Fukushima - Why did Unit 2 release so much more radioactivity than Units 1 and 3?

tsutsuji

http://www3.nhk.or.jp/news/genpatsu-...00_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...hima-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

As Masamichi Chino made a presentation at the workshop of 23/24 July ( http://www.nisa.meti.go.jp/shingikai...723/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/2...127241800.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...4659_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

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?

tsutsuji

It is rather vague. Perhaps he means that, roughly speaking, for every particle released into the atmosphere 99 particles remain in the water if the wet venting is done properly.

zapperzero

That's what I thought too - but I know better than to rely on my linguistic intuition wrt a translation from language I don't speak at all. So, thanks.

SteveElbows

Thanks so much for that, it is good to see this information. Its an explanation we have come up with ourselves before, and have seen so briefly mentioned in a few reports, but usually not very well explained in such reports, or only acknowledged in a very round about manner.

Likewise we have also heard more recently about the concerns about reactor 2 at the time, due to the publication of its of the TEPCO videoconferences, and reactor 2 got quite a lot of press attention because I am under the impression that the footage from the 14th March was the only one with sound, and because there is still controversy and questions about TEPCOs worker evacuation plans.

Again the concerns about not being able to vent and the state of the suppression chamber on the 14th have been mentioned before in other reports. However there still exist a range of possibilities as to which concern the various people in charge were most afraid of, eg that made them consider evacuation more at this moment than prior ones. Was it the fact they hadnt been able to scrub via wet venting, the possibility of the meltdown being very bad due to inability to inject water for a very long time due to high pressure, fear of dramatic or explosive s/c damage or d/w damage? In one report the emphasis was on the site managers concerns about the suppression chamber, but from the teleconference it sounds like at least one persons big fear was for the drywell. It is unclear whether they feared the sort of drywell leak that actually ended up happening, or an even more dramatic one that could rip the drywell apart. These more explosive imagined scenarios might explain the attention on evacuation.

As for the exact path of release for the reactor 2 releases at the various times, there is still disagreement. We can be fairly confident about the drywell releases that happening on the morning of the 15th, but the path of release from 21:20 on the 14th and throughout much of the night is less clear. We had questions about whether any of the vents may actually have happened (rupture disc status unconfirmed and shared stack contaminated so cant be sure). At least one of the tables showing estimated magnitude of releases at different times has the night of the 14th large release coming from the height of the stack, and only the later releases from the height of the building. Others such as Matsumoto that you just quoted are suggesting the releases came from the drywell that night. And the author of the study behind a paywall ( at http://www.tandfonline.com/doi/abs/1...669237#preview ), thinks it was the suppression chamber that leaked on the night of the 14th and the drywell didnt start to spew significant quantities of stuff till the morning of the 15th. The question is further complicated by those who have theories that damage had occurred days earlier, and although no prior damage is necessary to explain the later emissions, we cannot utterly rule these possibilities out at this point.

I dont necessarily expect to learn much more of interest about the top of the drywell, so to narrow down some of the above possibilities I will be hoping to learn more about the stack contamination, rupture discs, and the exact nature of any suppression chamber damage at some point.

tsutsuji

What would be the path if the theory that the timing of radiation peaks at the Daini plant is correlated with the timing of SR valve operation is true? Under that theory, at that time, something was damaged downstream the SR valve, while the RPV upstream was still OK? For the contaminated gasses to go from the SR valve to the outside without passing through the wet well, you need two kinds of damage : a damage of the pipe between the SR valve and the wet well, and a damage of the dry well. That sounds rather complicated, or am I missing something ?

I have the same question for this Tanabe theory. What is the meaning of "suppression chamber... leaked" ? I can understand that if there is a hole through which water escapes, the water level might become too low, and the "wet well" not being wet enough, the scrubbing function fails. Otherwise, I don't see the difference if the gasses escape through a hole in the upper part of the wet well after being scrubbed. Does that make any difference with being released through the stack ? In other words, does Tanabe say that the suppression chamber released a) liquids b) unscrubbed gasses or c) scrubbed gasses ?

SteveElbows

He mentions large amounts of steam with radioactive materials flowing into the S/C water pool. And that large amounts of radioactive materials which escaped pool scrubbing could then have escaped through S/C breach. However he does not go into any detail about the breach, or how the materials escaped pool scrubbing.

Various people have already criticised his theories here, and its also worth noting that his study was in part a response to analysis modelling where, in order to get the modelling estimates for D/W pressure over time to match the actual recorded values, they had assumed both a drywell breach and a S/C breach of certain sizes occurring days before the 14th. But since then they have developed a different theory, to do with cooling effects of water in torus room, and thermal stratification of the S/C. eg pages 11-16 of this http://www.nisa.meti.go.jp/shingikai...723/AM-1-2.pdf and then pages 27-35 dealing specifically with how this may apply to reactor 2 data. So I think his study is already out of date.

In other aspects his study was still of interest to me, just because it lays out a possible timetable for various sorts of melting in the RPV & drywell in a fairly clear way which many official reports somewhat avoid or get the timing wrong on. In particular he is not afraid to point out the various times that RPV pressure went too high for pumped water to reach the RPV, which ends up offering an explanation for why we didnt see steam coming from upper part of reactor 4 till after 8am, and a possible remelting of core material on the afternoon of the 15th.

SteveElbows

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...723/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/betu...20524j0105.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

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:
Table 8: "Assessed Values for Periods Where Air Dose Rates Fluctuated" on page 9 (11/89) of http://www.tepco.co.jp/cc/press/betu...20524j0105.pdf (Japanese) is translated into English on page 14 (16/104) of http://www.tepco.co.jp/en/press/corp...20524e0205.pdf (English).


http://www.tepco.co.jp/en/press/corp...20524e0205.pdf page 20 (22/104) says:

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 happend in Unit 2:

http://physicsforums.com/showpost.ph...ostcount=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 cant 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?