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

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

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 dont 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 thats 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/...apter2_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 dont really dwell on that using the same narrative they used for reactors 2 & 3. I think thats 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 learnt 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 havent 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

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.

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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...5638_1870.html

As we might have expected, from what I've read so far it doesnt 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...20620e0102.pdf

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...20620e0104.pdf

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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.