Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[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.

 

[NUCENG]

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.

I would urge a little caution as there are other questions to answer. The F1 operators tried multiple workarounds in venting and other actions they took in the early hours. Isolation signals can be reset or jumpered out. Alternatve power sources can be rigged. Were any of these part of the actions that night?

The information provided by Tsutsuji has been timely and useful, but in our voracious appetite for answers, we should keep in mind that answers can raise more questions.

 

[rmattila]

I would urge a little caution as there are other questions to answer. The F1 operators tried multiple workarounds in venting and other actions they took in the early hours. Isolation signals can be reset or jumpered out. Alternatve power sources can be rigged. Were any of these part of the actions that night?

The information provided by Tsutsuji has been timely and useful, but in our voracious appetite for answers, we should keep in mind that answers can raise more questions.

While all that is true, one must remember that there may be less than an hour to initiate core cooling in order to prevent fuel uncovery. It is very unlikely that workarounds can be fixed to locate the correct instrumentation cabinets, return the DC for the measurement circuits and AC for the inner isolation valves, and then steer the inner valves open in such a short time period. It's complicated enough to get a grip of what's going on and manually open the outer valves within an hour or so - simultaneous spurious closure of the non-hand-manageable inner valves makes the task too challenging to be reliable.

In ASEA BWRs, the logic has been to use check valves as the inner isolation valves whenever possible (=most ingoing lines), since they don't need any electricity or instrumentation to function - and for the outer isolation valves, there's a hand wheel to enable opening and closing of the valve. The isolation condenser relies on relatively small pressure differences, and a check valve in the return line may thus be impossible to arrange, but taking into account the importance of the safety function, loss of both cooling circuits due to a single loss of a DC measuring voltage supply just seems too thin.

 

[NUCENG]

While all that is true, one must remember that there may be less than an hour to initiate core cooling in order to prevent fuel uncovery. It is very unlikely that workarounds can be fixed to locate the correct instrumentation cabinets, return the DC for the measurement circuits and AC for the inner isolation valves, and then steer the inner valves open in such a short time period. It's complicated enough to get a grip of what's going on and manually open the outer valves within an hour or so - simultaneous spurious closure of the non-hand-manageable inner valves makes the task too challenging to be reliable.

In ASEA BWRs, the logic has been to use check valves as the inner isolation valves whenever possible (=most ingoing lines), since they don't need any electricity or instrumentation to function - and for the outer isolation valves, there's a hand wheel to enable opening and closing of the valve. The isolation condenser relies on relatively small pressure differences, and a check valve in the return line may thus be impossible to arrange, but taking into account the importance of the safety function, loss of both cooling circuits due to a single loss of a DC measuring voltage supply just seems too thin.

I agree, just trying to keep everybody up on a questioning attitude.

 

[tsutsuji]

Since the description of the event was complete blackout in some control rooms, and the control room lights are an essential electric load with emergency AC or DC backup, TEPCO may not even have had exit lights in the control room. So when the emergency DC and AC supplies were flooded all lighting was lost. US fire codes would not allow that omission of the emergency exit lights, so it is a lesson that needs to be corrected where it exists.

Early reports also indicated they had to scrounge for flashlights and batteries. But although loss of lighting was a complication, I really doubt it made the difference between success and meltdown.

Actually, what they say in http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110810e21.pdf is that in the unit 1 & 2 main control room, "only the emergency lighting remained on the unit 1 side and the unit 2 side was in total darkness". So these emergency lights were installed. Why those on the unit 2 side of the room remained dark is a mystery, though.

 

[NUCENG]

While all that is true, one must remember that there may be less than an hour to initiate core cooling in order to prevent fuel uncovery. It is very unlikely that workarounds can be fixed to locate the correct instrumentation cabinets, return the DC for the measurement circuits and AC for the inner isolation valves, and then steer the inner valves open in such a short time period. It's complicated enough to get a grip of what's going on and manually open the outer valves within an hour or so - simultaneous spurious closure of the non-hand-manageable inner valves makes the task too challenging to be reliable.

In ASEA BWRs, the logic has been to use check valves as the inner isolation valves whenever possible (=most ingoing lines), since they don't need any electricity or instrumentation to function - and for the outer isolation valves, there's a hand wheel to enable opening and closing of the valve. The isolation condenser relies on relatively small pressure differences, and a check valve in the return line may thus be impossible to arrange, but taking into account the importance of the safety function, loss of both cooling circuits due to a single loss of a DC measuring voltage supply just seems too thin.

That makes sense, and extended, reliable operation of ICs, ECCS systems, and Venting, are already on the action lists.

 

[tsutsuji]

The other day I was watching the following NHK video about manhole covers being ejected　by the tsunami and the new type of covers being designed to remain assembled with the hole even in case of a tsunami : http://www.dailymotion.com/video/xmsci6_3-11-yyyyyyyyyyyy_news

Perhaps this is not off-topic in a nuclear power plant thread as http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110810e21.pdf says "the work was made very difficult due to the darkness, pools of standing water from the tsunami, scattered debris obstructing the roads, missing manhole covers on the roads" (page 5) and "the severe working environment (darkness, scattered obstacles, missing manholes on roads)(...) prevented the work from progressing as expected" (page 44).

http://www.nikkansports.com/general/news/f-gn-tp0-20111002-844020.html According to the records of solar-powered seismometer(s), explosion happened only once on 15 March at 06:12 AM. It is inferred that it is the explosion at unit 4. The reason why no hydrogen explosion occurred at unit 2 is that, by chance, [unit 2's] blowout panel was removed by unit 1's explosion, enabling the hydrogen gas to be released to the outside.

The details on Tepco's seismometer analysis are available in the internal investigation interim report　http://www.tepco.co.jp/cc/press/betu11_j/images/111202c.pdf

*The map on page 87/140 shows the locations of the five seismometers (A, B, C, D, E)
*The records of seismometer D for the unit 1 explosion on 12 March 15:36 and for an earthquake on 12 March at 10:13 are provided as examples on the top of the next page. Then you have plots of the distance from unit 1 in function of the arrival time of P-wave and S-wave at each seismometer during unit 1's explosion. Then you have the same kind of plot for the unit 3 explosion. You can see that wave arrival time and distance are perfectly linearly correlated.
*On page 89/140 you have the plot for the 15 March 6:12 event, showing the distance from unit 2 on the left plot and the distance from unit 4 on the right plot. You can see that only the plot with the distances from unit 4 provides linear correlations.

 

[etudiant]

The other day I was watching the following NHK video about manhole covers being ejected　by the tsunami and the new type of covers being designed to remain assembled with the hole even in case of a tsunami : http://www.dailymotion.com/video/xmsci6_3-11-yyyyyyyyyyyy_news

Perhaps this is not off-topic in a nuclear power plant thread as http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110810e21.pdf says "the work was made very difficult due to the darkness, pools of standing water from the tsunami, scattered debris obstructing the roads, missing manhole covers on the roads" (page 5) and "the severe working environment (darkness, scattered obstacles, missing manholes on roads)(...) prevented the work from progressing as expected" (page 44).


I would guess that it might be safer to simply substitute open grids to cover the man holes, thereby avoiding any pressure differentials. Of course that will mean the service area covered will no longer be dry under normal circumstances, but the pressures generated by the tsunami are surely enough to rupture any such containment or piping if it is enclosed.

 

[tsutsuji]

In the second half of the video you can see the new design and how it is tested.

 

[tsutsuji]

I translated attachments 10-2, 10-3 and 10-4 of the internal investigation interim report on https://www.physicsforums.com/showpost.php?p=3674463&postcount=11913

Now I will translate the report's main text at http://www.tepco.co.jp/cc/press/betu11_j/images/111202c.pdf from page 99/140 to 104/140.

I am wondering about the following:
* Isn't the report of radiations higher than normal at reactor building entrance at 17: 50 the earliest radiation release record for this accident ?
* Why didn't they try to start IC (B) after 18:25 when IC (A) seemed to be broken with no steam observed ? Isn't IC (B) supposed to be the backup of IC (A) ? Could they not open the MO-2B and MO-3B valves as they did with other motor-operated valves ?
* What is the cause of the DC power restoration at 18:18 ?
* What were the plant operator's plans concerning the IC before the 18:18 DC power restoration ?

Translation:

3. Examination of the isolation condenser
As mentioned in the above plant behaviour sequence of events, it can be thought that core damage progressed within a short time interval after the arrival of the tsunami, so that it can be thought that it is possible that the isolation condenser status, as an equipment performing reactor cooling in the initial stage after shutdown, brought consequences on the progression of events. The sequence of events which drew our attention concerning the isolation condenser is collected below.

- - - - - -
Reference: outline of isolation condenser (see construction in attachment 10-2)
* The isolation condenser being for cooling the reactor when the reactor has been isolated, it is an equipment which extracts steam from the reactor, and returns it to the reactor as water after exchanging heat with the coolant water accumulated inside. It is installed in unit 1 only.
* The isolation condenser is composed of two systems, system A and system B, and the steam circuits are built with 4 valves. Isolation condenser entrance and exit are equipped two valves each, in a configuration where the primary containment vessel is interposed between them. The valves inside primary containment vessel are driven by AC power, and those outside by DC power.
* Normally, it is in standby with the valves outside primary containment vessel (valve 3A and valve 3B) being closed, and all the others being fully open. The starting and shutdown of the isolation condenser is performed by opening or closing the 3A and 3B valves.
* Reactor pressure is controlled by intermittent opening and closure of the aforementioned valves.
- - - - - -

< Sequence of events related to the isolation condenser >

11 March 14:52 ; automatic start of isolation condenser
Together with the loss of external power, the power source of the emergency bus was lost, the main steam isolation valve was automatically closed. Due to the "high reactor pressure (7.13 MPa [gage])" signal, both isolation condenser systems started automatically, and as reactor depressurisation and cooling began, reactor pressure started to decline.

Around 15:03 ; manual shutdown of isolation condenser
As the drop of reactor pressure that resulted from the start of the isolation condenser was quick, it was judged that it would not possible to respect the 55°C/h reactor coolant temperature variation speed specified in the operation manual, and the isolation condenser return line valves (MO-3A, 3B) were momentarily turned to "fully closed". The other valves being open, a normal standby status was obtained. As a result, reactor pressure rose again.
After this, in order to regulate reactor pressure at about 6 or 7 MPa, it was judged that one isolation condenser system was enough, and deciding to regulate with system A, and by opening and closing the return line isolation valve (MO-3A), the regulation of reactor pressure began.

15:37 ; loss of electric power
Because of the tsunami flood, all AC power was lost at unit 1. Moreover, DC power was also lost. For that reason, in the central control room, not only lighting but also monitoring instruments and all indicator lamps were extinguished. It created a situation where the isolation condenser's valves open/closed indicators cannot be checked and the isolation condenser's valves cannot be operated.

Around 16:42 ; temporary recovery of water level system
From around 16:40 to around 17:00, it became temporarily possible to check the until then unavailable reactor water level (wide band) (at 16:42, equivalent to TAF (top of active fuel) + 250 cm), and it was confirmed that it had declined since the tsunami arrival.

17:19 ; attempt to check the isolation condenser on location
Because it was impossible to check the isolation condenser from the central control room, it was decided to go to the location where the isolation condenser is installed, and to check such things as the level of condenser shell water, which is the isolation condenser coolant. A plant operator headed for the location, but because the radiation level there (at the entrance of the reactor building) was higher than normal, at 17:50 he temporarily came back.

18:18 ; recovery of DC power for A system outer side isolation valves / opening of A system outer side isolation valves
Whether or not because the DC power had become temporarily unstable in consequence of the tsunami, part of the DC power was later restored and operators found that the isolation condenser's feed line isolation valve MO-2A's and return line isolation valve MO-3A's "closed" green lamps were lit. As the normally open feed line isolation valve (MO-2A) was closed, it might have been thought that all the isolation condenser's isolation valves had been closed following the emission of the "isolation condenser pipe rupture" signal, which is an action toward the safe side following the loss of the DC power used for the detection of "isolation condenser pipe rupture". However, the operators expected that the isolation valves on the primary containment vessel's inner side (MO-1A, 4A) would be open, they performed the valve opening operation of isolation condenser return line isolation valve (MO-3A) and of feed line isolation valve (MO-2A), and the status indicating lamps changed from "closed" to "open".
After valve opening, as the monitoring instruments were not working due to the loss of electric power, and as they had no way to check if the isolation condenser is running, the operators confirmed steam generation from the isolation condenser venting pipe based on the steam generating sound and on the steam that could be seen beyond the reactor building.

18:25 ; A system outer side isolation valve closure
Because steam generation stopped after a while, they closed the isolation condenser's return line isolation valve (MO-3A) and they shut the isolation condenser down.
Moreover, as a response that can be operated in the central control room, they advanced the construction of a water injection line with the fire extinguishing system.
In the midst of unpredictable events occurring one after another, the operators thought about the primary containment vessel's inner side isolation valves (MO-1A, 4A) being closed by the isolation signal, but they worried about the possibility that the shell water, which is the isolation condenser's coolant, had disappeared for some reason. While thinking that the isolation condenser is not functioning, conscious that the construction of the line which is necessary to replenish the shell with water, was not ready, they temporarily closed the return line isolation valve (MO-3A).

Around 20:50 ; construction of reactor water injection line with the fire extinguishing system
The construction of the reactor water injection line with the fire extinguishing system being completed, the diesel driven fire extinguishing pump was started. This brought the prospect of replenishing the isolation condenser shell with coolant water. Later, when operators checked the operation status of the isolation condenser, they found that the closed status indicating lamp of the return line isolation valve (MO-3A) was unstable and starting to fade out.

21:19 ; temporary recovery of reactor water level gauge
It was discovered that the until then unavailable reactor water level was indicating TAF (top of active fuel) + 200 mm.

Around 21:30 ; Opening of valve 3A (start of system A)
Although the reactor water level is above top of fuel, the steam driven high pressure water injection system pump (HPCI)'s electric power faded out and it became impossible to run it. At that time, the isolation condenser was the only high pressure cooling system that could be expected to run. Normally, even if there is no shell replenishment, the isolation condenser can run for about 10 hours. As the diesel driven fire extinguishing pump has been started, it has also become possible to respond to the replenishment of the isolation condenser shell, and as the worry of a lack of water in the shell is diminishing, considering that in the present situation it is not known when the the isolation condenser can be operated again, as the running of the isolation condenser, a high pressure cooling system, is being expected, the opening operation of the return line isolation valve (MO-3A) that had been temporarily closed was performed at around 21:30, the valve opened, and the steam generation was confirmed with the steam generation sound and with the observation of the steam beyond the reactor building. Furthermore, the electric power plant response headquarters' electric power team, going outside of the seismic-isolated building, also confirmed steam generation.

29 March ; recovery of the shell water level gauge
The isolation condenser's shell water level gauge was recovered.

1 April ; check of valves' open or closed status using the isolation condenser's valve control circuit
Forming a part of recovery work, the valves' open or closed status was checked using the conduction status of the isolation condenser's valve control circuit. Due also to the overheat during the accident, it was not possible to check the status of the valves on the inner side of the primary containment vessel, but it was possible to determine the status of those on the outer side. Isolation condenser system A's 3A and 2A valves were open, and isolation condenser system B's 3B and 2B valves were closed.

3 April ; check of isolation condenser shell side water level
As checked with the isolation condenser water level indicator in the central control room, the A system's water level was 63% and that of B system was 83%.

18 October ; inspection on location
The status of the isolation condenser on the outer side of the primary containment vessel could be checked based on a visual inspection on location. Damages were not observed on the main body or on the main pipes, and the valve status was the same as the one checked on 1 April with the circuit inspection. Furthermore, the isolation condenser's water level gauges on location indicated 65% for system A and 85% for system B, and on the same day the values indicated in the central control room were the same.

(to be continued)

 

[tsutsuji]

The examinations noted below are based on the above sequence of events and on the previously presented analysis results.

< evaluation of the action of the isolation condenser immediately after the earthquake >

* Based on the sequence of operations before tsunami arrival, it can be thought that the status of valves at the time of tsunami arrival was (for system A) that valve 3A was closed, and the 3 other valves were fully open. Concerning system B, valve 3B was closed and the 3 other valves were fully open.

* Also, concerning system A, it was confirmed at around 18:18 that valve 2A, which had not been operated until then, was fully closed. Also, concerning system B too, the circuit inspection performed on 1 April confirmed that valve 2B, which had not been operated, was fully closed. (This fact was also confirmed by the inspection on location of degree of openness indicators on 18 October). It follows that although they were in open state before tsunami arrival and although they were not operated later, valves 2A and 2B were later confirmed closed.

* It is possible to confirm the action of valves 2A and 2B from the open/closed records of the transient recorders up to the initial shutdown operation, so that the possibility of an operator mistakenly operating the valves is ruled out. On the other hand, the design of the logic circuits ensures that in case of loss of the DC power supplying that circuit, the interlock is activated and all 4 valves in each isolation condenser system perform valve closure operations. In the present case, it is thought that due to the tsunami, the logic circuit's DC power was lost, and the valve closing order was activated by the aforementioned interlock. [attachment 10-3]

* Furthermore, the time needed to go from valve fully open to valve fully closed is 15 seconds in the case of the outer side valves, and 20 seconds in the case of the inner side valves. DC power was lost because of the tsunami flooding, but during the interval between the activation of the interlock, which is due to the consequences of tsunami flooding on instrumentation DC power, and the loss of driving DC power, the valves automatically performed valve closure operations.

* If driving DC power is lost during closure operation, an intermediate degree of openness is obtained, but as mentioned above, as it was confirmed that valve 2A and valve 2B were fully closed, there is a high probability that power panels were inundated by the tsunami flooding, the isolation signal was sent to the isolation condenser's valves, and they automatically performed full closure before the driving DC power was lost.

* Also, the valves on the inner side of the primary containment vessel are driven by AC power, and their open or closed status depends on the timing of the loss of instrumentation DC power and the loss of AC power. It is not possible to determine the inner side valves' open or closed status, and everything is possible between fully open and fully closed.

* As a result, the isolation condenser's status after tsunami is not determined by its status before tsunami. [attachment 10-4]

< connection with core damage >

* Due to the electric power loss caused by the tsunami, the isolation condenser's automatic isolation interlock was activated, it became impossible to operate the isolation condenser, and its function was lost. According to the accident analysis code (MAAP)'s analysis results, because it was immediately after reactor shutdown when the decay heat is the highest, it can be thought that reactor water level decreased in a short time, and this lead to fuel exposure (at around 17:46 top of active fuel is reached).

* Then, the isolation condenser (system A)'s DC power was restored, at 18:18 the isolation condenser (system A)'s isolation valves (valves 3A and 2A) were opened, steam generation was confirmed, but as steam generation stopped, at 18:25 valve 3A was closed. According to accident analysis code (MAAP), at that point the core was already exposed, and it is estimated that regardless whether the isolation condenser continued to operate or not after 18:18, core damage would have resulted anyway.

< estimate of inner side isolation valve status after tsunami >

* On 18 October, an onsite inspection of the isolation condenser was performed, and based on the onsite water level gauges it was confirmed that A system's water level was 65% and that B system's water level was 85%. It was also confirmed that the same values were displayed in the central control room.

* Because the water level values indicated on the onsite indicators and those read in the central control room are the same, it can be thought that the data transmission is accurate. Hence it can be thought that the values read in the control room in the past were also indicating the output of the onsite indicators.

* It follows that it can be thought that the values obtained in the central control room on 3 April (A system: 63%, B system: 83%) are also reflecting the values of the onsite indicators. Those results are different from those obtained on 18 October, but since April, it is thought that, for some reasons, the indicated value changed by 2%.

* The isolation condenser's 3A valve was open after the tsunami from 18:18 to 18:25 and after 21:30. Due to measuring instrument error, etc. it is difficult to calculate an accurate estimate, but the water level indicated on system A's water level gauge amounts to more than the consumption necessary to cover the heat generated by the reactor between the earthquake and the tsunami arrival. Hence, although it is not possible to determine the degree of openness of A system's inner side valves, it can be thought that they are open. After the tsunami, some amount of heat removal was performed during the running of the isolation condenser, and it is thought that, as a result, the water level declined to 65%.

* This fact is also in accordance with the result of the witness hearing confirming that steam was generated by the isolation condenser's vent pipe when valve 3A was opened at 18:18 and at 21:30.

* However, as a considerable amount of water is remaining in the shell, it can be thought that, as a result, the heat removal performed by isolation condenser system A was limited. [Attachment 10-5]

4 - Summary of plant behaviour

* Due to the electric power loss caused by the tsunami, the isolation condenser's automatic isolation interlock was activated, it became impossible to operate the isolation condenser, and its function was lost. Then, reactor water level decreased in a short time, and this lead to fuel exposure (top of active fuel is reached) and core damage. During that period, it was a situation where, due to loss of electric power, grasping plant status was difficult.

* The isolation condenser (system A) was operated on 11 March at 18:18 and at 21:30, but according to analysis results, it is estimated that regardless whether the isolation condenser continued to operate or not after 18:18, core damage would have resulted anyway.

* On the other hand, on 11 March after 21, when a temporary power source enabled the recovery of the water level gauge, the indication that reactor level was above top of active fuel was obtained, but at that time there is not enough information for generally judging that this is a wrong indication. At the emergency response headquarters (at the plant, at the main office), nothing lead to the awareness that the isolation condenser was not running. Because of the rise of radiation dose in front of the reactor building airlock on 11 March at around 23, and because the drywell pressure measured on 12 March at around 0 midnight was extremely high, there was an awareness of the possibility of core damage.

* On 12 March at around 3, because reactor pressure declines although pressure reduction operation has not been performed, the possibility of reactor coolant pressure boundary damage, resulting from core damage, is being shown and this is a hint that in a short time core damage made a considerable progression.

* Also, according to accident analysis code analysis results, top of active fuel was reached 3 hours after earthquake, core damage started about 4 hours after earthquake, which means that the accident progressed at high speed toward core damage, and this is in accordance with the sequence of real measured phenomenons.

* When venting the suppression chamber, the radiation measured in monitoring car rose temporarily, but the rise of the background level was limited. It is estimated that the hydrogen generated together with core damage could not be perfectly contained in the primary containment vessel, leaked into the reactor building, and was the cause of the reactor building explosion.

(end of translation)

See also attachment 6-8 (3) with the photographs of IC body, pipes, valve openness degree indicators, water level gauges on http://www.tepco.co.jp/cc/press/betu11_j/images/111202f.pdf page 157/314.