Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[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.

 

[SteveElbows]

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

I did consider this, but the gap shown there is quite wide. And the big problem is that if you look on another version of this diagram you will see that the same kind of opening is shown on the other side, but I do not think there are two access points into the pedestal area.

(for example page 4 of http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120228_04-e.pdf )

 

[SteveElbows]

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)).

I should also point out that the diagram that shows nothing in the sump pits of 2 & 3 is misleading. Take a look at pages 23 & 24 of the following document and you will see that they do have estimates for core material in the sumps of reactors 2 & 3.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_08-e.pdf

I shall also take this opportunity to point out that I do not like TEPCO analysis of reactor 2 in particular, since they have very long times for fuel relocation to lower plenum, and damage of RPV. Take a look at the graph on pages 10 & 11 to see how these estimates do not seem like a great fit. NISA cross-check analysis gave timings that seem better, but not so many graphs or follow-up studies & analysis are performed with these. I believe this issue potentially impacts on the quality of TEPCOs later core-melt & concrete analysis. (NISA cross-checking of TEPCO analysis was seen in the government reports to IAEA, I need to refresh myself as to some details and will post again if anything relevant comes up)

 

[Bandit127]

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 couldn't find the original of this chart so a good resolution copy that I found on a blog is linked. It states that the radiation next to the core of Chernobyl was 50 Sv in 10 min (300 Sv/hr).

Since the 72.9 Sv/hr result came from 4 m above the bottom of the PCV can we now conclude that there is a significant amount of the core down there? Or is there another explanation for the very high readings?

 

[jim hardy]

I haven't ruled out yet shine from a core still in the vessel lower plenum.

But I'm no expert. You should go with the experts. I'm an eternal optimist.

 

[Rive]

I could not download/watch all the vids, but most of them don't really worth the effort as I see.

Theoretically: could they gather some real data if they switched off the light for some moments during the 'mission'? Even the corium one year old and washed should produce some Cherenkov light and the cam should be able to pick it up (or at least some fraction of it as it shines through that 'slit').

Some weeks after the accident that faraway hilltop cam could pick up the lights of a SFP as I recall.

 

[SteveElbows]

You are not missing anything spectacular by not watching the videos.

One of the problems they will have with trying to get clearer images, either in the manner you suggest or some other way, is that the radiation really interferes badly with the image when they point the camera in interesting directions.

 

[tsutsuji]

I should also point out that the diagram that shows nothing in the sump pits of 2 & 3 is misleading. Take a look at pages 23 & 24 of the following document and you will see that they do have estimates for core material in the sumps of reactors 2 & 3.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_08-e.pdf

Pages 23 & 24 are the results of simulations based on the assumptions written on page 20. Page 20 says "It was assumed that the fuel debris had accumulated at the two (equipment/floor) drain sumps in the pedestal." It is logical that the simulation results are consistent with the assumption, but the question remains whether the assumption is correct in the first place.

 

[Yamanote]

Actually no big news from this investigation.
The main question remains: Is all of the corium inside the PCV or not? And also the next question: How to find that out?

 

[zapperzero]

Actually no big news from this investigation.
The main question remains: Is all of the corium inside the PCV or not? And also the next question: How to find that out?

The water level is pretty big news.

 

[SteveElbows]

Pages 23 & 24 are the results of simulations based on the assumptions written on page 20. Page 20 says "It was assumed that the fuel debris had accumulated at the two (equipment/floor) drain sumps in the pedestal." It is logical that the simulation results are consistent with the assumption, but the question remains whether the assumption is correct in the first place.

Well yes but there are other assumptions at work here, any number of which could be wrong.

For example I don't know if it is possible that fuel could go in the sump pits but not go on to contaminate the RCW system like seems to have happened in reactor 1.

And there are many assumption made in earlier simulations which could affect later simulations, as I mentioned earlier when complaining about the timing of certain events shown on the TEPCO graph. This evening I have done my further research on this, at least in relation to reactor 2.

To be clear, I am looking at the earlier studies of how much of the core melted, because the results of these will affect the later analysis about core deposition within the drywell and core-concrete reactions, etc.

Firstly I throw away TEPCO case 1 as usual, because it was far too optimistic, estimates from the model failed to match recorded data, and it concluded that no RPV damage had occurred.

TEPCO case 2 is a better fit. This one matches the measured pressure etc data much better, and its conclusions are more realistic in terms of RPV damage. But when the government cross-checked this case, by running the model simulation themselves, they had differences in timing of RPV failure. This is what I was complaining about on the graph used in TEPCO's later analysis, they still had the graph showing RPV damage failure 109 hours after the earthquake, which is not till the 16th March. Government analysis put the failure much earlier, at 22:50 on the 14th, which seems like a better fit with events and measured data.

So, I am concerned if TEPCO have fed a different time for some core falling into containment than seems reasonable.

Then there is the question of how successful the water pumping into reactor 2 was during the first hours of attempting this stuff. And here too I see some possible contradictions between different analysis. I will do a bit more research and checking before commenting on this in detail.