Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[SteveElbows]

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120322_01-e.pdf

Can somebody please enlighten me about this 'TIP room'?

They are about to get neutron activity readings, or these are new holes (reachable from a relatively low dose area) on the containment for more borescope investigations?

Im pretty sure the borescope mission is going through the same penetration as last time, so nothing to do with this room.

My guess is that this room is where existing penetrations that are used by normal neutron measuring equipment are located. And perhaps they have surveyed it now because these penetrations are one of the potential failure points that could leak or otherwise enable contaminated substances to travel through some of this equipment. But as with many other parts of the reactor 2 building, there is no sign of high levels of contamination here.

Alternatively perhaps they would like to start using this equipment again to monitor the situation, although I am not familiar with the technology or whether it would actually be any use once the reactor vessel is so badly damaged.

 

[SteveElbows]

TIP = traversing in-core probe. A gamma detector driven into the core every month or so in order to calibrate the in-core neutron detectors, which slowly burn out in the neutron flux. The TIP is driven from a room next to the containment, routed to one TIP channel at a time, and driven through the channel (=a small pipe) from the bottom of the RPV into the core and back.

Thanks for the info, I hadn't seen your post when I wrote my previous one.

 

[jim hardy]

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120322_01-e.pdf

Can somebody please enlighten me about this 'TIP room'?

They are about to get neutron activity readings, or these are new holes (reachable from a relatively low dose area) on the containment for more borescope investigations?

I'm not BWR guy

but i do know BWR's have "Travelling Incore Probes" which are miniature, moveable neutron detectors that traverse the core vertically in hollow tubes coming in from the bottom..
This is similar in principle to the PWR "Flux Mapper" and i do have experience with that system.
Our detectors were miniature fission chambers coated inside with oralloy, about two inches long by 3/16 diameter.
The vessel has penetrations on bottom for the hollow tubes in which the detectors travel. My reactor had five detectors that shared fifty tubes via indexing machanisms. We cleaned our tubes with a .22 caliber rifle bore brush soldered to a cable in place of the fission chamber. But i digress.Seems to me the TIP equipment room must be the room housing the mechanical detector drives and indexing machanisms, at the terminus of the tubes.

A natural place to launch an endoscope exploration.

Perhaps a BWR person can confirm.

EDIT Wow I'm fifth in line!

 

[SteveElbows]

Well like I said earlier I am pretty sure its not for this borescope mission, since they published details of what they will do and its very similar to the last mission.

However I suppose its possible they might have some ideas to use the TIP pipes for something else in future. Perhaps the future mission to install new temperature measurement equipment may consider using these penetrations for example.

 

[Rive]

Wow, thanks for everybody

The next borescope mission will be on the penetration used for the previous mission, that's in the linked document.

But such pipes (even if they are supposedly damaged close to the RPV) holds some possibilities for measurements, borescoping and such... And the starting point of the pipes are in a clean, nice room with low radiation.

Nice. Can't wait to see how will they use them.

 

[jim hardy]

Well like I said earlier I am pretty sure its not for this borescope mission, since they published details of what they will do and its very similar to the last mission.

However I suppose its possible they might have some ideas to use the TIP pipes for something else in future. Perhaps the future mission to install new temperature measurement equipment may consider using these penetrations for example.

AND

But such pipes (even if they are supposedly damaged close to the RPV) holds some possibilities for measurements, borescoping and such... And the starting point of the pipes are in a clean, nice room with low radiation.

Here's a description of the TIP system

http://www.google.com/url?sa=t&rct=...n9CCBg&usg=AFQjCNE3hTkaEtLnbVOEqBcNsW_Dh9PnrA

if link doesn't work search on "ML11258A339.pdf"

If no probes were inserted at time of accident, the fail closed isolation valves should have kept the room clean and dry when the far end of tubes melted with the core.

IF there ARE probes inserted, AND the room is clean, it would say something surprising about conditions in core, , ie core end of tubes didnt melt. Unless they actuated the explosive actuated shear valves before station batteries gave out..

Pushing limits of speculation here. Will say no more, but await further reports.

old jim

 

[rmattila]

In the BWRs I know, there's only three pipes going through the containment wall, and the multiplexers routing the TIP to different pipes (30 or so) within the core are located inside the containment (and are probably useless by now). But at least three of the pipes should be accessible after opening the containment isolation valves.

Disclaimer: I am mostly familiar with the ASEA BWRs, but I suppose the TIP system in those is more or less the same as in the American-designed BWRs.

EDIT: Fig 5.6-1 of old Jim's post confirms the designs are the same (and that the "multiplexer" I mentioned appears to be more properly called an "indexing mechanism".)

 

[Rive]

By this picture: http://www.tepco.co.jp/en/news/110311/images/120322_11.jpg
they have four equipment of the same kind and one more which is different.

 

[tsutsuji]

http://www.jiji.com/jc/c?g=soc_30&k=2012032200956 As a result of checking the TIP room located on unit 2's reactor building first floor with a robot, Tepco announced on 22 March that there is no pipe or equipment damage there. This is one of the inspections aimed at installing alternative thermometers to replace the broken thermometers. With 2 mSv/hour the radiation is relatively low. The room contains pipes which are directly connecting with the RPV. As no conspicuous damage was found, Tepco said that installing an alternative thermometer is possible.

The short list of alternative thermometer routes Tepco is considering, after having studied all the routes through all the RPV nozzles, are listed in the 2 March 2012 report http://www.tepco.co.jp/cc/press/betu12_j/images/120302a.pdf table 1-2 page 17/91 and 18/91. Route names are in the second column from left, and priority numbers are in the right most column. The circled numbers are references further used on the floor layout maps on the following pages.

On page 18/91, the Jet Pump instrumentation line is marked as "priority=1" but the TIP is marked as priority "2". The radiation for TIP was then marked as "unknown" in the table.

While there is only one priority 1 item (or you may say that there are as many as jet pump instrumentation lines - that must be 24), there are 2 other "priority 2" routes:

Water level instrumentation line: nozzles N16A and N16B; check valve = no; T-junction= no ; radiation=unknown; reactor building 2nd floor 4.5 m above floor; priority = 2

Boric acid injection system (differencial pressure sensor line) Nozzle N10 (in the case you do not insert further than the T on the reactor outside) check valve = no; T-junction= no ; radiation=unknown; reactor building 1st floor 4.8 or 4.5 m above floor; priority=2

 

[tsutsuji]

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_120323_01-j.pdf Results of onsite surveys for the purpose of installing alternative thermometers, Fukushima Daiichi unit 2. English is here: http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120323_01-e.pdf

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_120323_03-j.pdf One of unit 1's RPV thermometers was not connected because it was broken (since long ago, before the accident, I guess) and instead of recording the temperatures from that thermocouple, there was a jumper connecting with the nearby thermometer, so that what was recorded was a duplicate of the temperatures of the nearby thermometer.

 

[duccio]

IF there ARE probes inserted, AND the room is clean, it would say something surprising about conditions in core, , ie core end of tubes didnt melt. Unless they actuated the explosive actuated shear valves before station batteries gave out..

Jim, I'm not familiar with those instruments. Is it possible to tell if the probes are interted just from the videos? ( such as this one http://www.youtube.com/watch?feature=player_embedded&v=_dvf-Qg-IqI )

 

[SteveElbows]

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_120323_01-j.pdf Results of onsite surveys for the purpose of installing alternative thermometers, Fukushima Daiichi unit 2. English is here: http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120323_01-e.pdf.

WellI guess they won't be using penetration X-29B/C on the 2nd floor (last page of document) since there is 1640mSv/h measured at the stop valve.

 

[tsutsuji]

http://mainichi.jp/select/weathernews/news/20120323ddm008040202000c.html On 22 March, Tepco announced that one more unit 2 RPV bottom thermometer is broken. There were 6 RPV bottom thermometers. One of them had its temperature readings rising in February and it was understood that it was broken. Then on 3 March, another one was abnormal and was removed from the surveillance data. Thermometer failures are continuously occurring in other locations too, and only 14 RPV thermometers are remaining out of a total of 21. Tepco is making progress in its study of how to install new thermometers from outside the RPV via pipes, etc.. On 21 March, a robot surveyed radiations etc. in the reactor building for that purpose.

http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/2012parameter/120323_data_2u_g-e.pdf ※ "Vessel bottom above skirt jot (TE-2-3-69F3)" has been deleted since it is valuated as "Failure" by the reliability evaluation of the thermometer.

In the table http://www.tepco.co.jp/cc/press/betu12_j/images/120302a.pdf p.88/91 we see that TE-2-3-69F3's resistance had risen from 122 Ω in September to 143 Ω in February. Did the resistance continue to rise until reaching the "rose by more than 30%" criteria mentioned on http://www.tepco.co.jp/cc/press/betu12_j/images/120302a.pdf page 48/91 ?

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_120323_03-j.pdf One of unit 1's RPV thermometers was not connected because it was broken (since long ago, before the accident, I guess) and instead of recording the temperatures from that thermocouple, there was a jumper connecting with the nearby thermometer, so that what was recorded was a duplicate of the temperatures of the nearby thermometer.

http://news24.jp/nnn/news8653348.html The wire connection mistake was done 11 years ago. During a regular inspection performed 11 years ago, the thermometer was judged to be broken [short-circuit, according to the newsreader in the video]. And it was found yesterday that another thermometer's wires were being connected [to the data recorder]. So the data from the other thermometer were being recorded, while the temperature was not actually measured. Tepco is removing this thermometer from the reactor surveillance data, and is inspecting the detailed sequence of events through which this [jumper] connection remained overlooked until now.

 

[SteveElbows]

Given the deletion of another reactor 2 thermometer, I thought I would mention what the IAEA said about this sort of thing when it got a lot of attention in February:

http://www.iaea.org/newscenter/focus/fukushima/statusreport230212.pdf

IAEA comments on the temperature increase monitored inside Unit 2
The TEPCO assessment concludes that the temperature increase observed at one point near the bottom of the RPV most probably results from an instrumentation failure. The IAEA Safety Assessment and Evaluation Team considers this conclusion likely to be correct.
Nevertheless, it is prudent to take into account the possibility that the temperature measurements at the bottom of the RPV may have been correct in the recent past. The most common thermocouple failure modes result in low readings. Furthermore, the RPV bottom head temperature is only one of several temperature measurements that began to behave differently on 27 January. These readings and their trends might reflect a modification in the cooling paths in the reactor. This possibility should be further monitored and investigated.
Upscale failures of thermocouples can result from environmental effects. Consequently, if the bottom head thermocouple failed, additional failures may be expected over time. Furthermore, some thermocouples already seem to be failing down scale. Therefore, the IAEA Safety Assessment and Evaluation Team recommends that alternative measures to confirm continued uniform cooling of core debris should be considered.
Finally, the TEPCO assessment concludes that there has been no re-criticality based on available measurements in gas samples taken from the Primary Containment Vessel (PCV). The IAEA Safety Assessment and Evaluation Team agrees with this conclusion, but recommends that evaluating PCV gas samples be continuously taken and evaluated..

 

[jim hardy]

Jim, I'm not familiar with those instruments. Is it possible to tell if the probes are interted just from the videos? ( such as this one http://www.youtube.com/watch?feature=player_embedded&v=_dvf-Qg-IqI )

I don't think so, ours had covers that must be removed to see inside.

Remember mine were PWR, made by Teleflex. I don't know who made these for GE.

 

[tsutsuji]

http://www.jiji.com/jc/c?g=soc_30&k=2012032301023 On 23 March, Tepco announced that one of the three unit 2 RPV bottom thermometers used for surveillance rose by 7°C from the evening of 22 March to 5 PM on 23 March, reaching 51.4°C. It is unclear whether a phenomenon causing a temperature rise occurred near that thermometer, or if the thermometer is broken. [I guess this is about TE-2-3-69H2 which is seen rising on the plot at http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/2012parameter/120323_data_2u_g-e.pdf ]

 

[SteveElbows]

Ah yes, that one shows up on the latest graph:

http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/2012parameter/120323_data_2u_g-e.pdf

Also note that both that temperature sensor and the one that's been higher for ages that they finally stopped showing on that graph (last seen on this one http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/2012parameter/120322_data_2u_g-e.pdf ), were the focus of the March 3rd thermometer survey document http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120303_03-e.pdf

Regardess of the cause, they are now dangerously low on thermometers in these locations. I think that if this one is declared faulty then they are down to one sensor for the vessel wall bottom head and one for, as they label it, the vessel bottom skirt jot.

 

[tsutsuji]

I was wondering when and why TE-2-3-69F1 was excluded from the surveillance thermometer status.

In the table http://www.tepco.co.jp/cc/press/betu12_j/images/120302a.pdf (2 March 2012) p.88/91 TE-2-3-69F1 was already excluded and marked as "reference" instead of "surveillance". But it is not marked as "broken" (broken thermometers are those with a pink A1,A2,B1 or B2) in that table.

The plot on http://www.tepco.co.jp/cc/press/betu12_j/images/120302a.pdf page 75/91 shows that TE-2-3-69F1 is subject to a large amplitude oscillation, though it seems to follow the same temperature trend as the other thermometers, with an average value close to the measurements by TE-2-3-69F2.

 

[westfield]

The sky on Fuku was quite crowded in those days... very interesting pics! Doubt: is the exhaust stack pipe of R3 missing? It is still there on the pic in which reactor 4 is still intact. Do you believe Tepco removed it, or it was blown away during the explosion of R4?

I think I know what you mean but no, only part of the duct is still there in that image taken just after RB3 exploded, the other half of it that ran along the southern side of RB3 was smashed off in the RB3 explosion, as seen here on the 20th Mar 2011 Image. There was definitely no connection to the stack from the moment RB3 blew up.

The section running north-south to the stack itself was later removed by tepco presumably to allow better access for the pumping efforts.

 

[NUCENG]

I think I know what you mean but no, only part of the duct is still there in that image taken just after RB3 exploded, the other half of it that ran along the southern side of RB3 was smashed off in the RB3 explosion, as seen here on the 20th Mar 2011 Image. There was definitely no connection to the stack from the moment RB3 blew up.

The section running north-south to the stack itself was later removed by tepco presumably to allow better access for the pumping efforts.

For clarity, the picture shows the shared vent stack between Units 3 and 4 that is used for SBGT system venting and the hardened vent system. TheRe is a similar shared stack between units 1 and 2. The third stack at Fukushima Daiichi is a shared stack foR the offgas treatment system over by the offgas building. The offgas stack takes processed effluent from turbine building air ejectors and gives it an elevated release point. If you look at the top of the offgas stack there appear to be four separate pipes at the top of the stack so it is possible that the release paths are actually separate from each plant.

If they had used a similar design For the vent stacks there would have been little chance that hydrogen could have been forced from Unit 3 to Unit 4. Apparently, the systems for units 3 and 4 not only connected at the base of the stack, but there was no backflow prevention either. That was clearly an unrecognized design deficiency. Any multi-unit nuclear plant needs to address the issue if there are shared release pathways.

To accept the explanation that the explosive damage in Unit 4 came from hydrogen from Unit 3 you have to believe the path remained intact and there was either a pressure difference between the units before the explosion or the explosion on Unit 3 itself caused sufficient hydrogen transfer to unit 4 before the piping was blown off.

 

[SteveElbows]

When considering the pipes between reactors 3 & 4, I believe we need to consider that the official documentation about this actually points to the smaller pipes, not the most visible large one which we can easily see was broken.

On page IV-97 of this document you can see that they have annotated the photo with yellow arrows, and these are pointing at some very small pipes that can only just be seen on the photo.

http://www.kantei.go.jp/foreign/kan/topics/201106/pdf/chapter_iv_all.pdf

 

[hellbet]

Is there any curving along the PIPS / LPRM guides? Quince's picture (a really nice one) shows the pipes apertures going mainly "down" that is with a 225° angle. Now, isn't the RPV location "upper" if the TIPS room location is right on the R/B 1st floor?

Are LPRM inserted right along the pipe as soon the reactor is diverging or only for transitional measuring purposes?

Thanks in advance,

 

[MadderDoc]

Comparing the same position in the SFP of Unit 4 in an early, and in the most recent video, there's been some change:

It appears some assemblies are 'passive' and have simply developed a uniform caking on their tops, obscuring the view to any detail below, The assemblies seen in the montage above are of that type. Other assemblies are to a differing degree 'active', and have been popping/cracking the caking developed on their tops, particular so over their (empty) center tubes.

The caking has a somewhat tainted surface, but it seems in the main to be composed of a whitish mineral substance. I wonder, what it could be. Would someone with nuclear knowledge know if there could be a presence of objects made of aluminium alloy in a BWR spent fuel pool?

 

[westfield]

<snip >

The third stack at Fukushima Daiichi is a shared stack foR the offgas treatment system over by the offgas building. The offgas stack takes processed effluent from turbine building air ejectors and gives it an elevated release point. If you look at the top of the offgas stack there appear to be four separate pipes at the top of the stack so it is possible that the release paths are actually separate from each plant.

<snip>

You raise a salient point regarding the separate (presumably) duct for "hard vent" from Unit 3 to the common unit 3\4 stack.

Also, it would make sense to do as you surmise with the common offgas treatment stack but this image of the common waste treatment building stack seem to indicate that the four ducts within the stack serve different areas of the common waste treatment buildings, turbine hall ducting from Unit 4's unique TB ducting (on it's roof) and of course the common ducting from the SGTS's of the 4 (or perhaps only 3) RB's. Interestingly it's not readily evident that Unit 1 had any connection to the common waste treament plant like the other three. Also of interest is that the common facility wasn't even there in the early days of Fukuichi.

Image of base of common waste treament facility stack (during tsunami) -

Image of fukuichi 1975.
There's no common SFP, no common waste treatment buildings, no cask storage building amongst many yet to be constructed buildings. Also evident is that the Units were not completed in numerical order. Unit 5 is complete while Unit 4 is still under construction for example.

 

[cphoenix]

That caking in SFP4 kinda' worries me. Without water circulation, sufficient heat buildup could cause a geyser.

Months ago I posted a theory that a large geyser was responsible for the building 4 explosion. There'd certainly be enough energy in the pool to destroy the building, if circulation of water were largely blocked.

Given that the radiation distribution in the vent filters shows that gas was injected into building 4 through the vent, I'm assuming that this was a source of hydrogen and that the explosion was in fact a hydrogen explosion and not a steam explosion.

But I'm still concerned about future geysers in SFP4. Are any countermeasures being taken?

 

[Rive]

Are any countermeasures being taken?

Till there are enough cold water around - and now they has cooling for the pool - small/bundle sized heat buildups cannot become dangerous on the bottom of that pool.

 

[duccio]

I think I know what you mean but no, only part of the duct is still there in that image taken just after RB3 exploded, the other half of it that ran along the southern side of RB3 was smashed off in the RB3 explosion, as seen here on the 20th Mar 2011 Image. There was definitely no connection to the stack from the moment RB3 blew up.

The picture you show is after explosion of R3&4. I was curious about the dynamic of the various explosions, because unless it escaped back to the reactor building, the pipe should have been filled with hydrogen (the R3 part has a downward curve just before connecting to the exhaust tube). So were they broken because of the reactor 3 building explosion, or because it blew by itself once ignited?

BTW: did they hire a roller coaster engineer to design that exhaust pipe?

To accept the explanation that the explosive damage in Unit 4 came from hydrogen from Unit 3 you have to believe the path remained intact and there was either a pressure difference between the units before the explosion or the explosion on Unit 3 itself caused sufficient hydrogen transfer to unit 4 before the piping was blown off.

R4 filled with hydrogen probably during the venting of R3, at that time everything was intact.

On page IV-97 of this document you can see that they have annotated the photo with yellow arrows, and these are pointing at some very small pipes that can only just be seen on the photo.

That's very strange. In the document they refer to the big one, but then they put yellow arrows to the small pipe. Is anyone familiar with BWR that can tell us what that small pipe is?

 

[NUCENG]

When considering the pipes between reactors 3 & 4, I believe we need to consider that the official documentation about this actually points to the smaller pipes, not the most visible large one which we can easily see was broken.

You raise a salient point regarding the separate (presumably) duct for "hard vent" from Unit 3 to the common unit 3\4 stack.

The picture you show is after explosion of R3&4. I was curious about the dynamic of the various explosions, because unless it escaped back to the reactor building, the pipe should have been filled with hydrogen (the R3 part has a downward curve just before connecting to the exhaust tube). So were they broken because of the reactor 3 building explosion, or because it blew by itself once ignited?

That's very strange. In the document they refer to the big one, but then they put yellow arrows to the small pipe. Is anyone familiar with BWR that can tell us what that small pipe is?

I learned something so it is a successful day. Steve is right about the smaller pipe. Apparently the Fukushima plants also used that stack for the normal reactor building ventilation exhaust release point (large pipes). The yellow arrows are correct. The plants where I have worked had separate normal exhaust vent stacks on the reactor building and looking back at pre explosion pictures there weren't any such stacks at Fukushima.

So my concern about when the large pipe blew was off point. It still would have taken positive pressure which could have come from venting or the explosion to get hydrogen into Unit 4, but that now seems much more feasible.

As to the common offgas stack you may be right in describing the configuration of that stack near the waste processing building. But my point about maintaining independence between release paths by running them all to the top of the stack is also valid. By having that common connection in SBGT piping between units 3 and 4 there was a path between buildings.

Thanks all!

 

[westfield]

The picture you show is after explosion of R3&4. I was curious about the dynamic of the various explosions, because unless it escaped back to the reactor building, the pipe should have been filled with hydrogen (the R3 part has a downward curve just before connecting to the exhaust tube). So were they broken because of the reactor 3 building explosion, or because it blew by itself once ignited?

<snip>

That's very strange. In the document they refer to the big one, but then they put yellow arrows to the small pipe. Is anyone familiar with BWR that can tell us what that small pipe is?

Yes it is an image from the 20th, after RB 4 has blown up, but it shows the same as any other image available (to date) about the state of the large RB 3 SGTS ducting after RB3 blew up. I.E. RB4's explosion didn't visibly do further damage to the RB 3's remnant of SGTS ducting. It's just one of the better images to show the state of that ducting.The yellow arrows in the linked document are presumably pointing the "hardened vent" lines as they mention that system in the document and we know they are separate from the SGTS ducting at least up until they enter the stack. (I read somewhere that apparently the whole idea of the "hardened vent" system largely came about when it was realized that SGTS's ducting would fail badly inside the buildings if high pressure venting was attempted via it, so it's a separate system specifically for that reason...and because it was a retrofit. Someone who knows might clarify that.)SGTS in RB 3 was inoperative and the linked document hints that the hydrogen came to the stack from the RB 3 "hard vent" line but entered RB 4 via the larger SGTS ducting.
Two separate systems that only become common at the shared stack.

Maybe I'm only just catching up but that was a subtly that hadn't occurred to me until now.
Up until now I was just thinking SGTS ducting from RB3 to RB4.

So - It occurred to me after reading the document and NUCENGS & your posts that the state of the separate "hard vent" pipe from RB 3 to the stack isn't clear at all because it doesn't appear to run next to the route the SGTS ducting takes, it appears in that image to "dive" underground in the direction of RB3. It may be still connected even now for all those images tell us. On that basis could it have continued to fill RB4 with hydrogen right up until RB4 exploded? Again, sorry if I'm only catching up on this detail.

Was the state of the RB3 hard vent lineup known after the RB3 explosion?

Could it still have been venting to the common 3\4 stack via the hard vent post the RB3 explosion?

Continuing on from that -
So if it's plausable for RB4 to get filled by hydrogen via the RB3 hard vent line wouldn't it then also be plausible that the hydrogen that filled RB3 came from the same source? Wouldn't that then negate any requirement for a hydrogen leak to have occurred direct from containment into RB 3? I.E. if the hydrogen came back into RB3 via is own, non functioning, SGTS ducting?I havn't seen this mentioned before, maybe I missed it or else missed something that makes it far less plausable than RB4 getting filled by RB3 hydrogen.

Edit : Having written that - The idea that the hardened vent delivered the hydrogen via the common stack just makes the whole idea even less plausable to me than transfer via the SGTS only, just look at the plumbing at the stack.
Instead of simply rising up out of the stack the gases must do a 90 degree turn and move horizontally into the SGTS ducting.
Really?

At least with the SGTS to SGTS scenario the gases don't have to actually reach the stack.

Edit : just to muddy it up some more perhaps - Is this a "hardened vent stack within the stack"

 

[SteveElbows]

As we are on the subject of vent pipes, I shall repeat something I said on the reactor 2 thread not so long ago.

If you look at the following picture, you can see that for reactors 1 & 2 the smaller pipes run parallel to the larger ducting.

http://cryptome.org/eyeball/daiichi-npp/pict9.jpg

Now look at page 2 of the following document about the very high radiation detected in pipes at the 1 & 2 stack:

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110802_01-e.pdf

Look closely and both the large duct pipe (grey) and the smaller pipe (white) for reactor 2 can be seen in that photo, heading away and to the right of the photographer, and then angling upwards. And the gamma blob which is to the right could be showing the source is material stuck in the smaller pipe.

I looked at this again recently because the possibility that a very short dry-vent could actually of happened at 0:02 on the 15th for reactor 2 (status of rupture disc is unknown).

However I cannot rule out the possibility that this material came from reactor 1, even if it ended up in pipes heading for reactor 2. I should look again at the test results for SGTS equipment they did inside reactor 1 building to see if it gives any clues.