Jeffrey LewisLocalized Criticalities at Fukushima?

My colleague at MIIS, Ferenc Dalnoki-Veress has written a paper, What Was The Cause Of The High Cl-38 Radioactivity In The
Fukushima Daiichi Reactor #1
.  Ferenc  concludes that we cannot exclude the possibility that little pockets of melted fuel have gone critical, even if for a short period of time.  The possibilty of localized criticalities could severely hamper on-site operations.

Comments are welcome.



  1. Red_Blue (History)

    It contains at least the error of setting the starting point of seawater injection as “2:30 am on March 23rd”, but this was the time seawater injection to the reactor pressure vessel, which had been going on for over 11 days (since March 12th 2020 JST according to NISA) was switched from the fire fighting line to the feedwater line.

    If I understood it correctly, any radiation time exceeding 0.5 days would not increase the Cl-38 fraction, so this error is probably not significant. More important is the date when the core injection was switched from seawater to fresh water in reactor 1, which was March 25th 1537 JST (also according to NISA). The sample which according to TEPCO contained Cl-38, was taken on March 25th, so the water must be from the time when the reactor pressure vessel contained pretty much only seawater.

    Also the paper makes no justification for assuming (for scenario 1) that the core has melted to such extent, that the RPV has been breached and the core (corium?) is now lying on the primary containment floor. AFAIK nobody has come up with any evidence of this and also IAEA seems to think the RPVs are probably intact.

    As far as I can tell scenario 2 is also not representative of a situation where the core is mostly still intact (with about 2/3 of it covered with water) and inside the RPV with significant water flow along the normal spaces between the rods and assemblies.

    However, my expertise is insufficient to argue that these assumptions (if incorrect) are sufficient to distort the end result, that spontaneous fission is unable to reach the level of chlorine activation (I would say “alledgedly” considering the serious measurement errors of similar samples of unit 2 turbine hall water acknowledged by TEPCO) observed.

    I think more firm conclusions could be drawn only when TEPCO or someone more competent resamples and remeasures this water.

  2. Seb (History)

    I notice that she uses the mass of chlorine in fresh sea water, but what is in the reactor after days of distillation is going to be much more concentrated, and there may in fact be solid salt crusts over the reactors.

    Might this work to increase the possibility of spontaneous fission providing the same amount of Cl-38?

    • Seb Tallents (History)

      Excuse me, no idea how that he ended up a she in the final post! Anyone know how many tonnes of sea water has been put into the reactor? If we assume crudely that no significant mass of chlorine has left, then we can work out the total chlorine inventory and see if that would reduce the required neutron flux enough.

  3. bks (History)

    If only we had better data than reports in the Kyodo News to being the analysis!


  4. Eve (History)

    Perhaps all the more reason to monitor Na24 in workers and the plant.

  5. JamesL (History)

    I appreciate Mr. Dalnoki-Veress’s knowledgable pursuit of this line of thought and want him to continue. I too have been preoccupied by Fukushima and have followed a parallel line of reasoning, albeit with much less background knowledge. But there are valid questions that do not require his level of knowledge before they may be posed (and hopefully answered).

    1) The neutron beams are cited as suggestive that plutonium and unanium have escaped from the primary containment vessel, implying that the neutron beams were observable because the reaction occurred outside the PCV. Whether or not such a reaction within the PVC could be observed is a question that deserves clarification because it means such episodes could be ongoing.

    2) Fuel mass concentrations of melted nuclear rod portions, and concentrations of precipitated or dissolved salt are probably not homogenous. But that does not mean that concentration densities and heterogeneity of concentrations are static, particularly in immediate proximity to a melted fuel mass. Dynamic change in these variables supports a prediction of future episodic events. Given the amorphous and dynamic nature of amounts and concentrations, prediction of possibilities unfortunately lies more in the area of theory than in calculation. In the absence of more extensive calculations pursuant to those offered by Danoki-Veress, our grasp of present and future events is more likely to come initially from closely reasoned and subsequently critiqued theory.

    3) The point of criticality is only a single point. Dalnoki-Veress’s theory involves an episode of a localized mass moving from sub-critical to critical, and then back again due to decay. But it begs the question of why the mass stopped precisely at, or very near, the point of criticality, where (supposedly) natural decay was a sufficient brake to what had been an increase of activity–sub-critical moving to critical. If localized criticality is in fact occurring, or has occurred, the immediate next question would be to what degree and in how many ways might localized change occur that would negate the apparent self-limitation and permit the mass to move to super-criticality.

    4) The immediate next question is: what is the most probable, theoretical possibility of a super-critical mass covered by water? Hyman’s paper and probably others would be pertinent to this question. Is it possible for someone to provide a link?

    5) Localized (or perhaps equally accurate: temporary, transitional, or episodic criticality) suggests that a relatively simple correlation exists to observed episodes to date. Publication of the times of the neutron beam events would be most helpful in establishing (or not) correlations to observed Fukushima events.

  6. John Schilling (History)

    There’s a dog not barking here. Applying the same methodology to other components of sea water, and assuming a brief criticality event (or other neutron source), we should be seeing:

    ~3.4E5 Bq/cc of Na-24
    ~6.2E4 Bq/cc of Br-82
    ~3.3E4 Bq/cc of Mg-27
    ~1.9E3 Bq/cc of K-42

    Not as much activity as from the Cl-38, but enough to be readily detectable. And note that if this is a steady-state process rather than an instantaneous criticality event, the Na-24 would go up to ~1.8E7 Bq/cc. Similarly, a delay between activation and measurement would produce relative enhancement of Na-24.

    Either TEPCO is so narrowly focused on Cl-38 as to have completely missed some very large radiation signatures of obvious interest, or there’s some process other than neutron activation of sea water at work, or the reported Cl-38 numbers are wrong.

    Is there more than one distinct measurement showing significant Cl-38?

    • Red_Blue (History)

      “Is there more than one distinct measurement showing significant Cl-38?”

      To my knowledge, no. This measurement was also before TEPCO acknowledged that they have haphazardly “identified” other isotopes such as I-134 by detecting only one energy peak in gamma spectroscopy assigning it to the first isotope (or isotopes) that match the pattern. Incidentally, Cl-38 is also roughly the same decay energy range as other isotopes of interest, such as I-134 and Y-94. I-134 detection was later ruled to have been spurious for the unit 2 turbine building sample.

    • gepay (History)

      Speaking of other radioactive elements, why in any of the radiation measurements have I not seen any strontium 90 detected? Its not on any lists, not there with either a number or Not Detected. Do these nuclear plants not produce it? my readings tell me that along with cesium 137 it is generating a good bit of the decay heat.

  7. JamesL (History)

    Video of concrete pumper in action pumping water at un-named reactor as per entries here several days ago. The video pretty clearly shows that the water being ‘injected’ is more like a heavy rain with little accuracy. Three legs of the pumper are used just to reach over the top-most structure of the damaged building, so water is falling some distance. As per agricultural field irrigation knowns, the evaporation rate is very high compared to water delivered.

    • Tim (History)

      I believe the video you posted shows pouring water on the spent fuel pools, not injecting water into the reactor vessels. Two different things.

    • BabyStrangeLoop (History)

      “unnamed reactor”? Look at the left side of the corner of the building and a big “4” is visible.

  8. Katabatics (History)

    I agree with Red_Blue. The Cl-38 was mis-identified.

    Red_Blue | March 29, 2011

    “Is there more than one distinct measurement showing significant Cl-38?”

    To my knowledge, no. This measurement was also before TEPCO acknowledged that they have haphazardly “identified” other isotopes such as I-134 by detecting only one energy peak in gamma spectroscopy assigning it to the first isotope (or isotopes) that match the pattern. Incidentally, Cl-38 is also roughly the same decay energy range as other isotopes of interest, such as I-134 and Y-94. I-134 detection was later ruled to have been spurious for the unit 2 turbine building sample.

  9. anonymous (History)

    First, why are there no neutron detector measurements from the reactors? They were installed in the rpv and substitute ones should have been installed so that the controllers would know if there has been criticality.

    Second, why is NISA not giving daily plant reports even in japanese. The last one (in japanese) is from 0600 JST march 29th. Unit 1 rpv was at 600 kpa, obviously close to the pressure limits, and climbing 50 kpa every 12 hours. I would like to know what happened.

    Thirdly, by now they must have high resolution photos of the open units 1,2 and 4. Why have they not released pictures showing the pools?

    I am thinking information is being withheld.

    • FSB (History)

      Are you folks not familiar with Japanese culture?

      Everything is hello-kitty “bon” until the shit hits the fan and the management commits harakiri.

      That is the Japanese MO.


      I predict Management harakiri within the next 10 days.

    • Wramblin' Wreck (History)

      I don’t know if information is being withheld or just not taken in the first place. It seems to me that the Japanese are just conceptually overwhelmed by this situation and are treating it as a BFMI operation (Brute Force/Massive Ignorance.)

      On one hand the plant management is saying that the sun is shining and the birds are singing. On the other hand the media is foaming at the mouth about how the world is coming to an end and everybody will die.

      Some common sense is desperately needed here.

    • Red_Blue (History)

      Mark IV RPV operating pressure is 6,990 kPa (~7 MPa) and safety relief pressure 8,030 kPA. I think you are confusing it with the postulated failure pressures of Mark I primary containment, which are in the range of 700-900 kPa. The highest reactor 1 CV pressure has gone was 606 kPa on March 13th and 745 kPa for reactor 2 on March 15th. Also reactor 2 RPV peaked at over relief valve pressure of 8,109 kPa on March 14th.

  10. twt (History)

    Management harakiri within the next 10 days.that is damn enough! I suspect partial meltdown in core during earthquake.
    The fuel assembly design need to go review as well the control rod system design. Until their electric supply fully reconnect back, and monitoring system come on, no one knows the rod control all fully re-insert, and always intact in this position?(bottom up) Does fuel bundle structure intact,no crack, no bent? after 9 Richter
    The design flaw are hydrogen absorption system nor collection equipment What in my mind is how can the core and those spent fuel fully cool down faster, than the place can be bury?

  11. Eve (History)

    There could be a serious casualty at Daiichi…

    “Meanwhile, the president of the Fukushima nuclear plant operator Tepco has been admitted to hospital. Shimizu is being treated for high blood pressure and dizziness, a Tepco spokesperson said. Mr Shimizu has barely been seen in public since the earthquake and tsunami on 11 March which damaged the Fukushima plant.”

    • FSB (History)

      I change my forecast to harakiri in 7 days.

    • justin (History)

      admitted to the hospital? it’s a common ploy among japanese execs and intimates a far-too-harsh media spotlight.

  12. Eve (History)

    Maybe he should lie down and sleep on lead mats, I’ve heard they are comfortable and really warm.

  13. Sez Who (History)

    Speaking of criticality, has anybody seen any analysis or thought that the explosion that wrecked #3 reactor building might have involved a criticality? If you look at the video of the explosion, you can see a flash at the upper right corner of the reactor building that is followed by a very strong vertical explosion, even as the shock wave spreads around the building. Minus that (possibly secondary?) explosion, the explosion that destroyed Unit #3 looks a lot like that for Unit #1.

    Just wondering about the possibility of some configuration of earthquake jumbled fuel rods, low water levels, fuel melting, that coupled with a powerful hydrogen gas explosion might have slammed rods together and caused a criticality in either reactor cores or spent fuel pools.

    Has this been definitively ruled out as a possibility in the explosion of Unit #3, and/or the other reactor buildings?

    • jim hardy (History)

      Thank you for asking that.

      We are focused on thermal reactors, of course. It’s all we are taught, and you gotta have a moderator.
      I have asked this at several places, maybe somebody here will answer. I simply lack the education.

      Now there’s an awful lot of fuel in that pit.
      Given Boron’s transparency to fast neutrons (Google Boraflex),
      Might such a large pool,
      with so much fuel in it,
      when drained of moderator,
      approximate an infinite fast reactor?

      Sure would appreciate some comfort on that one.

      Oh by the way i tried his formula for flux.
      I too thought about a short excursion .
      Plugging in one sec for time i got 1.7E14 nv, a worrisome number. It’s about right for a 1 sec excursion to ~200% power.

      i sure hope the Cl38 report was bogus.

      old jim

  14. Spruce (History)

    While the paper is excellent in technical details, it also shows how even an expert can mislead himself by concentrating on evidence that supports the hypothesis while not taking into account the evidence that argues against it. While there are no errors in the paper that I can see, it fails to consider the several points that argue against recriticality.

    First, the lack of other short-lived fission product nuclides. Like John Schilling posted above, there should have been several other short-lived nuclides present if there had been been recriticality event. And several of those other nuclides have gamma energies that make them easier to (correrctly) identify than Cl-38.

    Second, the lack of any further observations of Cl-38. If there was a criticality event, it should’ve been detectable in more than one measurement.

    Thirdly, the lack of reports of unexpected heat production. Even a brief and local recriticality should generate heat that it would create unexplained and noticable increase in heat production.

    Fourth, the lack of credible scenario where a reactor which has been huge concentration of boric acid could reach criticality. The seawater pumped in has contained relatively large amounts of boric acid and the process of pumping in water, letting it boil, and venting steam has concentrated the aboric acid in the water remaining in reactor. And add to this the fact that the control rods are and remain fully inserted.

    Fifth, the lack of controlling effects if there had been criticality. In light of the previous points, any criticality would’ve required a considerable malformation of fuel. Should it been extensive and unexpected enough to cause recriticality, it’s highly doubtful that the geometry had been intact enough for the normal negative feedbacks be enough to again stop the reaction. Or, said other way: if conditions allowed criticality, what stopped it going supercritical?

    When you consider all of those points, it’s hard to belive that recriticality would be more likely than misidentification of the nuclide.

    • virtualnomad (History)


      That is an excellent analysis of the Ferenc Dalnoki-Veress paper. You should work for IAEA public information! You could have provided insight to the volte-face the agency did from its March 18 briefing when they said re-criticality was not an issue.

      I have several question:
      1. Given the dearth of information from Nisa/Tepco, what level of confidence do you have that there was even screening for the “several other short-lived nuclides present” that would have indicated a re-criticality.
      2. Have they continued testing for Cl-38 in unit-1 or elswhere (unit-4 for example, where the neutron absorbers almost certainly melted/dissolved before potential neutron moderating water/coolant was introduced)?
      3. Heat numbers are scarce. NHK-commissioned geothermal satellite imagery is the best I can find. Japan’s DoD has also produced some slides.
      4. Boric acid: well, I asked this question 10 days ago and IAEA wasn’t certain whether Boric acid was actually being mixed with the sea water spray. It sure would be a relief if that were true.
      5. I’m not sure if I completely understand you point #5. Are those neutrons having an orgy in there or what?

      My final point: There’s not enough information. The signals that are public point to a situation that may be regrettably graver than it is. Our best minds are baffled.

    • Eve (History)

      Can we be assured that the boric acid went into solution in the initial mixing stages? It’s not the best in the lab.

  15. John Neely (History)

    The decay sequence from the original fission is well known, with all the yields going down each chain. As a result there should be a known ratio of isotopes at any one time after a sample of fuel has stopped being part of a nuclear reaction. It would be messy because you would be looking at a long steady state of fuel fissioning followed by a shutdown. Messy, but easily done by computer (monte-carlo?) analysis. As a result, you should be able to tell by isotope analysis whether radiation came from spent fuel that has been out of the reactor for months or from reactors shut down on March 11, or from something more recent. More recent, of course, would point to a criticality after the shutdown. I think you can even determine a mixture of these sources.

    It’s more complicated than looking for Iodine 134 or Chlorine 38, but the information should be there in the data.

    The rest is left as an exercise for the student. Sorry about that, but I don’t have the data or tools to do this myself in a reasonably timely manner.

    • LucyLuLu (History)

      >>The rest is left as an exercise for the student. Sorry about that, but I don’t have the data or tools to do this myself in a reasonably timely manner.

      I have seen all the data they have released. None is sufficient to perform the type of calculations you propose because they introduce unknown variables. If the unknown variables could be ignored, the data would undoubtedly demonstrate that fission has occurred on more than one occasion since SCRAM. Not the least of which, is the strong possibility that the Cl-98 levels were a reading error by the testing facility.

  16. Peter Raffaele (History)

    What happens if there is another tsunami and it tops the existing seawall at the Diiachi site? How come the Japanese authorities have done nothing so far to secure that site fromn such an event. If one were to eventuate any time in the next few weeks it is good by Charlie. Then you might end up with super criticality in several hundred tonnes of material.

    What about the US firm about to test its aramid glue globules on the site to try and control particulate dispersal. Sharp as a marble some folks! Nice little bouyant balls floating across the Pacific to be gobbled up by the top order of the food chain.

    Send robots in to check out how things are going? Another great idea from sit on your hands dot com!

    The reactors need to be opened up as soon as possible folks and the massed-slumped rods torn apart in a boric acid bath to increase surface area asap. Yes the beam activity is worrisome but pretending hopefully that the assemblies can be cooled down is a joke.

    What can go wrong will. And if you are not paranoid you are niave.

  17. Major Variola (History)

    Would someone explain how you get so much of a 70 minute
    halflife element Te139 in unit 1 esp. compared to others.

    And why is reactor 3 Cs137:I131 ratio off?

    • jim hardy (History)

      i had same question a few days ago on a different forum..

      A kindly radiochemist offered me explanation it comes from that Te139 meta (Te139M)with the 39 day T1/2.
      so it’ll show up for a few months.

      its not my field so i accepted it.

      old jim

  18. Worth Noting (History)

    Very interesting discussion going on here.

    Please Google: “Hanford Nuclear Reservation”

    There’s been ongoing documented leakage there for quite some time now.
    As well as, fluid nuclear wastes being dumped directly into unlined trenches, so that the nuclear wastes could “disappear” into the soil (this has finally stopped supposedly.

  19. hwf (History)

    Very interesting hypothesis, but:

    the quoted isotope data show a variability in Cs activity by a factor of about 100, but they should be quite similar. So I assume that the gamma spectrum was misinterpreted, and I wouldn’t trust the Cl-38 data either.

  20. jim hardy (History)

    Re: criticality:
    i am told by a radiochemist that it is REAL easy to mistake I134 for Cl38.

    So that’s hearsay. But i believe it.

    The flash was indeed worrisome for it was color of sodium vapor lights.

    But even if it happened it didn’t scatter fuel everywhere else there would be no photos of people walking and driving fire trucks about .

    So if it has happened we got through it.

    I am awaiting the Lessons Learned report.

    Re pool vs reactor water in basement:
    I chased ratios of Cs137 to Cs136 for a few days, to try and distinguish pool from reactor water. Picked them because both have precursors short enough they might as well be fission products. That is if i read Chart of Nuclides right, and that’s not my specialty.

    Their ratio i think is 2.5 at end of cycle and should decrease at 13 day halflife of Cs136 from there.

    I found samples indicated months old, presumably from pool, and twenty four days old . nothing real conclusive.

    old jim

  21. jim hardy (History)


    OOPS make that INCREASE at halflife of Cs136

    please edit if you can
    thanks, old jim himself

  22. Sheridan Collins (History)

    I only see one comment here related to Japanese culture. This is a critical component no matter what happened. I predicted last week that the honor (or dishonor) suicides would start shortly.

  23. Eric (History) blistering-128-degrees-celsius-zone-reactor-3 Reactor 3 also seems to show a very hot leak.. like a breach in reactor core, and possible meltdown. Of course the whole thing is covered with a pile of debris, so it is hard to tell.

    Two of the reactors show a zero pressure reading according to TEPCO, so does that mean a breach or just a leak? They also say at least three reactors have partial meltdown of fuel rods, within the reactors, even though water is also inside of them.

    Also, three types of plutonium have been detected on plant grounds.

    Finally, what happens to a spent fuel pool when most or all of the water is gone?

    Feel free to post interesting items on

    • old jim hardy (History)

      i like to see the hot steam coming out. Here’s why.

      You can cool a core by pumpimg water through and letting the water carry away heat. But the amount of water is staggering and your basement fills up.

      If instead you let it turn to steam, two things happen:
      1. you reduce the amount of water needed by 90%. That’s because it takes a lot more heat to boil water than to just warm it up.

      2. You reduce the contamination getting out.
      That’s because you “distill” the water inside the reactor vessel.

      The steam that comes out is a whole lot cleaner than the water left inside.
      It is way better to let the wind carry away fairly well cleaned up steam than to have ten times as much horribly contaminated water run into the ocean.

      I think the guys over there are making progress.
      And they are heroes.

  24. Tom Elliott (History)

    Chernobyl’s reactor 4 was a 1.5 megawatt design and yet readings at the end read 33 gigawatts(34 gW by another account). That was far and away far more energy than I would have thought possible. I wonder what is possible for the amount of energy release from these damaged and even the undamaged units 5 and 6?

    Any ideas, theories, comments on that question?

    • skylark (History)

      There is some evidence that the Chernobyl core went prompt critical and the second, large explosion was a fission explosion that consumed 0.01-0.1% of the fissile core material. That is why the power excursion was so large. (Pure and Applied Geophysics 167 (4-5): 575-580 DOI: 10.1007/s00024-009-0029-9)

      I’m not sure what you are asking as far as what is possible in Japan. I’m NOT an expert in this stuff like many of the people around here, but Ferenc addresses the question of super criticality briefly in his reply below this.

  25. Ferenc Dalnoki-Veress (History)

    Thanks everyone for all your excellent comments. I wrote this paper because I wanted to rule out criticality in reactor #1 and with the scenarios that I invoked came to the uncomfortable conclusion that I could not. A colleague of mine (Patricia Lewis) had been wondering about the temperatures of the molten core and whether we could any longer think of the integrity of the fuel. We had investigated already the melting point of reactor grade steel, the effect of heating on volumes and the possible viscosities of the molten fuel and the reported 13 “beams” of neutrons also added into our concerns; so we were aware that transient critical masses could not be ruled out. In addition, we were concerned about the basing the entire analysis on one reported measurement of Cl-38 but that amount of Cl-38 activity would have been a red flag to any physicist and I did not have a reason at the time to dispute it. A healthy type of skepticism in all of the measurement numbers coming from Fukushima is absolutely necessary, but, I share Peter Raffaele’s statement “What can go wrong will. And if you are not paranoid you are naïve”. What is the probability that the reactor will not go critical again, if only for an instant? If your answer to that question is well it’s not zero, then you are where I am. In emergency preparedness we have to stretch our mind a bit further than we might want to. We have to imagine the impossible even if it goes against every instinct until it is ruled out absolutely.

    Can We Believe the Cl-38 Number?

    TEPCO has made many measurement errors, from mixing up I-131 and I-134 to adjusting the numbers for Tc-99m etc. So it is absolutely reasonable to question whether TEPCO correctly measured the Cl-38. Red_Blue is correct that the start time of seawater injection was March 12th, not March 23rd as I surmised. However, this does not change the analysis much because of the equilibrium that is established due to production and decay. I whole-heartedly agree with Red_Blue and many others that conclusions could be drawn when TEPCO re-samples and measures the water. Although, that would only be true if there have been further high neutron flux incidents. If these had been transient criticalities over a given period of time, the short half-life of Cl-38 would mean that resampling would not necessarily help us.

    I also agree that a valid criticism of my analysis is the assumption that the seawater is pumped through the system in a continuous flow with a salt concentration identical to that of seawater. However, we can relax this assumption and take the maximum salt concentration (40 g/mL) which is a factor of 20 more salt/mL than my assumption. However, this is not large enough to account for the large Cl-38 concentration of 1.6 MBq/cm3. We are talking six orders of magnitude difference. That is quite a significant difference.

    I agree with Eve that TEPCO should have also monitored the Na-24 1.368 MeV and 2.754 MeV gammas. I don’t presume to know why they didn’t or why they didn’t redo the Cl-38 measurements considering that these should have triggered an alarm. I agree with JamesL that it would have been nice to be able to time-correlate the reported observation of the “neutron bursts” with other measurements and I would urge TEPCO to publish them. I would also recommend TEPCO to publish the time when all samples are taken as well as when they are measured so the time difference can be accounted for. However, I disagree with Red_Blue and Old Jim Hardy that there are many fission product gammas that the Cl-38 could have been confused with. The spectra for the two isotopes are very distinct: Cl-38 has two gammas a 1.64 MeV and 2.17 MeV whereas I-134 has prominent emissions at 0.847 MeV and 0.884 MeV and no significant gammas at higher energies (see INL Spectra Catalog at . Beta spectroscopy is a little bit more complicated but Cl-38 has a 56% probability high 4.92 Q-value whereas I-134 has a complicated scheme with the highest beta endpoint at 2.2 MeV and most betas < 1.2 MeV. However, I come back to where I started: if you are absolutely certain that the reactor won’t go critical then by all means dismiss the Cl-38 number. However, if you think the probability is non-zero then it is prudent to consider all possibilities since the consequences could be serious.

    With respect to my esteemed colleague, I have to disagree with Dr. Jim Rushton of Oak Ridge National Laboratory’s assertion “Even if they [inadvertent criticalities] did occur briefly, they would not add much radioactivity or heat beyond what workers are already dealing with from the radioactive material that accumulated when the reactors were running at full power (see New Scientist at . ” Neutrons affect human tissue so very differently than gammas. We know from other transient (and non-explosive) criticality accidents that people have died very quickly from large neutron bursts – even recently in Japan, there have been fatal consequences of criticality accidents.

    Consequences of Possible “Inadvertent Criticalities”

    So now let’s assume that “inadvertent criticalities” do occur in reactor #1. What could the consequences be and how might they manifest themselves? Many of you brain-stormed on this topic and I will reserve comments and summarize what you have all said across different blogs and fora. Damfuzzy reminds us of the fact that new BWR fuel assemblies are located on the refueling floor which at least in one Fukushima reactor is exposed to the atmosphere and may have been disrupted from explosions. He suggests the possibility that the “spontaneous fires” that have been reported may be due to criticality excursions. Roger T Crouch postulates that rod-collapse could lead to loose material where vibrations, water flow and structural collapse of the assembly grid and control rod systems could result in a self-sustaining chain reaction. Many of you have worried about the possibility of starting a chain reaction that becomes super-critical rather than turning itself off due to negative feedback effects. The main explanation for not going super critical is that – if transient criticalities have taken place – they are probably due to small globules of fissile material that is expanding and moving in a viscous soup of molten metals and oxides, thus continually changing their mean free path.

    What Must be Done Right Now?

    The purpose of the Cl-38 calculation was to exclude the possibility that an “inadvertent criticality” can occur at the Fukushima reactor #1 which I was unable to do. Therefore, it is prudent that TEPCO takes seriously the possibility of criticality excursions and monitors the neutron flux with independent neutron detectors close to the core. A sudden increase in the neutron flux would be immediately measurable above the background due to the spontaneous fission of the different actinides in the fuel. TEPCO must continue to mix Boric acid with the fresh cooling water to ensure that no criticality excursions can occur especially in reactor #1. All efforts must be made to protect the workers when the probability for “inadvertent criticalities” are non-zero. I suggest that TEPCO takes the following actions:

    – Install a neutron detector to monitor the core of Fukushima Daiichi reactor #1
    – Keep mixing neutron absorbers with the cooling water for cooling reactor cores and spent nuclear fuel ponds
    – Give complete gamma spectra rather than just the summaries of the results
    – Include not only sampling times but also measurement times for all measurements and repeat measurements to increase confidence in the results

    Let’s keep the conversation going. We owe it to all those heroic Fukushima Daiichi workers.

    • Spruce (History)

      The nuclides that you can confuse Cl-38 with are actually Co-58 and Co-60. They have weak peaks at the energies that Cl-38 would be, but at least according to my back-of-envelope calculations, there’s a plausible case for both being present in high enough concentration to explain the observation.

      However, the main reason I cannot take belive the recriticality based on that observation is the total lack of other fission products. Namely, if you think that TEPCO could’ve missed the amount of Na-24 that must’ve been present assuming recriticality, how on Earth could they recognice Cl-38 correctly? Thinking that there’s Cl-38 present when in reality it isn’t is much smaller error than missing missing Na-24 completely. Since TEPCO commited one of those errors, pure logic would suggest that the misidentification of Cl-38 is much more likely one. And that’s without considering other short-lived fission products. Every such missing product increases the likelyhood that it was the Cl-38 identification that was the error, not missing all of the other nuclides.

      I agree that the probability of recriticality is non-zero (it always is once you get a partial meltdown), but I have hard time seeing that the “Cl-38” observation changes that probability at all. It’s still minuscule – and knowing the non-minuscule risks that are still present at Fukushima, there are bigger and likelier risks present.

      But one thing I completely agree with is that it would be nice for TEPCO to release more complete data, especially the spectra. It should not be that hard. Or in any way damaging even if they were completely public. And I’m sure there are spectroscopy analysts who would love to spend days on analysing the spectra throughly, it’s not often (luckly) that you get an opportunity to analyse that kind of complex spectra.

  26. Dan Yurman (History)

    TEPCO says it made a mistake in the radiation reading for tellurium 129

    “At the site of Fukushima Daiichi Nuclear Power Station, as part of our investigation for the pathway and the volume of emission, which contains radioactive material, we have been conducting nuclide analysis for air inside the plant, seawater near the plant and water puddle in the turbine building. We have been informing you the result of the nuclide analysis. It is ascertained that the result of nuclide analysis of tellurium 129 (half life : about 70 minutes) conducted on March 30th, for water puddle collected near the trench and ground water collected near the turbine building are doubtful.”

  27. old jim hardy (History)

    Thank you Dr Veress

    indeed i too wanted to disprove it .

    So I plugged 1 second into your flux formula got a worrisome result 1.7E14, just about right for a 1 sec excursion to 200 % power.
    That should be a boisterous event that would steam clean the area where it happened and leave it in disarray.
    Please check my arithmetic for my sliderule is old and i’m in bifocals.

    re neutron monitors, after TMI US plants installed post-accident monitors including widerange excore neutron detection. I assume Tepco did same. Surely that system is up and running by now, provided the ion chambers survived the earthquakes and post accident environment.
    As an old plant guy i cannot imagine that plant operators didn’t have their portable battery powered neutron survey meters out after their station batteries ran down..
    They know for sure and the story will unfold

    and thank you for recognizing those heroes.

    old jim himself

  28. TokyoVP (History)

    I’m no nuclear expert, but I live in Tokyo and have followed the TEPCO live news conferences, and one week ago the VP mentioned the discovery of Cobalt 50 several times. This was not reported in any Japanese press (no surprise there). I hope this information will assist in your analysis.

  29. old jim hardy (History)

    Cobalt 50 is a strange material, it decays away with a halflife around 1/20 second so it’s unlikely to last long enough to measure.

    might he have meant Cobalt 60? That’s made in the reactor when an occasional natural cobalt59 atom picks up an extra neutron.. that’s why they use low cobalt steel for reactor parts, to minimize production of Co60.

  30. rockstarholic (History)

    This is a spirited debate, but I can’t help note feeling that something is being over looked. The simple fact that the result was released at all, without comment from TEPCO officials.

    If the Cl-38 is spurious: This implies that under-trained operators and analyst are certifying the results prior to release, unaware of the implications of the release. For the greatest nuclear disaster in Japanese history, in a nation famous the world over for their technical acumen, it is troubling to think that they might not fully grasp what is going on.

    If the Cl-38 result is valid: Much of what I said before could still be true or it’s that TEPCO’s omission of inadvertent criticalities speaks directly to their PR campaign to minimize the extent of the casualty at every turn. Not that I blame them, panic in the streets would suck for everyone.

    “Give complete gamma spectra rather than just the summaries of the results” This really would put most of these issues to rest.

    Either way you cut it the reactors are “operating” at the limits of their design envelopes, let’s hope the workers efforts keep them caged succeed.

  31. jeff (History)

    well, they remeasured and went from 5 to 7

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