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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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MadderDoc
#19
Jun16-11, 05:48 PM
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Quote Quote by SteveElbows View Post
<..>
Im stilla bit confused about the exact timing of the release to the environment, because cetain graphs that I posted in the main thread just the other day, seem to suggest that a lot of stuff got into the environment in the hours leading up to the explosion, rather than after the explosion. Thats what their reactor analysis seems to show. But other analysis, based on radiation readings at certain locations on site, seems to have the highest magnitude releases happening after the explosion.
Assuming both analyses represent reality, it would seem to imply that (some) radioactivity was released from unit 2 before the explosion in the unit (due to venting), and quite a lot more radioactivity was released after the explosion in unit2 -- however not from unit 2 but from one or more of the other units.

First graph shows stuff going into environment after valve opened, before explosion.

http://www.physicsforums.com/attachm...8&d=1308073787
Perhaps I do not fully understand the graph, but it bothers me that it seems to show that all the compartments had the fraction 0 of fission products, until the safety release was opened at about 78h. Also (caveat: judged visually) in the following period there are times when the sum of increases in compartmental fractions seem to be not matched to the sum of decreases.

This graph of estimated total release amounts and the time periods they cover, has highest magnitude releases happening for just a few hours during the 15th, at a time after the explosion:

http://www.physicsforums.com/attachm...2&d=1308080200
There's certainly a peak in the release rate at that time, but the graph also shows the release rate to remain high for days thereafter. in particular as regards I-131. If we think in accumulated releases, the release during the peak would seem dwarfed by the sum of releases followingly.
zapperzero
#20
Jun17-11, 03:38 AM
P: 1,042
Quote Quote by SteveElbows View Post
That TEPCO data & post you link to are about the first hour or so after the earthquake hit. For a guide to what may have happened on the 14th and 15th, I use some other TEPCO & NISAs analysis, mostly via the english version of report to IAEA.

http://www.kantei.go.jp/foreign/kan/...ttach_04_1.pdf

Table on page 23 of Attachment IV-1 tells us:

9.20pm on 14th they open 2 SRV's and reactor pressure decreases.
11pm it is presumed that the 1 SRV was closed because reactor pressure increased.

The 2nd graph on page 35 shows these reactor pressure trends.

Graphs on page 36 show the D/W and S/C pressures, both measured and those predicted by the model. Note that S/C pressure readings start to go down well before the explosion, at a time when D/W pressure readings are going up.

Graph on page 32 of the document shows us the reason they have also assumed a D/W leak really early on, approx 21 hours after earthquake. This version of graph does not factor in this assumption, and as a result the model says that D/W and S/C pressures would have gone shooting up to very high levels days earlier. The other graphs I already mention do have the assumption of 21 hour D/W damage in them, and as a result the model D/W and S/C pressure values track the ones they actually measured pretty well.

Note that I have chosen analysis case 2 because thats the one that reflects what they learnt about bad water level readings at reactor 1, implying less water at the other reactors. This leads to conclusions such as RPV damage having happened. Case 1 was the more optimistic version of events where fuel was only partially melted and RPV damage didnt happen, so I've paid much less attention to it.

Also note that this document is TEPCOs version of analysis, and their versions of the FP existence ratio graphs are not like the ones I posted recently, theirs have only around 1% of stuff getting outside containment.
Here's my proposed sequence:

- quake, tsunami, blah blah
- SRV starts cycling
- a small D/W leak appears, steam goes who knows where, in any case some coolant loss happens.
- S/C wall goes pop, pressure drops (you can see it on your graph), water level drops too
- operators prop open two SRVs. Meltdown is now in progress.
- huge pressure spike in the D/W
- some of the new, hydrogen-laden steam makes it out of the S/C out of the water and through the crack. boom.
- water starts gushing out of the S/C
- a short time later, D/W pressure drops, as it is now venting to the outside, through the broken torus.
Quim
#21
Jun17-11, 02:53 PM
P: 84
Quote Quote by zapperzero View Post
Here's my proposed sequence:

- quake, tsunami, blah blah
- SRV starts cycling
- a small D/W leak appears, steam goes who knows where, in any case some coolant loss happens.
- S/C wall goes pop, pressure drops (you can see it on your graph), water level drops too
- operators prop open two SRVs. Meltdown is now in progress.
- huge pressure spike in the D/W
- some of the new, hydrogen-laden steam makes it out of the S/C out of the water and through the crack. boom.
- water starts gushing out of the S/C
- a short time later, D/W pressure drops, as it is now venting to the outside, through the broken torus.
That sequence makes sense to me, but I have a couple of questions.
Do you (also) assume that a " huge pressure spike in the D/W" was caused by a hydrogen/oxygen reaction? Why did #2 have so much more oxygen in it than 1&3?

Are you thinking that the " boom" was outside of containment?
Why no damage to the building?
zapperzero
#22
Jun17-11, 03:48 PM
P: 1,042
Quote Quote by Quim View Post
That sequence makes sense to me, but I have a couple of questions.
Do you (also) assume that a " huge pressure spike in the D/W" was caused by a hydrogen/oxygen reaction? Why did #2 have so much more oxygen in it than 1&3?

Are you thinking that the " boom" was outside of containment?
Why no damage to the building?
By the numbers from the top:
- no, I think that was the water from the RPV boiling off and two open SRVs
- I don't understand that question. Why would it have more?
- yes, somewhere right outside the torus. I don't know if there was damage to the building... no-one went into the sub-basement where the S/C is to check.
Quim
#23
Jun17-11, 04:58 PM
P: 84
Quote Quote by zapperzero View Post
By the numbers from the top:
- no, I think that was the water from the RPV boiling off and two open SRVs
- I don't understand that question. Why would it have more?
- yes, somewhere right outside the torus. I don't know if there was damage to the building... no-one went into the sub-basement where the S/C is to check.
I obviously made a false assumption; when you described a "- huge pressure spike in the D/W" I assumed that you believed that to be from an Oxygen/Hydrogen reaction, but since you think the spike was a result of boiling water, my question about where the oxygen came from was superfluous. I apologize for the confusion.

When you say the boom occurred "outside the torus" you were meaning outside the torus but still inside the containment structure.

I wondered if you meant outside both.
clancy688
#24
Jun17-11, 05:11 PM
P: 546
Quote Quote by zapperzero View Post
- yes, somewhere right outside the torus. I don't know if there was damage to the building... no-one went into the sub-basement where the S/C is to check.
I think damage to the building is very likely. At least if we take those "Unit 2 released over 90% of the radioactivity" claims by TEPCO into consideration.
I don't know the exact building structure, but if an explosion occured deep down in the torus room AND on the eastern side (speculation), the shock wave may have dispersed in the lower structures of the reactor building and the the turbine building, without triggering the overpressure panels at the roof.

I'm not very familiar with the overall building structure of the Fukushima reactors. I only know that part of the torus is in fact under the connection to the turbine buildings. Is it possible for a blast coming from the torus room and failing its walls to enter the turbine buildings?
NUCENG
#25
Jun17-11, 05:55 PM
Sci Advisor
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Quote Quote by Quim View Post
I obviously made a false assumption; when you described a "- huge pressure spike in the D/W" I assumed that you believed that to be from an Oxygen/Hydrogen reaction, but since you think the spike was a result of boiling water, my question about where the oxygen came from was superfluous. I apologize for the confusion.

When you say the boom occurred "outside the torus" you were meaning outside the torus but still inside the containment structure.

I wondered if you meant outside both.
You may understand this, if so, this is for other readers. To clarify - The drywelll and torus make up primary containment connected by the 8 large vent lines. Everything outside the drywell, vents, and torus in an intact RB is basically part of secondary containment. Units 1 through 4 have apparently had failures in both primary and secondary containment. In the case of Unit 4 the primary containment was opened for maintenance. In the other units failures were due to either overpressure, explosion or overtemperature.
Quim
#26
Jun17-11, 06:33 PM
P: 84
Quote Quote by NUCENG View Post
You may understand this, if so, this is for other readers. To clarify - The drywelll and torus make up primary containment connected by the 8 large vent lines. Everything outside the drywell, vents, and torus in an intact RB is basically part of secondary containment.

Units 1 through 4 have apparently had failures in both primary and secondary containment.

In the case of Unit 4 the primary containment was opened for maintenance. In the other units failures were due to either overpressure, explosion or overtemperature.
We are just fumbling with words here, but there is some built in confusion of terms and it wasn't me who started it.

Originally, when I was first learning about NPPs, I was taught that the first layer of containment for the uranium fuel was the zirconium sheath. Once I realized the clash of naming conventions that brought on, I took on the personal theory that there are but two containments, the RPV and the concrete structure in which it sits. (I do not consider the building itself to be part of the containment.) I see the RPV as the "primary" containment.

I know that I am out of sync with some, but I assume there are some who see it the same way I do.

So knowing this potential for confusion exists I tried to use the term "containment" to mean the concrete shell. But I see that won't work because some apparently consider the building itself to be part of the containment.

So I'll avoid the subject or be very specific in the future.
jim hardy
#27
Jun17-11, 08:32 PM
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in any field of study one starts with vocabulary and progresses to principles.
When one does not use accepted vocabulary communication goes poorly.

in this industry "Defense in Depth" (DID) is a vocabulary term for multiplicity of protections.
There are multiple protective barriers to keep fission products where they belong.
There are also multiple "Engineered Safety Features" which are machines whose job it is to protect the barriers.

WRT fission product barriers DID means these three layers:
1. fuel clad (your Zr sheath),
2. RPV (surrounds fuel & clad),
3. reactor vessel primary containment (usually a metal membrane) surrounding RPV and attachments
(in some texts the UO2 ceramic itself is considered the zeroth layer of defense.)

you might think the RPV would be called "Primary Containment" but it's not.
Primary containment encloses the whole primary system which is the reactor vessel plus its pumps & valves.

In power reactors the primary containment is a thin metal skin (the BWR light bulb) surrounded in turn by a concrete building that's mostly a missile barrier.
The concrete building provides a little bit of "secondary containment" but is basically analogous to a pair of coveralls. It's often called the "Containment Building" because that's what is inside it, the primary containment .

here's a decent paper with drawings..
https://netfiles.uiuc.edu/mragheb/ww...Structures.pdf

But, after all, the sciences have made progress, because philosophers have applied themselves with more attention to observe, and have communicated to their language that precision and accuracy which they have employed in their observations: In correcting their language they reason better.
http://web.lemoyne.edu/~giunta/lavpref.html
zapperzero
#28
Jun20-11, 11:56 AM
P: 1,042
Quote Quote by clancy688 View Post
Is it possible for a blast coming from the torus room and failing its walls to enter the turbine buildings?
Water and steam certainly can travel between the two buildings so why not a shock wave? But why do you ask?

Also, the torus rooms' walls are backed with earth, I don't think they could have failed. Ceilings, maybe.
SteveElbows
#29
Sep24-11, 08:33 PM
P: 630
I remain somewhat frustrated that this aspect of the disaster has not received more focus.

I have just read several details from the second report to the IAEA which are relevant to this topic.

Chapter 2: http://www.meti.go.jp/english/earthq...1/chapter2.pdf

Towards the end of this document the topic turns to estimated releases. Much is discussed, including a re-evaluation of March 12th-15th release estimates that JAEA conducted in light of NISA's June 3rd disclosure of emergency monitoring data from this period. They added some estimates based on the explosions, but also did something to the march 15th estimates, described as follows on page II-394

Also, as for the release on March 15, by taking into consideration the contribution to the air dose rate of tellurium-132 and its progeny nuclide, iodine-132, which were previously ignored, the estimated release rate of iodine-131 and cesium-137 decreased in relative terms.
Then, on page II-396 there is a table showing estimated release fractions into the environment, which they came up with using MELCOR. I've seen very similar in the past, buried in other reports, except this time there is a Tellurium group.

For Unit 1 they have approx 0.7% Iodine group, 1% Tellurium group, 0.3% Cesium group.

For Unit 2 they have approx 0.4%-7.0% Iodine, 0.4%-3.0% Tellurium and 0.3%-6.0% Cesium.

For Unit 3 they have approx 0.4%-0.8% Iodine, and Tellurium and Cesium at 0.3%-0.6%


Obviously reactor 2 sticks out with these figures compared to the other reactors, just as it did with previous version of these figures. Much higher possible percentage release of core material to the environment, but also a much wider range of approximation, with the lower end comparable to the estimates for units 1 & 3.

We know what sort of environmental monitoring data leads them to believe that bad things happened on March 15th, which forms a different part of their estimated release analysis, but Im really in the dark about what aspects of reactor 2's status, data or other observations post-earthquake cause this wider range of numbers to come out of MELCOR. Suppression chamber damage or some other details?

I also note that elsewhere in this document it seems to indicate that when the first estimates of how the disaster may unfold were being modelled late on march 11th/early march 12th, reactor two was the first one they did this for. This may have nothing to do with what happened later, I haven't spent too much time thinking about this yet.
westfield
#30
Sep24-11, 09:54 PM
P: 145
From the 2nd report to the IAEA, Summary Chapter

(Quoted text below is OCR extracted from an image of the page, there may be minor differences to the original)

"- PCV venting
In order to create a situation in which PCV venting can be performed, operations to
open a PCV vent valve (MO valve (motor operated valve)) (open 25% of the stipulated
procedure) was performed at 8:10 on the 13th, and the operation of opening the large
valve of the SC vent (AO valve (air operated valve)) was performed at 11:00 of the
same day to complete the vent line configuration and await the blowout of a rupture
disk. However, after that, the S/C large valve was closed and unable to be re-opened,
affected by the explosion of the reactor building of Unit 3 at 11:01 on the 14th;
nevertheless, efforts were continued to form a line. At around 21:00 on that day the
small valve of the S/C vent (AO valve) was opened slightly, making the vent line
configuration successful again. However, a policy of drywell venting was adopted
because the pressure on the S/C side was lower than the working pressure of the rupture
disk and the pressure on the drywell side was increasing, and an operation to open the
small valve of the drywell vent valve (AO valve) was performed once at 0:02 on the
15th; however, it was confirmed several minutes later that the small valve was closed.
After that, drywell pressure maintained a high level of values; large sounds of impact
occurred between around 6:00 and 6:10 of the lS, while S/C pressure indicated O MPa
abs. Lower drywell pressure was also confirmed at around 11:25 on that day."



My question is - does the abovementioned Drywell direct venting constitute a possible explanation for Unit #2's alleged large contribution to site releases?

This also seems to shed more light on why TEPCO seemed fairly sure something broke in the wetwell area? They were not able to vent from S\C?
zapperzero
#31
Sep25-11, 05:43 AM
P: 1,042
Quote Quote by westfield View Post
From the 2nd report to the IAEA, Summary Chapter

(Quoted text below is OCR extracted from an image of the page, there may be minor differences to the original)

" a policy of drywell venting was adopted
because the pressure on the S/C side was lower than the working pressure of the rupture
disk and the pressure on the drywell side was increasing, and an operation to open the
small valve of the drywell vent valve (AO valve) was performed once at 0:02 on the
15th; however, it was confirmed several minutes later that the small valve was closed.
After that, drywell pressure maintained a high level of values; large sounds of impact
occurred between around 6:00 and 6:10 of the lS, while S/C pressure indicated O MPa
abs. Lower drywell pressure was also confirmed at around 11:25 on that day."

My question is - does the abovementioned Drywell direct venting constitute a possible explanation for Unit #2's alleged large contribution to site releases?

This also seems to shed more light on why TEPCO seemed fairly sure something broke in the wetwell area? They were not able to vent from S\C?
The report raises interesting questions. Why would S/C pressure be lower than drywell? 0 MPa abs means the sensor was off or broken somehow. It measures using a water column? Maybe the water drained?

also
large sounds of impact occurred between around 6:00 and 6:10
sounds. plural.

To your question: yes, the drywell would have been "dirtier".

EDIT: I should probably revise my earlier proposed accident sequence.
clancy688
#32
Mar6-12, 03:37 AM
P: 546
Via EX-SKF:

http://ex-skf.blogspot.com/2012/03/d...-released.html

It shows the spacial dose of radiation at the front gate of Fukushima I Nuke Plant and the timeline of events at the plant in March. The largest spike seems to be around or after 12AM on March 15, and the label on the spike says "Dry vent at reactor no.2".
What's the difference between a "dry" and a "wet" went? Morever, according to the INPO report, the Unit 2 containment was never vented because the venting line-up wasn't completed (a rupture disk failed to break).
zapperzero
#33
Mar6-12, 06:11 AM
P: 1,042
Quote Quote by clancy688 View Post
What's the difference between a "dry" and a "wet" went?
Wet venting is venting steam from the RPV through the water in the suppression chamber (aka wetwell) and then out of the suppression chamber into the stack. This scrubs most of the particulates and some of the gasses out of the vented steam, so the radioactive release is diminished.

Dry venting is when the steam goes directly into the outside air by some route or another. Obviously it is very far from being clean.
SteveElbows
#34
Mar6-12, 01:02 PM
P: 630
It is good to see that this topic receives attention again, as I am still extremely dissatisfied that reactor 2's possible majority contribution to the total environmental contamination does not get much focus in the wider world. I've been fascinated by this subject for a long time now, but with little new info to help my understanding.

The ICANPS report was certainly much more readable than previous reports, made it easier to be confident about thing we already thought we knew, and added some detail that I don't think was discussed elsewhere.

It doesn't explain the venting contradictions, since it says in detail what we already heard from previous reports, that they had a range of nightmares when trying to vent, and failed. However I read with interest some of the comments on the ex-skf article, where someone has a theory that venting from the drywell may have occurred very briefly around midnight, not for long enough to reduce pressure in a noticeable way, but enough to allow substances to escape. To explore this theory further we would need to know why reports seem so certain that the rupture disk didn't fail.

The report isn't very helpful when it comes to why the measured pressure in the D/W and S/C diverged so much, but the report does emphasise the possibility that the S/C pressure readings developed a fault, especially as a reading of 0 as shown shortly after 6am on the 15th indicated less than atmospheric pressure, which makes no sense.

I think that the main thing from that report that was new to me was the explanation as to why the S/C was in a state that was inappropriate for receiving a load of stream from the reactor. They switched the RCIC to use the S/C water as a source early in the morning of March 12th, but they didn't pay any attention to S/C temperature & pressure until March 14th, nor did they attempt to complete an alternative to the RCIC until the RCIC was on its very last legs. Combine this with the multiple problems when trying to vent, fire truck running out of fuel without anyone noticing for a while, and some reactor pressure fluctuations which would have prevented water injection from working at various points, and we start to get a better idea of just why reactor 2s failure had such large implications for the environment. It will be interesting to perhaps learn one day as to how the fuel damage at this reactor compares to the others, but have a very long wait for that, if ever! Not sure how much we will really find out about the state of the drywall or S/C either.
SteveElbows
#35
Mar6-12, 01:10 PM
P: 630
Quote Quote by zapperzero View Post
Dry venting is when the steam goes directly into the outside air by some route or another. Obviously it is very far from being clean.
I expect that the term dry venting is supposed to mean deliberate venting by one very specific route, as opposed to unplanned escapes from drywell containment due to some kind of containment failure. Most importantly this route still involves the stack, but as you mentioned does not include scrubbing via the S/C so it has larger environmental implications.

Sadly unlike the initial vents at reactors 1 & 3, we have no visual confirmation of anything leaving the stack during any of the reactor 2 venting attempts, mostly because the attempt at dry venting happened at night, and the old 'one published image an hour' camera was no use at night.

What I find especially annoying is that we don't even know exactly when steam was first noticed to be coming from the blowout panel hole in the side of reactor 2. We certainly know that plenty of steam etc came out of this hole later, as we have footage, but not knowing when it began is frustrating. Assuming it was from the reactor rather than the fuel pool, which is a reasonable bet given much later footage showing steam from the reactor arena and the robot-measured radiation readings showing high levels by one side of the floor above the reactor well, we may well imagine that this started on March 15th. But it would be good to know for sure, especially as events at reactor 4 building on the 15th also got in the way of really clear explanations of radiation data on that day.
zapperzero
#36
Mar6-12, 02:00 PM
P: 1,042
Quote Quote by SteveElbows View Post
I expect that the term dry venting is supposed to mean deliberate venting by one very specific route, as opposed to unplanned escapes from drywell containment due to some kind of containment failure.
I feel a bit guilty for being unclear - I meant to say that there is a number of routes by which one could achieve this kind of venting

One could route the steam through the SGTS or not; if the suppression pool water level is lower than that of the steam pipes for some reason, dry venting is also achieved, by default; there is also a choice of RPV valves that could be opened.


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