Last updated on March 1, 2013
Two days ago we first got the report that there is highly radioactive water in the turbine building basement of reactor 1. NISA released this list of isotopes found in the water and its activity per cubic centimeter. I added half lives to the list to make it more understandable.
| nuclide | activity(Bq/cm³) | half life | Q value (keV) |
| Cl-38 | 1,60E+006 | 37,24 min | 4916,53 |
| As-74 | 3,90E+002 | 17,77 days | 2151 |
| Y-91 | 5,20E+004 | 58,51 days | 1544,82 |
| I-131 | 2,10E+005 | 8,025 days | 970,86 |
| Cs-134 | 1,60E+005 | 2,065 days | 1233,38 |
| Cs-136 | 1,70E+004 | 13,04 days | 2548,22 |
| Cs-137 | 1,80E+006 | 30,08 years | 1175,63 |
| La-140 | 3,40E+002 | 1,679 days | 3762,22 |
We see that there is one peculiar things, a high activity of Chloride-38. The only way for Chloride-38 to be created is by Chloride-37 in seawater absorbing a neutron and turning into Cl-38. This would indicate there is still a considerable neutron flux in the core. Forum discussions have been going on about it. The other peculiar thing is the activity levels, the activity per cm³ is just crazily high. I suspect they are really measuring activity per liter and mistakenly report it as cubic centimeter and I will give 2 arguments for that.
1. The activity of the water
They reported that 3 workers had been working in the water for more than an hour, their dosimeters showed a dosage close to 200 mSv and two of the workers didn’t wear rubber boots so they got a large surface dose on their ankles and feet from beta radiation(some reports say up to 6 gray). But this does not match with the activity levels seen in the table I pasted above. If one multiplies the activity in Bq/cm³ with the Q value, i.e the energy released in the decay of one atom, and massage the numbers a bit to get the numbers in joules instead of keV and in cubic meters instead of cubic centimeters. Then one finds that the water is putting out 1.68 Watts per cubic meter of water. Something like 20% of that energy is from gamma rays, the rest is beta rays and recoil energy in the decaying nuclei.
1.68 Watts doesn’t sound like a lot given that a normal water kettle can have a power of 1000 W. But in terms of ionizing radiation 1.68 W is just crazy. Radiation dose is measured in Grays, one Gray is equal to one joule absorbed by one kg of tissue and a Watt is as we know one Joule per second. A one Gray whole body dosage is enough to get radiation sickness, 10 Grays will kill you slowly(over a few days) and 100 Grays kills you instantly.
These workers where working in water that is putting out watts of radiation, that means they should have gotten hundreds of Grays in an hour. It was stated they where knee deep in this water! Now if we instead consider that perhaps they mean activity per liter, misstakenly reported as cubic centimeters, then everything goes down by a factor of 1000(there are one thousand cubic centimeters in a liter). Instead of hundreds of Gray the workers would have gotten hundres of milligray, that is consistent with their dosimeter readings.
2. The neutron flux needed to create Cl-38
The probability that a Cl-37 atom will absorb a neutron and turn into Cl-38 is quite low, 1 barn(this probability is measured in a unit called barns, one barn=10^-24 cm², uranium has as comparison a fission probability of about 600 barns). So one needs a lot of neutrons to create any significant quantity of Cl-38. If one assumes the water concentration of Cl-38 has dropped by a factor 1000 when traveling from the reactor vessel to the water in the turbine hall basement(dilution, decay etc). Then one can make a rough approximation of the neutron flux needed in the core in order to create the Cl-38 concentration seen in the turbine hall(technically solving the Bateman equation for a equilibrium case with one group cross sections).
Neutron flux ends up being around 10¹² neutrons per square centimeter and second. A reactor running at full power produces on the order of 10^14 n/cm²*s. The core would have to be running at about 1% of its full power(10-20 MW thermal) to produce that flux, but I might as well be off by a factor of 10 in my estimate of dilution, which would put the power in the rage from 1-100 MW. We can exclude 100 MW since we have not seen a pressure and temperature spike that high in reactor number 1. We can not exclude 1-10 MW power levels because it is on the same order as the decay energy. But if we also again here assume they mean Bq/liter instead of Bq/cm³ then the needed flux to produce the Cl-38 goes down to something around 1*10^9 neutrons/cm²*s. That kind of neutron flux could plausibly be around from radioactive decay or a very very low power level.
Those 2 factors together makes me believe they are reporting activity levels wrongly. Lets see if I will have to change my opinion when new information arrives.
Notes for the interested.
To convert from the unit eV(electron volt) to joule one multiplies 1 eV with 1.602*10^-19
The Bateman equation describes the time dependent concentration of radionuclides. For the case of a nuclide created by a neutron flux and destroyed by decay one gets this equation
dN/dT= N_37*sigma_37*flux – N_38*lambda_38
Where:
N_37=number of Cl37 atoms per cubic centimeter(I assumed saturated salt water at 100 degres, which gives 0.39 grams of salt per cm² which gives 9,74*10^20 Cl37 atoms per cm³)
sigma_37=neutron capture cross section, around 1*10^-24 cm²
flux=neutron flux in neutrons per cm² and second
N_38= Number of Cl-38 atoms per cm³.
lambda_38 = decay constant for Cl-38(equals to Ln(2)/half life)
If one assumes equilibrium then dN/dT=0 and one gets that
flux=N_38*lambda_38/(N_37*sigma_37)
One get N_38 from dividing activity with decay constant and then multiplying by a dilution factor.
Here are all the numbers I plugged in in case someone can spot an error.
| Find flux | |||||
| Avogadros number | 6,02E+023 | ||||
| Salt density(gram per cm3) | 3,90E-001 | Dilution factor(from vessel to puddle) | 1000 | ||
| Molar mass sodium | 2,30E+001 | Cl37 capture cross section | 1,00E-024 | ||
| Molar mass chlorine | 3,55E+001 | Cl37 number density (per cm³) | 9,74E+020 | ||
| Moles of NaCl | 6,67E-003 | Cl38 lambda | 3,10E-004 | ||
| Fraction Cl-37 | 2,42E-001 | Cl38 number density (per cm³) | 5,16E+009 | ||
| Moles Cl-37 | 1,62E-003 | ||||
| Number of Cl-37 per cm³ | 9,74E+020 | Flux | 1,64E+012 | ||
Links(english)
Kyodo News Woes deepen over radioactive waters at nuke plant, sea contamination
Reuters Soaring radioactivity deals blow to Japan’s plant
BBC Radiation soars at japan reactor
NHK extreme radiation detected at number 2 reactor
Links(swedish):
DN Arbetare evakueras efter akut strålning
SvD Fler svenskar misstror kärnkraft
SVD Nya bakslag vid Fukushima
Aftonbladet tio miljoner gånger högre strålning
Röda berget
Jinge
In your face
Kunskapssamhället
Svensson
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Why don’t you try to contact NISA about this?
I would I guess if I could find some email address to contact them through(nothing can be found on their website). But I doubt they have time to answer questions right now. If my conclusion is right then someone there must have drawn it by now as well. Otherwise I guess I am wrong 🙂
IAEA has mail addresses. Maybe ask them instead? They have contact with NISA.
I will try to contact IAEA, thanks for the advice! Didn’t even think of them.
I very much appreciate the detailed response, with the calculations laid out for all to see.
I suggest posting part of this post on http://arstechnica.com/civis/viewtopic.php?f=26&t=1139141
Your calculation seems to be more detailed than the others I have seen. (From dio82 and pietkuip)
Pietkuip”s note says he belongs to Växjö tribe, you could even get on the phone with each other
Your welcome,
Unfortunately I don’t have time to engage in a forum discussion. But feel free to post a link in the forum thread to this blog post or even copy the entire post along with a link.
The new measurements of short lived iodine hints to production of fission products going on, but again, if the concentrations are given in erranous volume unit it might not mean a very high power level.
There seem to be some measurements, that are not translated fully into english yet.
http://www.meti.go.jp/press/20110327001/20110327001-4.pdf
For reactor 2:
I-134 has 2.9 * 10*9 /cm3
From a press report:
“The plant operator, known as TEPCO, says it measured 2.9-billion becquerels of radiation per one cubic centimeter of water from the basement of the turbine building attached to the Number 2 reactor.
The level of contamination is about 1,000 times that of the leaked water already found in the basements of the Number 1 and 3 reactor turbine buildings.
The company says the latest reading is 10-million times the usual radioactivity of water circulating within a normally operating reactor.
TEPCO says the radioactive materials include 2.9-billion becquerels of iodine-134, 13-million becquerels of iodine-131, and 2.3-million becquerels each for cesium 134 and 137.”
http://www3.nhk.or.jp/daily/english/27_12.html
Is it possible to get this high levels unless the reactor have been critical very recently? The half-life is only 52 minutes.
That number makes no sense, the people that goes in to sample the water would be dead before filling a sample. A cubic meter of water with that concentration of I-134 would be radiating with 2000 watts. Lethal dose in seconds.
They measured wrongly on the I-134 http://www3.nhk.or.jp/daily/english/27_24.html
Well, the guys standing in front of the whiteboard are saying: (NHKs translation)
“By taking new samples, we are trying to determine if the leaked material is Iodine-134. As of now it is not certain it is iodine-134”
Depending upon how rigorous their analysis is, they may be misidentifying the 1600 keV line from La-140 as the 1640 keV line from Cl-38.
That sounds quite reasonable. It wouldn’t explain the high activity, but it could explain the detection.
Hm, I would expect far better accuracy than 40 keV in this energy range.
And by the way, there is no 1600 keV gamma from the decay of La-140, though there is a 1596.2 keV gamma in the decay to Ce-140. Close enough, you might say, but I would expect an energy resolution of 1-2 keV, so there should be no mismatch with a peak at 1642.7 keV. Maybe they have worse energy resolution, but even then 40 keV is a lot with a High Purity Germanium (HPGe) detector.
On the following link you can see other candidates around 1642.7 keV: http://ie.lbl.gov/toi/Gamma.asp?sql=&Min=1641.7&Max=1643.7
I see no obvious candidate from the usual suspects, maybe somebody else sees something that I am missing. You can modify the search, and the width of the uncertainty, on this link: http://ie.lbl.gov/toi/radSearch.asp
Regarding missidentification of the gamma energies: What other similar energies are there for Cl-38 and I-134? A link to a chart would be appreciated.
Someone besides TEPCO should be measuring too by now. Keep them honest.
By now, I would have expected a good measurement of the Cl-38 levels
This whole thing is quite a headache I have to say. I think we are going to have to wait a day or two for them to sort out their measurements. There is probably a lot of trace isotopes giving all sorts of gamma energies and analyzing that under time pressure must be very hard.
Hi kitty,
On the following link you get access to an online version of Table of Isotopes: http://ie.lbl.gov/toi/
Click on Radiation search for searches of possible isotopes for a given gamma energy (there are other options as well).
Click on Nuclide search for searches of specific nuclides. If you put “38” in the box for mass number and “Cl” for element you will get two options, Cl-38 and the meta-stable state Cl-38m. Click on Cl-38 and you will get a bunch of information about this isotope. If you are lucky (doesn’t work for me, so I use other sources for level schemes etc) you can get some nice tables and plots. A bit further down, below the label “TORI data (1999)” you can see the most prominent gamma energies. The second column “Ig (%)” gives the relative intensity of the gamma in percent. For Cl-38 you can see a 1642.7 keV gamma with 31.9% relative intensity, and a 2167.4 keV gamma with 42.4% relative intensity. There are about 6 more gamma transitions possible for the decay, but they are so weak (i.e. not very likely to be detected) that they have not even been listed here.
I hope this helps.
Very helpful Lantzelot!
Now I am getting curious how good precise you can measure the gamma levels. If you can get a very precise measurement, it looks like you can unambigously pick out wich isotope you are dealing with.
(What is the technical term for this btw?)
I did not realize you had THIS many gamma energies lying so close together.
For a very detailed determination of the energy of the gamma transition I guess you need a stable laboratory situation with good energy calibration from several know sources such as Na-22, Co-60 and Cs-137. Please note that the energy of the gamma transition is not the same as the excitation level in the nucleus. In order to avoid being too technical we should focus on the gamma transitions, i.e. the gamma rays that are registered in the detector.
On the following link you find a lot of information about gamma spectroscopy: http://www.nucleonica.net/wiki/index.php/Help:Gamma_Spectrum_Generator
Please focus on Fig. 4.3, i.e.: http://www.nucleonica.net/wiki/images/8/83/Spectrum_Eu-152_GC-6020.jpg
It shows the pulse height spectra (energy spectra) for a Eu-152 source. As seen there are many different peaks from this single nuclide. Fig. 4.4 gives you a much simpler spectra, for Co-60: http://www.nucleonica.net/wiki/images/0/07/Spectrum_Co-60_GC-6020.jpg
Please note that for each peak (photo-peak) there is a Compton continuum at energies below the peak. This is a sort of background from gammas that have not deposited all their energy within the detector. For the Co-60 spectrum there are two Compton continua, one from each peak, that are superimposed on each other down to zero energy. At about 0.95 MeV and 1.1 MeV there are some bumps, called Compton edge. They are correlated to the photo-peaks at 1.17 MeV and 1.33 MeV. At 0.2 MeV there is another peak, called back-scattering peak. All these features of the spectra comes from the two gamma energies 1.17 MeV and 1.33 MeV when Co-60 decays, they just display themselves in different ways depending on how they happen to interact with the detector. Now, go back to the Eu-152 spectra and try to identify all these features for all the possible gamma energies… They are all there, but slightly masked by being super-imposed on each other.
In the real world, outside of the laboratory, you will have a combination of different sources, and in Fukushima there are a lot of different sources, so you can imagine the problem of identifying some peaks to come from a specific isotope. There are peak-searching programs that will do most of the job for you (if you have calibrated your detector correctly), but if the program picks the wrong isotope when there are two peaks at about the same energy, then you need a skilled person to realize that the identified peak may be incorrect.
Now I will try try to answer your question: In order to precisely identify each gamma level (in a research situation), you need to have at least two detectors so that you can see how the peaks are related to each other; one gamma transition is usually coupled to one or more other transitions that will occur shortly after (of the order of pico-seconds). By “gating” on one peak one can in this way identify which other peaks that are related to it, and build up a tree of excitation levels. The accuracy can be down to single eV or even better, but it will certainly not happen during a slightly stressful field study such as the one going on in Fukushima.
In Fukushima there is no need for such setups, they “only” want to identify isotopes from the observed peaks, so a standard calibration with a few keV energy resolution should be good enough.
Sorry for rambling on… 🙂
Thank you for the ramblings Lantzelot 😀
Working on absorbing the data and playing around a bit with the webtools you provided.
Still, it seems TEPCO could use some curious scientists with little robots for taking good measurements.