Fukushima: Unit 2 Discharge - Why Differs from Units 1 & 3?

[zapperzero]

I expect that the term dry venting is supposed to mean deliberate venting by one very specific route, as opposed to unplanned escapes from drywell containment due to some kind of containment failure.

I feel a bit guilty for being unclear - I meant to say that there is a number of routes by which one could achieve this kind of venting

One could route the steam through the SGTS or not; if the suppression pool water level is lower than that of the steam pipes for some reason, dry venting is also achieved, by default; there is also a choice of RPV valves that could be opened.

 

[Yamanote]

Probably the venting of unit 2 happened in the way Arnie Gundersen explained in one of his videos:

http://vimeo.com/36492960

 

[clancy688]

Nope, most definitely not.

That's only a "containment vent", not a reactor vent. Moreover, it doesn't vent high pressure. It only ensures, that, at high pressure, the pressure doesn't get any higher. And that only goes for the containment.
So when pressure inside the containment reaches a certain value (I think it was twice the normal pressure), the containment cap lifts and pressure escapes. The pressure falls slightly and then the cap closes again. So the pressure can't go over twice the normal pressure, but it also can't go below twice the normal pressure.
But that still doesn't vent the reactor pressure vessel. And the unfiltered release ("dry venting") can't happen that way. Because for reactor pressure to escape via a lifting containment cap, the gases have to travel through the wetwell (torus). Which they obviously didn't, that's why we have such a dirty release for Unit 2.
So there must've been an other path.

 

[zapperzero]

Because for reactor pressure to escape via a lifting containment cap, the gases have to travel through the wetwell (torus). Which they obviously didn't, that's why we have such a dirty release for Unit 2.
So there must've been an other path.

We don't know that. The torus may have been dry.

 

[SteveElbows]

Now you are confusing me a bit clancy688, either I misunderstood or else I don't think I agree with you about several things.

I thought that the terms wet and dry venting applies only to how the gases are released from drywell containment to the outside world, not how those gasses got into containment in the first place. If the gases were vented from the reactor vessel to the s/c, but then ended up in the drywell, and the drywell was then vented directly, I think that would still count as a dry vent.

A quote from the interim investigation report gives us some info about the venting of the reactor vessel into containment, with emphasis on how poorly the s/c seemed to handle this:

According to the reactor pressure gage, Unit 2 reactor pressure indicated 6.998 MPa gage at around 16:34 that day. It indicated still 6.075 MPa gage at around 18:03, more than one hour after they had started depressurizing.
They continued trying to open the SRV to depressurize the reactor. However, they had trouble in keeping the SRV open and the steam from the RPV barely condensed in the S/C because of high temperature and pressure in the S/C. Consequently, it took time to depressurize the RPV to the sufficient extent.
The reactor pressure was finally lowered to a level where water injection was possible at around 19:03 that day, when the reactor pressure gage indicated 0.630 MPa gage.

and

According to the reactor pressure gage, Unit 2 reactor pressure indicated higher than 1 MPa gage from around 20:54 until 21:18 that day (it indicated 1.463 MPa gage at around 21:18) and then it decreased due to depressurization. It again exceeded 1 MPa gage from around 22:50 until 23:40 that day (it indicated 3.150 MPa gage from around 23:20 until 23:25 that day) and then decreased again as a result of further depressurization. From around 00:16 until 01:11 on March 15, it again rose to over 1 MPa gage (it indicated 2.520 MPa at around 01:02 that day). At least during those periods of high values, Unit 2 reactor pressure seemed higher than the discharge pressure of the fire pumps and therefore it was highly likely that water had not been injected into the reactor.

and

From around 01:00 on March 15, Unit 2 reactor pressure indicated steadily staying above 0.600 but below 0.7 MPa gage and continuous water injection into the reactor became possible.

(from pages labelled as numbers 257,258 and 259 of http://icanps.go.jp/eng/120224Honbun04Eng.pdf )

Obviously what we can't tell from this is whether the bulk of the depressurisation of the reactor was achieved through the deliberate operation to depressurise the reactor vessel, as opposed to the fuel falling out of the bottom of the reactor vessel. Its been ages since I looked at the pressure charts for this period, I will have to refresh my knowledge on this front and comment again if anything seems relevant.

In any case I find the approach of looking at containment failure in terms of a very simple model of a cap rising and falling at certain pressures to be just a bit too flawed. We know that there are a range of potential failure points in containment, such as gaskets & flanges around not just the cap but also pipework points and equipment & personnel airlocks. And we know that heat contributes to degradation of such seals, so we aren't just dealing with pressure. Now it may turn out that the cap rising was a main culprit at one or all of the affected reactors, but its going to be a long journey to discover this. Certainly we saw some visual evidence of stuff escaping from locations around the top of containment at all three of the reactors. Reactor 3 offered the clearest view of this, could see it emerging from at least the area where the drywall concrete pit gates meet the floor area above the reactor. But we saw some signs at reactors 1 and 2 as well, along with the robot readings around that area of reactor 2 that I mentioned earlier. But this isn't proof that the cap lifted, cap or another part of containment could have degraded in a different manner.

 

[SteveElbows]

Does anybody know where a Japanese version of the following document might be found?

It is described on Tepco's English website as '- Fukushima Nuclear Accident Investigation Report (Interim Report Supplementary Volume) (PDF 212KB) '

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111202e16.pdf

The reason I ask is that in the section dealing with reactor 2, on page 39 it says the following, which as you can see is suffering from poor wording just at the moment when it could reveal something about the status of the rupture disk:

At 00:02 on March 15, the AO valve (bypass valve) on the vent line from the D/W was opened, and it was thought that the vent line, with the exception of the rupture disk, was completed, however several minutes later it was discovered that the AO valve (bypass valve) in the vent line from the D/W was closed. As a result it was not possible to determine whether venting was successful (ruptured status of the rupture disk ruptured).

 

[Sorai]

Does anybody know where a Japanese version of the following document might be found?

It is described on Tepco's English website as '- Fukushima Nuclear Accident Investigation Report (Interim Report Supplementary Volume) (PDF 212KB) '

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111202e16.pdf

This one: http://www.tepco.co.jp/cc/press/betu11_j/images/111202g.pdf

Seems to be the same info. They couldn't confirm whether or not the rupture disk was open:

3月15日0時02分頃、ドライウェルからのベントラインにあるAO弁(小弁)
の開操作を実施し、ラプチャーディスクを除くベントライン構成が完了した
と思われたが、数分後にはドライウェルからのベントラインにあるAO弁
(小弁)が閉状態であることを確認した。結果として、ベントの成否
(ラプチャーディスク開放の有無)は確認出来ていない。

 

[etudiant]

There has been speculation on EX-SKF that the rupture disk did rupture, but that debris from within the RPV clogged the piping for the vent. Given the violence of the events, this does seem plausible.

 

[SteveElbows]

This one: http://www.tepco.co.jp/cc/press/betu11_j/images/111202g.pdf

Seems to be the same info. They couldn't confirm whether or not the rupture disk was open:

Thanks very much, this is perfect, just what I needed. The Japanese version is much clearer than the English version. If I google translate the relevant sentence I get this:

As a result, the success or failure of the vent
(The presence or absence of open rupture disk) has not been confirmed.

Whereas the English version fails to make that point clearly at all, due to using rupture words three times.

So now we have heard them say that the rupture disk status is unknown, the theory on ex-skf that there may have been a vent for a couple of minutes, is not incompatible with official statements.

 

[SteveElbows]

I've just been saying elsewhere that it might be useful to look again at some graphs in this document:

http://www.kantei.go.jp/foreign/kan/topics/201106/pdf/attach_04_2.pdf

Specifically the CsI distribution graph for reactor 2 Tepco Case 2 on page 36, and Cs graph on page 37.

Note the presence of reactor building, FHB (fuel handling building I guess) and environment in these graphs, as well as the timings. Then compare to the other reactors.

 

[Yamanote]

It seems to me that successful venting through the designated path is almost impossible under accident conditions and without electricity, at least for the Mark I containment (don't know if it would work out better for other containment designs).

Is the "blow out panel" opened at unit 2 actually an improvisation or a feature?
Because units 1 and 3 explosions showed us that trying to contain gases and Hydrogen (which must be released anyway) within the secondary containment might not be the right approach in an emergency.

 

[zapperzero]

Is the "blow out panel" opened at unit 2 actually an improvisation or a feature?

Design feature. But it was popped from the outside, by hand, as it were, to avoid the accumulation of hydrogen. Seems to have worked, to a point.

Because units 1 and 3 explosions showed us that trying to contain gases and Hydrogen (which must be released anyway) within the secondary containment might not be the right approach in an emergency.

Nothing is supposed to be released into the reactor building. The very worst case scenario designed for is to vent some steam from the RPV through the S/C, then through the hardened vent (bypassing the undersized SGTS) and out the stack.

 

[rowmag]

Design feature. But it was popped from the outside, by hand, as it were, to avoid the accumulation of hydrogen. Seems to have worked, to a point.

Are you sure? My recollection is that they announced that they intended to open it, but then discovered that it had already popped off by itself. (As a result of the Unit 1 explosion?)

 

[zapperzero]

Are you sure? My recollection is that they announced that they intended to open it, but then discovered that it had already popped off by itself. (As a result of the Unit 1 explosion?)

No, I am not sure. This is my recollection however. I would be grateful if you can dig up a source, of course.

 

[Yamanote]

Nothing is supposed to be released into the reactor building. The very worst case scenario designed for is to vent some steam from the RPV through the S/C, then through the hardened vent (bypassing the undersized SGTS) and out the stack.

I agree that it was not supposed but happened for all three units. And like Steve Lochbaum said, even unit 4 paid sympathy to the others and exploded as well.

So in the future we have to suppose that unintended venting into reactor building will happen in such kind of accidents. Opening the blow out panel in advance might be a good idea and venting into another unit through shared equipment must be avoided.

 

[zapperzero]

shared equipment must be avoided.

You got that right, I think.

 

[rowmag]

No, I am not sure. This is my recollection however. I would be grateful if you can dig up a source, of course.

See the third slide (page-numbered "2") of this NISA presentation:

http://www.nisa.meti.go.jp/shingikai/800/28/006/6-3.pdf

"The reason there was no hydrogen explosion at Unit 2 is conjectured to be that by luck, the blow-out panel was opened as a result of the explosion at Unit 1, releasing accumulated hydrogen to the outside and thereby avoiding an explosion."

(The date on the title page of the above is 20 Jan. 2011, presumably a typo for 2012.)

 

[zapperzero]

See the third slide (page-numbered "2") of this NISA presentation:

http://www.nisa.meti.go.jp/shingikai/800/28/006/6-3.pdf

Thank you.

 

[SteveElbows]

Now that we can consider the possibility that reactor 2 could have dry vented for a few minutes, I would like to suggest that reactor 2 may be the source for the >10 Sv/hr contamination at the pipes at the bottom of the reactor 1 & 2 stack.

So I took another look at the gamma picture taken in August, on page 2 of this document:

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110802_01-e.pdf

I am interested in the gamma blobs on the right of the image. By looking at some other photos of the area, including the early high-ish res overhead ones from early on, I believe that this area of the photo is heading east. And we can just make out the large ducting pipe heading in that direction. But either side of that duct pipe are the smaller pipes, running parallel to the larger pipe, and their height above ground is near the bottom of the larger ducting. I believe the smaller pipe on this side is one from reactor 2, and the gamma blob fits with a point in this pipe.

You can see the pipe I mean in the last 3 photos on this page, it stands out fairly well because although it is small it looks pretty white in the photos. Download the zip of the images to see it even more clearly.

http://cryptome.org/eyeball/daiichi-npp/daiichi-photos.htm

 

[jim hardy]

Here's a somewhat scholarly report from last summer regarding release rates.

They suggest a ~30fold increase in I131 release rate "was probably caused by the damage of the suppression chamber of Unit 2 on march 15."

https://docs.google.com/file/d/0B-lM5qg9ztErMzY1NDAxNmUtNmFkNi00OWEwLWI0MzMtMGI5ZjFjZTk0NTBi/edit?hl=en_US&pli=1

i hope link works okay.
Please excuse if it's old hat, just i hadn't seen it before despite having tried to keep up.

 

[SteveElbows]

Thanks Jim. The documents detail is new to me but its conclusions are old news, having been used to come up with the official release estimates and that graph of release rates over time which we have sometimes referred to when talking about reactor 2. Incidentally that graph was updated slightly at some later point, refining in particular events around the 15th, so somewhere there is likely to be an update of that paper I would think, or a different but related study.

It was good to learn a bit more about the data they used. It matches pretty well with what we had gleaned from less wordy sources, that a lack of certain kinds of sampling at various points in time made some of the estimates rather uncertain. For example they mention that certain data was only available when the march 15th plume met wet weather in the north west , giving them a much better look at a narrow range of time than the 15th as a whole.

And for example their focus on the suppression chamber is very understandable given the explosion report, but since the report was written they have become less certain as to what exactly happened in the region of the suppression chamber, backing away from the explosion theory somewhat whilst not denying the possibility that the suppression chamber was damaged.

These are also some of the reasons why we have only recently had this discussion about the possibility that reactor 2 did actually manage to dry vent for a few minutes around midnight. We can find data that shows radiation levels rising at locations south of the plant in the hours after midnight, but they aren't of the type most used in the report, which gets its best glimpse of reality much later on the 15th when things have gone to the north west and fallen to the ground with the wet weather.

Given the stuff that fell to Earth later on the 15th, and the radiation levels on site during the daytime on the 15th, I don't think anybody is trying to suggest that the possible midnight dry vent was more significant than what came out of the reactor for hours during the day. But the vent is of interest because it has largely been discounted in the past, many of us thought that it was reasonably certain there had been no vent due to the wording used in various reports, lack of pressure drop, etc. But if it did happen then its of interest both in terms of demonstrating the differences in what came out of reactor 2 compared to the other reactors, and in possibly explaining the story of the contamination that went towards Tokyo as opposed to the stuff which affected the ground in the north west so much which is better understood. Mind you even though its better understood I am sure reactor 2 still manages not to get star billing when discussing the north west contamination in most reports, which often focus more on the weather and previous emissions from reactor 1 than the bad story of reactor 2 on the 15th.

When looking at what happened during the daytime of the 15th I no longer make the assumption that the stuff came mostly from the suppression chamber, since we don't actually know how much came out of the drywell as it depressurised that morning. Much later we saw signs of stuff emerging from refuelling floor level in the area of the drywell top, but that may have been responsible for relatively little emissions compared to, for example, the drywall depressurising via the suppression chamber failure point. I probably need to look at those graphs again that showed estimates for where substances may have gone, the one that mentions the FHB, but I don't expect to learn much and that stuff was based on modelling anyway. If we don't learn much more about reactor 2 from the next endoscope mission, then I am prepared for a very long wait before getting more detail that could shed light on nature of suppression chamber failure and its role in the emissions of the 15th.

Another reason why I try not to assume too much that the suppression chamber damage was the main release pathway, is that it was about the only form of containment damage that they have wanted to mention much, especially at the time, so it provided a very simple bit of narrative at that moment, 'here is the event that has taken things to another level'. Even later we only got vague comments that the possibility of containment damage at the other reactors could not be denied. We've seen them taking air samples from above likely failure points of refuelling floor level containment top areas, and sometimes steam emerging from such areas on videos, primarily at reactor 3 but occasional glimpses from others.

So for me right now, if I am looking for a shorthand way to describe why reactor 2s emissions were so bad, I'll look more broadly than the suppression chamber. The possible dry vent didn't change the pressure in a notable way, and so I can still speculate that reactor 2 was different from the others because one or more parts of containment failed and released substances whilst the containment was under rather high pressure, unrelieved by scrubbed venting. Whereas at the other 2 reactors, for all we know containment failures may not have had such an opportunity to release substances, due to venting or the damage to containment happening due to heat or explosions at a time when the pressure wasn't at such a peak. However as I write this I have forgotten some key data trends from other reactors so that last statement may be easy to disprove.

 

[jim hardy]

backing away from the explosion theory somewhat whilst not denying the possibility that the suppression chamber was damaged.

You clearly have a better grasp of how all the pieces of information fit together than i have.

I'm still at "The water hammer from safety valve blowdown into SC ripped out a pipe connection" stage of thinking.

Time will tell.

 

[SteveElbows]

You clearly have a better grasp of how all the pieces of information fit together than i have.

I'm still at "The water hammer from safety valve blowdown into SC ripped out a pipe connection" stage of thinking.

Time will tell.

I don't suggest that my thinking leads to any better answers than your stuff, or that it makes any of that irrelevant. I have a reasonable understanding of much of the released reports, and the ways that some of the factors may interplay with each other, But there is precious little data with which we can form solid conclusions.

There are a number of different ways that the suppression chamber may have failed. The committee report that was written in clearer and more critical language than other reports was very informative. It said that one of the main mistakes made at reactor 2 in the first days was that they switched the cooling system to use the suppression chamber water, and then failed to put a priority on getting pressure & temperature data from the suppression chamber for several days. They go into detail about how concerned the site manager was that the suppression chamber would not be able to handle SRV release from the reactor, a release which they really needed to do in order to get reactor pressure down to levels where water could be pumped from firetrucks. He thought that they needed to wet-vent first to give the suppression chamber a chance, but they couldn't wet vent and so resorted to dry vent attempts in a desperate bid to save containment. None of this happened in time so they had to depressurise the reactor anyway, and depressurisation was slow which indicated the suppression chamber was losing its ability to lend a useful hand. Later either its relationship with the drywall changed and then it failed, or at the very minimum its pressure gauge failed.

 

[jim hardy]

I don't suggest that my thinking leads to any better answers than your stuff,

It's not a competition so no sweat there.

There was understandable reluctance to vent their families live downwind.

depressurisation was slow which indicated the suppression chamber was losing its ability to lend a useful hand.

I wonder still, did it get too hot to quench more steam or was it being handed noncondensibles?

As i said in another forum, I'm waiing for the Nova show on it.

 

[SteveElbows]

I wonder still, did it get too hot to quench more steam or was it being handed noncondensibles?

Not sure, this evening was the first time I actually went looking for proper historical studies of such things. I hadn't realized that some problems of this nature were known as the 'Würgassen effect' because of what happened at that power plant decades ago.

I found reference to this stuff in this paper:

http://www.osti.gov/bridge/servlets/purl/5338909-FPWlHy/5338909.pdf

And a fair bit more more detail in understandable form in this document, from page 30 to 33:

http://wikdahl.se/Filer/Korr_3_07_Marviken_eng.pdf

I know this topic has been covered on the forum in the past but I don't know to what extent or whether that 2nd document I link to was referenced at the time.

 

[SteveElbows]

I am following up on some of the things I said in recent days with a bit more clarity.

I tried to learn more about the MELCOR system which they used to do the analysis of the accident. I mentioned that analysis graphs for reactor 2 showed contamination in additional areas compared to the ones for reactors 1 & 3.

I have learned that the FHB (Fuel handling Building) does indeed seem to simply mean the upper refuelling floors of the building. The reactor building label on these charts does not include these refuelling floors.

I have also seen, by looking at an old study of Peach Bottom, that MELCOR is based on treating the different parts of the reactor & buildings as nodes, and defining the possible pathways that substances can take through the network of nodes. In that particular old study, I note with interest that their node map didn't actually have a pathway for the suppression chamber to emit stuff directly to the environment, instead it goes via the drywell, which then goes to the torus room, then to the various rooms in the reactor building. Once it reaches reactor building rooms it can then travel to other rooms, to the environment, or to the refuelling floor. From the refuelling floor it can then go to the environment, and looking at the graph we can see that their analysis gave them results where about half of the environmental release came via the refuelling floors, and the other half escaped at some earlier stage of the pathway that cannot be determined from that graph.

Note that this is of course a model rather than how things actually work out in reality, but as its the model they used to come up with one of the very few sets of figures we have to work with for this thread about reactor 2, its worth understanding. Also note that as far as I know we don't actually know what the node map for Fukushima looks like, we don't know exactly what they put into the analysis. For example their version may have a node map that allows for direct release from suppression chamber to environment, rather than the via drywell path that I just mentioned, I have no way to know.

Anyway for the reactor 2 analysis I've decided that it can be easier to refer to the following NISA document rather than the government report to the IAEA, the file is more manageable and on a few pages presents the sequence of events in a more understandable way. Its the same analysis as we saw in the other versions, and in some areas it is worse, but for the stuff I am talking about right now its pretty handy.

http://www.nisa.meti.go.jp/english/press/2011/06/en20110615-5.pdf

For example on page labelled as 7 it reminds us that according to this analysis:

in terms of how events developed, it is surmised that the RPV was damaged at the time when there was a substantially elevation of containment pressure as recorded around 0:00 on the 15th, hence a large elevation in the containment pressure and temperature.

The release of radioactive materials from Unit 2 is considered mainly due to leakage caused by rise in containment pressure as melted fuel is believed to have moved beginning at 21:00 on March 14, as well as the PCV vent, and release due to leakage from the suppression chamber and other factors assumed in relation to the large impact noise in the vicinity of the suppression chamber

And there is quite a nice chart showing the timing of various things along with data from that period. Its on what would be labelled page 11, but is probably actually page 12 of the document as the first page is an additional coversheet. I will come back to these timings later as they will help clarify some of the stuff that has been mentioned before in this thread.

 

[SteveElbows]

The other really important thing the document reminded me of was something that was mentioned in this thread many months ago, but that I had forgotten about when I was going on about drywell pressure the other day. In order to make their analysis spit out drywell pressure estimates that are close to the real data that was actually measured, they had to assume some sort of containment leak starting within a day of the earthquake & tsunami hitting, days earlier than the 'main event' at reactor 2.

This is shown very clearly in the graphs that are on pages labelled 2-15 and 2-16. Case 1 is where they assume no leak. Case 2 assumes a leak in PCV of 50 cm² and case 3 assumes a leak in S/C of 300cm². Case 2 fits most closely with the observed data.

Anyway the new twist I wanted to put on this old detail, is whether it is possible that the blow-out panel at reactor 2 was actually blown out by containment leak quite early on, rather than the explosion at reactor 1 building. I don't think it will be very easy to draw any conclusions on this seeing as we only have limited detail about the containment leak estimation used in the analysis, but it doesn't take too much pressure for one of these panels to blow out does it? So perhaps this possibility deserves a little more attention?

 

[SteveElbows]

I will have to return to the timing of reactor 2 events another day, sent all my time this evening looking at a document that is quite handy for getting a sense of how MELCOR works. There are other documents too but this one has a better node diagram of the different parts of the Peach Bottom MELCOR model than the one I saw the other day.

http://prod.sandia.gov/techlib/access-control.cgi/2007/077697.pdf

The diagrams are really great, and can be found on pages 93-96. There is plenty else in this document that's of interest but I shall not go on about that now. How I wish I had these diagrams handy during many discussions over the last year.

 

[NUCENG]

Nope, most definitely not.

That's only a "containment vent", not a reactor vent. Moreover, it doesn't vent high pressure. It only ensures, that, at high pressure, the pressure doesn't get any higher. And that only goes for the containment.
So when pressure inside the containment reaches a certain value (I think it was twice the normal pressure), the containment cap lifts and pressure escapes. The pressure falls slightly and then the cap closes again. So the pressure can't go over twice the normal pressure, but it also can't go below twice the normal pressure.
But that still doesn't vent the reactor pressure vessel. And the unfiltered release ("dry venting") can't happen that way. Because for reactor pressure to escape via a lifting containment cap, the gases have to travel through the wetwell (torus). Which they obviously didn't, that's why we have such a dirty release for Unit 2.
So there must've been an other path.

The vessel has pressure relief valves that go to the torus (wetwell). If the coolant piping or vessel are breached the vessel releases pressure to the drywell and through the Drywell to Torus Vent pipes. If the pressure in the drywell is below the pressure in the torus the Torus to Drywell Vacuum breakers will mjnjmjze pressure differences between the torus and drywell. There are also vacuum breakers from atmosphere to the torus to prevent containment from being below atmospheric pressure.

Containment can be filtered and vented to the stack from either the drywell or the wetwell through the SBGT system if power is available. The hardened wetwell vent system is intended to vent from the torus to the stack. In this case the vented effluent is scrubbed through the water in the wetwell to retain soluble and particulate products. At least some plants can also use a dry vent path from the drywell through the hardened vent path without the scrubbing of the wetwell path.

The design uses a pressure suppression mode of operation. The water volume in the suppression pool absorbs the energy and condensessteam. As s it heats up there are cooling systems (sprays and heat exchangers) to keep the suppression pool below saturation temperatures. At Fukushima the cooling systemslost power to the pumps and eventually the suppression pools were no longer available to keep the containment pressure low.

Unit 1 IC systems were lost due to possible human error and there were no low pressure ECCS systems due to loss of AC power. Units 2 and 3 also had no low pressure ECCS systems available due to loss of AC power. In units 2 and 3 the heatup of the Pool also resulted in loss of the steam exhaust path for the RCIC and HPCI systems (battery backed High Pressure ECCS systems). In all three plants, the heatup also meant that vessel relief valve discharge to the torus could no longer be condensed.

Bottom line, they couldn't reduce the pressure so they could use the fire trucks and lower pressure pumps to cool the reactor fuel. That was unrecoverable.

In the specific case of Unit 2 the containment failure was apparently in the torus providing a vent pathdirectly to the building. This would have resulted in direct release of non-condensibles and probably further releases due to rapid boiling in the suppression pool due to the pressure release. If the unit 1 and Unit 3 failures were initially throughthe stretching of the drywell caps they would have eventually reseated as you described. The assumed torus failure in Unit 2 would have remained open allowing a larger release path.

In all three units there were also probably failures of the containment penetrations for cables and piping systems due to high temperatures and pressures. The potential that the torus failure in Unit 2 remains open even today is my best guess for why Unit 2 releases are assumed to be greater than the other units.

 

[NUCENG]

There has been speculation on EX-SKF that the rupture disk did rupture, but that debris from within the RPV clogged the piping for the vent. Given the violence of the events, this does seem plausible.

The Hardened vent path does not connect directly to the RPV. It is connected to the airspace in the top of torus (above the suppression pool). If the drywell vent path is used it is from the drywell above the level where blockage is likely.

 

[NUCENG]

It seems to me that successful venting through the designated path is almost impossible under accident conditions and without electricity, at least for the Mark I containment (don't know if it would work out better for other containment designs).

Is the "blow out panel" opened at unit 2 actually an improvisation or a feature?
Because units 1 and 3 explosions showed us that trying to contain gases and Hydrogen (which must be released anyway) within the secondary containment might not be the right approach in an emergency.

Blowout panels on the reactor building are a design feature initially designed to relieve from pipe breaks or ruptures in the secondary contaiment or possibly during low pressures from a tornado.

The reactor safety design basis is to prevent the fuel temperatures that can result in fuel damage and hydrogen generation. Blowout panels were never designed for hydrogen control.

 

[SteveElbows]

Im trying to come up with a simple set of factors that could be involved in making the reactor 2 release so substantial.

Im thinking along the lines of one or more of the following, some of which are related to lack of venting or the amount of time that water was not pumped into the reactor:

The state of the suppression chamber at the time of core melt (pressure, temp, amount of water).
The pressure that the primary containment was under at the moment of containment failure.
The pressure that the primary containment was under at the time the core left the reactor.
The pressure of the reactor vessel at the time the core left the vessel.
The location or nature of the containment failure. (Including possibilities such as the blowdown of core material into the torus room).
Brief dry venting.
The pathway that the radioactive material was able to travel through the reactor building and perhaps refuelling floor, and/or turbine building before reaching the environment.

When considering the above it would be rather helpful to know the status of a variety of other blowout panels. I understand that some other nuclear reactors of similar design have panels or other stuff that will fail under pressure in various other places, such as between lower main reactor building and the refuelling floors, between the reactor building and the turbine building, etc. Not sure what Fukushima had but knowing more about these would give further clues about any release pathways, at least ones that may have happened quite dramatically under pressure.

 

[etudiant]

The Hardened vent path does not connect directly to the RPV. It is connected to the airspace in the top of torus (above the suppression pool). If the drywell vent path is used it is from the drywell above the level where blockage is likely.

The speculation was based on the idea that lagging material dislodged by the earthquakes and subsequent reactor excursions, which would have created much stronger than nominal steam bursts, was the culprit in blocking the vents. The pipe is not very big apparently, about 6 inches in diameter.

 

[NUCENG]

Im trying to come up with a simple set of factors that could be involved in making the reactor 2 release so substantial.

Im thinking along the lines of one or more of the following, some of which are related to lack of venting or the amount of time that water was not pumped into the reactor:

The state of the suppression chamber at the time of core melt (pressure, temp, amount of water).
The pressure that the primary containment was under at the moment of containment failure.
The pressure that the primary containment was under at the time the core left the reactor.
The pressure of the reactor vessel at the time the core left the vessel.
The location or nature of the containment failure. (Including possibilities such as the blowdown of core material into the torus room).
Brief dry venting.
The pathway that the radioactive material was able to travel through the reactor building and perhaps refuelling floor, and/or turbine building before reaching the environment.

When considering the above it would be rather helpful to know the status of a variety of other blowout panels. I understand that some other nuclear reactors of similar design have panels or other stuff that will fail under pressure in various other places, such as between lower main reactor building and the refuelling floors, between the reactor building and the turbine building, etc. Not sure what Fukushima had but knowing more about these would give further clues about any release pathways, at least ones that may have happened quite dramatically under pressure.

Good list/

I don't know of other blowout panels in the Reactor Building proper. There is a series of hatches from the first floor to the refueling floor for movement of spent fuel casks and other heavy loads. These hatches are not designed to be air tight. Many US plants have open web cargo nets on these hatches as fall protection. Neither are there airtight doors on stairwells. Finally the elevator shaft is not airtight. The SBGT system (emergencies) and reactor building ventilation system (normal operation) are designed to keep the reator building at a negative pressure compared to the atmosphere. This ensures the leakage in the building and the exhaust is through monitored paths to detect possible radioactivity releases.

 

[NUCENG]

The speculation was based on the idea that lagging material dislodged by the earthquakes and subsequent reactor excursions, which would have created much stronger than nominal steam bursts, was the culprit in blocking the vents. The pipe is not very big apparently, about 6 inches in diameter.

There have been significant regulatory requirements to address the potential for lagging and debris to clog PWR sump strainers and BWR ECCS system strainers in the torus suppression pool. The debris would have to defy gravity to clog the vent pipes. Remember there was no attempt to start venting until well after the earthquake.

It is possible that debris could have been generated by blowdown after the RPVs were breached. And it is possible some made its way into the torus. But unless there was some significant wave action and simultaneous vent flow to hold the debris in the vent pipe penetration, it would have no reason to plug the opening. I think that is unlikely.