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

clancy688

Thanks to tsutsuji for providing this paper:

http://sciences.blogs.liberation.fr/...hima-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

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.../111202e14.pdf

MadderDoc

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 Im 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

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.

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 :-)

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

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/betu...20524j0105.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

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

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

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

Sharma SK

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

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

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.