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

[SteveElbows]

The period before the evening of March 14th would seem irrelevant. Unit 2 can't claim to have vented large amounts of radioactivity, hidden by going seawards, during the period preceding it's experiencing fuel damage.

True but there are other reactors to consider, and some hours before 21:30 to think about too.

I do expect that dramatic events that were occurring at that time had a big influence on the site monitoring point, but I still have to include wind direction when analysing the data for clues about precise timing of events.

Anyway I am closer to talking about this period in detail, but first I will just go back to cover the earlier pressure blip.

In my timeline of earlier events, I have the following sequence of events on the 13th from reports:

March 13th 08:10 PCV vent valve opened 25%
March 13th 10:15 Order to complete vent line path (except rupture disc)
March 13th 11:00 PCV vent line complete but rupture disc pressure not reached

If I look at reactor & drywell pressure measurements from this date I see the following:

05:30-09:25 reactor pressure falls slightly from 6.14 to 6.08
09:55 reactor pressure now measured as being 1.283
15:30 reactor pressure back up to 5.85

05:30-09:25 D/W pressure rises slightly from 0.33 to 0.365
09:55 D/W pressure now 0.16
10:30 D/W pressure now 0.1
10:35 D/W pressure now 0.01
11:35 D/W pressure back up to 0.38
15:30 D/W pressure 0.4

This could be some problem with the measurements or the recording of them, but as there is some overlap between these readings and vent preparation I thought I should mention them before moving on to the more interesting periods of 14th & 15th March.

 

[MadderDoc]

True but there are other reactors to consider, and some hours before 21:30 to think about too.

I do expect that dramatic events that were occurring at that time had a big influence on the site monitoring point, but I still have to include wind direction when analysing the data for clues about precise timing of events.

Anyway I am closer to talking about this period in detail, but first I will just go back to cover the earlier pressure blip.

In my timeline of earlier events, I have the following sequence of events on the 13th from reports:

March 13th 08:10 PCV vent valve opened 25%
March 13th 10:15 Order to complete vent line path (except rupture disc)
March 13th 11:00 PCV vent line complete but rupture disc pressure not reached

If I look at reactor & drywell pressure measurements from this date I see the following:

05:30-09:25 reactor pressure falls slightly from 6.14 to 6.08
09:55 reactor pressure now measured as being 1.283
15:30 reactor pressure back up to 5.85

05:30-09:25 D/W pressure rises slightly from 0.33 to 0.365
09:55 D/W pressure now 0.16
10:30 D/W pressure now 0.1
10:35 D/W pressure now 0.01
11:35 D/W pressure back up to 0.38
15:30 D/W pressure 0.4

This could be some problem with the measurements or the recording of them, but as there is some overlap between these readings and vent preparation I thought I should mention them before moving on to the more interesting periods of 14th & 15th March.

All right. Here is a plot of the doserate measurements on March 13th. Note: log scale.

General caveat: there is little to no data published from monitoring posts north of the reactors until about 8 am on March 13th.

This lack of data is particularly a pity here, since the wind in the morning of March 13th appears to have come mainly from the south. However, the earliest data from northern posts we have do seem to indicate a relatively recent additional deposition in the morning hours up to about 8 am. There could be a window of opportunity here for some reactor or other to make claims.

However, after 8 am, during the morning of March 13th, Unit 3 might have been the major player involved in the observed raising of doserate: there is operation record of venting in unit 3 about 9 am, and satellite photo, and webcam evidence, show emission from its stack at 9:50 and10:00 respectively.

Similarly, during the afternoon, the webcam shows emission from the unit 3+4 stack at 13:00 and at 14:00. Unit 3 could reasonably claim responsibility for any incremental contamination seen at monitoring posts during this period.

 

[tsutsuji]

Reading the summary, the author proposes a two stage release. First the suppression chamber (torus) fails due to either earthquake or hydrodynamic loads. This pathway is then open for release of the radioactivity being transported to the suppression chamber from the SRV valve operation.

The second stage is direct release from the drywell through failed electrical pentrations from high temperatures caused by corium outside the reactor vessel.

It sounds like he is assuming that the other units did not have the large early releases from the suppression pool and that explains the higher releases from unit 2. But that is based on reading only that first page.

http://mainichi.jp/select/jiken/news/20120403ddm016040021000c.html Eastern Japan mega seismic disaster: Fukushima Daiichi NPP accident: Was unit 2's PCV damaged by the earthquake?

Among the discussions about Tokyo Electric Fukushima Daiichi nuclear power plant accident, there is a controversy regarding whether unit 2's PCV was broken by the earthquake. Large amounts of radioactive substances were released from unit 2 in the morning of the 5th day of the accident, the 15 March of last year, and there is a high probability that Earth contamination spread wider to Iitate village, Fukushima prefecture and other places. Elucidation of causes is crucial to recurrence prevention. We present the discussion that took place last month at the Japan Atomic Society meeting in Fukui.

(...)

However, in December of last year, Tokyo Electric revised its position, saying that the seismometer data generated at the same time as the damage came from unit 4's hydrogen explosion. Concerning the suppression chamber's pressure gauge, it judged that "there is a high probability that it is broken". It inferred that radioactive substances leaked after part of the PCV top part, whose resistance to high temperatures is weak, was damaged. Actually, the PCV pressure, which was 7.3 atmospheres at 7:20 AM on 15 March declined to 1.5 atmospheres in 4 hours' time. Plant premises surrounding radiations also rose.

In contrast with this, former JAEA senior chief researcher Fumiya Tanabe (66) insists that "there is a phenomenon which cannot be explained solely [1] by the melting of the junction parts in the PCV top part". This phenomenon is that, after the core melt down after 01:00 AM on 15 March and during the 5 hours that ensued, in consequence of the fuel that melted and fell from the RPV to the PCV, while the radiation dose rose by a factor of 2.1, the suppression chamber's radiation dose, conversely, declined by 40%.

This is where Mr Tanabe explains: "If one assumes that the suppression chamber was damaged by the earthquake, it is possible to explain the reason why the suppression chamber radiation declined". As water is contained in the suppression chamber, it is difficult for the suppression chamber to reach high temperatures, and to suffer damage by melting, he says.

About the status of the suppression chamber, Tepco says "minimal damage might have taken place, but if it had suffered major damage it would not have been able to maintain a 7.3 amosphere pressure".

[1] Edit: I had forgotten to translate this one word : "solely".

 

[MadderDoc]

<..> after the core melt down after 01:00 AM on 15 March and during the 5 hours that ensued, in consequence of the fuel that melted and fell from the RPV to the PCV, while the radiation dose rose by a factor of 2.1, the suppression chamber's radiation dose, conversely, declined by 40%.

<..>

I didn't think that is a mystery, but is it? What I thought is that while the fuel melted down, RPV was loosing pressure, so the safety relief valve naturally stopped transporting radioactivity to the suppression chamber, and the natural decay of shortlived isotopes took over . But the RPV of course kept leaking to the PCV only more than ever.

 

[SteveElbows]

However, after 8 am, during the morning of March 13th, Unit 3 might have been the major player involved in the observed raising of doserate: there is operation record of venting in unit 3 about 9 am, and satellite photo, and webcam evidence, show emission from its stack at 9:50 and10:00 respectively.

Similarly, during the afternoon, the webcam shows emission from the unit 3+4 stack at 13:00 and at 14:00. Unit 3 could reasonably claim responsibility for any incremental contamination seen at monitoring posts during this period.

I agree, I mention the pre March 14th stuff in regard to reactor 2 only because there are a couple of questions about possible damage well before it melted-down, but I wouldn't expect any notable radioactive emissions from reactor 2 during this period.

But as its more than a year since the disaster I think any time we want to refresh our understanding of events it is worth discussing detail, even if it doesn't turn out to be directly relevant.

 

[SteveElbows]

In contrast with this, former JAEA senior chief researcher Fumiya Tanabe (66) insists that "there is a phenomenon which cannot be explained by the melting of the junction parts in the PCV top part". This phenomenon is that, after the core melt down after 01:00 AM on 15 March and during the 5 hours that ensued, in consequence of the fuel that melted and fell from the RPV to the PCV, while the radiation dose rose by a factor of 2.1, the suppression chamber's radiation dose, conversely, declined by 40%.

This is where Mr Tanabe explains: "If one assumes that the suppression chamber was damaged by the earthquake, it is possible to explain the reason why the suppression chamber radiation declined". As water is contained in the suppression chamber, it is difficult for the suppression chamber to reach high temperatures, and to suffer damage by melting, he says. .

Interesting stuff. I am not sure that I think very much of this analysis though.

Firstly I don't think that possible evidence of suppression chamber leaking between 1am and 6am is evidence that damage must have happened during the earthquake. e.g. the suppression chamber may have been damaged at 9.20PM on the 14th, or midnight, or some other time well after the earthquake but before 6am on the 15th.

I don't quite understand his point about suppression chamber water and damage by melting. They had been using the suppression chamber as water source for RCIC for several days, which is going to affect conditions of the suppression chamber, amount of water etc. And melting is not the mode of failure that we often think of for the suppression chamber anyway (rather over pressure, hydrogen burn, or forces caused by steam) although from the article I can't quite tell what sort of melting he means (melting of chamber itself or melting of seals etc).

I don't know enough about CAMS readings to say how useful they are in a severe accident. I am sure this is the data he refers to when taking about a rising trend in the drywell but falling radiation trend in suppression chamber. But if he wants to use CAMS as a guide, I don't know why he picks 1am as a start time. Because actually the CAMS D/W readings are first recorded as being rather high earlier, at 21:55 on the 14th its recorded as 5.36 Sv/h. By 23:54 the D/W CAMS has reached 24.5 Sv/h, and when it is next measured at 01:18 15th its risen higher but not a huge leap, 28.88 Sv/h. Its actually the pressure readings of the reactor that could point to a significant failure around 1am, not the CAMS, so either his timing is wrong or CAMS is actually not a good guide to explain events on this level.

But perhaps lower S/C CAMS readings at reactor 2 do mean something, as you can tell I do not place too much faith in CAMS measurements as being able to tell the whole story, but if the S/C CAMS at reactors 1 & 3 were far more consistent with the D/W CAMS at those reactors, then this is a difference that should be noted I guess. But it certainly doesn't help that in these first days the CAMS readings have some gaps where measurements were not recorded, and also only 1 D/W and one S/C reading are recorded, rather than both A & B sensors.

 

[SteveElbows]

An additional area of doubt regarding S/C CAMS comes from the fact that when they finally started reporting & publishing data from the S/C CAMS B sensor on April 15th 2011, it was very much higher than the A sensor they had been recording for the prior month. But the values of this CAMS S/C B sensor fluctuate quite a lot, so its not clear if it is faulty. It still leaps up sometimes to this day, I think it has been higher again for weeks, but I cannot take this data very seriously.

 

[SteveElbows]

I checked the CAMS data for the other reactors and unfortunately they did not start receiving CAMS data soon enough for us to see any trends during the initial melting events at those reactors. Reactor 3 data shows much higher levels for D/W CAMS than S/C CAMS. At reactor one the S/C CAMS reading is lower than the D/W CAMS but not by very much order of magnitude, much less difference than at reactors 2 & 3.

Anyway for now I will only safely assume that CAMS is good for alerting you to fuel damage, but perhaps not much else.

 

[MadderDoc]

<..> for now I will only safely assume that CAMS is good for alerting you to fuel damage, but perhaps not much else.

CAMS is designed for normal as well as post-accident _monitoring_ of gamma radiation, it is certainly meant to yield quantitative measurements, not just a binary yes/no answer to the question whether there is fuel damage. The uncertainty of CAMS data should be reflected by the number of significant digits given. Assuming invalidity of such data is a serious matter and one must have specific reasons to do so. In the case of the CAMS readings from unit 2 in the period we are talking about there are afaics no such reasons.

 

[SteveElbows]

OK well they used CAMS data to estimate fuel damage ratio and only time will tell if these estimates are any good. At this stage all we can say is that some people will accuse these estimates of being part of the 'much too positive' spin from the company, but I cannot judge what the reality is. The damage ratios sound a bit low given the estimates about meltdown we have seen since. Obviously this is old data and I mention it now only in connection with the way this particular discussion has evolved.

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110427e19.pdf

The graphs in that document are another good visualisation of what I was saying about reactor 3 S/C CAMS being low too. And reactor 1 isn't exactly seeing huge numbers in the S/C.

I am left assuming that the analysis I questioned is using only the differing trends of D/W and S/C CAMS at a key moment to make a point, rather than the difference in magnitude of D/W and SC measured levels, and that it is to be expected that S/C levels will be lower than D/W ones? In this case the lack of measurements during crucial times for reactors 1 & 3 make it impossible to look for similar or dissimilar patterns at the other reactors, making it harder for me to evaluate the plausibility of his argument.

 

[MadderDoc]

OK well they used CAMS data to estimate fuel damage ratio and only time will tell if these estimates are any good. At this stage all we can say is that some people will accuse these estimates of being part of the 'much too positive' spin from the company, but I cannot judge what the reality is. The damage ratios sound a bit low given the estimates about meltdown we have seen since. Obviously this is old data and I mention it now only in connection with the way this particular discussion has evolved.

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110427e19.pdf

The graphs in that document are another good visualisation of what I was saying about reactor 3 S/C CAMS being low too. And reactor 1 isn't exactly seeing huge numbers in the S/C.

I am left assuming that the analysis I questioned is using only the differing trends of D/W and S/C CAMS at a key moment to make a point, rather than the difference in magnitude of D/W and SC measured levels, and that it is to be expected that S/C levels will be lower than D/W ones? In this case the lack of measurements during crucial times for reactors 1 & 3 make it impossible to look for similar or dissimilar patterns at the other reactors, making it harder for me to evaluate the plausibility of his argument.

Well, I don't accept his argument at all. It is a double non sequitur: One cannot conclude from the observation of a decreasing trend of radioactivity in the suppression chamber that it must be leaking. And even if one could, as you have pointed out, one cannot conclude from observing a current leak in the S/C , that it was also leaking at some time well before that observation.

 

[SteveElbows]

Yeah. Mind you I am still interested in possible earlier leaks at reactor 2, but for different reasons.

Firstly there is that stuff I already went on about to do with them having to assume a leak to make their disaster modelling give pressure results that are in line with measured pressures.

And then there is the question of the reactor pressure in the period before fuel melting. How come reactor 3's RPV pressure was above 7 MPa for large periods of time, but reactor 2's was only 7.0 MPa the first time they measured it at 20:07 on the 11th, and was then only 6.3 or lower until the time that water level dropped and melting started?

 

[MadderDoc]

<..>
And then there is the question of the reactor pressure in the period before fuel melting. How come reactor 3's RPV pressure was above 7 MPa for large periods of time, but reactor 2's was only 7.0 MPa the first time they measured it at 20:07 on the 11th, and was then only 6.3 or lower until the time that water level dropped and melting started?

Yes. unit 2 and 3 seem to be very much twin sisters as regards size and equipment. Comparable periods for your question on pressure developments could be, say, from the earthquake until noon on March 14 in the case of unit 2, and from the earthquake until noon on March 12 in the case of unit 3. Overall it seems to me that somehow reactor 2 utilised its limited heat sinks more efficiently than reactor 3 during these first periods after the earthquake, and therefore was able to be held at a somewhat lower pressure. (Either that, or unit 2 had 'extra' sinks for the decay heat at its disposal)

 

[clancy688]

Thanks to tsutsuji for providing this paper:

http://sciences.blogs.liberation.fr/files/irsn_rapport_complet-fukushima-1-an-.pdf

On page 46, there's a table where IRSN lists its release estimates. For Cs-137: Unit 1 1 PBq, Unit 2 6 PBq, Unit 3 14 PBq. The NISA estimate which makes Unit 2 responsible for 90% of the release is right below it, so IRSN has to be aware that the japanese think otherwise. Perhaps they are explaining why somewhere in the text...?

Unfortunately, I don't understand a single word french...

 

[etudiant]

Thanks to tsutsuji for providing this paper:

http://sciences.blogs.liberation.fr/files/irsn_rapport_complet-fukushima-1-an-.pdf

On page 46, there's a table where IRSN lists its release estimates. For Cs-137: Unit 1 1 PBq, Unit 2 6 PBq, Unit 3 14 PBq. The NISA estimate which makes Unit 2 responsible for 90% of the release is right below it, so IRSN has to be aware that the japanese think otherwise. Perhaps they are explaining why somewhere in the text...?

Unfortunately, I don't understand a single word french...

The study does not give a specific explanation of the differences, but blandly notes it and moves on to note that the overall results are comparable.
However, it is possible to infer that because the winds were onshore while #2 was emitting, shifting to offshore when #3 was the focus ( discussion on p50), reactor 2 was responsible for most of the land pollution damage.
The estimates given in Science were for the total site emission, yet even those cover a factor of 3 range, so much better estimates will be laborious to obtain.

 

[MadderDoc]

From the recent torus room survey, the radioactivity of the room is very high everywhere, but particularly so in two of the surveyed areas, one area at the door entry level to the NE corner of the torus room, and another at the inspection level at the NW corner.

The simplest explanations I can think of are, respectively, that the door entry level is just closer to the surface of the accumulated highly radioactive water below, while the hotspot in the NW corner could be because there's a leak to the torus room from the PCV, somewhere above the waterline of the torus room, either a leak from the torus, or directly from the PCV.

Also interesting is that we see some damage to insulation around pipes several places, the shape of which seems to indicate they've been exposed to some low key event of internal overpressure, or external vacuum, relative to the insulation capping of those pipes. But really nothing serious of an explosive nature seems to have happened in the room.

 

[SteveElbows]

Unless interesting new information emerges, for now I am going to stick with the conclusion that reactor 2 released more radioactive substances because of a failure to wet-vent.

On page 152 of Tepco's interim report, they manage to make this point without actually drawing attention to high releases from reactor 2:

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111202e14.pdf

[Action plan 3: Impact mitigation measures after core damage]
A large amount of radioactive materials will not be released if during PCV venting is conducted before core damage. In Fukushima Daiichi Units 1 and 3, the release of radioactive materials was reduced by releasing radioactive materials by wet well (S/C) venting through a water filter after core damage occurred.

It can be said that taking measures to enhance the certainty of venting in strategy 2 will also be effective after core damage has occurred.

Also, in order to cool the PCV, procedures that enable water injection to the PCV will need to be prepared in addition to water injection to the reactor through fire engines, etc.

 

[MadderDoc]

Unless interesting new information emerges, for now I am going to stick with the conclusion that reactor 2 released more radioactive substances because of a failure to wet-vent.<..>

Can you sketch out, please, the main pieces of evidence and the thoughts that gave them order, which made you come to the conclusion that Unit 2 released more radioactive substances than Unit 3?

 

[SteveElbows]

I won't be repeating all of that since I posted so much in this thread already. I have acknowledged that it is possible to underestimate the release from reactor 3 due initially to wind direction, and then because it could have got mixed up with events occurring at reactor 2.

So no, I cannot be absolutely sure that reactor 2 released much more than reactor 3. We have a range of reasons why it might of, some of which I found quite compelling. Given the timing of the melt, failure to vent, pressure the containment was under and likely contents of its atmosphere when it started to leak in a vigorous fashion, I have no trouble with this conclusion, although I am always ready to change it or learn more if the chance presents itself.

One way which we may further explore the differences between reactor 2 & 3 is by looking at the timing of the release of substances from the fuel, and the timing of vents. If we would expect the bulk of the substances to enter the atmosphere of containment during the main initial fuel melting, then scrubbing that via a wet vent before containment leaks in a major way should make quite a large difference to how much stuff enters the environment. This is not a subject I have expert knowledge in so I would like to know more about the timing of releases from fuel.

 

[MadderDoc]

I won't be repeating all of that since I posted so much in this thread already. I have acknowledged that it is possible to underestimate the release from reactor 3 due initially to wind direction, and then because it could have got mixed up with events occurring at reactor 2.

Yes, it would seem unfair to neglect giving unit3 some credit from the observed significant higher on-site deposition in the track of the unit 3 explosion cloud, towards the ocean. It is difficult to imagine that deposition stopped at the waterfront, but it is difficult to quantify that emission, to come at same terms as the measurements from the on site monitoring points. However, no unit would be a serious contender to the title if it could not bid in on the large contamination events that followed and showed up in those measurements. As you indicate, unit 3 as well as unit 2 could do that.

So no, I cannot be absolutely sure that reactor 2 released much more than reactor 3. We have a range of reasons why it might of, some of which I found quite compelling. Given the timing of the melt, failure to vent, pressure the containment was under and likely contents of its atmosphere when it started to leak in a vigorous fashion, I have no trouble with this conclusion, although I am always ready to change it or learn more if the chance presents itself.

I understand your point, but how can we be certain, that melting of the fuel did not proceed in stages in either or both of the reactors, and continued after the point when both reactors had more or less free exhaust from the PCV to the atmosphere. (Unit 3 will of course claim to have had the freeest and most vigorous exhaust, and to have colored it in all nuances from white to black :-)

One way which we may further explore the differences between reactor 2 & 3 is by looking at the timing of the release of substances from the fuel, and the timing of vents. If we would expect the bulk of the substances to enter the atmosphere of containment during the main initial fuel melting, then scrubbing that via a wet vent before containment leaks in a major way should make quite a large difference to how much stuff enters the environment. This is not a subject I have expert knowledge in so I would like to know more about the timing of releases from fuel.

It would seem to me that the 5th floor of unit 2 should have functioned as a condenser and retained a significant fraction of radioactive material within its walls, on the assumption that the route of the assumed bulk of the total radioactive material emitted from the plant did pass though that space.

 

[SteveElbows]

It would seem to me that the 5th floor of unit 2 should have functioned as a condenser and retained a significant fraction of radioactive material within its walls, on the assumption that the route of the assumed bulk of the total radioactive material emitted from the plant did pass though that space.

Well I believe that this and numerous other details that are of interest to this thread, have been at least briefly mentioned by TEPCO in their new report of release estimates.

However at the time of writing it is only available in Japanese, and computer translation of this document in particular gave quite horrible results in many places. So I really need to wait for better translation before discussing any of these details further.

http://www.tepco.co.jp/cc/press/betu12_j/images/120524j0105.pdf

 

[SteveElbows]

OK I've had to make do with machine translation because it doesn't sound very likely that TEPCO will be translating the full document, only the shorter ones.

I will talk on the main thread about the report as a whole as it obviously deals with all reactors, but from what I think I've been able to make sense of in relation to reactor 2:

Even though they are unsure whether the vent took place, they seem to be focussing far more on the March 14th wet vent attempt, rather than the very brief dry vent attempt just after midnight on the 15th. Probably because of ability to speculate a link between increased site dose rates and the wet vent attempt, quite a large amount of emissions are pencilled in as occurring during this time and coming from reactor 2 (see table on page 9). That table is interesting for many other reasons also, including the fact that these figures are probably more in line with some peoples expectations in terms of which reactors are to blame for radioactive release - reactor 3 (and to a lesser degree reactor 1) gets a much larger share of the blame in this table, reactor 2 totals are still rather large but don't make other reactors irrelevant by comparison.

When they talk of soil contamination, March 15th reactor 2 emissions are the main focus for reasons we already discussed before. They have various diagrams showing wind direction and presumed plume path during variety of different venting operations, and they have the rain radar images from late on the 15th to the north west.

In regards to non-vent leaks, they talk about such matter quite a bit. They acknowledge steam escape from top of containment. Here is the original Japanese for a key part of the detail on this as it pertains to reactor 2:

1.3.2 2 号機原子炉直上部からの蒸気確認
平成 23 年 9 月 17 日に、2 号機 R/B のブローアウトパネル開口部からダストサ
ンプリングを実施した際に動画を撮影したところ、原子炉直上部から蒸気発生が 確認された(別図 3)。具体的に蒸気がどこから漏えいしているのか現時点では確 認出来ていないが、動画映像から蒸気の漏えい箇所としては原子炉の上部に位置 する PCV トップヘッドフランジ上部の原子炉ウェル上蓋付近からであると推定さ れる。また、使用済燃料プールの温度が高くない(平成 23 年 9 月 17 日時点で約 34°C)ことから、確認された蒸気は PCV から漏えいしてきたものであると考えら れる。PCV トップヘッドフランジ(及び原子炉ウェル上蓋)の構造は 1 号機、3 号機とも 2 号機と同様であり、1・3 号機でも 2 号機と同じ PCV 漏えいパスが存 在している可能性が考えられる。
さらに、現在判明しているR/B内の空間線量測定結果によると、R/B5階で確認 された高線量(約200mSv/h)は、4階以下では確認されていない(別図4)。2号機 はR/B構造が維持されており、線量の分布と漏えい経路には相関性があると考えら れることを踏まえると、5階からの漏えいが主たるものであったことを示唆してい る。

 

[MadderDoc]

<..>
Even though they are unsure whether the vent took place, they seem to be focussing far more on the March 14th wet vent attempt, rather than the very brief dry vent attempt just after midnight on the 15th. Probably because of ability to speculate a link between increased site dose rates and the wet vent attempt, quite a large amount of emissions are pencilled in as occurring during this time and coming from reactor 2 (see table on page 9).

I think the reason is much simpler: there's a measured peak in dose rate at the main gate MP in the evening on March 14th, but none after midnight. When there is no peak in published MP data Tepco can freely assume there is no emission. Otherwise of course, when there is a published peak.

As regards the peak in the evening of March 14th, using Google Translate, I glean from the note that Tepco is assuming it came from exhaust stack 1+2, because the emission point is unknown.

If that is the evidential standard, I am pretty sure I can mount better evidence for the proposition. that the March 14th evening peak was due to emissions from unit 3.

 

[jim hardy]

I am pretty sure I can mount better evidence for the proposition...

Why Doc - surely you don't think attention was ever directed away from unit 3 !

old jim

 

[Sharma SK]

Lot of discussion is going on why maximaum radioactive material was released from unit 2 of Fukushima. Pirma facie it looks surprising even unbelivable as unit 2 building is least damaged, no hydrogen explosion took place in unit 2, its cooling system (RCIC) lasted for 70 hrs after the earthquake, core damage started later as compared to other units and no venting was done from unit 2 containment. Then why more radioactive material got released from unit 2 than oter units. Answer of this is very simple, that is, because no venting was done or could not be done from unit 2 containment.
As no venting could be done from unit 2, its containment got pressurised and it failed due to overpressure. Venting could not be done as rupture disc in hardened vent header did not rupture. Containment failure took place at weakest links such as penetrations, seals and gaskets. Activity released from drywell through these leaky points and came into reactor building. From reactor building it came into atmosphere through blowout panel of reactor building which had got opened during unit 1 hydrogen explosion on 12th March. Since release was from drywell, it was unfiltered release and that too it was ground level release. It is a case of 'suppression pool bypass'.Because of failure of containment penetrations, water injected for core cooling came into turbine building through pipe trenches/cable trenches and it contributed in release through water route. In contrast, containment venting could be done in unit 1 and unit 3. This venting was done from suppression pool air space. Radioactive material coming to suppression air space has to pass through suppression pool water. During this passage much of the radioactivity (around 99%) is filtered out. Water acts as filter in this case, since most of the volatile fission products like cesium, iodine, telerrium get dissolved in it and particulates get suspended in it. Remaing 1% contains mostly noble gases which get dispersed in atmosphere, do not dissolve in water/rain and thus do not fall out on the ground.
One line answer to the question why maximum radioactivity was released in atmosphere from unit 2 is that "Activity released from unit 2 was mostly unfiltered while from unit 1 &3 it was mostly filtered."

 

[zapperzero]

One line answer to the question why maximum radioactivity was released in atmosphere from unit 2 is that "Activity released from unit 2 was mostly unfiltered while from unit 1 &3 it was mostly filtered."

Too bad we have no evidence for this most parsimonious and elegant hypothesis which has been discussed. Or do we?

 

[Sharma SK]

Too bad we have no evidence for this most parsimonious and elegant hypothesis which has been discussed. Or do we?

Pl give rationale for your comments. If you want to prove someone wrong then prove it by logic not by nasty comments.
Sharma sk

 

[MadderDoc]

Lot of discussion is going on why maximaum radioactive material was released from unit 2 of Fukushima. Pirma facie it looks surprising even unbelivable as unit 2 building is least damaged, no hydrogen explosion took place in unit 2, its cooling system (RCIC) lasted for 70 hrs after the earthquake, core damage started later as compared to other units and no venting was done from unit 2 containment. Then why more radioactive material got released from unit 2 than oter units. Answer of this is very simple, that is, because no venting was done or could not be done from unit 2 containment.
As no venting could be done from unit 2, its containment got pressurised and it failed due to overpressure. Venting could not be done as rupture disc in hardened vent header did not rupture. Containment failure took place at weakest links such as penetrations, seals and gaskets. Activity released from drywell through these leaky points and came into reactor building. From reactor building it came into atmosphere through blowout panel of reactor building which had got opened during unit 1 hydrogen explosion on 12th March. Since release was from drywell, it was unfiltered release and that too it was ground level release. It is a case of 'suppression pool bypass'.Because of failure of containment penetrations, water injected for core cooling came into turbine building through pipe trenches/cable trenches and it contributed in release through water route. In contrast, containment venting could be done in unit 1 and unit 3. This venting was done from suppression pool air space. Radioactive material coming to suppression air space has to pass through suppression pool water. During this passage much of the radioactivity (around 99%) is filtered out. Water acts as filter in this case, since most of the volatile fission products like cesium, iodine, telerrium get dissolved in it and particulates get suspended in it. Remaing 1% contains mostly noble gases which get dispersed in atmosphere, do not dissolve in water/rain and thus do not fall out on the ground.
One line answer to the question why maximum radioactivity was released in atmosphere from unit 2 is that "Activity released from unit 2 was mostly unfiltered while from unit 1 &3 it was mostly filtered."

It does not seem to me to follow from the fact that there were filtered vents from e.g. unit 3, that the majority of activity released from that unit was filtered. Looking at things from the plant monitoring posts, any contender to the title of most polluting unit would need to claim the lions share of the major emission observed during the morning of March 15th. Photos show Unit 2 as well as Unit 3 steaming at that time presumably directly from their leaking containments. I can see no good reason to think that the steam from unit 3 was at that stage particularly more 'clean' just because successful filtered vents were done in that unit on March 13th. This would seem to me to be like assuming that most of the core damage in Unit 3 should have preceded those vents.

 

[zapperzero]

Pl give rationale for your comments. If you want to prove someone wrong then prove it by logic not by nasty comments.
Sharma sk

MadderDoc is much more polite and well-spoken than me and he has spelled it out. I will point out also that, although it does not appear to have done anything spectacular on the 15th, Unit 1 is the only one which had to be covered with a tent. Might the most contaminated unit have been also the most polluting, overall?

Point is, without extensive information that would allow us to piece together something close to the real accident sequence, we can't really tell. Speculation is (VERY uncharacteristically for this forum) allowed by the mods in the Fukushima technical threads. But, it should be clearly understood as such.

 

[SteveElbows]

I think this particular stuff goes beyond the usual unsubstantiated speculation though. Yes there are many questions remaining, but those who are not very keen on the idea that reactor 2 released more into the environment than the others might want to investigate the following sort of thing further:

Given that the failure to wet-vent at an important moment is key to this theory, can't someone look into details about when the most significant amounts of radioactive material is expected to be present in the atmosphere and steam from containment?

Because left to a common-sense rather than proper scientific view of this issue, I certainly don't have any trouble believing that the time during and immediately after the core is completely exposed is a rather crucial one. And at reactors 1 & 3 we had wet-venting operations conducted in the aftermath of the core uncovering, and before we saw evidence of steam rising from upper containment failures.

The other factor is the pressure of containment at the time of failure & mass ejection of steam etc.

These are the reasons I am not going to reject the hypothesis that reactor 2 had far more significant environmental implications, although as discussed in the past this is complicated by the fact that weather factors meant any releases from reactor 2 were far more likely to have an impact on Japanese land.

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20120724/2000_roatsu.html Shinichiro Kado of Tokyo university and Masamichi Chino of JAEA studied accident response records and radiation doses in the surroundings and concluded there is a high probability that radiations were released from unit 2 while the operators decreased reactor pressure by opening the S/R valve. 3 radiation peaks were observed at the Fukushima Daini plant, located 10 km away in the South, during the 5 hour lapse of time after 10:00 PM on 14 March 2011. Each peak happens about one hour after the S/R valve was operated. According to the specialists, there is a high probability that radiation was released through damaged parts of the PCV and reached Fukushima Daini by being carried by northerly winds. According to simulation results, the release from unit 2 is about 10 to 20 times greater than that after unit 1's hydrogen explosion and it was carried to the Kanto region in the morning of March 15.

http://enformable.com/2012/07/tepcos-do-or-die-mission-to-save-a-nuclear-reactor-may-have-caused-largest-radiation-leaks-during-fukushima-disaster/ An English article about this same Masamichi Chino study.

 

[SteveElbows]

Thanks for the info, that study does seem to match some of the speculation we engaged in, in regards to venting and radiation levels further south over that crucial time period. I wish I could find more reports about this study, or even the study itself, any ideas where to look?

 

[tsutsuji]

Thanks for the info, that study does seem to match some of the speculation we engaged in, in regards to venting and radiation levels further south over that crucial time period. I wish I could find more reports about this study, or even the study itself, any ideas where to look?

As Masamichi Chino made a presentation at the workshop of 23/24 July ( http://www.nisa.meti.go.jp/shingikai/700/14/240723/AM-3-3.pdf ), I was hoping there would be more details in the workshop papers, but it seems to be something different. I tried to google both names (Shinichiro Kado and Masamichi Chino) in Japanese writing, but google did not provide helpful answers.

 

[tsutsuji]

There is a bit more here:

http://genpatsu-watch.blogspot.com/2012/07/20127241800.html Transcript of Tepco's press conference on 24 July 2012:

NHK's Gotoh:

Concerning unit 2 it is said that the greatest amount of radioactive releases took place 4 days after the accident on 15 March. A variety of reports have been compiled, but specialists say that when they check the records of the response and Tepco's records, exactly from 3 days after the accident in the night of 14 March to the next day 15 March where large quantities are supposed to have been released, it is recorded in Tepco's documents that the SR valve was pretty repeatedly operated. Also, at that time, in the SPEEDI records, winds are found to be blowing toward the south, and when one checks the radiation doses at Fukushima Daini nuclear plant at that time, in the night of 14 March, against the normal 0.03 μSv/h, at around 10:00 PM it was 7.95μSv/h, then at 00:00 on 15 March it became 95μSv/h. After that, the radiation never stopped step by step sharply rising, and it is pointed out that radiations rose one hour after SR valve changing operations, and when one checks the wind directions after that, by looking at the accident response records and so on, the people who make that remark suggest that there is a high probability that due to the operation of unit 2's SR valve, the radiations blew toward the south and this raised unit 2's monitoring post values. Are you grasping this point?

Matsumoto:

I am not aware that the point that you said etc. was raised, but if we look at the time series of radiation releases in our previous reports, we infer that the releases from unit 2 became greater on 14 March at 21:20, well, from late in the night.

From a little after 21:00 on 14 March to 15 March, basically on 15 March over nearly one full day, we suspect that it was a release from unit 2. From the perspective of the accident situation at unit 2 at that time, core damage was quickly progressing, large volumes of radioactive substances were inside the RPV... well, it is a situation where they come out of the so-called cladding tubes. As a result, when the safety relief valve was opened, at that time, gas substances were released into the PCV. As the PCV has high temperature and high pressure, for example the flange, the seal parts suffer damage, and we think the radioactive substances have probably rapidly come out into the building. Then, at that time, the blow-out panel was already open, and the route is from there toward the reactor building eastern side, so it was probably released that way into the atmosphere.

NHK's Gotoh:

At present it is considered only as a possibility, but temporally, unit 2's radiation doses are said to have risen sharply in the night about one hour after. In the surveys you did until now, have you been matching [these data] with Fukushima Daini nuclear plant's radiations ?

Matsumoto:

No we haven't. In the future, well... I think it is probably necessary to compare the trend of radioactive releases with the meteorological conditions.

According to the radioactive release report released on 24 May ( http://www.tepco.co.jp/en/press/corp-com/release/2012/1204659_1870.html ), if we say that the release started at 21:00 on 14 March, as the wind was blowing toward the south, the Fukushima Daini plant beeing right in the south, that the radiation dose rose is... well... in a sense... it is matching. That's our line of thought. Well, it is after it. On 13 March at 09:00, 12:00, 20:00 here we compared the venting timing of unit 3, and well, we did a little analysis. Well, basically, it seems that it was a wind blowing to the south, so it is quite logical that Fukushima Daini's monitoring post radiation doses rose, I think.

NHK's Gotoh:

Do you think that it is necessary to perform this kind of data matching/comparison in the future ?

Matsumoto:

I think this kind of survey, or rather verification, will be necessary in the future. However, the analysis code we have, "Diana", is not able to perform analysis over such a wide area, so I think we will need to receive the cooperation of an outside laboratory.


NHK's Gotoh:

At that time it was necessary to operate the SR valve, but as a result of operating the SR valve...

Matsumoto:

... what do we feel about the fact that there is a high probability that it caused the outside release of large quantities of radioactive substances?

Concerning Fukushima Daichi unit 2, of course we suspect that venting the PCV, especially the wet well venting via the pressure suppression chamber was not sufficiently performed...

Matsumoto:

Well, I think this is probably the main point. At units 1 and 3 we succeeded in wet well venting, and as a result the concentrations were about one hundredth. At unit 2, well..., as regard the causes, we suspect that an important factor is the fact that things did not go smoothly with the connection of batteries and compressed air taking a lot of time.

NHK's Gotoh:

Thank you.

 

[zapperzero]

At units 1 and 3 we succeeded in wet well venting, and as a result the concentrations were about one hundredth.

A hundredth of what? Does he mean a hundredth of what was seen at unit 2? Why is he talking "concentration"? Does this mean someone was measuring the steam, in real time?