Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Shinjukusam]

Latest update from NHK says TEPCO retracts claim of criticality, states that the Xenon level is consistent with spontaneous fissioning of curium-242 and -244.

 

[Sorai]

Latest update from NHK says TEPCO retracts claim of criticality, states that the Xenon level is consistent with spontaneous fissioning of curium-242 and -244.

Did they ever claim that there was a criticality or they just said that they couldn't discard the possibility? I know we already have the tendency to eliminate all the qualifiers from the Japanese reports, but it seems in this case both Tepco and NISA meant it when they said that "the possibility couldn't be excluded." The hypothesis of spontaneous fission being the cause was also mentioned from the beginning.

 

[Shinjukusam]

NHK themselves said "retracts", but that might have been media overreaction. I think a few of the networks were pretty content to make the criticality assumption, and I know a number of netizens around the world were patting themselves pretty hard on the back yesterday.

 

[tsutsuji]

http://www.yomiuri.co.jp/science/news/20111103-OYT1T00346.htm (3 November 01:12 PM) Tepco said that Xenon can be explained by spontaneous fission of curium. It was found in a more detailed analysis that in the case of a small scale criticality, ten thousand more Xenon would be detected. The fact that xenon was still found after the boron injection also provides ground for believing that criticality is not occurring.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111103_01-e.pdf "Schematic diagram of the nuclear fission reaction"

http://jp.wsj.com/Japan/Companies/node_336401 (3 November 05:09 PM) A a ten thousand higher value resulted of a calculated estimation of Xe-135 concentration "based on the output energy in case of criticality". Also, a calculation of xenon concentration in case of spontaneous curium fission almost matched the present measured values.

 

[elektrownik]

What is the source of those Cs spikes in last few days ?
(page 5, 10/30)
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111102e10.pdf
(pages 2-5, 8, 10/30)
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111102e5.pdf

 

[rmattila]

Attached is my rough sketch to estimate the amount of Xe-135 to be expected as a consequence of spontaneous fission in Pu-240, Cm-242 & Cm-244. I made it really quickly and have not been able to double-check my figures, so if you see errors, please point them out.

Some conclusions:

• The main SF contributor indeed seems to be Cm-244.
• However, the majority of fissions occurs in U-235 due to subcritical multiplication - at a rate highly dependent on the assumed multiplication factor.
• Thus boron should have an effet on the Xe-135 concentration even though it would be due to subcritical multiplication.
• The main uncertainty lies in the release fraction of Xe-135 from the fuel/corium. The measurement data given by TEPCO conincides with release fractions between 0,001 .. 0,01 % in my sketch - a plausible figure assuming the fuel (in whatever state it is) is cool.
• Effect of nitrogen injection in the containment was neglected in my sketch - however, its effect is smaller than one decade, and thus smaller than the uncertainty in the Xe-135 release fraction.

All in all, it appears that the measured Xe-135 concentration would be well in line with the figures to be expected due to spontaneous fission, and no criticality is needed to explain them.

 

[NUCENG]

Attached is my rough sketch to estimate the amount of Xe-135 to be expected as a consequence of spontaneous fission in Pu-240, Cm-242 & Cm-244. I made it really quickly and have not been able to double-check my figures, so if you see errors, please point them out.

Some conclusions:

• The main SF contributor indeed seems to be Cm-244.
• However, the majority of fissions occurs in U-235 due to subcritical multiplication - at a rate highly dependent on the assumed multiplication factor.
• Thus boron should have an effet on the Xe-135 concentration even though it would be due to subcritical multiplication.
• The main uncertainty lies in the release fraction of Xe-135 from the fuel/corium. The measurement data given by TEPCO conincides with release fractions between 0,001 .. 0,01 % in my sketch - a plausible figure assuming the fuel (in whatever state it is) is cool.
• Effect of nitrogen injection in the containment was neglected in my sketch - however, its effect is smaller than one decade, and thus smaller than the uncertainty in the Xe-135 release fraction.

All in all, it appears that the measured Xe-135 concentration would be well in line with the figures to be expected due to spontaneous fission, and no criticality is needed to explain them.

Your work is spot on. I just logged into post my results and you stole my thunder. So I will simply say congratulations and independently confirm your results. Only minor difference is that I believe Pu-238 is actually a larger source than Pu-240, but two orders of magnitude less than the Cm isotopes. Good job!

 

[NUCENG]

Questions have been asked by several posters about why radioiodine has not been measured in the gas samples. This may mean that the pH of the water in the containment and vessel have remained alkaline and is keeping the iodine in solution until it decays. Acidic pH allows radioiodine to release as iodine gas. Thus only the iodine generated by beta decay in of Xenon in the containment atmosphere would be available in samples. Calculating the equilibrium iodine concentrations from that source will provide an answer if it is less than the detection threshold.

 

[SteveElbows]

Todays status update has info about a further rise in hydrogen density, and measures taken.

Unit 2
11/3 12:40 In order to improve reliability of nitrogen injection, additional flow gauge is started to install at the Unit 2 nitrogen injection line.

Around 14:00 The work was finished. Although nitrogen injection was stopped for approx. 10 minutes during the work, there were no significant changes in the parameters

11/3 16:50 Since some increase at the hydrogen density (hydrogen density of 2.9% [As of 16:30 on 11/3]) in the exhaust gas from PCV was observed from the last time of nitrogen injection amount change (hydrogen density of 2.7% [As of 18:10 on 10/30]), nitrogen injection amount was changed from 21m3/h to 26m3/h.

Taken from http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111103_02-e.pdf

 

[rowmag]

Questions have been asked by several posters about why radioiodine has not been measured in the gas samples. This may mean that the pH of the water in the containment and vessel have remained alkaline and is keeping the iodine in solution until it decays. Acidic pH allows radioiodine to release as iodine gas. Thus only the iodine generated by beta decay in of Xenon in the containment atmosphere would be available in samples. Calculating the equilibrium iodine concentrations from that source will provide an answer if it is less than the detection threshold.

Thank you, NUCENG. That makes sense.

My other question was whether the krypton and xenon could possibly account for the occasional synchronized transient spikes seen around Kanto. The spikes are pretty small, around 0.05 uSv/h above steady-state levels, but the krypton and xenon concentrations measured here look so darn low (barely above detectable limits even when sniffing gas directly from inside the reactor) that it seems hard to believe they could be responsible for measurable signals a couple hundred kilometers away. Unless somehow even very low levels of these isotopes can trick detectors calibrated for cesium into reporting much higher values somehow? (I.e., if this gas were wafted past a GM counter, what kind of reading would that counter report?)

Otherwise, maybe the radon-daughters-kicked-up-from-the-soil-by-rain theory is the correct one?

 

[tsutsuji]

Todays status update has info about a further rise in hydrogen density, and measures taken.

Unit 2
11/3 12:40 In order to improve reliability of nitrogen injection, additional flow gauge is started to install at the Unit 2 nitrogen injection line.

Around 14:00 The work was finished. Although nitrogen injection was stopped for approx. 10 minutes during the work, there were no significant changes in the parameters

11/3 16:50 Since some increase at the hydrogen density (hydrogen density of 2.9% [As of 16:30 on 11/3]) in the exhaust gas from PCV was observed from the last time of nitrogen injection amount change (hydrogen density of 2.7% [As of 18:10 on 10/30]), nitrogen injection amount was changed from 21m3/h to 26m3/h.

Taken from http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111103_02-e.pdf

I don't remember if the following 30 October pdf was already mentioned in this forum :

Regarding the hydrogen concentration in the gas emission of the gas management system of the reactor containment vessel in Unit 2, we confirmed that it increased to approx. 2.3vol% at 5:00pm on 29 October, which was approx. 1vol% at the beginning of the operation. Therefore we adjusted the amount of injecting nitrogen gas from approx.14 m3/h to 16.5 m3/h in order to avoid exceeding the combustible threshold concentration (4vol%).
http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111030_01-e.pdf

http://www.asahi.com/national/update/1030/TKY201110300269.html (30 October) 1% is the hydrogen concentration measured at the start of the gas extracting system on 28 October. A Tepco official, Susumu Kawamata, said "Didn't hydrogen get out because an air flow occurred ?".

 

[NUCENG]

I don't remember if the following 30 October pdf was already mentioned in this forum :


http://www.asahi.com/national/update/1030/TKY201110300269.html (30 October) says that 1% is the value on 28 October. It quotes a Tepco official, Susumu Kawamata, giving some kind of explanation but I don't really understand : "Didn't hydrogen leak out because an air flow occurred ?".

It is unlikely that temperatures are causing any further zirconium water reactions. Radiolysis and corrosion processes will generate Hydrogen. Lots of carbon steel is exposed to warm wet conditions. As corrosion layer builds up rate should gradually decrease. May need to control pH to stabilize the corrosion layer.

 

[NUCENG]

Thank you, NUCENG. That makes sense.

My other question was whether the krypton and xenon could possibly account for the occasional synchronized transient spikes seen around Kanto. The spikes are pretty small, around 0.05 uSv/h above steady-state levels, but the krypton and xenon concentrations measured here look so darn low (barely above detectable limits even when sniffing gas directly from inside the reactor) that it seems hard to believe they could be responsible for measurable signals a couple hundred kilometers away. Unless somehow even very low levels of these isotopes can trick detectors calibrated for cesium into reporting much higher values somehow? (I.e., if this gas were wafted past a GM counter, what kind of reading would that counter report?)

Otherwise, maybe the radon-daughters-kicked-up-from-the-soil-by-rain theory is the correct one?

Can you give me a reference? I am a little confused if they are reporting a dose rate in place of activity for a specific isotope.

 

[dezzert]

I'd like to ask this question again. If no answer that's OK, but I am very concerned about the fact that Tepco has not been made to do more sampling of gaseous emissions. The only other gas sampling done, on Aug 9, they found Xe-131m at 3.8 X 10¹ to 4.7 X 10¹ Bq/cm3 in the lower floors of Unit 2. At that time little significance was placed on these readings. However, they now have readings of Xe-131m at 10-⁴, and concerns are raised.

I am confused. Were the Aug findings significant. And if so, with there only being two tests for gaseous emissions that would indicate fission occurring, would this not imply ongoing fission and criticality, and not spontaneous fission, especially as it appears the ongoing fission is winding down.

Wouldn't these findings necessitate/force further testing per regulations.

And lastly, what would be the requirements of a plant in America experiencing similar conditions.

And again, if these questions don't need answering, I apologize.

 

[rmattila]

Only minor difference is that I believe Pu-238 is actually a larger source than Pu-240, but two orders of magnitude less than the Cm isotopes.

Is it really so? In my reference inventory, the alpha activity of Pu-238 is about 5 times larger than that of Pu-240, but its SF probability is so much lower (1,8e-9 vs. 5,5e-8) that the SF rate would actually seem to end up lower by a factor of 5 (9e6 vs. 5e7):


[nuclide] [Bq] [T12] [T12(s)] [SF T12] [SF t12(s)] [SF prob.] [nu] [SF rate] [SF neutron production]
Pu-238 5,00E+15 87,7a 2765707200 4,90E+10 1,55E+18 1,79E-09 2,2 8,95E+06 1,97E+07
Pu-240 9,00E+14 6550a 2,06561E+11 1,20E+11 3,78E+18 5,46E-08 2,2 4,91E+07 1,08E+08
Cm-242 3,00E+16 162,8d 14065920 7,20E+06 2,27E+14 6,19E-08 3 1,86E+09 5,58E+09
Cm-244 4,00E+15 18,11a 571116960 1,40E+07 4,42E+14 1,29E-06 2,8 5,17E+09 1,45E+10

Even though the conclusion is the same, it would be nice to know where and why our results differ in this respect.

 

[tsutsuji]

http://www.meti.go.jp/press/2011/11/20111104007/20111104007-2.pdf Report from Tepco to NISA about the xenon detection. (4 November 2011, Japanese, 18 pages)

 

[thebluestligh]

http://www.meti.go.jp/press/2011/11/20111104007/20111104007-2.pdf Report from Tepco to NISA about the xenon detection. (4 November 2011, Japanese, 18 pages)

Thanks Tsutsuji as always, I found a report from TEPCO in English version as well

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111104e19.pdf

 

[rmattila]

Thanks Tsutsuji as always, I found a report from TEPCO in English version as well

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111104e19.pdf

If I understood the TEPCO report correctly, they have

(a) completely neglected the subcritical multiplication, justifying this by conservativity
(b) assumed all Xe-135 created by SF is immediately released in the containment airspace, which I think is a huge error in the non-conservative direction, since the cool fuel/corium should certainly be able to contain a significant amount of the fission products

Did I miss something? Can someone make better sense of the report?

 

[zapperzero]

If I understood the TEPCO report correctly, they have

(a) completely neglected the subcritical multiplication, justifying this by conservativity
(b) assumed all Xe-135 created by SF is immediately released in the containment airspace, which I think is a huge error in the non-conservative direction, since the cool fuel/corium should certainly be able to contain a significant amount of the fission products

Did I miss something? Can someone make better sense of the report?

I'm more interested in why they keep harping on about Curium. It's certainly not the only fissile in there.

To your first question: it is strange indeed.
As to your second question, they are probably assuming that the corium is saturated, so naturally all excess production goes into the containment immediately.

 

[tsutsuji]

Thanks Tsutsuji as always, I found a report from TEPCO in English version as well

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111104e19.pdf

Thanks for the English version.

The following translation, while not wrong, was forgetting to mention that the ratio is found using the krypton 85 measurements:

Collection efficiency of the charcoal filter is 1566 times as determined from the ratio of the values of Kr-85 measured in the charcoal (5.3E-1 Bq/cc) and [STRIKE]the value measured[/STRIKE] in a vial (8.3E2 Bq/cc).

 

[dezzert]

The following is from
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111104e19.pdf

If the Unit 2 fuel is in the critical state, Xe135 concentration is observed 4 orders of magnitude larger than current level. Therefore, present state is not considered critical.

OK, so on Aug 9 they found Xe-131m at 3.8 X 10¹ to 4.7 X 10¹ Bq/cm3 in the lower floors of Unit 2. This is 5 orders of magnitude greater. According to Tepco's analysis criticality has occurred. Am I missing something?

 

[rmattila]

Am I missing something?

The contribution of the residual I-131 (remaining in the containment water since March) on the Xe-131m production.

 

[clancy688]

Via http://ex-skf.blogspot.com/2011/11/xenon-detection-in-reactor-2-different.html" :

Comparison between the three gas tests on October 28th, November 1st and November 2nd. Looks quite funny. With the detection limits being very funny.

So... what the heck? No wonder they didn't detect any Xenon on October 28th.

 

[dezzert]

The contribution of the residual I-131 (remaining in the containment water since March) on the Xe-131m production.

Thanks rmattila. Woudlnt the residual I-131 be gone by then. And certainly not in numbers to produce those levels of Xe-131m.

 

[NUCENG]

Is it really so? In my reference inventory, the alpha activity of Pu-238 is about 5 times larger than that of Pu-240, but its SF probability is so much lower (1,8e-9 vs. 5,5e-8) that the SF rate would actually seem to end up lower by a factor of 5 (9e6 vs. 5e7):


[nuclide] [Bq] [T12] [T12(s)] [SF T12] [SF t12(s)] [SF prob.] [nu] [SF rate] [SF neutron production]
Pu-238 5,00E+15 87,7a 2765707200 4,90E+10 1,55E+18 1,79E-09 2,2 8,95E+06 1,97E+07
Pu-240 9,00E+14 6550a 2,06561E+11 1,20E+11 3,78E+18 5,46E-08 2,2 4,91E+07 1,08E+08
Cm-242 3,00E+16 162,8d 14065920 7,20E+06 2,27E+14 6,19E-08 3 1,86E+09 5,58E+09
Cm-244 4,00E+15 18,11a 571116960 1,40E+07 4,42E+14 1,29E-06 2,8 5,17E+09 1,45E+10

Even though the conclusion is the same, it would be nice to know where and why our results differ in this respect.

I am using a different source term list from a US BWR4 and the fission branch percentage and isotopic yields I used from an old textbook. The textbook data includes data uncertainty ranges which I used to produce conservative (low) results. I used the same source term and updated values for fission branch and yield from http://atom.kaeri.re.kr/index.html and now Pu-240 is about 4 times larger than Pu-238. Since the data I got online has no uncertainty data I classify the new results as a best estimate case.

My analysis included 4 Uranium isotopes, 1 neptunium isotope, 6 Plutonium isotpes, 3 americium isotopes, and 5 curium isotopes that can spontaneous fission. All of my results are well within one order of magnitude of yours which is what I believe is reasonable agreement based on different data and the uncertainty of that data. Switching to the new values did not change that result, because the effects of curium seem to control the data.

Other than that I made several assumptions and some additional data:

• I developed and solved a linear first order differential equation based on fission and spontaneous fission sources and removal by decay and leakage. It demonstrated that the isotopes of concern are appropriately treated as equilibrium values due to the long half lives of the fission sources.
• I assumed that the relative sources for spontaneous fission
• My inventory lists 50 transuranic and actinide isotopes and is based on a typical equilibrium core. Japanese operating cycles are probably shorter and so transuranic sources may be overestimated.
• I assumed a single fission product yield curve for U-235.
• I assumed a single value for neutron yield from fission.
• I assumed zero out-eakage, but ran cases for 5, 10, and 20% keakage rates.
• I included cases for Keff and subcritical multiplication from 0.7 t0 0.999. I used the results for Keff -0.8 for consistency.
• I included release of Xe from I Beta decay.
• I used a smaller free volume for containment to account for a water level inside containment.
• I uswd source terms based on 8 months decay and buildup.

 

[rmattila]

I did not have a suitable BWR inventory right at hand and no time to calculate one either, so I had to do with a VVER EOC inventory and scale it according to the thermal power level. Using an EOC inventory, I probably overestimated the transuranics, thus I believe your results to be better than the quick sketch I made in order to have at least something to base the estimates on.

 

[NUCENG]

The following is from
http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111104e19.pdf

If the Unit 2 fuel is in the critical state, Xe135 concentration is observed 4 orders of magnitude larger than current level. Therefore, present state is not considered critical.

OK, so on Aug 9 they found Xe-131m at 3.8 X 10¹ to 4.7 X 10¹ Bq/cm3 in the lower floors of Unit 2. This is 5 orders of magnitude greater. According to Tepco's analysis criticality has occurred. Am I missing something?

Via http://ex-skf.blogspot.com/2011/11/xenon-detection-in-reactor-2-different.html" :

Comparison between the three gas tests on October 28th, November 1st and November 2nd. Looks quite funny. With the detection limits being very funny.

So... what the heck? No wonder they didn't detect any Xenon on October 28th.

clancy 688 - what are the units of those measurements? dezzerts values (Bq/cm^3) from Unit 2 are significantly different.

 

[clancy688]

clancy 688 - what are the units of those measurements? dezzerts values (Bq/cm^3) from Unit 2 are significantly different.

Sorry, but what do you mean?

All of these values have the unit of Bq/cm³. http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111102_04-e.pdf" the original.

 

[LabratSR]

New results from Unit 2 containment released. Using different filters. Released Nov. 4, sampling date Nov. 1

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111104_04-e.pdf


Sampling System
http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111104_03-e.pdf

 

[zapperzero]

the effects of curium seem to control the data.

And there's my answer. Thank you.

 

[dezzert]

All of these values have the unit of Bq/cm³. http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111102_04-e.pdf" the original.

And this is the original that showed both Aug 9 readings (reported on the 10th) as well as Nov 1. Lower left box is Aug. Why they only tested for Xe-131m and not 133 or 135 is a mystery.
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111102_01-j.pdf

 

[clancy688]

http://spectrum.ieee.org/energy/nuclear/24-hours-at-fukushima" - report by IEEE.

Probably nothing new for most of us who're following this thread regularly but it's still interesting to read.

 

[tsutsuji]

TBS video showing a robot performing debris removal on unit 3's first floor under radiations up to 650 mSv/hour.

 

[LabratSR]

TBS video showing a robot performing debris removal on unit 3's first floor under radiations up to 650 mSv/hour.

Shows removed by user.

 

[LabratSR]

TEPCO Video - Fukushima Daiichi Water Treatment - Second Installment

http://youtu.be/ylgiQ1KWm48