www3.nhk.or.jp/news/html/20111209/t10014537511000.html (9 December) A JNES study was presented at a hearing of specialists organised by the NISA on 9 December. According to the JNES study, if unit 1's IC had been started at 4:15 PM (which is 45 minutes after the tsunami), cooling could have started before fuel exposure takes place, the water level could have been kept stable, and meltdown would not have occurred. But that would have required replenishing the IC with water. If the IC had been started at 6:15 PM (which is 2 hours 45 minutes after the tsunami), the fuel would be already exposed and it would not have been possible to restore the water level. A Hokkaido university professor who attended the meeting commented that "If the cooling equipment had been continuously used, the situation would not have been so severe. Retrospectively, unit 1's hydrogen explosion had consequences on unit 2 and unit 3. The complacency of the early response is an important soul searching item. How much the cooling equipment was in people's mind and how much the response had been prepared must be investigated". Tepco's Junichi Matsumoto commented that "searching one's way in the dark with a flashlight, and given the fact than only a limited number of valves can be manually operated from outside the PCV, recovering the cooling equipment's function was difficult. (...) The equipment is designed so that the valves are automatically closed when battery power is lost, so it was a situation where operation is impossible. Assuming that it had been possible to recover cooling function within the time mentioned in the [JNES] study, perhaps the water level could have been maintained, but wasn't it realistically difficult given the design of the equipment ?"

http://www.tepco.co.jp/en/news/topics/11120901-e.html "The isolation condenser was out of control and lost its function via the automatic isolation interlock operation due to the blackout caused by the tsunami."

http://www.tepco.co.jp/en/press/corp-com/release/11121513-e.html Report with regards to "Policy on the mid term security" for the Units 1 to 4 of Fukushima Daiichi Nuclear Power Station to Nuclear and Industrial Safety Agency at the Ministry of Economy, Trade and Industry (part 3) (15 December 2011, English press release linking to a 125 page Japanese report)

http://www.nisa.meti.go.jp/english/press/2011/12/en20111214-2.html "Regarding Evaluation of the Report from Tokyo Electric Power Co., Inc. (TEPCO) on Leakage of Water Containing Radioactive Materials from Evaporation Concentration Device"

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111215/0825_hairo.html The decommissioning of Fukushima Daiichi will take a maximum of 40 years. Tepco and the Ministry of Economy and Industry have prepared a middle and long term schedule based on a report by the NSC. Fuel removal from unit 4 will start within two years, which is one year earlier than the NSC's report was recommending. The removal of melted fuel from units 1, 2, and 3 will be performed within 25 years, and the dismantling of the reactors and buildings will take until a maximum of 40 years from now. It implies difficult operations without precedent in the world and the design of new robots for remote operations is included in the plan. The "cold shutdown" and completion of step 2 will be announced by the government on 16 December. The middle and long term schedule will be released at the end of December.

 

[rmattila]

http://www.tepco.co.jp/en/news/topics/11120901-e.html "Comments concerning the page 2 article in the December 9 morning edition of the Mainichi Newspaper titled, "Had the reactor condenser begun running within an hour following the tsunami, the Unit 1 reactor meltdown would not have occurred. (Japan Nuclear Energy Safety Organization (JNES) analysis)" "

Thank you, Tsutsuji-san for this piece of information! Behind the link, there's a statement by TEPCO that the isolation condensor at unit 1 was out of order because of an "automatic isolation interlock operation due to the blackout caused by the tsunami". My understanding of neither the Japanese nor the English language is not sufficient to be able to say for certain, but to me this appears to be an explanation that it would not have been possible to open the IC valve between the tsunami and 18:18. At 18:25, the steam production by the IC is said to have stopped, which may be due to the hydrogen blocking the IC pipes.

Two questions to those more familiar with the GE design:

How is the valve in the IC return line operated? Is it just motor operated or would it be possible to open it manually in case of loss of DC?

Is there a way to release non-condensible gases from the IC?

As far as I'm aware, so far TEPCO has blamed the loss of measurements for the inability to engage the IC. This is the first time I've seen them suggesting "it would have been done if only it was possible".

Clearly, the control room personnel did what they were instructed to do and it would not be fair to put blame on them, but this yet another version of "why the IC was not used" seems somewhat confusing.

 

[NUCENG]

One of the assumptions here is that significant amounts of gas from a corium-concrete reaction would necessarily end up in the cooling water. Is this true, I wonder?

I used http://www.scienceaccelerator.gov/ advanced search with keywords “corium” and “concrete” to search for references. Here is a summary of what I found.

Gaseous products of Corium/Limestone Concrete Interaction:

http://www.osti.gov/bridge/servlets/purl/6439801-y4LnD0/6439801.pdf

Similar, but includes Basaltic Concrete and lots of math details:

http://www.osti.gov/bridge/servlets/purl/5080398-ulvUFv/5080398.pdf

Experimental Results

http://www.osti.gov/bridge/servlets/purl/5073998-ELWgMT/5073998.pdf

Mk I BWR specific:

http://www.osti.gov/bridge/servlets/purl/6572846-bJT5Ss/6572846.pdf

Theoretical model with crust and overlaying water pool

http://www.osti.gov/bridge/servlets/purl/10136693-iaggOp/10136693.pdf

Discussion of DW Liner Failure (beyond what has been discussed at Fukushima reactors):

http://www.osti.gov/bridge/servlets/purl/6476557-e9NERk/6476557.pdf

Experimental Heat transfer to liquid layers with entrainment of gases:

http://www.osti.gov/bridge/servlets/purl/7100357-d83aw9/7100357.pdf

NUREG /CR-5978 Probably best answers are in this document for aerosols retained in water including corium/concrete interactions in a BWR:

http://www.osti.gov/bridge/servlets/purl/10186838-I0dgvH/native/10186838.pdf

I have skimmed these documents and the NUREG seems to quantify the gas retention in the watter pool best. Bon Apetit! (BTW, There are lots more references in the search described above.)

 

[zapperzero]

I have skimmed these documents and the NUREG seems to quantify the gas retention in the watter pool best. Bon Apetit! (BTW, There are lots more references in the search described above.)

Oh this is good stuff, thanks a bunch.

one early catch (I'm still skimming through this stuff):

http://www.osti.gov/bridge/servlets/purl/10136693-iaggOp/10136693.pdf

efficient bulk cooling of melt will occur as long as the concrete
decomposition gas velocity lies above 6 cm/s. When the gas velocity falls below
this value, an interstitial crust is predicted to form: Thereafter, heat
transfer from the MCCI zone to the coolant will be crust-limited.

iow, it appears to be possible for MCCI-produced gasses to stop going into the cooling water, as the melt cools somewhat and crusts over.

 

[NUCENG]

Oh this is good stuff, thanks a bunch.

one early catch (I'm still skimming through this stuff):

http://www.osti.gov/bridge/servlets/purl/10136693-iaggOp/10136693.pdf


iow, it appears to be possible for MCCI-produced gasses to stop going into the cooling water, as the melt cools somewhat and crusts over.

Correct, if you assume the crust is stable. Gases are still generated and tend to build up in the liquid corium which cold cause ruptures in the crust. It may be a iterative process of formation and rupture of crust for some period until the heat removal stabilizes the crust. Also consider that the presence of a water pool overlaying the corium. may be periodic due to boiling and makeup cycles. If the water has boiled off. crust may remelt. This appears to be an extremely dynamic process with a lot of variables.

 

[zapperzero]

This appears to be an extremely dynamic process with a lot of variables.

I don't see how I could extract values for many of those variables using only the data that TEPCO is releasing.

 

[NUCENG]

I don't see how I could extract values for many of those variables using only the data that TEPCO is releasing.

True, but what does that matter? They are measuring offgassing, and temperatures and indications are that the plants are reasonably stable now. It will be quire some time before they start to disassemble and remove core debris. I have asked this question before: What would we do differently if we had all the parameters you feel are missing? The original question you asked was whether water retention of gasses was a reasonable assumption. The information I provided indicates that the assumption is theoretically and experimentally valid. It seems that TEPCOs measurements (limited thogh they may be) also are consistent.

I guess the point is that if the current behavior is consistent with theory and experiment, the conclusion that the plants are stabilizing is reasonable. That is progress. We should continue to watch for divergence from the expected behavior, but lack of divergence is good news.

 

[zapperzero]

True, but what does that matter? They are measuring offgassing, and temperatures and indications are that the plants are reasonably stable now.

Their thinking is "the corium concrete melt must have crusted over because we are seeing no CO2 gas in the water, therefore it must be at so-and-so temperature, therefore MCCI ceased and all is well".

But the models are more complex than that, are they not? Lots and lots of variables. Maybe there is a "stable" crust, but it will break again tomorrow? Maybe a stable crust formed earlier than the models suggest, for some reason (such as insufficient/unreliable data), so the melt is way hotter than TEPCO believes?

Maybe there wasn't any crust at all, ever, because fuel was sprayed under pressure from the RPV and is now laying in the form of small flakes and powder on the floor of the PCV, amply cooled but in danger of going critical again because there is so much water around?

They have temp sensors in places where there certainly is no corium now, and they are estimating the corium temp based on that. They are measuring the gas content of the cooling water and inferring stuff about how MCCI is proceeding, but have no explanation for the continued production of hydrogen and its presence in the PCV. They know of breached containments in three reactors, have zero information as to the shape and position of the corium yet work is proceeding as if a new major release of radioactive gasses and dust is impossible.

I have very little faith in their modelling and how it represents the physical reality. I suppose that makes me a skeptic? Or paranoid, perhaps?

 

[tsutsuji]

I was waiting until Tepco would provide a full translation of the document, but such a translation does not seem to be coming. So here is my question: the other day I translated a few tables :

I attach a translation of the figures on pages 12 and 15 of http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_09-j.pdf

In https://www.physicsforums.com/attachment.php?attachmentid=41476&d=1322928125 we have a list of assumptions made by Tepco before launching its simulations. I was disappointed to learn that the 0.81 m deposit thickness inside the sump is not a result but a mere supposition. Perhaps it is a good educated guess, but doesn't that make the 0.65 m concrete erosion depth result and the "PCV steel lining is still OK" conclusion look unsure ? Should not Tepco have simulated with a worse assumption, like assuming the sump is full with corium (which means using a 1.2 m thickness instead of 0.81 m) ? I tend to imagine liquid corium like liquid water. If enough liquid corium falls on the pedestal, the liquid would flow into the sump until the sump is completely filled, wouln't it ? So, conversely, does it mean I am wrong to imagine corium as a liquid which fills every hole like liquid bronze fills a statue mold (http://www.pbs.org/newshour/art/blog/2010/01/wednesdays-art-notes-6.html)?

I have another, most likely stupid question, but I can't find the answer myself. Why do they need that big Areva/Kurion/Sarry water purification facility in the first place ? I understand that they need to remove salt because salt and steel do not get on very well with each other. But why remove cesium ?

 

[Astronuc]

I have another, most likely stupid question, but I can't find the answer myself. Why do they need that big Areva/Kurion/Sarry water purification facility in the first place ? I understand that they need to remove salt because salt and steel do not get on very well with each other. But why remove cesium ?

The salt interferes with the collection of Cs and other isotopes. The water in the reactor building contains soluble fuel, transuranics and fission products. The objective of AREVA/Kurion/Sarry is to process the liquid, i.e., collect and concentrate in solid form the radionuclides in order to reduce the release of radionulcides. Basically, they set up a system similar to a fuel reprocessing stream. Ideally, the cleaner water is sent back to flush the reactor buildings. I would expect the goal is to remove all the soluble fission products leaving only those insoluble in containtment. However, I understand the fission products have leached into the concrete containment structures, so decontaminating them will would be virtually impossible, although the radioactivity levels would be lower than without the current processing program.

 

[tsutsuji]

The objective of AREVA/Kurion/Sarry is to process the liquid, i.e., collect and concentrate in solid form the radionuclides in order to reduce the release of radionulcides.

You mean that injecting the contaminated water without purification would create other clouds of contaminated steam which would then be spread into the atmosphere ? Yes that makes sense. Thank you for your answer. I was too much focused on the idea "those reactors need cooling, so all kind of water, including dirty contaminated one can do the job".

 

[rmattila]

You mean that injecting the contaminated water without purification would create other clouds of contaminated steam which would then be spread into the atmosphere ? Yes that makes sense. Thank you for your answer. I was too much focused on the idea "those reactors need cooling, so all kind of water, including dirty contaminated one can do the job".

There's also the issue that the systems pumping water to the reactors would become extremely radioactive and impossible to approach for e.g. maintenance/repair work, and all leaks (that will eventually occur no matter what in such a temporary arrangements) would spread contamination with the site. And work would not be possible in those areas of the site where water injection lines are located.

 

[Astronuc]

You mean that injecting the contaminated water without purification would create other clouds of contaminated steam which would then be spread into the atmosphere ? Yes that makes sense. Thank you for your answer. I was too much focused on the idea "those reactors need cooling, so all kind of water, including dirty contaminated one can do the job".

There is still decay heat to remove. If the water was stagnant, it woud heat up, just like the spent fuel pool, which is cooled. The stagnant water would also dissolve radionuclides from the fuel.

So there are two objectives - cool the fuel material that is present (get to 'cold shutdown'), and extract the radioactive material (fuel, transuranics and fission products) in order to mitigate dispersion and release. At some point, the closed system would transport the mobile/soluble radionuclides to a collection vessel which can then be removed and sent to some disposal site.

I order for people to access the reactor buildings at some point, the contamination/radioactivity there has to be reduced to some level. However the complication is the fuel material that remains (which is mostly insoluble) and the radionuclides that have leached into the concrete structure. Ultimately, they have two choices - entomb the containments such that no leaching of radionuclides is possible, or demoslish the containment structures (while mitigating the release of radioacitive dust) and shipping the contaminated concrete to some final repository. I suspect there is pressure to go with the former option.

 

[tsutsuji]

There's also the issue that the systems pumping water to the reactors would become extremely radioactive and impossible to approach for e.g. maintenance/repair work, and all leaks (that will eventually occur no matter what in such a temporary arrangements) would spread contamination with the site. And work would not be possible in those areas of the site where water injection lines are located.

I was having this idea in mind, but aren't they already having long lines with contaminated water between the turbine buildings and the waste processing facility? So somehow, they have no other choice at present than coping with long lines of highly radioactive water. On the other hand, if they reinjected the water "as is" (including salt, including cesium), they would need only a short line between building basement and reactor. But I understand your point that they need to create the safest environment for the workers working in the reactor building and refrain from adding extra radioactive sources in addition to those already created by the accident.

 

[Astronuc]

I was assuming that they have a closed loop for filtering the contaminated water. Actually there are two choices: close or open. In a closed loop processing, one simply recycles the water which moves a mass of nuclides from one volume (the containment/reactor building) to the other volume (filters), then recycle the water (ideally cooled) back to the reactor building. The distance between containment and filters should be as short as possible, although that puts maintenance workers closer to the source of radiation. The alternative is to use an open loop in which one injects fresh cold water into the containment which then warms up and collects (inevitiably) soluble radionuclides, and that water is collected and sent to the filters. The filters collect the radionuclides with less than 100% efficiency, so that water has to be discharged somewhere - either to some larger storage volume or discharged to the ocean.

The closed loop is ideal in terms of minimizing the amount of contaminated water that must be dispositioned. The disadvantage would be the recycling of slightly contaminated water, although it is much less contaminated than what is being extracted from containment.

 

[rmattila]

They do have a "closed" loop in the sense that water is recirculated back. However, they have a problem with the inflowing ground water that prevents them from pumping the turbine and reactor buildings to less than +3000 mm, which means that the amount of contaminated water in the circulation remains high.

The reported radioactivity level has dropped by more than a decade since June (see attachment), but until they are able to prevent groundwater from flowing into the basements (either by dropping groundwater lever around the complex by pumping or by building additional structures to prevent the inflow), the decrase rate of cesium within the buildings will remain rather limited.

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111216/0500_shorisui.html Tepco is postponing its plan to release treated water into the sea. On 8 December, Tepco announced a plan to release the decontaminated water into the sea after further reducing the concentrations of radioactive substances. This plan faced opposition from Zengyoren (federation of Japan fisheries cooperatives). On 15 December, Tepco presented a management plan for the next three years purporting that the sea release is postponed, while 3 countermeasures are undertaken 1) to reduce the seeping in of ground water 2) to increase the power of the treatment equipment [I don't know how this can reduce the quantity of water ?] 3) to increase the number of tanks. Tepco pledges not to release the water into the sea without the agreement of the concerned ministries such as the ministry of agriculture, forestry and fisheries, and is planning to "politely explain" its plan to concerned organisation, not only Zengyoren but also local fisheries organizations.

 

[Astronuc]

They do have a "closed" loop in the sense that water is recirculated back. However, they have a problem with the inflowing ground water that prevents them from pumping the turbine and reactor buildings to less than +3000 mm, which means that the amount of contaminated water in the circulation remains high.

The reported radioactivity level has dropped by more than a decade since June (see attachment), but until they are able to prevent groundwater from flowing into the basements (either by dropping groundwater lever around the complex by pumping or by building additional structures to prevent the inflow), the decrase rate of cesium within the buildings will remain rather limited.

Pretty much the only way to do that is to have wells outside of the containment that draw out the ground water. However, one has to balance the flow rate such that one does not draw out contaminated water from containment. The persistent ground water intrusion can mean that the seismic activity has changed the local hydrology such that flow toward containment has increased, and/or the containment has been damaged (cracked) due to seismic activity (earthquake) and so now, the containment is failing to perform its function.

 

[tsutsuji]

Cabinet investigation committee:

According to the committee's most recent findings, engineers operating the Fukushima No. 1 plant had stopped the high-pressure coolant injection (HPCI) system of the No. 3 reactor -- which was the final chance at cooling the reactor -- without authorization from higher ups. The main issue here is that TEPCO was not adequately prepared for such a crisis, forcing engineers to take such action based on their own judgment.
http://mdn.mainichi.jp/perspectives/news/20111216p2a00m0na005000c.html

 

[Astronuc]

Their Emergency Operating Procedures (EOPs) did consider long-term loss of power. A loss of offsite power (LOOP) was a possibility, but the assumption is that local emergency power is available - from batteries and EDGs. The unanticipated tsunami disabled the emergency power. IIRC, the concern with HPCI was the rising containment pressure. They needed a closed loop cooling system that didn't add pressure to containment, but for that they needed power.

 

[zapperzero]

http://www.yomiuri.co.jp/dy/national/T111215005428.htm

[former jp PM Yukio] Hatoyama also says that he and [Diet member Tomoyuki] Taira obtained data on samples of contaminated water TEPCO obtained from the basement of the plant's No. 1 reactor and asked an outside research institute to reanalyze them.

Results showed that radionuclide chlorine 38, one of the isotopes released during recriticality, "was indeed present," he claims.

 

[zapperzero]

Also,
http://latimesblogs.latimes.com/wor...lear-power-plant-damage-and-radiation-le.html

Japan declared Friday that the tsunami-stricken Fukushima Daiichi nuclear power plant has reached a condition that suggests a critical stable state known as a “cold shutdown” and has ceased to leak substantial amounts of radiation.

 

[matthewdb]

Two questions to those more familiar with the GE design:

How is the valve in the IC return line operated? Is it just motor operated or would it be possible to open it manually in case of loss of DC?

There are two valves in the return line. There is one outside containment is DC operated and normally closed. There is one inside containment that is AC operated and normally open. The inside one is only closed if there is very high flow indicating a ruptured pipe.

The valve outside containment can be hand operated. The one inside can not, but it shouldn't ever be closed.

Is there a way to release non-condensible gases from the IC?

The IC is normally vented to the main steam line so that it is continually purged of non-condensibles when the plant is operating. If the main steam isolation valves close, the vent to the IC will close too. This valve can be manually re-opened to vent if necessary.

 

[rmattila]

[former jp PM Yukio] Hatoyama also says that he and [Diet member Tomoyuki] Taira obtained data on samples of contaminated water TEPCO obtained from the basement of the plant's No. 1 reactor and asked an outside research institute to reanalyze them.

Results showed that radionuclide chlorine 38, one of the isotopes released during recriticality, "was indeed present,"

It's certain that there must have been some Cl-38 present due to neutrons produced by spontaneous fissions and subcritical multiplication. It's the amount that matters, just as in case of the Xe-135 observations, and the number given in the spring before being withdrawn was so high that it would have required the reactor remains to return more or less full power. And no Na-24 was reported, which should also have been present in comparable quantities.

This time, no numbers are given at all, which makes it impossible to put this news in perspective.

 

[rmattila]

There are two valves in the return line. There is one outside containment is DC operated and normally closed. There is one inside containment that is AC operated and normally open. The inside one is only closed if there is very high flow indicating a ruptured pipe.

The valve outside containment can be hand operated. The one inside can not, but it shouldn't ever be closed.



The IC is normally vented to the main steam line so that it is continually purged of non-condensibles when the plant is operating. If the main steam isolation valves close, the vent to the IC will close too. This valve can be manually re-opened to vent if necessary.

Thank you. So based on this, from technical point of view it would have been possible to re-engage the IC after the loss of DC, as well as to vent the hydrogen blocking the steam (although the latter would have been quite risky due to the risk of igniting the hydrogen). But of course, such operations need to be considered and tested in advance in order to be able to act quickly enough, if the need arises. A BWR core will uncover within an hour if cooling is lost soon after shutdown, and this is a very short time to consider options and make decisions in an emergency situation.

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111217/index.html In the morning of 15 December, 15 workers of a Mitsubishi subcontractor felt sick with diarrhoea and vomiting in the bus between hotel and the office close to Fukushima Daiichi. As of 16 December the number of people with the symptoms is 52. Among those, 3 have been diagnosed with a norovirus. Receiving treatment in hospital such as intravenous drip, many have recovered. Some are still in hospital. Many of them have traveled by bus to Fukushima Daiichi after gathering from different hotels at Mitsubishi's office in Hirono. Tepco is checking the cause and the infection route, suspecting a norovirus group infection. All of them are in charge of the installation of radioactive substance storing tanks and this work has been interrupted, but this has no consequences such as on the cooling of the reactors.

 

[zapperzero]

This time, no numbers are given at all, which makes it impossible to put this news in perspective.

Which is what makes it all the more annoying. I can't even get at the Nature article (paywall).

 

[~kujala~]

The persistent ground water intrusion can mean that the seismic activity has changed the local hydrology such that flow toward containment has increased,

The original ground water level was well above the current ground level. Surely because they knew this before building Daiichi they must have also built the waterproof systems in the reactor buildings to hold the ground water at least up to the current ground level.

If this is (was) the case then the change in the local hydrology wouldn't affect the situation. Even if the ground water level rose up to the current ground level the waterproof systems should still be able to hold the water outside the containtment.

http://varasto.kerrostalo.huone.net/lietekivi_7.png

and/or the containment has been damaged (cracked) due to seismic activity (earthquake) and so now, the containment is failing to perform its function.

There is also a third alternative: the waterproof systems have been cracked a long time ago but they have not been aware of it or they have not wanted the others to be aware of it.

By the way: aren't the waterproof systems regarded as a critical part of a nuclear plant? On the other hand TEPCO has said that no critical parts of the Daiichi plants have been compromised by the earthquake. On the other hand there is a big possibility that the waterproof systems in the reactor buildings have been compromised.

So is it "OK" for a nuclear plant to have non-waterproof reactor buildings before/just after earthquake? Isn't this considered a serious flaw in the design/building/testing of a nuclear plant?

 

[tsutsuji]

Cabinet investigation committee:

According to the plant's manual on how to respond to severe accidents, workers must first confirm that there are about seven atmospheres of pressure or less in a reactor core before using a substitute pumping system. The fire pump uses relatively low pressure to inject water.

However, the No. 3 reactor's pressure had jumped to about 40 atmospheres at that time, preventing the fire pump from injecting water to it.

Therefore, the workers tried to revert to the high-pressure core cooling system, only to find it could not be started due to a low battery charge.
http://www.yomiuri.co.jp/dy/national/T111216005550.htm

workers on site shut down the HPCI system out of fear that batteries would die, without the authorization of then plant chief Yoshida. (...) The government panel is expected to state in its midterm report this month that "it would have been preferable not to shut down the HPCI system."
http://mdn.mainichi.jp/mdnnews/news/20111216p2a00m0na020000c.html

It's certain that there must have been some Cl-38 present due to neutrons produced by spontaneous fissions and subcritical multiplication. It's the amount that matters, just as in case of the Xe-135 observations, and the number given in the spring before being withdrawn was so high that it would have required the reactor remains to return more or less full power. And no Na-24 was reported, which should also have been present in comparable quantities.

This time, no numbers are given at all, which makes it impossible to put this news in perspective.

Translated by me from the Japanese version published in Nature Asia:

On 26 March, the NISA announced that two days earlier 38Cl had been found in the accumulated water in unit 1's basement. (...) A number of scientists argue that it is possible to detect 38Cl even if 24Na is not detected. On 20 April, Tepco retracted its previous report, and announced that 38Cl and 24Na had not been detected, but did not release the data used for that analysis. We, the members of team B [a group of Diet members], obtained Tepco's data (germanium semi-conductor detector data) via the NISA, and performed a new analysis. It reached the conclusion that a concentration of 38Cl close to that of Tepco's original report (1,600,000 Bq/ml) existed. We think that the NISA's and Tepco's suspicion on this detection is groundless.

http://www.natureasia.com/japan/nature/specials/earthquake/nature_comment_121511.php

March 25th, 2011
Tokyo Electric Power Co.
Fukushima Dai-ichi NPS
Regarding the result of concentration measurement in the stagnant water on the basement floor of the turbine building of Unit 1 of Fukushima Dai-ichi Nuclear Power Station
Radioactive Nuclide Concentration (Bq/cm3)
Cl-38 1.6×10^6
http://www.nisa.meti.go.jp/english/files/en20110325-6.pdf

Press Release (Apr 20,2011)
Cl-38（approx. 37 minutes）Below minimum detectable density ; reason for change ①Identification and determination of radioactivity density were conducted based on main peaks,
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110420e11.pdf

 

[tsutsuji]

http://mainichi.jp/area/niigata/news/20111216ddlk15040081000c.html [Kashiwazaki-Kariwa] Tepco was unable to find some of the paper documents documenting the shipments of spent fuel from Kashiwazaki-Kariwa unit 1. "We are unable to say if we never wrote them or if we lost them". The documents have already been prepared again. The Kashiwazaki-Kariwa NISA office said it could be a breach of safety regulations. Tepco is adding two more sites, bringing to 9 the number of sites where it is planning to investigate tsunami vestiges.

http://mainichi.jp/select/jiken/news/20111218ddm002040092000c.html [Fukushima Daiichi] At 10:23 AM, 17 December, Fukushima Daiichi unit 1's pool cooling system made an automatic stop triggered by an abnormal flow detection signal. A 100 litre leak of water was found. A valve was not fully closed. The system was started again at 1:39 PM. With 13°C, pool temperature was the same as before the stop. Being from pipes which are not in direct contact with fuel, the leaked water is not contaminated.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111218_01-e.pdf "12/18 At 10:00 am, during the patrol activity, a TEPCO employee found an accumulated water (...) The depth of the water was estimated to be 50 cm and the amount was to be 125 m³. The radiation dose at the water surface was 3 mSv/h (provisional value)."

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111218_02-e.pdf "Overview of the accumulated water in the trench between process main building of Centralized Radiation Waste Treatment Facility and Miscellaneous Solid Waste Volume Reduction Treatment Building (High Temperature Incinerator Building)"

 

[tsutsuji]

There are two valves in the return line. There is one outside containment is DC operated and normally closed. There is one inside containment that is AC operated and normally open. The inside one is only closed if there is very high flow indicating a ruptured pipe.

The valve outside containment can be hand operated. The one inside can not, but it shouldn't ever be closed.

I translate some bits of Tepco's internal investigation interim report:

Attachment 10-2 ( http://www.tepco.co.jp/cc/press/betu11_j/images/111202f.pdf page 281/314 )

About the Isolation Condenser

1. Status before earthquake (idle)
* There are two systems: system A (left part of diagram below) and system B (right part of diagram below)
* In normal time it is idle, A system's MO-3A valve is closed, and B system's MO-3B valve is closed.
< diagram > [happens to be the same as the one on http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111122_03-e.pdf page 2]

2. When it is running
* Due to manual operation, or to automatic operation signal (due to high reactor pressure), A system's MO-3A valve and B system's MO-3B become opened.
* As a result, all the valves in the RPV-->IC-->RPV lines (valves 1A, 2A, 3A, 4A for system A, valves 1B, 2B, 3B, 4B for system B) are open.

3. When the isolation signal has come
* The isolation signal is emitted upon detection of rupture by the pipe rupture detection circuit, or upon a loss of electric power (DC power) suffered by that circuit.
* When the isolation signal is received, the interlock, which closes the valves installed in the line (valves 1A, 2A, 3A, 4A for system A, valves 1B, 2B, 3B, 4B for system B), is activated.

Attachment 10-3 ( http://www.tepco.co.jp/cc/press/betu11_j/images/111202f.pdf page 282/314 )

Isolation condenser - electric-operated valves interlock block diagram

Attachment 10-3 English translation

Attachment 10-4 ( http://www.tepco.co.jp/cc/press/betu11_j/images/111202f.pdf page 283/314 )

Fukushima Daiichi unit 1 - Isolation condenser valve status chronology

Attachment 10-4 English translation

 

[matthewdb]

Isolation condenser - electric-operated valves interlock block diagram

Attachment 10-3 English translation

Thank you tsutsuji

You middle diagram could explain why the IC wasn't able to operate after the tsunami. It states that the IC received a isolation signal so the AC valves inside containment were commanded to close.

Since they are inside containment they are impossible to open (containment is inerted with N2). It will probably be more than a year before they can be inspected.

 

[rmattila]

Thank you tsutsuji

You middle diagram could explain why the IC wasn't able to operate after the tsunami. It states that the IC received a isolation signal so the AC valves inside containment were commanded to close.

Since they are inside containment they are impossible to open (containment is inerted with N2). It will probably be more than a year before they can be inspected.

I was just about to write the same reply, but you were faster..

Finally this isolation condenser mystery is starting to make sense: loss of DC results into a (spurious) system isolation due to the fail-safe direction of the valves, and depending on whether or not there was AC available at the time of loss of DC, the inner IC valves (4A and 4B) may have closed at that time.

Since steam was reportedly observed at 18:18 upon opening the 3A valve, it might suggest that the 4A valve would have remained open in spite of the closure signal, but that remains to be seen.

Once again, thank you, Tsutsuji-san - you're helping many people to get understanding of the situation.

Deciding the fail-safe mode of different valves is always a difficult optimization task. In the GE BWR:s, it seems that fail-close has been a very dominating design principle (thinking of the difficulties in lowering the reactor pressure and now this issue of possibly losing the isolation condenser due to the return lines fail-closing). Possibly worth a thought or two at other NPP:s as well.