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

by clancy688
Tags: fukushima, radioactivity, release, unit, units
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Yamanote
#37
Mar6-12, 03:13 PM
P: 68
Probably the venting of unit 2 happened in the way Arnie Gundersen explained in one of his videos:

http://vimeo.com/36492960
clancy688
#38
Mar6-12, 04:14 PM
P: 546
Nope, most definitely not.

That's only a "containment vent", not a reactor vent. Moreover, it doesn't vent high pressure. It only ensures, that, at high pressure, the pressure doesn't get any higher. And that only goes for the containment.
So when pressure inside the containment reaches a certain value (I think it was twice the normal pressure), the containment cap lifts and pressure escapes. The pressure falls slightly and then the cap closes again. So the pressure can't go over twice the normal pressure, but it also can't go below twice the normal pressure.
But that still doesn't vent the reactor pressure vessel. And the unfiltered release ("dry venting") can't happen that way. Because for reactor pressure to escape via a lifting containment cap, the gases have to travel through the wetwell (torus). Which they obviously didn't, that's why we have such a dirty release for Unit 2.
So there must've been an other path.
zapperzero
#39
Mar6-12, 05:10 PM
P: 1,041
Quote Quote by clancy688 View Post
Because for reactor pressure to escape via a lifting containment cap, the gases have to travel through the wetwell (torus). Which they obviously didn't, that's why we have such a dirty release for Unit 2.
So there must've been an other path.
We don't know that. The torus may have been dry.
SteveElbows
#40
Mar6-12, 07:19 PM
P: 630
Now you are confusing me a bit clancy688, either I misunderstood or else I don't think I agree with you about several things.

I thought that the terms wet and dry venting applies only to how the gases are released from drywell containment to the outside world, not how those gasses got into containment in the first place. If the gases were vented from the reactor vessel to the s/c, but then ended up in the drywell, and the drywell was then vented directly, I think that would still count as a dry vent.

A quote from the interim investigation report gives us some info about the venting of the reactor vessel into containment, with emphasis on how poorly the s/c seemed to handle this:

According to the reactor pressure gage, Unit 2 reactor pressure indicated 6.998 MPa gage at around 16:34 that day. It indicated still 6.075 MPa gage at around 18:03, more than one hour after they had started depressurizing.
They continued trying to open the SRV to depressurize the reactor. However, they had trouble in keeping the SRV open and the steam from the RPV barely condensed in the S/C because of high temperature and pressure in the S/C. Consequently, it took time to depressurize the RPV to the sufficient extent.
The reactor pressure was finally lowered to a level where water injection was possible at around 19:03 that day, when the reactor pressure gage indicated 0.630 MPa gage.
and

According to the reactor pressure gage, Unit 2 reactor pressure indicated higher than 1 MPa gage from around 20:54 until 21:18 that day (it indicated 1.463 MPa gage at around 21:18) and then it decreased due to depressurization. It again exceeded 1 MPa gage from around 22:50 until 23:40 that day (it indicated 3.150 MPa gage from around 23:20 until 23:25 that day) and then decreased again as a result of further depressurization. From around 00:16 until 01:11 on March 15, it again rose to over 1 MPa gage (it indicated 2.520 MPa at around 01:02 that day). At least during those periods of high values, Unit 2 reactor pressure seemed higher than the discharge pressure of the fire pumps and therefore it was highly likely that water had not been injected into the reactor.
and

From around 01:00 on March 15, Unit 2 reactor pressure indicated steadily staying above 0.600 but below 0.7 MPa gage and continuous water injection into the reactor became possible.
(from pages labelled as numbers 257,258 and 259 of http://icanps.go.jp/eng/120224Honbun04Eng.pdf )

Obviously what we can't tell from this is whether the bulk of the depressurisation of the reactor was achieved through the deliberate operation to depressurise the reactor vessel, as opposed to the fuel falling out of the bottom of the reactor vessel. Its been ages since I looked at the pressure charts for this period, I will have to refresh my knowledge on this front and comment again if anything seems relevant.

In any case I find the approach of looking at containment failure in terms of a very simple model of a cap rising and falling at certain pressures to be just a bit too flawed. We know that there are a range of potential failure points in containment, such as gaskets & flanges around not just the cap but also pipework points and equipment & personnel airlocks. And we know that heat contributes to degradation of such seals, so we aren't just dealing with pressure. Now it may turn out that the cap rising was a main culprit at one or all of the affected reactors, but its going to be a long journey to discover this. Certainly we saw some visual evidence of stuff escaping from locations around the top of containment at all three of the reactors. Reactor 3 offered the clearest view of this, could see it emerging from at least the area where the drywall concrete pit gates meet the floor area above the reactor. But we saw some signs at reactors 1 and 2 as well, along with the robot readings around that area of reactor 2 that I mentioned earlier. But this isn't proof that the cap lifted, cap or another part of containment could have degraded in a different manner.
SteveElbows
#41
Mar6-12, 08:13 PM
P: 630
Does anybody know where a Japanese version of the following document might be found?

It is described on Tepco's English website as '- Fukushima Nuclear Accident Investigation Report (Interim Report Supplementary Volume) (PDF 212KB) '

http://www.tepco.co.jp/en/press/corp.../111202e16.pdf

The reason I ask is that in the section dealing with reactor 2, on page 39 it says the following, which as you can see is suffering from poor wording just at the moment when it could reveal something about the status of the rupture disk:

At 00:02 on March 15, the AO valve (bypass valve) on the vent line from the D/W was opened, and it was thought that the vent line, with the exception of the rupture disk, was completed, however several minutes later it was discovered that the AO valve (bypass valve) in the vent line from the D/W was closed. As a result it was not possible to determine whether venting was successful (ruptured status of the rupture disk ruptured).
Sorai
#42
Mar6-12, 11:03 PM
P: 16
Quote Quote by SteveElbows View Post
Does anybody know where a Japanese version of the following document might be found?

It is described on Tepco's English website as '- Fukushima Nuclear Accident Investigation Report (Interim Report Supplementary Volume) (PDF 212KB) '

http://www.tepco.co.jp/en/press/corp.../111202e16.pdf
This one: http://www.tepco.co.jp/cc/press/betu...es/111202g.pdf

Seems to be the same info. They couldn't confirm whether or not the rupture disk was open:

3月15日0時02分頃、ドライウェルからのベントラインにあるAO弁(小弁)
の開操作を実施し、ラプチャーディスクを除くベントライン構成が完了した
と思われたが、数分後にはドライウェルからのベントラインにあるAO弁
(小弁)が閉状態であることを確認した。結果として、ベントの成否
(ラプチャーディスク開放の有無)は確認出来ていない。
etudiant
#43
Mar7-12, 06:42 AM
PF Gold
P: 858
There has been speculation on EX-SKF that the rupture disk did rupture, but that debris from within the RPV clogged the piping for the vent. Given the violence of the events, this does seem plausible.
SteveElbows
#44
Mar7-12, 09:07 AM
P: 630
Quote Quote by Sorai View Post
This one: http://www.tepco.co.jp/cc/press/betu...es/111202g.pdf

Seems to be the same info. They couldn't confirm whether or not the rupture disk was open:
Thanks very much, this is perfect, just what I needed. The Japanese version is much clearer than the English version. If I google translate the relevant sentence I get this:

As a result, the success or failure of the vent
(The presence or absence of open rupture disk) has not been confirmed.
Whereas the English version fails to make that point clearly at all, due to using rupture words three times.

So now we have heard them say that the rupture disk status is unknown, the theory on ex-skf that there may have been a vent for a couple of minutes, is not incompatible with official statements.
SteveElbows
#45
Mar7-12, 03:31 PM
P: 630
I've just been saying elsewhere that it might be useful to look again at some graphs in this document:

http://www.kantei.go.jp/foreign/kan/...ttach_04_2.pdf

Specifically the CsI distribution graph for reactor 2 Tepco Case 2 on page 36, and Cs graph on page 37.

Note the presence of reactor building, FHB (fuel handling building I guess) and environment in these graphs, as well as the timings. Then compare to the other reactors.
Yamanote
#46
Mar7-12, 03:51 PM
P: 68
It seems to me that successful venting through the designated path is almost impossible under accident conditions and without electricity, at least for the Mark I containment (don't know if it would work out better for other containment designs).

Is the "blow out panel" opened at unit 2 actually an improvisation or a feature?
Because units 1 and 3 explosions showed us that trying to contain gases and Hydrogen (which must be released anyway) within the secondary containment might not be the right approach in an emergency.
zapperzero
#47
Mar7-12, 04:31 PM
P: 1,041
Quote Quote by Yamanote View Post
Is the "blow out panel" opened at unit 2 actually an improvisation or a feature?
Design feature. But it was popped from the outside, by hand, as it were, to avoid the accumulation of hydrogen. Seems to have worked, to a point.

Because units 1 and 3 explosions showed us that trying to contain gases and Hydrogen (which must be released anyway) within the secondary containment might not be the right approach in an emergency.
Nothing is supposed to be released into the reactor building. The very worst case scenario designed for is to vent some steam from the RPV through the S/C, then through the hardened vent (bypassing the undersized SGTS) and out the stack.
rowmag
#48
Mar8-12, 07:56 AM
P: 209
Quote Quote by zapperzero View Post
Design feature. But it was popped from the outside, by hand, as it were, to avoid the accumulation of hydrogen. Seems to have worked, to a point.
Are you sure? My recollection is that they announced that they intended to open it, but then discovered that it had already popped off by itself. (As a result of the Unit 1 explosion?)
zapperzero
#49
Mar8-12, 08:32 AM
P: 1,041
Quote Quote by rowmag View Post
Are you sure? My recollection is that they announced that they intended to open it, but then discovered that it had already popped off by itself. (As a result of the Unit 1 explosion?)
No, I am not sure. This is my recollection however. I would be grateful if you can dig up a source, of course.
Yamanote
#50
Mar8-12, 01:36 PM
P: 68
Quote Quote by zapperzero View Post
Nothing is supposed to be released into the reactor building. The very worst case scenario designed for is to vent some steam from the RPV through the S/C, then through the hardened vent (bypassing the undersized SGTS) and out the stack.
I agree that it was not supposed but happened for all three units. And like Steve Lochbaum said, even unit 4 paid sympathy to the others and exploded as well.

So in the future we have to suppose that unintended venting into reactor building will happen in such kind of accidents. Opening the blow out panel in advance might be a good idea and venting into another unit through shared equipment must be avoided.
zapperzero
#51
Mar8-12, 03:32 PM
P: 1,041
Quote Quote by Yamanote View Post
shared equipment must be avoided.
You got that right, I think.
rowmag
#52
Mar8-12, 04:37 PM
P: 209
Quote Quote by zapperzero View Post
No, I am not sure. This is my recollection however. I would be grateful if you can dig up a source, of course.
See the third slide (page-numbered "2") of this NISA presentation:

http://www.nisa.meti.go.jp/shingikai/800/28/006/6-3.pdf

"The reason there was no hydrogen explosion at Unit 2 is conjectured to be that by luck, the blow-out panel was opened as a result of the explosion at Unit 1, releasing accumulated hydrogen to the outside and thereby avoiding an explosion."

(The date on the title page of the above is 20 Jan. 2011, presumably a typo for 2012.)
zapperzero
#53
Mar9-12, 01:36 AM
P: 1,041
Quote Quote by rowmag View Post
See the third slide (page-numbered "2") of this NISA presentation:

http://www.nisa.meti.go.jp/shingikai/800/28/006/6-3.pdf
Thank you.
SteveElbows
#54
Mar10-12, 09:34 PM
P: 630
Now that we can consider the possibility that reactor 2 could have dry vented for a few minutes, I would like to suggest that reactor 2 may be the source for the >10 Sv/hr contamination at the pipes at the bottom of the reactor 1 & 2 stack.

So I took another look at the gamma picture taken in August, on page 2 of this document:

http://www.tepco.co.jp/en/nu/fukushi...10802_01-e.pdf

I am interested in the gamma blobs on the right of the image. By looking at some other photos of the area, including the early high-ish res overhead ones from early on, I believe that this area of the photo is heading east. And we can just make out the large ducting pipe heading in that direction. But either side of that duct pipe are the smaller pipes, running parallel to the larger pipe, and their height above ground is near the bottom of the larger ducting. I believe the smaller pipe on this side is one from reactor 2, and the gamma blob fits with a point in this pipe.

You can see the pipe I mean in the last 3 photos on this page, it stands out fairly well because although it is small it looks pretty white in the photos. Download the zip of the images to see it even more clearly.

http://cryptome.org/eyeball/daiichi-...chi-photos.htm


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