Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Samy24]

Is it plausible, that 3 sensors fail at the same time.
I have attached a list of temperatures from Tepco reports and marked the three sensors in doubt.

No 1 (the "failed sensor") has nearly the same temperature as No 2. So No 1 and No 2 are correct or both failed in the same way.
No 3 is now at 3.6 °C. It's going downwards since the same time No 1 did rise. Is this temperature possible at that location?

I can think of two possibilities:
1. All three sensors faild at the same time. Reason?
2. There is a new hole in the buttom of the RPV. Injected water flows directly to sensor no 3 without taking heat form the fuel. What is the fresh water temperature being injected?

 

[tsutsuji]

Yeah, is there a simple to understand diagram showing where on the bottom of the vessel this sensor is located?

might be nice to have a general idea of what is above/beside it.

There is a diagram on http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120212_07-e.pdf page 2

The company says an inspection showed that a cable inside the thermometer is probably cut, resulting in a false reading.
Monday, February 13, 2012 21:17 +0900 (JST)
http://www3.nhk.or.jp/daily/english/20120213_28.html

http://mainichi.jp/select/wadai/news/20120214k0000m040062000c.html The thermometer temporarily read 400°C after 3 PM and around 275°C in the evening on 13 February. Tepco said "we have a nearly definitive judgement that it is broken" (...) "The atmosphere inside the PCV has a high humidity. We suspect that as time goes, the circuit line connected to the thermometer is cut or has an insulation defect" (...) "We will check when the thermometer started being broken, bring together conclusions and explain those to the NISA". If other thermometers break down like this one in the future, knowing the reactor status might become extremely difficult.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120213_10-e.pdf latest temperature plot

 

[etudiant]

There is a diagram on http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120212_07-e.pdf page 2


If I interpret the diagram correctly, the high readings come from a location that is higher than the fuel would be, unless there was enough water circulating to wash fuel particles upward. That seems to support TEPCOs thesis of instrument failure as the most probable cause.
Of course, this also suggests that the remaining sensors may fail fairly soon as well, certainly well before the reactor cools down enough to end the cooling flow.
Is it sensible to insert additional temperature sensors using the port set up for the fiber optics camera or does that not help?

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20120213/index.html Tepco measured a thermometer resistance 1.7 times greater than normal. As this phenomenon occurs when part of a thermometer cable is cut, Tepco says that the probability that the thermometer has broken down is high. Tepco will use another method to find out if the cable is broken, and will reach a final conclusion. For the time being, the injection rate is kept stable, and the situation is carefully watched.

 

[zapperzero]

If I interpret the diagram correctly, the high readings come from a location that is higher than the fuel would be.

I know I'm nitpicking here but I just can't help it. We have no information about where the fuel is - just some educated guesses, which we dignify with the moniker "models". Some of the corium may have stuck to a wall and then inched downwards, for all we know.

 

[elektrownik]

http://www.reuters.com/article/2012/02/13/us-japan-nuclear-idUSTRE81C0FN20120213

Matsumoto said there was little sign of steam, which would be produced when water is at such a high temperature

So maybe readings from sensor were correct ?

 

[Gaffney]

The sensor might be broken but they aren't testing to make sure if the others are also broken? They have put in a lot of risk for just a broken sensor. Pumping in the max amount of water from 3.1t/h to 17.1t/h within two weeks, injecting boron, doing tests for a nuclear reaction quickly.

This might just be following protocol and the sensor could be broken, the worst case could be that the fuel has shifted and a small nuclear reaction is starting to build. It could lead to hydrogen being produced, causing another explosion/large radioactive release.

There have been indications that the plant has been releasing more radiation in February because of rises in radiation over japan. Could possibly be linked with the steam that was seen on the tepco cam in February which was coming from building three or four.

What we do know is that Tepco is using a lot of water with reactor two and there isn't a massive amount of space to store it left.

 

[jim hardy]

i'm no industry apologist, just an old instrument guy.

If i recall these sensors are thermocouples.
If thermocouples have one weakness it's this - moisture in their connecting wires.

A thermocouple is just two wires of dissimilar metals joined together at the end.
The dissimilar metals when heated produce tens of microvolts per degree , and one reads that miniscule voltage to infer temperature at the business end.

So in effect you have a microvolt source that's a function of(temperature) in series with the resistance of the wires . Most thermocouple wire has resistance that is substantially higher than copper. Make a thevenin equivalent out of that and measure it with instrument of your choice.

Now should the insulation of those wires get damaged and allow moisture to ingress,
you no longer have just a thermocouple, you also have dissimilar metals in an electrolyte which make a galvanic cell.
That places a galvanic cell electrically in parallel with your thermocouple junction.
Galvanic cells make thousands of microvolts not tens.
If there's enough wetted area on the moist part of the wires the galvanic cell will deliver enough current to overwhelm the thermocouple's meager voltage.
If you're lucky the polarity of the galvanic cell will cause a voltage indicating temperatures below freezing so you know immediately it's haywire.
If you're unlucky it makes a voltage that indicates high temperature and everybody wrings their hands until reading becomes impossibly high.

SO-- my point is this - they changed injection flow and one thermocouple departed from its neighbors. It bears watching but is suspect. I think they have a water leak above that thermocouple.

In my plant i checked for such things with a simple analog multimeter, which you don't ordinarily use on thermocouples.
The way you tell is read resistance, switch the polarity and read again. If the reading changes by more than about 1/4 inch needle deflection it's likely wet. (Modern digital meters don't have that reversing switch you have to swap the leads and infer from the numbers, which is much less intuitive)
Then you read the current it'll deliver into your multimeter on current scale and if it's more than a few microamps you know there's a substantial galvanic cell out there. If the thermocouple is the ungrounded type a simple resistance to Earth confirms insulation damage..

It is sometimes difficult to explain these details to people who aren't mildly autistic like me.
And you understand why those details aren't in the press release - they infer uncertainty which is not acceptable public image.

And while a lowly instrument technician like me might firmly believe in his results, there's no such thing as absolute certainty.
I'm sure they have better technnicians than me over there and that's why they are saying they think the sensor has failed. It'll probably come back when it dries out. I saw a report of resistance measurement "1.5X normal" and would love to know what it read with polarity reversed - 2/3 normal would be a telling answer. (Just as would "1.5X normal with no sign of wetness".)

Dont know why i rambled so - just want to help those guys over there but cannot.

Anyhow don't bet a lot of money or invest much worry on a lone thermocouple that's suffered the abuse those have. But keep an eye on it.

please advise if this post is out of line.

 

[Yamanote]

If there is no re-criticality or Hydrogen built-up and increased water injection dosn't lead to a temperature decrease, what more can be done than assuming this particular sensor is broken?

Btw, it's not the only temperature sensor in unit two showing this behaviour and some temperatures in unit one are rising as well, only unit three seems to be stable:

http://www.houseoffoust.com/reactors/graphs.html

It is likely that more and more sensors will fade away as time passes by.

 

[Yamanote]

Thanks a lot for your explanation Jim, I fully agree with you!

It also makes me wonder, why Tepco pays so much attention to this temperature sensor, perhaps they have some other signs that something might go on inside reactor number two.

Regarding the radiation releases - could it be, that higher release is just related to lower outside temperature?

 

[Gaffney]

One thing I don't understand is how the amount of water has been multiplied 6x over February but there has been no change in the reactor water level. Is the gauge broken, is the water evaporating quicker or is the water escaping the reactor?

 

[elektrownik]

One thing I don't understand is how the amount of water has been multiplied 6x over February but there has been no change in the reactor water level. Is the gauge broken, is the water evaporating quicker or is the water escaping the reactor?

I think that this is the case.

 

[Yamanote]

Yes, there is absolutely no change. Either the gauge is broken, or there is a leak at this level, letting escape even the big amount of water they are adding now.

 

[Borek]

It also makes me wonder, why Tepco pays so much attention to this temperature sensor

Regardless of what is happening, imagine the wave of criticism they would face if they ignore it.

 

[rmattila]

The main circulation pump seals have certainly leaked since mid-March, and if there's any hole in the shroud, water from the core region will escape that way. In addition, there most probably are holes in the RPV bottom penetrations caused either by molten core material or corrosion.

But as far as I know, we still haven't been told which pressure measurement locations the reported level measurement is actually based on.

 

[Yamanote]

Regardless of what is happening, imagine the wave of criticism they would face if they ignore it.

You are right Borek and I really appreciate their precaution this time. So let's hope for the best.

 

[etudiant]

One thing I don't understand is how the amount of water has been multiplied 6x over February but there has been no change in the reactor water level. Is the gauge broken, is the water evaporating quicker or is the water escaping the reactor?

Seen that the water level in the reactor was too low to be seen by the borescope, one can assume the RPV has some leaks, maybe large ones and that any reported reactor water level may be invalid. Obviously the increased injections will gradually raise the water level in the plant basement, but with 80,000 cubic meters there already, it will take some time to make a substantial increase.
The JAIF used to publish daily, then weekly situation reports that included the water levels in the various parts of the facility, but that stopped about a month ago when a more generic summary was substituted. TEPCO was the source of their data, so it may still be available there if one knows where to look.

 

[Yamanote]

TEPCO was the source of their data, so it may still be available there if one knows where to look.

If this is what you mean:

-> http://www.tepco.co.jp/en/nu/fukushima-np/index-e.html

-> Scroll down to "Situation of water level, transfer and treatment of the accumulated water"

This information is also provided with the daily press conference report and every now and then there is a more detailed report:

http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120201e3.pdf

 

[mheslep]

Unit 2 sensor is damaged because it is 250+ C now, and it is impossible

is it possible that the water boiled/drained away?

 

[etudiant]

If this is what you mean:

-> http://www.tepco.co.jp/en/nu/fukushima-np/index-e.html

-> Scroll down to "Situation of water level, transfer and treatment of the accumulated water"

This information is also provided with the daily press conference report and every now and then there is a more detailed report:

http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120201e3.pdf

Thank you, that is exactly what I was thinking of.
A very helpful reference that also gives some insight into the functioning of the water decontamination effort.

 

[zapperzero]

Anyhow don't bet a lot of money or invest much worry on a lone thermocouple that's suffered the abuse those have. But keep an eye on it.

It's great info from you as usual, jim. But it makes it sound to me as if the other thermocouples failing too is pretty much a given.

 

[elektrownik]

Strange temps in unit 2 (data from today):

 

[zapperzero]

Strange temps in unit 2 (data from today):

More sensors on the fritz? Maybe because of the extra water?

 

[gbettanini]

Strange temps in unit 2 (data from today):

Is a melt-through possible?

 

[elektrownik]

Interesting that all strange temperature data are from bottom of RPV or close to this part of RPV so it is possible that something bad is in progress.

 

[jim hardy]

Multiple sensors trending together is worrisome.

But do i see readings of 118, 251 and 4 ?
Those need to be cross checked against other indications such as visible steam, temperature and particulate analysis of air exiting building.

and ask the technnicians whether those tc's are wet.

 

[swl]

I have not followed the situation for a long time, so I am not sure about details. What pressures are we talking about? Temperature shouldn't go above boiling point, 276℃ means boiling unless pressure is over 60 atm.

I'm guessing it will be difficult to pressurize that RPV much above one atmosphere. And if the TC is away from water, there need not be any pressure.

 

[Borek]

I'm guessing it will be difficult to pressurize that RPV much above one atmosphere.

That's what I was aiming at - as the pressure is unlikely to be that high, if the sensor is under water it must be wrong.

And if the TC is away from water, there need not be any pressure.

Yes, but what is the water level and is the TC under water or above? Or do we simply not know?

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20120214/1350_ondokei.html The NISA instructed Tepco to write a report by 15 February, on such things as the cause of the high thermometer values and the way to measure temperatures by other methods.

http://www.mbs.jp/news/jnn_4953273_zen.shtml Tepco tested 15 of the 41 thermometers installed inside unit 2's PCV, and found that two more thermometers were broken. Nothing abnormal was found with the two thermometers located at the same height as the one that had abnormal values.

http://www.47news.jp/CN/201202/CN2012021401002271.html 8 thermometers are broken out of a total of 41 at unit 2's RPV. Adding to the thermometer that momentarily reached 400°C, two other thermometers were found with an abnormal electric resistance. Tepco had judged that 5 other thermometers were broken. Tepco is judging the RPV's temperature trend with the remaining 33 thermometers.

 

[elektrownik]

Xenon detected in unit 2: http://enenews.com/tepco-press-handout-xenon-detected-reactor-2-containment-vessel-9-hour-half-life
http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120214_07-e.pdf

 

[tsutsuji]

Xenon detected in unit 2: http://enenews.com/tepco-press-handout-xenon-detected-reactor-2-containment-vessel-9-hour-half-life
http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120214_07-e.pdf

These results are hardly different from those of 6 February, are they?

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_120208_07-e.pdf Fukushima Daiichi unit 2 charcoal filter nuclides, February 6

 

[Yamanote]

These results are hardly different from those of 6 February, are they?

Right. That's why I ignore all the stuff on enenews.

 

[etudiant]

Right. That's why I ignore all the stuff on enenews.

Well, ENENews has been in the forefront of the disaster focused reporting, so they deserve to be taken with more than a pinch of salt.
That said, we've not had afaik a nuclear reactor of this size experience as catastrophic a failure, so Fukushima is writing new chapters in the book.
The layman's understanding is that there is no plausible mechanism to achieve recriticality in the damaged fuel, but that residual decay heat is a serious issue. Is this incorrect and are there plausible ways the damaged fuel can be brought back to criticality in any substantial way as a consequence of this accident?

 

[Rive]

The layman's understanding is that there is no plausible mechanism to achieve recriticality in the damaged fuel, but that residual decay heat is a serious issue. Is this incorrect and are there plausible ways the damaged fuel can be brought back to criticality in any substantial way as a consequence of this accident?

ftp://ftp.cordis.europa.eu/pub/fp5-euratom/docs/09-sara.pdf

So they think it's likely happens in some circumstances.

But Tsutsuji is right about the Xenon levels, so in this particular case it's not likely happened.

 

[Yamanote]

Thanks Rive, very interesting!

Enenews compares two different measurement anyway...
So they seem to be more into headlines than into explanations and solutions. Not helpful to me.