Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[tsutsuji]

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110803_04-j.pdf Water treatment facility bypass line No. 1 and bypass line No. 2 diagram (not yet translated into English)

http://www.nikkei.com/news/headline/related-article/g=96958A9C889DE1E1E1E3E4E4EBE2E2E0E2EAE0E2E3E39797E0E2E2E2;bm=96958A9C93819595E2E3E2E0E58DE2E3E2EAE0E2E3E3E2E2E2E2E2E2　 On 1 August, Tepco announced that the ground water shelding wall construction would start seaside before January. It is an 800 m long, 30 m deep wall made of steel tubes and plates. The construction will take two years. Its cost will be announced together with the April-June financial closing. The decision to build or not to build a wall hillside will be taken by January.

http://www.meti.go.jp/press/2011/08/20110803004/20110803004-2.pdf The water treatment facility treated 6190 m³ during the 27 July-2 August week. Utilization rate 6190/(50*24*7) = 74%.

http://www.mbs.jp/news/jnn_4792306_zen.shtml A new independently acquired JNN video taken last week. The car passes close to the 10000 mSv/hour pipe. Then workers are shown being busy with hoses outside the water treatment facility. Their geiger counter reaches 4 mSv/hour as they are driving seaside close to unit 4 turbine building. This is higher than the 2.2 - 2.5 mSv values written on Tepco survey maps.

http://www.tepco.co.jp/cc/press/betu11_j/images/110803l.pdf This is the Japanese language press release version of Tepco's report to NISA about the stability of securing cooling to units 1,2,3. According to http://mainichi.jp/select/weathernews/news/20110803ddm008040042000c.html it contains details such as the earthquake safety of the system, or how much time it would take to recover from a blackout. I have begun to read some pages. Page 11-1 (pdf page number 33) tells how much time it would take to reach 1200°C if cooling stops : 15,14,13 hours respectively for units 1,2,3.

 

[zapperzero]

Using the Freedom of Information Act, UCS obtained the Draft State of the Art Reactor Consequences Analysis (SOARCA) Report

Any idea where they got their 10E-6/reactor-year probability for an accident?

 

[joewein]

Does this not mean that the stack emissions were the primary source of the contamination plumes that Japan is now starting to focus on?

I think the bulk of the radioactive release that came down over land was from units 2 and 3, not unit 1.

An earlier posting had noted that the stack emissions for the Swedish reactors are put through a large absorption bed. Should this not be a requirement generally, along with the hardened stack?

What I have read before, but can't provide the source of right now, is that the normal venting path uses filters / charcoal scrubbers, but the hardened venting system added in the 1990s (based on modifications installed in US reactors after TMI-2) uses an unfiltered vent path. From what was explained, the back-pressure from filters would have been too high for a quick release of large amounts of gases threatening the integrity of the containment vessel.

When the JP government first announced that some venting would take place at unit 1 they also said the vented gases would go through filters. The http://www.iaea.org/newscenter/news/2011/fukushima110311.html" :

Japanese officials have also reported that pressure is increasing inside the Unit 1 reactor's containment, and the officials have decided to vent the containment to lower the pressure. The controlled release will be filtered to retain radiation within the containment.

Christoph Mueller of http://www.tec-sim.de/" writes (translated from the German original by me):

1) Clarification about the high radiation figures in the pipe elbow. It is a typical phenomenon that tiny particles in a gas flow deposit at an elbow, as they have difficulties, flowing "around the corner". They fly straight on and hit the wall and remain stuck there. Therefore highly radioactive deposits form at elbows. This has nothing to do with rain or other effects, as some "experts" theorize in the press today. In all elbows, through which radioactively polluted gases gases flowed in a controlled venting such deposits can be found. It is only a matter of time when they are detected.
2) Clarification about the contaminated rooms in unit 1. Since at the point of controlled venting of the containments the containment atmosphere was highly contaminated because of the melt-down, a large portion of the radioactivity will have condensed in the filters, which because of that will radiate strongly. That's exactly what TEPCO has now found while inspecting the filter room in unit 1.

(See his original post: "Aktuell 3.8.: Klarstellung zu den heißen Stellen in der Abluftleitung und I am Filterraum" for the German text)

EDIT: I found my source for the statement that the new venting system was unfiltered. It was http://www.nytimes.com/2011/05/18/world/asia/18japan.html?pagewanted=2&_r=1":

The improved venting system at the Fukushima plant was first mandated for use in the United States in the late 1980s as part of a “safety enhancement program” for boiling-water reactors that used the Mark I containment system, which had been designed by General Electric in the 1960s. Between 1998 and 2001, Tokyo Electric followed suit at Fukushima Daiichi, where five of six reactors use the Mark I design.
(...)
The fortified venting system addressed concerns that the existing systems were not strong enough to channel pent-up pressure inside the reactors in an emergency. Pressure would be expected to rise along with temperature, damaging the zirconium cladding on the fuel rods at the reactor core and allowing them to react chemically with water to produce zirconium oxide and hydrogen gas.

The new vents were designed to send steam and gas directly from the reactor’s primary containment, which houses the reactor vessel, racing past the usual filters and gas treatment systems that would normally slow releases of gas and eliminate most radioactive materials.

 

[zapperzero]

The normal venting path uses filters / charcoal scrubbers, but the hardened venting system added in the 1990s (based on modifications installed in US reactors after TMI-2) uses an unfiltered vent path.

IOW, the operators got permission to release contaminants directly to the environment during emergencies instead of having to spend money on replacing their existing, severely undersized filters and scrubbers.

Neat.

 

[rmattila]

What I have read before, but can't provide the source of right now, is that the normal venting path uses filters / charcoal scrubbers, but the hardened venting system added in the 1990s (based on modifications installed in US reactors after TMI-2) uses an unfiltered vent path. From what was explained, the back-pressure from filters would have been too high for a quick release of large amounts of gases threatening the integrity of the containment vessel.

Regarding the Finnish/Swedish BWR:s referred to previously:

With the exception of the already-closed Barsebäck units, the Swedish BWR:s (and their two sister plants here in Finland) were backfitted with two separate severe-accident systems: the containment overpressure protection system (system 361 in ASEA coding) and the filtered venting system (system 362). 362 is filtered with so called SAM scrubbers, whereas 361 is unfiltered.

See page 12/51 on this document for the solution by TVO.

Both systems are equipped with frangible plates such that the filtered line (system 362) will open in lower pressure (typically around 4 bar) and the unfiltered system 361 at a couple of bars higher. The sole purpose of the unfiltered system 361 is to provide a pressure release path in a rapid overpressurization of the containment, caused by a large-break LOCA coincident with loss of the pressure suppression function of the containment. Depending on the plant, the manual valves of the unfiltered system 361 will close automatically or manually 10-20 minutes after the LOCA is detected, and will remain closed thereafter.

So the venting after fuel damages are to be expected is always done through the filtered-venting line (system 362). Since it has frangible plates, human decision will not be needed to initiate the first venting, but after the venting begins and the manual valves are closed for the first time, re-opening them will require manual action. Both lines are "hardened" in the sense that they are dimensioned for the severe accident conditions, and are separated from the normal containment venting lines used for atmosphere change etc. And as said earlier, they were installed in the late 80's following the TMI accident (which initiated the SAM system projects) and ultimately Chernobyl (which gave it more urgency).

 

[Bandit127]

Tepco have posted a video and PDF regarding the high dose rate (5 Sv/h) in Unit 1.

PDF:
http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110803_01-e.pdf"
Video (just a camera feed from the robot):
http://www.tepco.co.jp/en/news/110311/images/110803_01.zip"

i would guess it's a small but extremely hot piece of something
were the whole inside of that pipe filled with the stuff i doubt he'd be able get that close to it.
it's likely something that has been around neutrons(near reactor core) .
<snip>

old jim

My understanding is that the pipe that they found 10 Sv/h in is the same as the one inside Unit 1 and my interpretation of that is that it is coated or full of contamination.

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110802/index.html The robot found more than 5000 msV/hour in a room on unit 1 reactor building 2nd floor. The 10,000 mSv/hour pipe mentioned yesterday runs across that room.

 

[SpunkyMonkey]

http://www.mbs.jp/news/jnn_4792306_zen.shtml A new independently acquired JNN video taken last week. The car passes close to the 10000 mSv/hour pipe. Then workers are shown being busy with hoses outside the water treatment facility. Their geiger counter reaches 4 mSv/hour as they are driving seaside close to unit 4 turbine building. This is higher than the 2.2 - 2.5 mSv values written on Tepco survey maps.

Thanks! In the graphic in the video @ 4:24, it looks more like the 4 mSv/h reading is by the unit 3, not unit 4, turbine building. Right?

 

[radio_guy]

that 4 Sv/h source is really making the camera struggle.. I wonder what it looked like when they got closer? snow?

 

[etudiant]

Regarding the Finnish/Swedish BWR:s referred to previously:

With the exception of the already-closed Barsebäck units, the Swedish BWR:s (and their two sister plants here in Finland) were backfitted with two separate severe-accident systems: the containment overpressure protection system (system 361 in ASEA coding) and the filtered venting system (system 362). 362 is filtered with so called SAM scrubbers, whereas 361 is unfiltered.


So the venting after fuel damages are to be expected is always done through the filtered-venting line (system 362). Since it has frangible plates, human decision will not be needed to initiate the first venting, but after the venting begins and the manual valves are closed for the first time, re-opening them will require manual action. Both lines are "hardened" in the sense that they are dimensioned for the severe accident conditions, and are separated from the normal containment venting lines used for atmosphere change etc. And as said earlier, they were installed in the late 80's following the TMI accident (which initiated the SAM system projects) and ultimately Chernobyl (which gave it more urgency).

Would the venting still work if the power was out?
Presumably there need to be blowers to force the vented gasses through the filter media.

Separately, if the vent pipe is internally coated with enough material to provide a 10+ Sievert/hr dose, as suggested by the probes, does that not imply the stacks were a very large contributor to the wide dissemination of airborne contamination? Other than the initial blast at reactor 3, pretty much all of the site emissions were fumes from the SFPs and reactors, stuff that would be expected to redeposit locally. It is the plumes from the 200+ ft stacks that really disseminate emissions.
Joewein's reference to Sellafield seems quite apposite, that design did have filters on the stacks, which saved the country when the reactor graphite core burned. Would seem logical to re institute that precaution more broadly.

 

[SpunkyMonkey]

Would the venting still work if the power was out?

The vents did work. Units 1 and 3 containment vessels were vented by way of the stacks several times before they exploded. The exact time of each venting was recorded and can be found online. There are also Tepco-Camera stills showing stack emissions, one was just posted in this thread.

Separately, if the vent pipe is internally coated with enough material to provide a 10+ Sievert/hr dose, as suggested by the probes, does that not imply the stacks were a very large contributor to the wide dissemination of airborne contamination? Other than the initial blast at reactor 3, pretty much all of the site emissions were fumes from the SFPs and reactors, stuff that would be expected to redeposit locally. It is the plumes from the 200+ ft stacks that really disseminate emissions.

There's not much visible stack emissions at the time of the explosions. A faint wisp can be seen from the unit 3/4 stack when unit 3 blew.

What I wonder is why it's being said this new high reading is from residual emissions from the March 12 explosion. But it's not like the unit 1 stack was just discovered after months of looking for it. They've been all around unit 1 but only recently did this super-high reading arise. So imo it seems to reflect something new happening.

 

[rmattila]

Would the venting still work if the power was out?
Presumably there need to be blowers to force the vented gasses through the filter media.

Still regarding the ASEA plants in Finland:

Valves are manual (with a long shaft running through one room and a concrete wall to reduce dose rate), so no power is needed for venting. The running force is the overpressure in the containment, and only very small pressure difference is needed to force the gases through scrubbers, so the system has no blowers either.

So loss of power should not incapacitate the filtered venting system - since total loss of power is one of the most probable causes for a severe accident, it would not make much sense to make severe accident management systems depend on electricity. However, even though the system is equipped with the mechanical remote operation handles, it was not included in the plants' original design basis, and therefore is not quite optimally located within the plant. Thus the valve operations cause some burden to the workers in the severe accident conditions (climbing of several stairs etc.)

The operation philosophy is that the operators won't initiate the venting, but rather wait for the frangible plate to give in and start the venting by itself. After the pressure has been reduced to a given value, the operators will then close the valve manually. Thereafter, if the pressure would rise again to require venting, further ventings are done manually. For these later ventings the preferable route is from the wet well (the initial release being from the drywell) in order to take advantage of the scrubbing capabilities of the containment blowdown and spray systems.

 

[etudiant]

Still regarding the ASEA plants in Finland:

Valves are manual (with a long shaft running through one room and a concrete wall to reduce dose rate), so no power is needed for venting. The running force is the overpressure in the containment, and only very small pressure difference is needed to force the gases through scrubbers, so the system has no blowers either.

So loss of power should not incapacitate the filtered venting system - since total loss of power is one of the most probable causes for a severe accident, it would not make much sense to make severe accident management systems depend on electricity. However, even though the system is equipped with the mechanical remote operation handles, it was not included in the plants' original design basis, and therefore is not quite optimally located within the plant. Thus the valve operations cause some burden to the workers in the severe accident conditions (climbing of several stairs etc.)

The operation philosophy is that the operators won't initiate the venting, but rather wait for the frangible plate to give in and start the venting by itself. After the pressure has been reduced to a given value, the operators will then close the valve manually. Thereafter, if the pressure would rise again to require venting, further ventings are done manually. For these later ventings the preferable route is from the wet well (the initial release being from the drywell) in order to take advantage of the scrubbing capabilities of the containment blowdown and spray systems.

That seems like a well engineered design to reduce the impact of an accident.
It would be helpful to have some idea of the size of the installation needed, to ghet a sense of how easily it can be retrofitted.
Certainly it does give one pause, the idea that the hardened vents might in fact be the instrument for spreading the impact of a disaster over a much wider area. I am surprised that this design feature has not been discussed at all afaik in the US licensing program. What am I missing?

 

[NUCENG]

Any idea where they got their 10E-6/reactor-year probability for an accident?

See section 2.4 in the main report. Note that the Peach Bottom and Surry PRAs have been peer reviewed so this is really "state of the art."

Also note that seismic risk was already being reevaluated as described here:
http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/fs-seismic-issues.html.

The unmitigated Long Term SBO is the closest case to Fukushima Daiichi Units 2 and 3.
The effects of RCIC blackstart and black run and B.5.b capabilities added agfter 9/11 in the mitigated cases are significant.

 

[joewein]

What I wonder is why it's being said this new high reading is from residual emissions from the March 12 explosion. But it's not like the unit 1 stack was just discovered after months of looking for it. They've been all around unit 1 but only recently did this super-high reading arise. So imo it seems to reflect something new happening.

One thing that is different and that therefore comes to mind is that construction of the plastic cover around unit 1 started recently, meaning there is a lot more activity around the building and more chances to notice what was overlooked before.

Here is a link to http://www.nikkei.com/news/headline/article/g=96958A9C93819595E2E3E2E0E58DE2E3E2EAE0E2E3E3E2E2E2E2E2E2"

- the story says that the radiation was recorded on the outside surface of some external 'ventilation' plumbing near to the ground between reactors 1 & 2 by a worker clearing rubble. The upper limit of his meter is 10 Sv/hr and it maxed out, so the true figure is possibly higher. They speculate that radioactive materials released / leaked during the initial venting (pre-explosion) may have adhered to the pipe.

The "worker clearing rubble" quote to me suggests a connection to the cover construction.

Also, is it possible that recent rains could have washed condensate down the inside walls of the stack, concentrating them at the lower end, including the "elbow" joining it at the bottom?

[URL]http://tec-sim.de/images/stories/10svha.jpg[/URL]

 

[SteveElbows]

Does this not mean that the stack emissions were the primary source of the contamination plumes that Japan is now starting to focus on? It seems to indicate that the explosions may have played second fiddle to the stack emissions in propagating the radioactivity beyond the plant boundaries.

The picture that emerged from a variety of official data points to quite a collection of significant releases over a number of days early on. The releases that seem to be responsible for the plumes that caused significant land pollution were probably from reactor 1 and then later a much more significant one from reactor 2, which is blamed for around 90% of the total radioactive release into the air.

What I cannot say with any certainty is how much of this stuff came from vent stack rather than more directly from the reactor, turbine or waste processing buildings of each reactor after containment damage and explosions. Early contamination data from before reactor 1 explosion certainly points to some radioactive material having traveled off-site before the explosion, so I think its probably reasonable to expect that venting had a notable impact in this regard.

As for reactor 2, they had a lot of trouble trying to get venting procedures to work, so the picture as to whether venting happened successfully, to what extent, and at what time, is less than complete. And despite the official estimations for radioactive release putting so much blame on reactor 2, we never really got much more detail from company, government or media about this. But if we recall how the news of the suspected explosion near the suppression chamber was treated when it happened, it was considered a big deal and so it would be reasonable to think that this event, rather than venting via stack, caused a lot of the reactor 2 release. But I cannot rule out a large amount coming from the stack via venting either, not enough information & visual evidence to be sure.

I think Reactor 3 explosion stuff was lately carried away from land due to wind direction at the time, and I have not studied venting activity for reactor 3 or tried to connect it to times when site radiation levels rose.

Official data also shows that significant releases (albeit several orders of magnitude less than the peak release rate) continued for much of March, and it is possible that some of this was due to further venting.

The chart of estimated release rates over time may be of help when trying to get our heads round this stuff, or quite the opposite as it kills the simplified version of events:

 

[clancy688]

Official data also shows that significant releases (albeit several orders of magnitude less than the peak release rate) continued for much of March, and it is possible that some of this was due to further venting.

I think "albeit several orders of magnitude less" is correct but misleading. Actually, between March 30th and 31st 1900 TBq C-137 was released. That's 10-15% of the total number.
The discharge on March 30th was magnitudes smaller than the maximum discharge rate right after the tsunami, but it went on for a whole day. So it highly contributed to the total release.

https://www.physicsforums.com/showpost.php?p=3383355&postcount=10418

 

[etudiant]

The picture that emerged from a variety of official data points to quite a collection of significant releases over a number of days early on. The releases that seem to be responsible for the plumes that caused significant land pollution were probably from reactor 1 and then later a much more significant one from reactor 2, which is blamed for around 90% of the total radioactive release into the air.

What I cannot say with any certainty is how much of this stuff came from vent stack rather than more directly from the reactor, turbine or waste processing buildings of each reactor after containment damage and explosions. Early contamination data from before reactor 1 explosion certainly points to some radioactive material having traveled off-site before the explosion, so I think its probably reasonable to expect that venting had a notable impact in this regard.

As for reactor 2, they had a lot of trouble trying to get venting procedures to work, so the picture as to whether venting happened successfully, to what extent, and at what time, is less than complete. And despite the official estimations for radioactive release putting so much blame on reactor 2, we never really got much more detail from company, government or media about this. But if we recall how the news of the suspected explosion near the suppression chamber was treated when it happened, it was considered a big deal and so it would be reasonable to think that this event, rather than venting via stack, caused a lot of the reactor 2 release. But I cannot rule out a large amount coming from the stack via venting either, not enough information & visual evidence to be sure.

I think Reactor 3 explosion stuff was lately carried away from land due to wind direction at the time, and I have not studied venting activity for reactor 3 or tried to connect it to times when site radiation levels rose.

Official data also shows that significant releases (albeit several orders of magnitude less than the peak release rate) continued for much of March, and it is possible that some of this was due to further venting.

The chart of estimated release rates over time may be of help when trying to get our heads round this stuff, or quite the opposite as it kills the simplified version of events:

Thank you again for this excellent graphical summary of the initial releases.
My focus on the stack emissions comes from a background in air pollution. When standards were first set, it became rapidly clear to industry that tall stacks were a great way to meet standards by polluting a much larger area less intensively. Normally, emissions tend to redeposit within a few miles, but a multi hundred foot stack will lift the plume enough to diffuse through tens of miles. Unfortunately, this accident was so bad the dilution was entirely too little. That seems a serious weakness in the design, that it potentially magnifies the geographic scale of the damage considerably.

 

[tsutsuji]

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110803_04-j.pdf Water treatment facility bypass line No. 1 and bypass line No. 2 diagram (not yet translated into English)

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110804/0612_kasetsu_ho-su.html The hoses are being connected today, which requires shutting down the facility for half a day. SARRY will be launched on 8 August. With the increase of water treatment capacity, Tepco thinks they can achieve to bring the basement water levels to "safe levels" in unit 1 & 2 in the first decade of September, instead of the last decade of September as originally planned.

http://www.tepco.co.jp/cc/press/betu11_j/images/110803l.pdf This is the Japanese language press release version of Tepco's report to NISA about the stability of securing cooling to units 1,2,3. According to http://mainichi.jp/select/weathernews/news/20110803ddm008040042000c.html it contains details such as the earthquake safety of the system, or how much time it would take to recover from a blackout. I have begun to read some pages. Page 11-1 (pdf page number 33) tells how much time it would take to reach 1200°C if cooling stops : 15,14,13 hours respectively for units 1,2,3.

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110804/0600_saikai.html Tepco says they can recover from a blackout in 3 hours. If backup pumps and generators are available, they can recover in 30 minutes. If fire engines need to be brought, it takes 3 hours. When recovering from a loss of coolant the injection rate would be brought to a maximum to minimize the risk of hydrogen explosion.

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110803/index.html The middle-long term special committee had its first meeting yesterday. They listened to the explanations of Mr Yuichi Hayase who worked in a joint US-Japan research team on Three Mile Island. The committee president said he expects the work to remove the fuel from Fukushima Daiichi will take longer than at TMI, because the fuel damage is worse.

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110804/index.html He said it could be something like 20 years. According to Mr Hayase, it took one year until workers could enter TMI's containment vessel, and 3 and a half years until cameras could enter TMI's RPV. It took 3 years to complete water treatment at TMI. Then the work was delayed because of a growth of micro-organisms in the water. The fuel removal started 6 and a half years after the accident and was completed 11 years after the accident. However, with broken containment vessels and not only one but three reactors, the situation at Fukushima is worse than TMI.

http://www.meti.go.jp/press/2011/08/20110803004/20110803004-2.pdf The water treatment facility treated 6190 m³ during the 27 July-2 August week. Utilization rate 6190/(50*24*7) = 74%.

http://news.tbs.co.jp/newseye/tbs_newseye4792695.html It is a dramatic increase from the previous week's 58%, but the quantity accumulated in turbine buildings basements decreased by 60 tons only! This is due to either rainfalls or ground water seeping in.

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110803e2.pdf (English version of the above). Aren't they forgetting the extra 700 tons recently found in the Site Bunker Building ?

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110803e4.pdf This press release version of a Tepco report to NISA shows the various switching possibilities between the SARRY, KURION and AREVA systems.

On 28 July, Tepco reported to NISA the following SFP cooling systems design changes:

Unit 1
In order to maintain spent fuel pool water level and manage leakage, we have reported to monitor the water level of skimmer surge tank by existing skimmer surge tank water gauge. However, we will alter this method as the corresponding water gauge is broken.(ref. Figure 1)

Unit 2
We will alter the layout of weirs etc to prevent leakage of contaminated water outside building, as a result of consideration to reducing workers' exposures. We will partly amend the evaluation result of estimated leakage amount, in cases leakage occur. We will add method to treat accumulated water containing radioactive materials within buildings, in cases leakage occur (ref. Figure 2)

Unit 3
We will add method to treat accumulated water containing radioactive materials within buildings, in cases leakage occur

Unit 4
We will alter the area to install weirs, and alter method to drain leakage water, as a result of consideration to site conditions, working environment safety and constructing conditions (ref. Figure 3).
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110728e17.pdf

 

[rmattila]

It would be helpful to have some idea of the size of the installation needed, to ghet a sense of how easily it can be retrofitted.

Some idea of the design bases can be found in this presentation, pages 8 and 9. (Not the actual dimensions of the tanks, though.)

 

[nikkkom]

Still regarding the ASEA plants in Finland:

Valves are manual (with a long shaft running through one room and a concrete wall to reduce dose rate), so no power is needed for venting. The running force is the overpressure in the containment, and only very small pressure difference is needed to force the gases through scrubbers, so the system has no blowers either.

So loss of power should not incapacitate the filtered venting system - since total loss of power is one of the most probable causes for a severe accident, it would not make much sense to make severe accident management systems depend on electricity. However, even though the system is equipped with the mechanical remote operation handles, it was not included in the plants' original design basis, and therefore is not quite optimally located within the plant. Thus the valve operations cause some burden to the workers in the severe accident conditions (climbing of several stairs etc.)

The operation philosophy is that the operators won't initiate the venting, but rather wait for the frangible plate to give in and start the venting by itself. After the pressure has been reduced to a given value, the operators will then close the valve manually. Thereafter, if the pressure would rise again to require venting, further ventings are done manually. For these later ventings the preferable route is from the wet well (the initial release being from the drywell) in order to take advantage of the scrubbing capabilities of the containment blowdown and spray systems.

I imagine than if you are in SBO and all cooling is lost, it is actually better to vent (depressurize) early, while temperature in the reactor is not much higher than ~300 C - this will deprive Zr from the water to react with when/if temperature will rise later. Additional benefit is that in many cases, the vented steam will not be strongly radioactive yet, since fuel did not yet melt.

IOW: "dry" (waterless) meltdown is better than "wet" one. If you think you are heading towards one, try to at least make it "dry".

However, I do not know if (1) I am correct in my reasoning, and (2) whether current accident manuals and operator training include guidelines for such a severe accident case.

Can someone from industry comment on this?

 

[nikkkom]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110804/index.html He said it could be something like 20 years. According to Mr Hayase, it took one year until workers could enter TMI's containment vessel, and 3 and a half years until cameras could enter TMI's RPV. It took 3 years to complete water treatment at TMI. Then the work was delayed because of a growth of micro-organisms in the water. The fuel removal started 6 and a half years after the accident and was completed 11 years after the accident. However, with broken containment vessels and not only one but three reactors, the situation at Fukushima is worse than TMI.

The understatement of the year?

 

[tsutsuji]

The understatement of the year?

This is not a quote. It is my summarized translation. A more literal translation of the NHK article would be "It will take more time at Fukushima as the situation is deemed more serious/severe, compared with TMI" (instead of "worse than").

 

[rmattila]

IOW: "dry" (waterless) meltdown is better than "wet" one. If you think you are heading towards one, try to at least make it "dry".

However, I do not know if (1) I am correct in my reasoning, and (2) whether current accident manuals and operator training include guidelines for such a severe accident case.

Can someone from industry comment on this?

I'd say there is no such thing as a "wet meltdown": as long as at least half of the core is covered, the steam will provide some cooling to the top of the fuel to slow down the heat buildup, and cladding integrity is lost only after water will fall to -2m or so.

In the particular Finnish case, SAM procedures are entered when water level has been below +0.7 m for more than 30 minutes. At that time, the reactor pressure vessel is depressurized (to prevent high-pressure melt through, which the containment would not endure, and also to enable low-pressure core injection, if it would happen to be available) and the lower drywell is flooded in preparation for a melt-through. However, there's no point in venting the containment at that time, since there's a relatively high probability that no venting will be needed at all - venting will only be needed, if the power outage lasts for several hours after the core has melted, and the decay heat removal from the wet well can not be started.

In other words, venting is not a standard procedure to be applied in core melt situations, but rather an additional backup in case the containment heat removal has not been started after about 8 hours after the meltdown. If this can be done, then no venting will be needed to contain the core remains.

 

[Dmytry]

Regarding the filtered venting - just as Fukushima struck, NRC was in the final stage of preparation of a much lower impact estimate for core meltdowns:
http://www.physorg.com/news/2011-08-nrc-downsizes-deaths-nuclear-meltdown.html
, most interestingly, downsizing the Cs-137 release in a core melt from 60% of the inventory to 2% of the inventory.
My understanding is that it is the 60% estimate which justified Finnish/Swedish implementation of filtered high volume venting.
This reminds of Cockcroft's Folly...

 

[SteveElbows]

I think "albeit several orders of magnitude less" is correct but misleading. Actually, between March 30th and 31st 1900 TBq C-137 was released. That's 10-15% of the total number.
The discharge on March 30th was magnitudes smaller than the maximum discharge rate right after the tsunami, but it went on for a whole day. So it highly contributed to the total release.

https://www.physicsforums.com/showpost.php?p=3383355&postcount=10418

Yeah that's a fair point. Do we have any idea what happened on the 30th-31st? I don't remember hearing of anything specific in the past, is venting a possibility?

 

[MadderDoc]

Yeah that's a fair point. Do we have any idea what happened on the 30th-31st? I don't remember hearing of anything specific in the past, is venting a possibility?

I suggest the basis for the data estimate for that period may be in error. If there had in fact been any striking change or elevation in emissions during March 30th-31st, it would be expected to have shown up in the measurements from the site monitoring posts, but there is nothing there to be seen:

 

[nikkkom]

I'd say there is no such thing as a "wet meltdown": as long as at least half of the core is covered, the steam will provide some cooling to the top of the fuel to slow down the heat buildup, and cladding integrity is lost only after water will fall to -2m or so.

In the particular Finnish case, SAM procedures are entered when water level has been below +0.7 m for more than 30 minutes. At that time, the reactor pressure vessel is depressurized (to prevent high-pressure melt through, which the containment would not endure, and also to enable low-pressure core injection, if it would happen to be available) and the lower drywell is flooded in preparation for a melt-through.

ok...

However, there's no point in venting the containment at that time, since there's a relatively high probability that no venting will be needed at all - venting will only be needed, if the power outage lasts for several hours after the core has melted, and the decay heat removal from the wet well can not be started.

Now this doesn't sound good. What would be better - to vent a relatively cool (-> easier to scrub/filter) and relatively less radioactive steam earlier, or wait until after core melt and vent very hot, very radioactive steam/gas later?

In other words, venting is not a standard procedure to be applied in core melt situations, but rather an additional backup in case the containment heat removal has not been started after about 8 hours after the meltdown. If this can be done, then no venting will be needed to contain the core remains.

To me it looks that if core melt has started and progressed to a significant degree (say, 20% or more of core has melted), the reactor is not salvageable anyway, and attempts to reflood it actually may make the situation worse, not better: massive generation of VERY radioactive steam, possibly H2 from water/Zr reaction, explosions.

 

[MadderDoc]

<..>What would be better - to vent a relatively cool (-> easier to scrub/filter) and relatively less radioactive steam earlier, or wait until after core melt and vent very hot, very radioactive steam/gas later?<..>

It would seem to me as a lay man, that an early depressurization of the reactor pressure vessel into the S/C might have avoided much of the damage caused to the dry-wells by the excessive heat and pressure they must have been exposed to while the pressure and temperature were allowed to remain high in the RPV, while any cooling systems of the dry-wells became inoperable due to lack of power.

Naturally this would imply also an early need to do S/C venting, something one would have had to have the willingness as well as the technical ability to do in an efficient manner. Also it would have implied the assumption that the return of power to the plant would not be forthcoming in a relevant time frame.

As it worked out in reality, it appears to me, the RPV's were depressurized only with difficulty, and action to do so was postponed to become too late to avoid damage to the dry wells -- and in the end there proved to be major technical difficulties of doing any vents at all.

 

[robinson]

It's possible there actually are none, as in zero, plans in place for what to do when a meltdown is occurring. If there are plans, and drills done, what are they?

It seems like "run away" is the only actual response anybody is ready to do at a moments notice.

 

[MJRacer]

Regarding the filtered venting - just as Fukushima struck, NRC was in the final stage of preparation of a much lower impact estimate for core meltdowns:
http://www.physorg.com/news/2011-08-nrc-downsizes-deaths-nuclear-meltdown.html
, most interestingly, downsizing the Cs-137 release in a core melt from 60% of the inventory to 2% of the inventory.
My understanding is that it is the 60% estimate which justified Finnish/Swedish implementation of filtered high volume venting.
This reminds of Cockcroft's Folly...

Is there a reliable estimate of what percentage of Cs-137 inventory has been released so far at Fukushima?

 

[NUCENG]

It's possible there actually are none, as in zero, plans in place for what to do when a meltdown is occurring. If there are plans, and drills done, what are they?

It seems like "run away" is the only actual response anybody is ready to do at a moments notice.

Why would you say that when everything discussed here proves that isn't true?

US nuclear plants have Emergency Operating Procedures to preveny or limit core damage, These procedures are symptom based so it is directed at providing makeup to maintain water level, core cooling to remove decay heayt, rreactivity control to ensure shutdown, containment and presseure vessel pressure control and containment cooling to prevent containment failures. Once core damage is suspected (radioactivity increases, the Severe Accident management Guidelines are activated doing everything possible to protect containment and limit releases to the environment. These procedures are trained and drilled on simulators and are validated via walkthroughs in the actual plants.

Emergency drills are performed several times a year and exercise command control and communications for offsite response providers and regiulators. Evacuation plans are reviewed and approved for use and have been exercised on a limited basis.

After 9/11 a whole new set of requirements were included in US plants to address the effects of SBOs, aircraft attacks or large fires. These capabilitiies were reviewed and inspected at every US plant after March 11 to ensure they were operable and available.

Finally, TMI, Chernobyl, and Fukushima Daiichi have proven one thing - operators don't "run away." At TMI while they were trying to reduce the hydrogen bubble, they had to order people out of the control room becausae everybody wanted to help. Chernobyl operators died because they refused to abandon their responsibility. At Fukushima workers told us they were ready to die if necessary.

It is a legitimate point to say that the event at Fukushima did result in increased risk to the public and environmental and economic effects that will last for years. But we also know that more was involved in that failure that emergency procedures and planning. There was negligence on design basis, there were significant delays in venting containment, and there was deliberate understatement of risk after the event that contributed to the public risk.

Your flippant conclusion that there were no plans or drills is not helpful. They had plans and procedures at Fukushima. At one point there was a claim that they had to go look for them in another building. So my questions are:

What were the procedures?
Were they available?
Were Operators trained on their use?
Was everything available to execute the procedures (e.g., flashlight batteries, tools, etc.)?
Were they implemented?
Were they delayed?
Were they effective?
If they failed, why did they fail?

Look at a couple of specific points.

Operators at Unit 1 may have unintentionaly or inappropriately disabled the isolation condensers.
Uperator were not certain whether containment venting was successful at Unit 1.
Hardened Wetwell Venting may have been delayed by seeking approval until pressures exceeded even the design pressure for the hardened piping.
Fire trucks and pumps were used to inject water into the RPVs. Was the pressure within the capabilities of those pumps?

In short, it takes more than a one line zinger to add value to this discussion.

 

[NUCENG]

I suggest the basis for the data estimate for that period may be in error. If there had in fact been any striking change or elevation in emissions during March 30th-31st, it would be expected to have shown up in the measurements from the site monitoring posts, but there is nothing there to be seen:

Can you tell us more about that chart? Source of data, how many points were used, etc.?

 

[rmattila]

To me it looks that if core melt has started and progressed to a significant degree (say, 20% or more of core has melted), the reactor is not salvageable anyway, and attempts to reflood it actually may make the situation worse, not better: massive generation of VERY radioactive steam, possibly H2 from water/Zr reaction, explosions.

Since there's no oxygen in the containment, there is no risk of H2 explosions as long as the stuff remains contained. I'd rather say the uncertainties and different opinions related to the severe accident mitigation strategies in older BWR:s revolve around the issue whether to catch the molten core in dry containment and face the risk of core-concrete interaction and failure of lower penetrations, or flood the lower drywell and face the risk of steam explosions. The optimal solution varies according to the plant design and the assumed failure mode of the reactor pressure vessel. The Nordic approach of flooding the drywell before melt-through is based mainly on two arguments: the depth of the flooded drywell is large, and the core melt is expected to come through one failed penetration rather than as a large slump. Therefore the risk of steam explosion is analyzed to be smaller in comparison to the risk of containment failure due to melt-through to a dry containment.

New plants can (and at least here: will have to) be equipped with better core catchers in order to minimize the risk of steam explosions further.

But concerning early venting: to me it would not seem a sensible approach to backfit the containment to endure the severe accident conditions without any release, and yet vent it to the atmosphere at the early stages of the accident, when there are no signs of containment not being able to contain the radioactivity as designed. The need to vent arises only, if there's the additional failure of losing the capability to remove heat from the containment for an extended period of time after the core has already melted - i,e. even though the core would not be salvable and there was a risk of getting very high radioactivity in the containment, there would still not be need to do any venting as long as the heat removal from containment can be started within about 8 hours. Early venting would buy some more time - at the cost of making a release - in a situation where additional time is not necessary even needed, and additional time can be obtained also by other means such as initiating the water filling of the containment, which is a standard procedure in severe accident situations.

 

[robinson]

I was watching a TV documentary on the Fukushima disaster again. They interviewed an American who was there at the time of the earthquake. He described the terror and the running. The hallways were pitch black. Because the power was out. Battery powered emergency lights seemed to be missing from the plant.

He also described how everybody ran, twice. And described the cracks he saw in the buildings.

 

[Marita]

robinson , did you ever think the "running" was part of the the drill to get the unnecessary people to a designated area ... also some of the running was people going to stations to man equipment.

Think General Quarters on a Navy ship ... people running all over the ship , to an outsider it would look like total chaos.

Also if it was Running in Panic I would have expected to see a few more bodies found in the wreckage of the plant.