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

[SteveElbows]

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.

That was still an interesting report anyway.

Another one from the same collection should be of interest to those who want to see the potential emissions from reactor 3 properly considered and compared to those of reactor 2.

http://www.nisa.meti.go.jp/shingikai/700/14/240723/AM-3-1.pdf

In particular the table on page 28 shows how much wider the estimates for reactor 3 are in this updated study, with variables such as PCV leakage area and actual water injection rates being responsible for the uncertainty. The top end of many of these estimates are much closer to reactor 2 estimates than before.

On a similar note the table on page 9 of this Japanese TEPCO report, which shows estimated releases at all sorts of different moments throughout march, features some pretty big numbers for reactor 3. I added up the most significant ones for reactors 2 & 3 and again reactor 3 is closer to the reactor 2 totals than before, though still a bit lower.

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

If you have any time I would so appreciate a translation of the notes column for the most significant releases. This is the same table that I mentioned recently in reference to them putting the release height of the stack in for the 14th 21:20 entry, although the obvious explanation for this is they just went for the potentially erroneous theory that this was due to the venting attempt rather than a breach of containment.

 

[tsutsuji]

On a similar note the table on page 9 of this Japanese TEPCO report, which shows estimated releases at all sorts of different moments throughout march, features some pretty big numbers for reactor 3. I added up the most significant ones for reactors 2 & 3 and again reactor 3 is closer to the reactor 2 totals than before, though still a bit lower.

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

If you have any time I would so appreciate a translation of the notes column for the most significant releases. This is the same table that I mentioned recently in reference to them putting the release height of the stack in for the 14th 21:20 entry, although the obvious explanation for this is they just went for the potentially erroneous theory that this was due to the venting attempt rather than a breach of containment.

All the documents from the Tepco report of 24 May 2012 have been translated by Tepco on the page linked in the press conference transcript:

"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 )..." .

Table 8: "Assessed Values for Periods Where Air Dose Rates Fluctuated" on page 9 (11/89) of http://www.tepco.co.jp/cc/press/betu12_j/images/120524j0105.pdf (Japanese) is translated into English on page 14 (16/104) of http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120524e0205.pdf (English).


http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120524e0205.pdf page 20 (22/104) says:

5.5.3 Amounts Released from Reactor Buildings
In this accident, in addition to releases accompanying primary containment vessel venting and building explosions, it is believed that radioactive material was released to the atmosphere from R/Bs. The assessment results for the amount of radioactive material released from R/Bs are shown in Table 12. The amount released from R/Bs was larger than even the releases accompanying primary containment vessel venting and building explosions, but this is inferred to be due to radioactive material leaking without undergoing S/C pool scrubbing.

It is difficult to specify the path of release from the primary containment vessel to the R/B (leak location), but, from the results of field surveys and data on the design of the primary containment vessel, it is thought that the leak might have occurred at the seal of the primary containment vessel top head flange. (See Attachment 3)

 

[SteveElbows]

Thanks very much :) I admit to having got somewhat overwhelmed by the quantity of reports that are mounting up, and if they arent translated very quickly then I lose track. I will take a look at the detail later, but the bits you quote demonstrate why I thought this stuff was relevant for this thread. Cheers.

 

[clancy688]

A new NHK documentary, shedding some light on what happened in Unit 2:

https://www.physicsforums.com/showpost.php?p=4056805&postcount=13651

No mentioning of a dry vent though.

 

[SteveElbows]

I've just been moaning about it on the other thread. Mostly because I didnt find it too helpful in terms of shedding light. It was in some ways a mess that makes me feel like I need to reread one of the long official reports into the accident again in order to put events back into order in my mind. It had some interesting detail in places and I suppose I should be happy that it focussed in a different direction to the ones we often end up dwelling on, it made a change. But it does nothing for my thirst to see or read something that tells reactor stories using the amount of detail we've been able to ascertain from so many sources and talk about here.

Its kind of sad to see the focus still falling on failings that have been obvious since the start, such as the workers having to scramble to get car batteries. Official reports had other interesting stories to tell about failings. Turning the IC off at reactor 1 was not the only bad decision that could be turned into an interesting explanation in a tv documentary. OK I can't moan at them for picking up that a lot of the story about reactor 2 is about venting problems, but they could do a better job of putting it in context, and if they want to go into detail they should try to be clearer about it. It would also be nice if new documentaries made some effort to dispel some of the potential suppression chamber myths that have been with us since there was a bang after 6am on the 15th march 2011. Stories that end with this bang are not likely to satisfy us now that attention often turns to the top of the drywell.

I was hoping the TEPCO conference footage might give us some tiny new shred of info, especially as march 14th seemed to be one of the few times they had audio. But all there seemed to be was stuff to encourage the appetite of those who are interested in some non-technical stories about reactor 2 that have much media interest - whether TEPCO planned to evacuate everyone, and disagreements between people/government interference about the prioritisation of venting, srv opening and water pumping attempts.

 

[SteveElbows]

When reviewing this lengthy attachment to a final report I was very pleased to see that a lot of the stuff we were left to speculate about ourselves in relation to reactor 2 are covered ina decent amount of detail.

http://icanps.go.jp/eng/02Attachment1.pdf

For example:

Page 103+ Considers that pressure changes may not have been due to SRV operation, but rather vessel failures. Large CAMS differences between the D/W values and S/C values are used to offer possible answers to this question and some of the other I mention below.

Page 105 Problems with using CAMS readings to estimate percentage core damage.

Page 114+ Lengthy discussion about pressure changes including at 0:05 on the 15th march 2011

Pages 126-140 are a brutal look at the MAAP analysis from TEPCO and the MELCOR analysis, looking at multiple serious flaws with the models themselves and the data and assumptions fed into them. Includes many of the things I ranted about previously such as using too late a time for RCIC failure, too early a time for successful water injection.

Pages 160-161 Another brutal discussion about suggestions that a vent may have been successful, they are very unimpressed by the talk about rupture disc etc, and as with much of their analysis they point to the far more obvious failure of containment as an alternative explanation that makes much more sense.

Page 161 looks at whether something happening at reactor 1 during the morning of March 15th may have contributed to the increased radiation on site.

Page 163 discusses a worsening situation from around 07:20 that morning, the steam seen escaping from the blowout panel at 08:25 and rather interestingly mentions a site report that the amount of steam escaping had increased by 09:40.

There is a lot of other good stuff in the document but these were some of the highlights for me given my past interest in many of these specifics at the expense of some others.

 

[Hiddencamper]

Just some FYI here. I spoke with someone who was at the Chicago ANS conference over the summer, and apparently there is talk about the possibility of a previous weld flaw in unit 2 cracking during the overpressurization of the containment. This would lead to suppression pool bypass and prevent wet scrubbing of radioactive material released from the core during SRV lifts or a core bottom head breach. I've been trying to find more info but have been unable to. Anyone have any insight?

 

[rmattila]

Containment dome seal failure is - as far as I am aware - one probable bypass route, considering the rather high dose rates measured directly above it on the service floor.

 

[SteveElbows]

Yes, and although there are a range of possible failures that cannot be ruled out at this point, none of them are completely necessary to explain what happened at reactor 2.

For example there are a host of reasons why they struggled to vent reactor 2, and a number of reasons why SRV opening on the day in question may not have worked. And one obvious difference between reactor 2 and the others is that they had switched the source of water for the RCIC to the suppression chamber at reactor 2 and then failed to monitor the parameters of the suppression chamber, likely leaving the suppression chamber in a state different to the state of the s/c at the other reactors when they melted.

It is understandable that some will still question whether reactor 2 contaminated the environment more than the others, for a number of reasons. Firstly the weather conditions when reactor 2 containment failed were conducive to land contamination, and there is also the fact that the authorities were very keen to downplay the possibility of containment damage at the other two reactors during and for quite a while after the disaster. Combine this with the interest people had in the reactor 3 explosion, the discovery that the reactor 2 suppression chamber had not failed in a very dramatic explosive manner, and the authorities lack of interest in discussing the later smoke incidents and instability of reactor 3, and I cannot complain about people remaining keen on attributing plenty of environmental to the reactors other than 2. And indeed as time has passed we do see reports that are more willing to consider some of this stuff than the official reports of the first year were. Personally I remain especially interested in reactor 2 because of the failure to wet-vent, and the implications of this failure.

 

[Hiddencamper]

Yes, and although there are a range of possible failures that cannot be ruled out at this point, none of them are completely necessary to explain what happened at reactor 2.

For example there are a host of reasons why they struggled to vent reactor 2, and a number of reasons why SRV opening on the day in question may not have worked. And one obvious difference between reactor 2 and the others is that they had switched the source of water for the RCIC to the suppression chamber at reactor 2 and then failed to monitor the parameters of the suppression chamber, likely leaving the suppression chamber in a state different to the state of the s/c at the other reactors when they melted.

It is understandable that some will still question whether reactor 2 contaminated the environment more than the others, for a number of reasons. Firstly the weather conditions when reactor 2 containment failed were conducive to land contamination, and there is also the fact that the authorities were very keen to downplay the possibility of containment damage at the other two reactors during and for quite a while after the disaster. Combine this with the interest people had in the reactor 3 explosion, the discovery that the reactor 2 suppression chamber had not failed in a very dramatic explosive manner, and the authorities lack of interest in discussing the later smoke incidents and instability of reactor 3, and I cannot complain about people remaining keen on attributing plenty of environmental to the reactors other than 2. And indeed as time has passed we do see reports that are more willing to consider some of this stuff than the official reports of the first year were. Personally I remain especially interested in reactor 2 because of the failure to wet-vent, and the implications of this failure.

I'm not sure if RCIC drawing suction from the suppression pool would have caused a big difference. If anything, I would anticipate containment pressure to be higher in the suppression chamber as a result, and reaching HCTL (Heat Capacity Temperature Limit) sooner. There's always a chance the RCIC suction line from the suppression pool failed in some way, as I don't think the RCIC suction line is designed for high pressure (but I don't think 150 PSI [1MPa] would have caused that piping to fail either).

It's weird that they would stop monitoring suppression pool/chamber parameters. Those are critical safety function parameters for post accident conditions.

 

[SteveElbows]

I guess their need to prioritise and the numerous ways they became overwhelmed by the situation would explain the lack of measurements. Perhaps there was a specific difficulty with getting these measurements in particular, although I don't remember reading about it in reports, mostly it was just criticism of their failure to monitor it.

And yes you are right in that I cannot determine whether the state of the S/C made much difference, they had plenty of other problems that hampered venting and SRV operations.

I've now been looking at the detailed documents that form part of the other 'final report' that I was waiting forever to be translated and then forgot about until recently. One of them does go along a path that's been pursued on this thread from time to time, the failure to vent and why this lead to the idea that reactor 2's containment failure was especially bad for the environment.

Pages 34-40 of this one: http://warp.da.ndl.go.jp/info:ndljp...nt/uploads/2012/08/NAIIC_Eng_Chapter2_web.pdf

They make it pretty clear that since reactor 3 was vented on numerous occasions, containment did not remain at high pressures for extended periods of time. They make the point that reactor 2 drywell pressure was way too high for seven straight hours, and that containment then failed in more than a minor way, leading to depressurisation without venting. The report is somewhat inconsistent in its discussions though, since they also talk about reactor 1 and from the data they've used the containment pressure for 1 was very high for even longer, 12+ hours, but they don't really dwell on that using the same narrative they used for reactors 2 & 3. I think that's because in the case of reactor 1 a large failure of containment sufficient to decrease pressure significantly didnt occur during this time, so despite the lengthy delay in venting it was still the vent that was eventually responsible for massively reducing pressure at that stage.

So despite the fact that containment ultimately failed in some manner at all three reactors, this does leave me with questions such as 'why did reactor 2 containment fail in a dramatic way more quickly than reactor 1 containment?' or to put it another way 'how did reactor 1 containment last so long under high pressure?'.

Of course to answer this we need to know more about the damage that has been done, and may be waiting a long time for that, especially as TEPCO & government agencies do not seem very keen to highlight every detail of containment damage that they have observed at any point. Mostly what we have learned so far is more about what did not happen, ie the suppression chamber of reactor 2 and the surrounding area does not sem to be damaged in a way people originally thought, and these days the s/c low pressure is assumed to be due to sensor failure.

Please note that I am only speaking of containment failure on a scale necessary to rapidly and significantly reduce pressure, and only about the first meltdown events at each reactor. Clearly there were more minor containment leaks at the reactors at early stages, and potentially further significant releases of radioactive material on subsequent days that haven't received enough attention.

Regardless, the areas I've drawn attention to support the earlier discussions on this thread regarding the reactor 2 release. Large quantities of containment atmosphere, including radioactive material from the initial melting stages left reactor 2 containment in a vigorous manner, at a time when the containment was at a high pressure. At the other reactors, despite some lesser containment leakage, broadly equivalent releases happened via wet-venting.

However I should be careful not to ignore later events at all three reactors, since these reports also touch on the possibilities that further melting events happened at 1 & 3 and venting was not the only release path for these. For example there is talk of reactor 1 doing bad things some days after it first melted, and with regards to reactor 3 in addition to various smoke events that people talked about on these forums at a lot at the time, both these reactors had periods where water injection was temporarily halted long after their original meltdowns. For this reason i would like to better understand how much radioactive material can get from the fuel into containment atmosphere and ultimately the environment at later stages, compared to the amount that can be generated and released during the initial melt. This is one way to better understand how much the failure to take into account later events may have distorted the picture that reactor 2 was responsible for the bulk of the environmental release.

 

[rmattila]

For this reason i would like to better understand how much radioactive material can get from the fuel into containment atmosphere and ultimately the environment at later stages, compared to the amount that can be generated and released during the initial melt. This is one way to better understand how much the failure to take into account later events may have distorted the picture that reactor 2 was responsible for the bulk of the environmental release.

In the attached picture there are half-times of different elements in the fuel matrix. For I, Xe, Cs and Te, the half-time at melting temperature is only 10 s, i.e. when the fuel reaches melting point, these substances are already out of the fuel matrix and in the containment.

 

[SteveElbows]

Thanks :)

Completing my look back at reports from 2012 that have now been translated, I am reading the TEPCO one. http://www.tepco.co.jp/en/press/corp-com/release/2012/1205638_1870.html

As we might have expected, from what I've read so far it doesn't explain technical problems clearly or with much in the way of interesting new detail. But here is what they said about release of radioactive materials on page 36 of http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120620e0102.pdf

12. Evaluation of the release of radioactive materials (Report [12])
(1) Release of radioactive materials into the atmosphere (Report [12.1]) Evaluation of each of the main phenomena in this accident when radioactive
materials were released into the atmosphere and the causes of high level contamination areas to the northwest of Fukushima Daiichi are as follows:
・The release of radioactive materials was restricted (not clearly established in regard to Unit 2) during the venting operations of Units 1 to 3 due to the scrubbing effect of the suppression chamber, and the amount released was smaller in comparison to that of Unit 2 reactor building, so TEPCO does not consider this to have been a major factor leading to the contamination.
・Judging from the nature of the monitoring data at the time of the explosions of Units 1, 3, and 4 reactor buildings, the amount of the release was quite small compared to that of Unit 2 reactor building, and TEPCO does not consider this to have been a leading cause of contamination.
・Monitoring data rose sharply on March 15. At the time, the Unit 2 PCV pressure dropped considerably, and white smoke was seen coming from Unit 2 reactor building. Winds blowing toward the north-northwest direction prevailed that day, and since the contaminated areas had rain at the time, it is possible that the contamination in the high contamination zones resulted from a release of radioactive materials from the Unit 2 reactor building on March 15. It is further hypothesized that the emission from Unit 2 bypassed the suppression chamber scrubbing effects (radioactive material water decontamination effect, which has
roughly the same effect as filtering).
・There was a large fluctuation in the air dose rate on March 16. From
meteorological data from around that time, it would not seem that it could have been a major factor in the contamination of areas to the northwest, but in regard to the fluctuation in the air dose rate a little after 10:00 on March 16, white smoke was seen emanating from the Unit 3 reactor building at 8:30 on the same day, and since there was a fluctuation in the dry well pressure at around the same time, it is believed that the emission could have come from Unit 3.

 

[SteveElbows]

Page 368 from this TEPCO document shows release estimates for different events. They have very high amounts from reactor 3 on March 16th, the same values as for reactor 2 on March 15th! Reactor 2 still 'wins' because there are also notable releases after 21:00 on March 14th, but even so I think this data will be of interest to people.

These figures are hardly likely to be perfect, to pick one example they miss out any releases from reactor 1 after the building explosions, whereas some non-TEPCO reports I talked about recently mention the possibility of this.

http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120620e0104.pdf

 

[ronaldkr]

Hi Steve, you might be interested in JNES' reevaluation of release rates: p. 11 of this document:
http://www.aec.go.jp/jicst/NC/sitemap/pdf/P-4.pdf

Basically, unit 3 did some discrete emissions, unit 1 had a long sustained one, and unit 2 started with an emission at least a magnitude larger than unit 1. The time frame is however insufficient.