Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20120326/2255_osensui.html There was a 120 ton leak of water decontaminated from cesium. 80 [liters] [1] are believed to have flowed to the ocean. This water contains strontium. Beta rays are 0.25 Bq/cm³.

According to this article they had previously expected a water level of about 3 metres.

http://www3.nhk.or.jp/daily/english/20120326_34.html

Found an article about the previous mission in January, and it states that they initially expected a water level of 4.5 metres, and after that mission failed to find the water they revised it to 4 metres or lower.

http://ajw.asahi.com/article/0311disaster/fukushima/AJ201201190067

In January I had found the following piece of news:

http://www.shimbun.denki.or.jp/news/main/20120120_04.html The endoscope was able to look down to the grating (PCV first floor) at OP 9.5 metre, so that the water level must be lower than this. As part of severe accident countermeasures, a water level gauge is installed at OP 8.3 m, which switches on when submerged, and this switched on signal is being received. For this reason, it is possible to estimate that the water level is between OP 8.3 and OP 9.5 m, but Tepco said : "the integrity of the water level gauge is unclear, so it is necessary to carefully evaluate that matter".

That water level switch must be broken.

[1] Edit "tons" was wrong. "liters" is correct. Thanks to Joffan. Sorry for the mistake.

 

[Joffan]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20120326/2255_osensui.html There was a 120 ton leak of water decontaminated from cesium. 80 tons are believed to have flowed to the ocean. This water contains strontium. Beta rays are 0.25 Bq/cm³.

tsutsuji can you please check the units for the amount going to the ocean, Google translate has it as 80 liters.

 

[clancy688]

Is it safe to say that there may be only a minute amount of corium outside the Unit 2 reactor vessel?

There's only 60 cm of water, but it is not boiling.

 

[elektrownik]

If you look on drywell drawings or models 60cm is equal to level where large pipes to suppression chamber are, so maybe suppression chamber is leaking ? It would be easier to damage suppression chamber (for example via hydrogen explosion) than big drywell walls.
But 60cm of water wouldn't cover corium enought and there would be big radiation, so maybe most of corium is still in RPV ?

 

[SteveElbows]

Yes there are certainly some interesting questions as a result of this stuff, but as I try to draw conclusions from it I struggle to find safe ones.

Yes the water could be draining into the suppression chamber that is then leaking to the torus room.

But various papers about drywell wall failure do tend to suggest that a possible failure point involves the area around the pipes that connect the drywell to the suppression chamber. If there is sufficient quantity of molten core, hitting the ground whilst at the right temperature, the corium can spread out, and eat through some of the drywell shell wall in an area that's near to the pipes to s/c. Material then blows down into the torus room from the drywell via the gap, it doesn't have to go via the pipes & suppression chamber.

So that's two different possibilities, and I have no particular evidence to favour one over the other, although I expect the core blowdown to torus room sounds like a wilder claim than a more basic suppression chamber leak. As someone mentioned its possible that not much core material is actually down at the drywell floor, which if true would certainly hugely reduce the possibility of drywall shell each-through and blowdown.

But I am not sure if todays info about the water level actually gives us much guide as to quantity of core in the drywell. If it spread out, or ate into concrete, then that would change the equation about how much water was needed to cover it.

And I am not sure what we learn from the temperature of the water either really. Doesn't it depend on how often the water is being replaced with completely new water at the bottom, i.e. how long it takes for 60cm worth of water to drain down to the torus room? And what temperature the water is just before it reaches the bottom of the drywell, which will be dependant on factors such as what core material it met and cooled as it journeyed from the reactor vessel downwards.

 

[Yamanote]

The results of today's survey is no surprise to me. Apparently this kind of containment isn't able to do it's job in case of a core melt-down accident.

If I recall correctly, the weak spot of the MKI containment was supposed to be the joints between PCV and SC. So all the water poured into the RPV escapes rather quickly through either damaged joints or a damaged torus and ends up in the torus room. And perhaps some corium has already taken the same path one year ago.
But to get confirmation about the location of the corium, further investigations will be needed.

PS: Considering the deformed door to the torus room in unit 3, I would expect much more damage to the torus there.

 

[elektrownik]

I found information that radiation in air was 6.1 Sv/h there, anyone know how big would be radiation from uncovered corium or from corium under 60cm of water ?

 

[SteveElbows]

The results of today's survey is no surprise to me. Apparently this kind of containment isn't able to do it's job in case of a core melt-down accident.

If I recall correctly, the weak spot of the MKI containment was supposed to be the joints between PCV and SC. So all the water poured into the RPV escapes rather quickly through either damaged joints or a damaged torus and ends up in the torus room. And perhaps some corium has already taken the same path one year ago.

Well speaking more broadly I think what Fukushima taught us is that containment doesn't actually do a job of containing everything, which is how it tended to be described in the past. Either temperature or pressure or both will cause it to fail under severe core melt conditions.

Rather, at least at these type of reactors & containment facilities, containment still contains a lot of stuff and keeps radiation levels down to the extent that people can at least work on site. But in the event of a core melt the containment will not keep everything inside.

If we discover that reactor 2 did release the majority of the substances that contaminated land, then I think the lesson here is one that was already known, that you have to release some of the nasties from containment via wet-venting. You release some substances but at least a lot of stuff is scrubbed or otherwise remains inside containment. Containment may then leak but at least a lot of the stuff that came out of the fuel at the height on the accident got scrubbed before release. But if you fail to wet-vent at all, like happened with reactor 2, then you risk these substances coming straight out from the drywell when it fails.

So on one level containment seems like a misnomer if you have to release stuff from containment via venting in an accident to save containment, what sort of containment is that? But in fact its much better to do that than not wet-vent at all.

And even the much discussed known flaws with containment due to heat or pressure have a silver lining. If we look at the worst fears over Fukushima, they involved the containment itself exploding or otherwise breaking in a rather dramatic way. But the sorts of leaks due to heat or pressure that seem likely to have occurred at Fukushima probably helped prevent this from happening. Not much consolation since a leaking containment vessel is still bad news for the environment, but I guess its not as bad as a more explosive loss of containment.

And no I don't think you can describe the MK-I containment weak spot as being the joints between S/C and D/W, simply because there are lots of weak spots and I don't think it would be fair to single one out in particular as being the weakest. All manner of seals etc will degrade at certain temperatures, as well as the containment cap, personnel & equipment access hatches and other pipework penetrations.

 

[SteveElbows]

I found information that radiation in air was 6.1 Sv/h there, anyone know how big would be radiation from uncovered corium or from corium under 60cm of water ?

Where did you find that? As far as I know they are doing this measurement on Tuesday.

Perhaps you've seen something based on CAMS drywell readings? Many of these readings have been described as potentially faulty by the company, so I will be glad to get a reading taken in a different way.

 

[elektrownik]

Where did you find that?

http://fukushima-diary.com/2012/03/water-level-was-only-60cm-from-the-bottom-in-reactor2/

 

[tsutsuji]

tsutsuji can you please check the units for the amount going to the ocean, Google translate has it as 80 liters.

The NHK says 120 tons leaked among which 80 liters flowed to the Ocean. Sorry for the mistake.

 

[SteveElbows]

http://fukushima-diary.com/2012/03/water-level-was-only-60cm-from-the-bottom-in-reactor2/

Cheers, but no clue where he got that number from, and this is not a source I place too much weight on. Maybe its right but I will wait until official document is published about this.

Anyway since the discussion of the water level brings up discussion of the corium, I went back to remind myself of how they got the initial estimate that only a small amount of the core of reactor 2 fell down to the drywell, and that lots remains in the reactor vessel.

Its based on modelling using the following data:

Reactor water level gauge
Heat decrease of core over time
Amount of time that water was not injected into reactor after water level dropped
The temperature measured by RPV sensors after the disaster
How this temperature changed when injection amounts or methods changed

Reactor 1 always creates the most extreme estimates because of how long they failed to inject water, and to a lesser extent because it happened sooner after reactor shutdown. And higher temperatures of RPV vessels of reactors 2 & 3 make them think more fuel is in those RPVs.

They might be broadly correct or, as the incorrect water level estimates show, they may have got it wrong. I suppose we shouldn't be surprised if a lot of the fuel at reactors 2 and/or 3 turns out to still be in the RPV, since it isn't necessary to have a large core release to the drywell to explain the other things that went wrong or the large levels of radioactive release.

This is the sort of document I sued to remind myself of how they did the estimates. http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_04-e.pdf

 

[SteveElbows]

OK I checked the data for reactor 2 and the 6.1 Sv/hr figure you found is indeed from drywell CAMS A, its not a new measurement done in a new way, and it was misleading of that site to throw it into the article as if it was data from Mondays investigation. But it will be interesting to see how it compares to the level they should measure on Tuesday as part of this investigation.

(CAMS data is part of the temperature data now released mostly in csv format http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/2012parameter/csv_6h_data_2u-e.csv )

 

[SteveElbows]

Video of the reactor 2 containment mission is now available:

http://photo.tepco.co.jp/en/date/2012/201203-e/120327-01e.html

I haven't had a chance to watch it yet.

 

[tsutsuji]

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_120327_02-j.pdf endoscope radiation measurements

http://www3.nhk.or.jp/news/genpatsu-fukushima/20120327/index.html Masanori Naito of the Institute of Applied Energy says "the fact that the radiation increases as you come closer to the water is evidence that melt down fuel is accumulated inside the water".

 

[SteveElbows]

Thanks very much!

So then, 72.9 Sv/h at the lowest point they measured, which is still quite far away from the water.

I knew we would be dealing with some rather large numbers, but I didn't really know how large. Even though containment failed to contain everything, I hope this number demonstrates that containment still serves a purpose, and what sort of radiation levels we might have expected on site if there had been a catastrophic (e.g. explosive) containment failure, or reactor 3 shooting into the air, you know the fantastical worst-case scenarios that some were obsessed with in the first months of the disaster.

 

[SteveElbows]

Can someone give me a sense of the shielding potential of water?

e.g. if I had some substance that was giving off 100 Sv/h and there was 5 metres of water above this source, what sort of radiation levels would we expect to measure at the top of the water?

 

[SteveElbows]

Radiation levels inside containment in english:

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120327_02-e.pdf

 

[MadderDoc]

Can someone give me a sense of the shielding potential of water?

e.g. if I had some substance that was giving off 100 Sv/h and there was 5 metres of water above this source, what sort of radiation levels would we expect to measure at the top of the water?

Gamma radiation through a 1 by 1 cm column of water, of length 5 meter would pass 500 cm3 cm of water, or 500 g. From dabbling around, it seems to me that as a rule of thumb, a radiation halving mass of matter close to 20 g/cm2 can be assumed, so the 500 g would seem to be capable of about 25 halvings of radiation traveling the length of the column. 1/2 times itself 25 times is an awfully small fraction, so it would seem safe :-)

 

[tsutsuji]

http://www.asahi.com/national/update/0326/TKY201203260611.html[About the fact that the previous (3.5 ~ 4 m above PCV bottom) water level estimate was wrong] Tepco said the pressure gauge that was used for the estimate was not indicating correct values.

http://www.tepco.co.jp/en/press/corp-com/release/2012/12031207-e.html directive from NISA regarding isolation condenser In Establishment Permit Application of Unit 1 at Fukushima Daiichi Nuclear Power Station

 

[Rive]

OK I checked the data for reactor 2 and the 6.1 Sv/hr figure you found is indeed from drywell CAMS A...

I wonder where is that CAMS sensor located within the drywell.

 

[NUCENG]

Can someone give me a sense of the shielding potential of water?

e.g. if I had some substance that was giving off 100 Sv/h and there was 5 metres of water above this source, what sort of radiation levels would we expect to measure at the top of the water?

Old radcon math thumbrule is that gamma tenth thickness of lead is 2 inches and water is 24 inches.

5 m of water is about 8 tenth thicknesses so the shielding factor is about 1E8 which means

100 Sv/hr / 1E8 or 1 microSv/hr.

 

[clancy688]

Radiation is well over 50 Sv/h several meters above the water. Is there any way to approximate the amount of barely water covered, one year old fuel present to create such values?

 

[zapperzero]

Radiation is well over 50 Sv/h several meters above the water. Is there any way to approximate the amount of barely water covered, one year old fuel present to create such values?

What use would that be? It is not at all clear where all that radiation is coming from. It might be shining from penetrations in the RPV, or whatever else.

 

[clancy688]

What use would that be? It is not at all clear where all that radiation is coming from. It might be shining from penetrations in the RPV, or whatever else.

It's rising the deeper and the closer to the middle you get. Which makes sense when there's a big blob of whatever radioactive material lying in the middle, as in fell out of the RPV.

 

[SteveElbows]

Thanks very much to you both for the water shielding info, I will make use of this info when I find time.

I've watched the videos from reactor 2 now. The second of the full videos is worth watching as we actually get to see a couple of objects in detail, this video gets more interesting as it progresses. The sixth video shows good detail of the wall beneath the water, with best images coming after the 27 minute mark of that video. The other videos are not so interesting, as the image quality is no good.

 

[tsutsuji]

What use would that be? It is not at all clear where all that radiation is coming from. It might be shining from penetrations in the RPV, or whatever else.

Which kind of openings are there in the pedestal wall (any doors, windows ?) through which the radiation might be shining ? Does someone know the dimensions of the "slit" shown on http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_08-e.pdf page 22 (23/29). How do people enter inside the inner side of the pedestal ? Is it through this "slit" ?

And which is the position (angle) of the slit, compared with the position (angle) of the dosimeter ?

 

[elektrownik]

Which kind of openings are there in the pedestal wall (any doors, windows ?) through which the radiation might be shining ? Does someone know the dimensions of the "slit" shown on http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_08-e.pdf page 22 (23/29). How do people enter inside the inner side of the pedestal ? Is it through this "slit" ?

And which is the position (angle) of the slit, compared with the position (angle) of the dosimeter ?

If you look on this: http://www.houseoffoust.com/fukushima/blueprint.html you will see on right drawing something with label "access" in the pedestal, this could be "Slit". From your link it would be 1m width and from my link around 1,5m height.

 

[SteveElbows]

I am reasonably sure that the 'slit' is like a doorway, although there is no actual door.

This photo of reactor 5 pedestal area seems to be taken from this area, it will give an idea about the size of the 'slit'. But discovering what angle the doorway is at will not be so easy. However I think it is possible that the angle is the same as that shown in the TEPCO document you linked to.

http://www.tepco.co.jp/en/news/110311/images/110924_05.jpg

 

[elektrownik]

But discovering what angle the doorway is at will not be so easy.

Page 20 could be also interesting: http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_08-e.pdf

 

[NUCENG]

Radiation is well over 50 Sv/h several meters above the water. Is there any way to approximate the amount of barely water covered, one year old fuel present to create such values?

Which water? On the floor of the Drywell I thought the water depth was less than that.

If you are talking about SFP the problem is how much radiation is from the pool and how mush from the contamination around the top of the pool.

If you are talking about the torus or the drywell it also needs to consider how much radioactivity is suspended in the pool versus at the bottom of the pool.

Given some some assumptions about how fuel debris is distributed, some data about water contamination levels. general area radiation levels, and some geometry assumptions we could make some rough estimates. But that may be so sensitive to the assumptions it could be a waste of time. Best advice is that if the level is 50 Sv/hr I don't want to disturb any shilding while people are anywhere close.

Time, Distance, Shielding and Ammount of radioactivity are the key parameters.

Decontaminate or shield sources of general area radiation. Clean up the water to reduce that source. Use remote control equipment to provide distance for people. Let time and decay work for you. Then when you start to remove fuel under water make sure there is plenty of additional shielding where needed.

 

[tsutsuji]

Page 20 could be also interesting: http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_08-e.pdf

Thanks.

Browsing through the 30 November report about the status of cores at units 1,2,3, I found something which perhaps was not already mentioned in this forum about the status of the corium at unit 2 (and unit 3).

The English summary at http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_04-e.pdf page 13 (14/28) contains some explanations about "Estimation the state inside the Primary Containment Vessel by the state of Reactor Cooling Water system (Unit 1)", saying that high radiations (220~260 mSv/h) near the heat exchangers imply that the RCW seems to be broken, which supports the hypothesis that there is corium in the drain sump pit. But this summary fails to mention that a similar analysis with different conclusions can be done at unit 2 and unit 3.

This is what is done in the full report http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_09-j.pdf attachment 11-11 page 160/207 where it says:

The distribution of radiations inside unit 2 and unit 3 reactor buildings are provided on figure 7 to figure 13 [pages 161/207 to 173/207]. The RCW heat exchangers both at unit 2 and at unit 3 are located on the second floor. The distribution of radiations on unit 2's second floor is provided on figure 10 and the distribution of radiations on unit 3's second floor is provided on figure 12. In both cases the radiation doses near the RCW heat exchangers are of the order of tens of milisievert [per hour], and extremely high doses such as those at unit 1 were not found. Therefore, there is a high probability that the RCW pipes inside the PCVs are not damaged, both at unit 2 and unit 3.

I think this is at least one of the reasons why the diagram for unit 2 and unit 3 in http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_04-e.pdf page 23 (24/28) shows only a small heap of corium beween the sump pits, with sump pits free of corium, instead of showing corium inside the sump pits like on the unit 1 diagram (page 20 (21/28)).

 

[SteveElbows]

Page 20 could be also interesting: http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_08-e.pdf

Yes that is the diagram I meant. But as a later diagram in that document shows the opening in a different place, I cannot assume these diagrams are accurate or that north = upwards.

One thing we do know is the angle of the X-53 penetration that they used for the borescope. If I use the same format for talking about angles as TEPCO used when describing the temperature sensors, then 0 degrees = west, 90 degrees = north, etc. X-53 is at approximately 40 degrees, so almost north-west.

(position of X-53 is shown in the document about the original January mission at http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120121_03-e.pdf )

 

[SteveElbows]

Thanks.
The English summary at http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_04-e.pdf page 13 (14/28) contains some explanations about "Estimation the state inside the Primary Containment Vessel by the state of Reactor Cooling Water system (Unit 1)", saying that high radiations (220~260 mSv/h) near the heat exchangers imply that the RCW seems to be broken, which supports the hypothesis that there is corium in the drain sump pit. But this summary fails to mention that a similar analysis with different conclusions can be done at unit 2 and unit 3.

Its mentioned in the english version of another document from the same date date:

Pages 15-17 of http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_07-e.pdf

As I mentioned yesterday I was looking at these documents again to remind myself why they had different estimates of core %age drop to drywall floor for reactors 2 & 3 compared to reactor 1. The length of time that no water was injected was the largest factor, but these other evaluations add to the picture, including the sump drain stuff you mention, as well as temperature of reactor vessel in the months following the disaster. I expect there is a large margin of error for these sorts of evaluations. For example they had various trouble getting water into reactor 2 during the evening of the 14th, and we know that the water level gauge readings became inaccurate at some point. So its not clear if the amount of time that core was uncovered that they fed into their model was accurate, and this would certainly have a big impact on core damage & melt estimates.

 

[tsutsuji]

(position of X-53 is shown in the document about the original January mission at http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120121_03-e.pdf )

Perhaps the pedestal wall and the slit are shown on this document. See my attachment.