FNPP1 reactor parameters

[updated 06/27  00:30]

Quick jumps to graphs


Here are the graphs depicting the evolution of the main parameters of Fukushima NPP1 reactors (unit1 to unit3) since the accident. You can find similar and additional interesting graphs on this other website from a Brazilian University, as well as, since 5/17, from TEPCO themselves. You may also find worth reading NISA’s own interpretation of what happened on the first stages of the accident.

Data Sources

  • NISA and METI press releases
  • TEPCO’s own website (EN, JP),
  • All data and graphics are available online at the following online Excel workbook, which I update more frequently than the graphs bellow. Most NISA/METI PRs can be downloaded here.

Evolution highlights

  • [04/14] Added “under investigation” tags to the graphs following NISA press releases. According to TEPCO they are simply not sure all indicators are working properly, and they are closely following a number of “suspect” ones.
  • [04/18] Added several new sensors as provided by TEPCO/NISA (noteworthy are those with PCV temperature info); Hope the graphs are still relatively easy to read, but I’m open for any suggestion so please don’t hesitate to comment.
  • [05/11] After gaining access to #1 Reactor Building and correcting RPV water level gauge A, TEPCO estimates the whole core had melted on the early stages of the accident. As of 6/27 a similar scenario is not ruled out on #2 and #3
  • [05/22] Added “water injection rate” graphs to help understanding other params evol.
  • [06/04] TEPCO installed a new temporary pressure measuring system on #1 RPV (details).
  • [06/21] Facing continuous problems with radioactive water purification facility, TEPCO decided to lower water injection on all 3 reactors in a bid to reduce storage filling pace.
  • [06/24] TEPCO gained access to #2 and installed pressure meas syst, corrected water level.

TEPCO’s recovery roadmap evolution

  • [04/17] 1st Version; 6-9 months to stabilize the situation ; don’t expect sub 100ºC core temp in the next 3 months
  • [05/17] 2nd Version; timeframe unchanged; countermeasures modified for 100% melt-down scenario.
  • [06/17] 3rd Version; no major changes from v2.0


Core Water Level

  • A and B are two different instrumentations
  • Core height is approximately 4.5m (here are the actual specs).
  • At the very beginning of the accident all 3 units spent some hours without any water on the core after losing their cooling systems. You can see more details on the evolution of the accident early stages in this PPT from Areva’s Dr Braun .
  • Since then all cores remain above half filled with water, which was sea-water at the beginning then progressively changed for fresh water. An early NRC report (thx Joe Wein) suggested half filled cores would indicate likely failure from recirculation pump seals, but as you can see most sensors are closer to 66% (normal value estimated by NRC) than 50%, so I wonder if that early assessment was accurate.
  • [5/11] After gaining access to the interior of n1, TEPCO operators said to have “corrected” #1-A gauge, which was found to be “down-scale” (i.e. core empty).  Since temperature is low all over the reactor the fuel is assumed to have completely melted down and reached the bottom of the RPV where it’s kept cool. As of 6/27 TEPCO keeps on repporting #1-B (half filled core) as well.
  • [6/06] Tepco is closely investigating the evolution of all water level gauges (except #1-A)
  • [6/22] Tepco gained access to #2 reactor building and tried to correct gauge level; corrected level [6/24] remains about 1/2 filled core, but TEPCO is still not sure of their “new calibration”.

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RPV Temperature

  • Properly cooled down reactor RPV temperature is ~ 20 to 30ºC.
  • “Cool shut down” is considered ~ bellow boiling point (100ºC at atm pres)
  • [6/09] After checking it, TEPCO has determined #3 FW-Nozzle is reliable again
  • [6/15] TEPCO believes all temperature readings are now reliable.

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RPV Pressure

  • Gauge pressure = Absolute Pressure – Atmospheric Pressure (atm pressure ~ 0.1MPa)
  • Properly cooled down reactor RPV pressure is ~ 0 to 0.1 MPa gauge
  • RPV normal operating pressure is ~ 7MPa, max. design pressure ~ 8.7 MPa
  • [06/04] TEPCO installed a new temporary measuring system on #1 RPV (details), showing a low pressure level, similar to that in #2 and #3. I removed previous data because it was obviously wrong; you can still find it here if you feel like it.
  • [06/24] TEPCO installed on #2 a temporary measuring system ~ #1.

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PCV Temperature

  • Data for PCV temperature started on 4/17 ; You can see the respective locations of each sensor here:
    • HVH: Heating Ventilating Handling Unit: ~ bottom part
    • Bellow Seal: ~ upper part

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PCV Pressure

  • Maximum DW design pressure is .528 MPa abs ;
  • Properly cooled down reactor PCV (DW or SC) pressure is just above atm pressure (~ 0.1 MPa abs)
  • Whenever pressure approaches design max. TEPCO is forced to “vent” to prevent any risk of containment failure.
  • Rise observed on #1 D/W after 4/7 is due to N2 injection to reduce the risk of H2 explosion within the PCV in case of RPV leak (IAEA, NISA). After a few days it seems pressure has been distributed between DW and SC

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CAMS absorbed dose monitoring

  • Containment Atmospheric Monitoring System (CAMS) measures H2 and O2 concentration, as well as gamma radioactivity, within the Dry Well (D/W) and Suppression Chamber (S/C).
  • You read it right, these are “Sieverts/h“, no “mili”, nor “micro”; keep in mind CAMS measures radioactivity within the containment: that is no place for people to go around
  • [04/08] Unit1 CAMS sudden increase happened just before instrument malfunction and it remains unclear whether it was part of such problem or an actual reading.
  • [04/17] TEPCO added many new sensors, but most of them have dubious readings (A & B are two different instrumentations measuring the same thing).
  • [06/09] TEPCO checked #2 SC(B) sensor and determined it was broken (→ removed from graphs; you can still see its pre-check evolution here if you want).
  • [06/15] TEPCO believes all SC sensors but #2-B are now reliable.

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Explanatory drawings


Here are some schematic drawings showing the relevant parts to understand the measurement points of the graphs bellow.


BWR RPV assembly (BWR4 vs. BWR6)

Reactor building main components (showing RPV, D/W, S/C)

You can find more details on the reactor building parts in this figure at wikipedia; I stripped it to show only the relevant parts for the graphs.

  1. James
    April 10, 2011 at 05:18

    Looks like something major happened on 04/08?

    I read the team were evacuated for 8.5 hours during this time!

    RPV failure?

    • April 11, 2011 at 00:35

      Updated the info today, but just in case what happened on late 4/7 is TEPCO started injecting Nitrogen on #1’s PCV; Here is NISA explanation (http://bit.ly/e5G6cB);

      About “evacuation”; AFAIK it wasn’t “all the workers”, nor “8.5hours”. On 4/7 23:32 largest replica since 3/11 quake hit Japan and a tsunami warning was issued on Tohoku area (including Fukushima prefecture). As a result TEPCO took 7 or 9 workers (forgot the exact number) that were on potentially dangerous areas to higher ground in administrative buildings. They said they would not return to their posts until the tsunami warning was lifted, which happened less than 2 hours after the quake.

      There were no changes on any plant parameter or radioactivity monitoring posts as a result of the quake, and both cooling and N2 injection were reported unaffected.

  2. April 12, 2011 at 05:54

    If I understand the core water level graph correctly, the water level remains well bellow the original top of the fuel in all three units?

    • April 12, 2011 at 14:09

      Yes, you got it right; Although obviously we are still far from a cold shut-down, as long as both temp. and pres. remain stable it’s not so much of a concern.

      Remember that on a perfectly cooled core, completely filled with water, RPV pressure is about atm pres. and temperature is bellow 100C (boiling point for water at atm pres).

      The fact that they aren’t full doesn’t mean they couldn’t fill them up, but that’s not necessarily a good idea right now. NRC already expressed concerns about the RPV condition after the accident. Pumping more water inside would add mechanical stress on the RPV, whose condition can’t be perfectly assessed right now.

    • April 15, 2011 at 14:44

      The NRC report states that the maximum water level coincides with the relative position of the water circulation pumps. They suspect that the seals of the pumps have failed so that any extra water would leak out at the pumps. That’s why they are unable to completely cover the fuel rods in units 1, 2 and 3.

      • April 15, 2011 at 15:06

        Thanks for the info; I’ll update the blog 🙂

  3. Roy Craft
    April 13, 2011 at 06:44

    On the CAMS “Containment Atmospheric Monitoring System” the units are in Sv/h. IS there some description of the system? I can understand the relative readings, but wondering about the total system, and use of Sv vs. Grays or evem Bq/unit.

    • April 13, 2011 at 09:06

      Sorry, I did a quick search before posting the graphs but didn’t find any specifics on CAMS; I’ll try to check later and update the post if I find it.

    • April 14, 2011 at 22:57

      I found some info on CAMS on this document from NRC (p246). Here are some bits of what you can read there:
      – “The CAM system provides the ability to monitor hydrogen, oxygen, and gross gamma radiation
      levels in the containment following a LOCA, and provides necessary indication and trip signals”.
      – “During CAM system operation, containment atmosphere is withdrawn through piping connected
      to primary containment penetrations for obtaining both a drywell and suppression chamber air

      Also, here you have the website of a CAMS manufacturer (link)

  4. Roy Craft
    April 13, 2011 at 10:34


  5. John Doe
    April 13, 2011 at 15:21

    Does the water level graph mean the reactors were completely dry before 3/15? Or just that no data were available from that time?

    • April 13, 2011 at 16:36

      First available data from NISA shows both n1 and n2 water levels were “downscale”, meaning gauges reached their bottom. It is my understanding that both cores had lost all water (though most likely steam was still being produced) after repported lose of cooling capability.

      You can also see in this presentation from AREVA(p19) how estimated time without water in the cores was:

      • – 27h on unit 1
      • – 7h on unit 2
      • – 7h on unit 3

      Accordingly estimated core damage is higher for unit1, which is likely to be one of the reasons why it’s proving to be the most difficult to cool down.

  6. April 15, 2011 at 16:02

    You have done a fine job in presenting the data, but as long as you do not have more detailed information on the location and type of the transducers, it is very hard to interpret the data.
    The data remind me of the TMI data in 1979, when we had also a hard time to tell what went wrong.
    Basically I have the feeling that the reactor cores will look like the one in TMI.
    I think that by now the accident is over and the reactors and the containments will be cooled by natural heat conduction without heat removal by pumps.
    I am still very concerned about radiolysis in the torus, where explosive pockets of hydrogen/oxygen may accumulate.

    • April 15, 2011 at 16:15

      Thanks for the compliment.
      That said I’m certainly not knowledgeable enough myself to interpret what’s going on in detail (with or without extra info).
      One of those H2 pockets in the torus (S/C) may have been the reason for the early explosion heard on unit2, believed to have damaged it (still not confirmed).
      I wonder if H2/O2 reading capabilities of CAMS are still working… ; guess they should since gamma measures do, but I haven’t seen any data.

  7. Bill Dorland
    April 15, 2011 at 19:23

    I really, really appreciate this website — thank you.

    Today in Kyodo there is a description of an analysis of the state of the melted fuel in the RPV(s). The reporting is not very clear and the conclusions, if correct, seem to apply only to units 2-3 as opposed to 1 and 4. The conclusions are that the fuel probably granulated and spread out at the bottom of the RPV as the corium fell back into the cooling water. However, in Unit 1, there would have been an extra 20 hours for the corium to drip down without this effect. Hence, the additional trouble you cite for Unit 1 could be associated with a more condensed blob of corium at the bottom of the RPV. Similarly, the fire in the SFP of Unit 4 was very likely associated with cladding oxidation and subsequent fuel melt. Has anyone seen an estimate for how long the Unit 4 reactor core (which is in the SFP) might have been without water?

    Thanks again.

    • April 16, 2011 at 20:29

      As you may read on the Areva presentation cited, unit1 spent 27h without water (vs. about 7h for unit2 and 3); as such it is expected that its core was considerably more damaged so for sure there are more bits of fuel at its bottom.

      AFAIK no precise info on unit4 spent fuel pool, but I think it was never emptied as such; recent TEPCO analysis of the water in the pool indicates little fuel damage according to NISA (live PR on NHK).

  8. April 15, 2011 at 22:58

    For reference, here’s a quote and the source URL where I found the NRC assessment:

    Based on the reports of RPV level at one half core height, the reactor vessel water level is believed to be even with the level of the recirculalion pump seals, implying these also have failed.


    • April 18, 2011 at 11:01

      Thanks again for your contribution; added the NRC document to the article body

  9. brc
    April 16, 2011 at 14:09

    What software did you use to create the graph images?

    — Guy who creates a lot of graphs at work

    • April 16, 2011 at 20:31

      Excel 2010, you can see the files and graphs on the online files (and download them…); some graphical elements do not show up while viewing with Excel web app.

  10. April 16, 2011 at 21:23

    I am not as optimistic as the Japanese experts as far the amount of molten core in the lower head is concerned. In TMI the core was uncovered for about two hours and in Fukushima much longer. In TMI 20 tons relocated to the lower head.
    Once the molten material has relocated to the lower head, chances are much higher that the melt progess will stop here as a forest of pipes in the lower head provides a substantial additional cooling.
    I have some doubts concerning the loop seal failure. The reactors I know in detail have a system to recirculate the loop seal leackage back to the cooling circuit. So, the leackage does not go into the containment.

  11. CRV9
    April 17, 2011 at 01:06

    Hi Danny-san,
    I’m so confused. There are so many different info out there. I have a few questions I’d like to ask you.
    1. Areva said #1 CCV is flooded with water, (NYT article said #2). Is it?
    2. All bottom temps are above 100c, though pressures are alittle above 1 atm? Are they boiling temps?
    3. Are seconderly cooling systems, seawater to cool fresh water from PCV, working for #1,2,3, and SFPs? I’m really confused on this one. I don’t think they are. I heard they’ve working on outside cooling system setups?

    When you have a chance. I’d appreciate it. I trust you. Regard.

    • April 17, 2011 at 22:56


      The secondary cooling systems are not working for either the reactors or the spent fuel pools because the corresponding pumps are not working. Without the pumps no heat will get removed in the heat exchangers.

      I presume all injected water is still providing cooling through boiling, but am not sure where the steam ends up (or any leaks for that matter). Is the containment filling with condensate? Last I heard they were injecting 6 cubic metres of fresh water per hour from a nearby dam via temporary electric pumps into unit 1 and 7 cubic metres per hour into units 2 and 3, which have more fuel rods than unit 1.

      Where does that water go, in the absence of a proper cooling circuit? There seems to be too little discussion about that.

      • April 18, 2011 at 11:25

        You know, I’ve been wondering about your final question for days; still couldn’t find an answer. Obviously some of it is going to the PCV (D/W and S/C), but the pumping rate is too high … The only guess I could make in the absence of any other info is that they are releasing the steam outside

    • April 18, 2011 at 11:15

      Hi again CRV9;

      1) #2 PCV cannot be filled with water because by now we are almost sure there is a leak on S/C. Most of the radioactive water that has leaked comes from it, but they had no choice as cooling the RPV is the top priority; if you check TEPCO’s countermeasure plan you’ll see this is being taken into account (i.e. cooling RPV with minimum water input);

      2) Yes, water in all RPVs is boiling at the top of the cores; liquid at the bottom. Remember though these are “Boiling Water Reactors”, and that’s part of the heat exchange process.

      3) So far all proper cooling (forced circulation) systems on the reactors are broken or unavailable. TEPCO is basically pumping water into the RPV/PCV through the feedwater line using external pumps. Same for spent fuel pool at #4 (others have very old fuel with very limited calorific power and pose no problem).

      PS: Being Japanese you have the luxury of understanding JP media; the best advise I could give you is “watch NHK and stop reading foreign stuff”.

  12. April 17, 2011 at 23:45

    Bill Dorland :
    Has anyone seen an estimate for how long the Unit 4 reactor core (which is in the SFP) might have been without water?
    Thanks again.

    Unit 4 was not operating when the quake struck. All its fuel had been unloaded some time ago. Its core is *empty*.

    The previously unloaded fuel elements are in the spent fuel pool, which sits on top of the containment. It is the spent fuel pool that all worries about unit 4 are about. The recent analysis of pool water suggests that damage to the rods in the pool is relatively minor. As long as Tepco can keep the pool topped up via the concrete pump truck for a couple of months (which assumes no major high level radiation leaks from other units) and there won’t be any fresh major earthquake damage to the unit, they should be able to stabilize unit 4.

  13. April 18, 2011 at 05:06

    If the water in the spend fuel pond is borated then the content of each damaged fuel rod will be dissolved in the boric acid water and reduce the shielding effect of the water on top of the fuel elements. After the refill it would be interesting to learn the radiation level on top of the spend fuel pond. If it is high, then there is a lot of damage.
    On the surveillance cam pictures you do not see a lot of steam and the temperatures measured indicate that the crew has installed closed cooling circuits.
    In the reactors that I know in detail they have mobile power generators, pumps and heat exchangers ready to connect to the reactor systems.

  14. alex
    April 18, 2011 at 05:11


    May I ask a question regarding the current situation in unit 1 – 3.

    Several people think we have a layor of salt in the bottom of the reactors and the corium is burning through it. From a physical standpoint of view its a realistic picture, because Sodium Cloride has a low melting point and it acts as an efficient thermal insulating material. What do you think?


    • April 18, 2011 at 11:41

      Again, I’m no expert, but indeed NRC (check the report link I added) expects salt accumulation to be an important issue hampering proper core cooling (especially at #1). Hope the situation improves with the continuous influx of fresh water; either way it’s encouraging to see temp is decreasing and pres. seemingly under control in the past few days.

      PS: Obviously it was a tough decision to use sea-water; yet TEPCO had nothing better at the time and haven’t heard any expert voice questioning that move.

  15. alex
    April 18, 2011 at 16:07

    Hi, Thanks for your answer, and thanks for monitoring the data, a nice piece of work. You might have seen the group in a Brazilian university doing the same and they have added recently new data for radioactivity. For unit 2 and 3 they gave values which are significantly higher than before.


    In Germany we have some people being highly concerned about the status of the plant at Fukushima. And using the WWW we are trying to figure out what is going on in the reactors. Our picture is a Corium wich is melting through a layor of salt.

    PS: It is not our intention to claim anything or to say something was done wrong in the past. It is a pure sientific interest which drives us.

    • April 18, 2011 at 19:12

      Yeah, I posted that link this (japanese) morning on top of my “article”; I specially like their plots relating for example pressure at RPV vs. PCV, and the “liquid/gas” phase ones.

      Also tweeted yesterday on the new values (which are on the Excel file), but I don’t have time to update the website more than once a day; I’ll make some “major” changes tonight when including the new available sensors.

      PS: I don’t think it’s possible that damaged assemblies gathered at the bottom of the RPV can be melting (with or without salt) with the current registered temperatures (~120º at the bottom head).

  16. April 18, 2011 at 19:34

    I am not really concerned about the salt, because it will mix well with the corium. I do not believe that the salt from the sea water has form a pure salt layer anywhere.
    Do not get confused by the properties of pure material, we are dealing with mixtures (solid solutions)!

    The time span during which enough decay heat was produced to melt through the vessel wall is over by now.

  17. April 18, 2011 at 22:36

    Christoph Müller :
    On the surveillance cam pictures you do not see a lot of steam and the temperatures measured indicate that the crew has installed closed cooling circuits.

    According to:

    as of April 18 the spent fuel pool cooling in units 1-4 is still happening via “water spray” or “water injection”, i.e. evaporation. The most recent figures for temperatures that I’ve seen are below the boiling point but higher than they would be if sea water cooling via a heat exchanger was available.

  18. Alex
    April 18, 2011 at 23:03

    Hi thanks for your answers.

    Yes we have mixed materials in front of us, no doubt, which makes the whole thing even more complex. The decay heat based on Way et al. is based on pure material either indicating 3MW, which is still a lot. So small particles could melt slowly through the salt without being measured by any sensor, the salt even the mixture of sea salt acts as an isolator. Why should salt, mainly NaCl, mix up with hot metals? Do you have any data or phase diagram?

    Let´s cross our fingers but I guess we are far away of any safe status.

  19. CRV9
    April 19, 2011 at 03:13

    Thanks for the replies.
    A stupid question if you don’t mind. The decay heat, it’s kind of a heat/time thing? You have to excuse my wording. I worry but I don’t know this nuclear stuff. Doest it matter if you keep the cooling water at 90c or 40c? Any difference in time to reach the cold shut-down? If cooler would it reach faster? Once it reachs the cold shut-down(?), you don’t have to cool it? I’d think natural decaying is still going on but not that much? Sorry for my ignorance, too.

    • April 19, 2011 at 08:47

      Adding on Alex post on decay heat.
      The water used to cool #FNPP1 reactors is for sure at ambient/sea water temperature (20-30ºC, maybe a bit colder right now), and if the plant cooling decay heat removal system was working properly it’d be about the same.
      – Injecting hotter water would decrease its capacity to remove heat from the fuel.
      – If core is hotter than that it’s because there is not sufficient water flow through it. Simply said, we refer to a “cold shut-down” the situation when core temperature is kept at a similar level to the water being injected, with all the decay heat being taken care of.

      Hope that helped.

  20. Alex
    April 19, 2011 at 04:27


    there are no stupid questions in general!

    The decay heat is based on the radioactive by-products which are formed during the fission reaction. The decay heat depends on the running time, capacity etc., time after you had stopped the fisson and you cannot influence it by larger cooling. It is one of the most critical issues for NPS. From a math point of view it is not a linear function and the value seems to be very low, but it is still there and after a year we are talking about 0.6 MW. More information could be found in wikipedia


  21. April 19, 2011 at 07:48

    Hi, congratulations for the plots!

    Two comments on the above posts:

    * From the little I know of chemistry, boric acid is a very weak acid (it was formerly sold in pharmacies for use in antiseptic eye drops). I don’t think it can dissolve any nuclear materials, even at boiling temperatures. Moreover most borates are insoluble (alkali metals being exceptions), so uranium borate would probably collect at the bottom anyway, like uranium oxide.

    * Molten sodium borate (borax), on the other hand, does dissolve most metal oxides very well. It may be formed from boric acid and molten salt.

    * I do not know whether molten salt by itself can dissolve uranium or zirconium oxides. I would bet it doesn’t, so it may not get incorporated into the corium but merely create a separate layer over it. Moreover it melts at a rather low temperature (800C) and boils at 1400 C, before steel melts (1500C) and well before zirconium melts (1800C).

  22. CRV9
    April 19, 2011 at 21:02

    Thanks again, not that I understand though. My knowlegde of radiation was from watching from “Godzilla” when I was a kid. The message was that if you were too casual with radiation it will become a monstor and come back to haunt you. It was a sentiment from Japan’s experiences from Hiroshima and Nagasaki.
    I was an impressionable child.

  23. April 20, 2011 at 21:48

    dgr4quake :

    You know, I’ve been wondering about your final question for days; still couldn’t find an answer. Obviously some of it is going to the PCV (D/W and S/C), but the pumping rate is too high … The only guess I could make in the absence of any other info is that they are releasing the steam outside

    By now I’m pretty much convinced that most of that water being evaporated is what’s filling up Turbine Buildings, etc. Injecting about 7 m3/h on 3 reactors for a month makes a about 15k tones; TEPCO has about 67k tones of water around the buildings so there is more than room enough for all the pumped water to be included there.

    Another indication: water level of trench under unit2 is constantly rising, and the pace is … well, a bit less than 10 tones/h 🙂

  24. April 23, 2011 at 03:53

    1 MW is equivalent to the evaporation of 0.5 liter per second. 7 m3/h per reactor makes about 2 liters/second or 4 MW per reactor and 5000 m3 per reactor in one month.
    The containment is loosing about 4 MW by natural heat conduction. If this water is not recirculated and does not leak from the containment there must be a pressure build up inside the containment which had originally a free space of about 10.000 m3 (+/-) initially filled with nitrogen. This has to happen, if you put into the containment 5.000 m3 of water. There is no indication for the pressure build up in the plots.

  25. April 23, 2011 at 04:04

    If you have a substantial core damage, the containment is filled with radioactive fission products. In case there is a leak in the containment, these fission products would be permanently leaking to the environment and you would see as permanent substantial radioactive release, which is not the case.
    If you argue that you do not have a substantial core damage, you have to explain where all the hydrogen came from.

  26. CRV9
    April 24, 2011 at 03:22

    I have another question if you don’t mind. There seems to be some experts here. Specially ones from Germany, We almost could have ruled the would.

    How things get radioactive if it ever? Could humans get to be radioactive after one got exposed to radiation? I thought that you need radioactive matters or isotopes to give off radiation. It may sound stupid to you but I need to know.
    People from Fukushima are already discriminated, even kids in kinergarten. People are afraid they might be radioactive.

    • April 24, 2011 at 09:22

      Things can get radioactive mostly from neutron radiation. The primary source of neutron radiation in a reactor is an ongoing chain reaction. This was stopped when the reactors where “scrammed” (emergency stopped) when the quake separated them from the grid. Since then the injection of boron along with the sea water should have prevented any chain reaction from resuming.

      Most of the radiation one has to worry about in the current situation is gamma radiation, which is like a harder (more energetic) version of X-rays. It does not cause secondary radioactivity in other materials.

      Most of the radioisotopes in nuclear waste undergo beta decay, which releases gamma along with beta radiation. A smaller fraction releases alpha particles, but these tend to be the less volatile materials that don’t escape as easily. Gamma is more penetrating than either alpha or beta radiation.

      The main risk is contamination, where small particles (dust, condensate) adhere to clothing, skin, etc or get ingested or inhaled into the body. You become radioactive because of radioactive things attaching themselves to you. Exterior contamination can be dealt with by showering and washing or changing clothes. Interior contamination is a danger to yourself but not to others.

      • CRV9
        April 24, 2011 at 10:34

        When you say things, dose that mean regular things? Not radioactive materials, like ulanium? Can nuetron make regular stable materials radioactive? Can nuetron make water radioactive? H2O is stable nucleus. To make it radioactive means what? Sorry for my ignorance.
        I am old but I feel like a 10 year old boy.

  27. Alex
    April 24, 2011 at 03:30


    May I could ask a question regarding the isotope distribution of Cs and 131I. 131I is found more or less in a constant ratio to Cs in sea water samples since weeks. 131I has a half life time of 8 days, Cs of years. Does somebody have any idea to explain this strange behavior?


    • April 24, 2011 at 09:10

      That would indeed be odd. Do you have sample numbers?

      The ratios could shift (other than through faster decay of I-131 vs Cs-134 and Cs-137) if the source of the pollution shifts over time. For example, older spent fuel in the unit 1, 2 and 3 spent fuel pool has less Iodine relative to Cesium, fresher spent fuel in the unit 4 pool has more and cores inside 1, 2 and 3 have the most.

  28. April 24, 2011 at 04:09

    There are no stupid questions. You are right. Radition, especially neutrons can turn a stable atom into a radioactive atom. This is what is happening with the oxygen in the coolant water. The same would happen with the water in a human being.
    BUT: the main concern is contamination. Some radioactive atoms may be on a worker after workig in the plant. This radioactivity has to be removed (decontamination) before this worker leaves the controlled area.

    • CRV9
      April 24, 2011 at 09:34

      Sorry, I didn’t understand what you mean by “nuetrons can turn a stable atom into a radioactive atom” and “This radioactivity has to be removed (decontamination) before this worker leaves the controlled area.”.t?

      Change a atom to what kind of radioactive isotpoes? It has to change it into some kind of radioactive isotopes?
      If it’s in atoms how can you remove(decontaminate)?

  29. CRV9
    April 24, 2011 at 09:25

    I have another question, again. According to Yomiuri news, #4 SFP’s water temp is about 90c. Cooling is done by evaporation of the water, so they said.
    My question is that evaporation is like distilling water? I know there are some radioactive isotopes which are water soluble but salt is soluble, too. Yet you can separate water from salt.

    Vapor from the water of #4 SFP is radioactive? Are there any radioactive isotopes in the vapor? The vapor can still be radioative?
    I’ll thank you for your help.

  30. April 24, 2011 at 11:21

    The evaporation from the pool is quite major. Here are the amounts of water added to the pool recently:

    21 Apr 2011: 100 m3
    19 Apr 2011: 40 m3
    17 Apr 2011: 140 m3
    12 Apr 2011: 195 m3

    The total pool volume is about 1800 m3 if I recall correctly. Here are the figures for radioactivity in the unit 4 pool water as they were on April 12:

    Caesium 137: 93,000 Bq/l (half life: 30 years)
    Caesium 134: 88,000 Bq/l (half life: 2 years)
    Iodine 131: 220,000 Bq/l (half life: 8 days)

    Multiply those figures by 1000 to get becquerel per m3. Since this was 12 days ago or 1.5 half lives the Iodine-131 levels should have dropped to about 35% of that value by now, or 77,000 Bq/l, assuming no extra radioactivity had been leached from the damaged fuel rods.

    The bulk of the radioactivity should stay behind when the water evaporates, just as salt water that evaporates gets more and more salty, with mostly water vapour escaping.

    There would probably be more air pollution if the water was boiling because when steam bubbles burst at the surface, some liquid is always thrown up into the air which could create a fine dust of previously dissolved minerals, some of which could be carried away by wind.

    Lowering the temperature of the spent fuel pool water by restarting the heat removal system or installing a new heat exchanger operated with sea water would help settle these worries as well as reducing the need for the continued large top-ups of pool water. Depending how hard the fresh water from the nearby dam is it could leave the fuel rods caked in limestone or calcium borate, which could lead to them getting hotter.

  31. April 24, 2011 at 11:50

    Neutron radiation can make naturally occurring stable elements like nitrogen or hydrogen radioactive. Neutrons can turn heavy hydrogen (deuterium) contained in water into tritium, which decays with beta decay with a half-life of 12 years. Neutrons can turn oxygen from water into radioactive nitrogen (N-16), which decays with beta decay back into table oxygen.

    Only relatively small amounts of these radioactive isotopes form that way in the reactor. They are far outweighed by fission products like Iodine and Cesium, the two halves of split uranium and plutonium atoms, which get generated in vastly larger quantities, and their decay products.

    The real problem now is not the spread of radiation (i.e. radiation making non-radioactive substances radioactive), but the spread of radioactive substances themselves, such as with wind or water or people coming into contact with radioactive dust.

    • April 24, 2011 at 11:52

      Joe Wein :
      …which decays with beta decay back into table oxygen.

      Sorry for the typo, that was meant to be “stable oxygen”.

  32. April 24, 2011 at 16:56

    In severe accident analysis there is a rule of thumb, that about one third of the fission products are volatile. If the fuel rods fail these fission products are released to the atmosphere. All other fission products will stay in the pool though the fuel rods have been damaged.
    But a very small amount of the medium and low volatile fission products will be continuously released. Just as it is on the sea side, due to the salty water the air smells salty.

  33. April 24, 2011 at 18:28

    I just did a quick calculation of the pumping hours and water tonnage for the unit 4 pool. Unfortunately the data is incomplete. Sometimes the pumping hours are listed, sometimes the amount in tons, sometimes both, sometimes only the starting time. Earlier on the pool cleaning system was also used for pumping, with no water amounts listed.

    It appears that over the past five weeks that the Putzmeister concrete truck has been in use there, at least 1200 m3 of water have been added to the pool. Once they didn’t pump for a week and level dropped 5 m below where it should have been.

    Spread over 24 hours, the pumping averages out at 1600 litres per hour or half a litre per second, which according to Christoph Müller is the equivalent of 1 MW of decay heat. It’s about a quarter of the feed into the RPVs of units 1-3.

  34. CRV9
    April 24, 2011 at 23:12

    Thank you, guys for your help, kindness and patience.
    Hey, your brains and my ignorance, we could try to rule the world, again.

  35. CRV9
    April 25, 2011 at 00:18

    According to Mainichi news on 23rd, there already are water in #1 CCV, up to 6m. TEPCO speculated that The steam from RPV had been acumilated in there. They have pumped ~7,000 tons of water into the #1 RPV. They don’t know if they are going to flood it all way up to 11.8m or the water level would rise when more water are added to the RPV.

    #4 SFP, from 22nd to 23rd, they added 280 tons of water to raise the water level to 4m above the SF and lowered temp to 66c from 91c.

    Hope this helps.

  36. April 25, 2011 at 01:30

    From the webcam you can see a lot of steam from block 4 in the early morning hours. This spend fuel pond is definitely cooled by evaporation. The other 3 SPF do not produce so much heat, so the webcam does not give any information.

    I still have a problem in understanding the cooling of the three reactors. As I said before, if they are only feeding water into the RPV which is evaporated and then condensed in the water or the wetwell, the rise of the water level in the containment should compress the remaining containment atmosphere and there should be a pressure increase inside the drywell. This cannot disappear by the natural leak rate of the containment, which is 0.3% per day at 0.4MPa (gauge). If the leakage is larger, there has to be a permanent release of radioactivity.
    The only good idea I have is that they have blown out a substantial amount of the nitrogen during controlled containment venting and now the atmopshere consists mainly of steam. In this case they can run into problems with vacuum in the drywell.
    Does anybody have a better idea?

    • CRV9
      April 25, 2011 at 03:04

      TEPCO knows there is bleach with #1 PCV because the pressure didn’t rise as much as supposed be when Nitrogen was injected. That is the reason why they don’t want to flood the PCV fully, yet. Their plan is to set up a heat-exchnger for PCV cooling water.
      If they did that now with 5m of water in PCV, would that be enough to keep RPV somewhat cool enough, say 60-70c and 25~30c in PCV? You could also use cooled water from the heat-exchanger to RPV. The thing you don’t want is that you don’t want mix two water RPV’s and PCV’s because the water in RPV must be really contaminated. But vapour from RPV and condesended to PCV would be better, less contamination?

  37. April 25, 2011 at 02:11
  38. Alex
    April 25, 2011 at 20:39


    Thanks for the link!

    Planned installation of heath-exchangers / desalting units turns me positive to avoid massiv contamination of the sea. A more precise time scenario would be great to see, because all these installations needs time and a lot of water will be produced in parallel.

    Just a quick question regarding politics. Within Germany approx. 70 – 80% of the people want to leave the NP-technology and local politicans are picking this up quickly. As a consequence further local activities might be a result. Those activities are influencing the cost position of any business and we have a global market now. A question could be, is it the time to say good bye to NP-technology on a global perspective? And in case yes, how to achieve it best?

    • May 15, 2011 at 18:28

      You know, I’m sure everyone here (people, TEPCO, gov) would like to have a more precise time scenario … yet for doing that we would need a “more precise idea of the actual status of the plant”, which at this point is still pretty far away. Don’t be surprised to see schedules updated several times in the months to come.

  39. April 25, 2011 at 21:11

    Alex :
    Planned installation of heat-exchangers / desalting units turns me positive to avoid massiv contamination of the sea.

    It will take quite a while just to remove enough highly radioactive debris around the units using remote controlled equipment.

    I heard the remote controlled bulldozers / power shovels need someone with a remote control unit to stand in the vicinity, which is hardly ideal. There were some bits near unit 3 that radiated at 900 mSv/h. First such hot spots need to be taken care of, before any other kind of construction work can take place.

    Radiation inside the reactors is quite high, about 50 mSv/h. That’s far too hot to spend much time doing construction work, such as setting up heat exchangers.

    So far all the cooling water injection work has been done from inside the turbine hall next door, as far as I know. Setting up a heat exchanger inside the reactor building would be a huge challenge because of radiation levels and you can’t get inside with a crane or other heavy equipment.

    I read that Tepco was planning to have the heat exchangers and filters quite far from the reactor to make them safer to operate for the crews and that for that they wanted to use 2 out of 5 existing pipes (?) that lead into the reactor.

    Any duct work they do would have to be able to withstand any expected quakes for a few years.

    Cooling work would be a lot easier at the spent fuel pools because they are a lot more accessible from the outside (i.e. unprotected). The major construction work in Chernobyl was done using remote operated cranes and concrete pump trucks with lead-armored cabins.

  40. April 26, 2011 at 04:10

    I have the impression that TEPCO intends to start the shutdown heat exchangers which are inside the reactor building on level 3. As TEPCO has been waiting so long before taking the necessary accident management measures the reactor building is more contaminated than in the first days due to the containment leak rate.

  41. CRV9
    April 26, 2011 at 20:23

    According to Yomiuri news on 26th, TEPCO will try to flood #1 PCV.
    On 26th they will use two robots to check possible leaks and damages, on 27th they will inject 10ton/h of water for first, then to 14ton/h to see the integrity of the contaiment. If everything looks good, after checking pressure, water level and using robots to see leaks and damages,they will flood it.
    However, NISA is still afraid of water leaks.

  42. CRV9
    April 26, 2011 at 20:39

    According to Yahoo Japan, 26th PM, TEPCO plans to flood the #1 PCV from 27th, because they did not see any visual signs of leaks from two robots.

  43. April 27, 2011 at 00:23

    This is the same containment vessel for which in 1992 Tepco falsified leak tests. It was pressurized with 3 bar of nitrogen and then checked to ensure pressure wouldn’t drop by more than 0.25% per day. Since it dropped by as much as 2.5% per day, Tepco would secretly inject air using a feed pipe and a compressor to balance the leakage while the government inspectors were testing. That scandal blew up in 2002, but with no lasting consequence.

  44. CRV9
    April 28, 2011 at 23:28

    According to Yomiuri on 28th, TEPCO continues to inject water to #! PCV at 10ton/h. They are seeing decreasing of pressure, 1.2atm and temp, 96.8c in RPV. They will continue it until the pressure reaches at ~1.1atm so that air would not come into to mix with Hydrogen. They are not seeing any increas of watre in the Turbine building’s basement. So they suspect there are no leaks.

    Tepco said that the Hydrogen explosion might had prevented the melt-down from #4 SFP. The explosion had damaged the side gate of #4 pool and water from (?, sorry i didn’t queit figure out exactly where, a pool at the other side or reactor-well or RPV? were there any water in RPV?) poured into #4 SFP to cool the rods.

    The evapolation from the pool is estimated at ~70 ton/day. There is no water leaking into PCV but leaking into the reactor-well(?) is suspected.
    Hope this helps.

  45. April 28, 2011 at 23:51

    CRV9 :
    The explosion had damaged the side gate of #4 pool and water from (?, sorry i didn’t queit figure out exactly where, a pool at the other side or reactor-well or RPV? were there any water in RPV?) poured into #4 SFP to cool the rods.

    That gate is used when moving fuel between the reactor core and the SFP, i.e. during refueling and unloading of the core. The fuel rods stay under water while being moved, to keep down the gamma radiation.

    The broken gate allowed water from above the top of the empty RPV to aid in cooling.

    The concrete pump truck can supply as much as 50 t per hour, so it should be able to keep up with a smaller leak, but if too much water runs off into the bottom of the building that could make the walls seismically vulnerable, which is not a good position to be in.

  46. May 1, 2011 at 01:10

    The strange thing about block 4 is that no explosion noise was reported. When the dawn came on March 15,the destroyed roof was shown by the webcam at 6:10. That was the first sign of the damage.

    My feeling is that the side gate failed for some reason or other and the reflooding of the spend fuel pool caused a steam explosion which distroyed the roof. This steam explosion does not produce a loud noise but a substantial pressure build up. A minor hydrogen explosion which distroyed the gate only may not have been detected as long as the secondary containment was undamaged.

  47. May 6, 2011 at 11:56

    Christoph Müller :
    The strange thing about block 4 is that no explosion noise was reported. When the dawn came on March 15,the destroyed roof was shown by the webcam at 6:10. That was the first sign of the damage.

    Do you have any source for that? It’s not what the TEPCO press release said:

    Press Release (Mar 15,2011)
    Damage to the Unit 4 Nuclear Reactor Building at Fukushima Dai-ichi Nuclear Power Station

    At approximately 6:00am, a loud explosion was heard from within the
    power station. Afterwards, it was confirmed that the 4th floor rooftop
    area of the Unit 4 Nuclear Reactor Building had sustained damage.


  48. May 7, 2011 at 18:32

    The temperature of the atmosphere inside the dry well of unit 3 and to a smaller extent the RPV has been rising, for example:

    May 6, 06:00:

    April 29, 13:00:

    Compare the fields in the “temperature related to reactor pressure vessel (RPV” and “D/W atmosphere temperature” rows for unit 3.

    The date can be found in the daily “Fukushima Dai-ichi Nuclear Power Station Major Parameters of the Plant” documents, indexed under “Seismic Damage Information (the xxxth Release)” at:

    An increase of over 100C (120C->238C) seems significant. The dry well atmosphere has been hotter before, for example 240C on April 18:

    Anyway, why is the dry well atmosphere hotter than the RPV? Is it because the RPV is cooled by water from below and through heat conduction, but the steam/nitrogen touching the exposed upper half of the fuel rods is not cooled as much?

  49. melt down most likely for unit 1
    May 14, 2011 at 00:20


    Let us assume all 3 reactors are broken, because they got not filled since weeks. In case this is true than a single information is missing – how much Plutonium is involved already.

  50. CRV9
    May 14, 2011 at 05:47

    I have been troubling last a couple of days. It turned out Areva was right on #1 unit being melted down through its RPV and PCV was flooded with water. They did this assessment a week or two after the accident. They had the same data as everyone else. Yet, many nuclear experts in Japan down played it. They must have know Areva’s assessment was at least probable.

    Isn’t this science? Melting through RPV or not, isn’t it a big difference? Were scientists not telling everything to public eventhough they knew? Do they know enough to have similer assessment with the same data? This makes me very uneasy.
    This is an ignorant’s rant.

    • May 15, 2011 at 18:39

      You may have got something wrong; Areva never predicted the full core to be molten, just that some of it did melt. That amount has been considered to be over 50% for unit1 since the beginning, though never fully molten since it was supposed to be “half filled” with water.

      Now I still don’t understand what’s up with that “corrected A” gauge, and why they keep reporting the “B” as well … Still read over and over again that “they believe”, never that “they know”.

      Finally, the problem with “science” and “scientists” is that normal people are taught in a way that 2+2=4 (i.e. there is always an exact and known answer to every problem), while in real life “we never know”, we just “guess”. Some wiser better informed people make “more likely guesses” than others, but they are not necessarily those getting it right at the end.

  51. Melt down
    May 15, 2011 at 01:52


    Just read this, and you know whats up. I waiting now for data on Plutonium contamination in the sea, nothing else to comment.

  52. May 15, 2011 at 17:40

    TEPCO has increased the amount of water it’s pumping into unit 3 from 7 m3/h to 15 m3/h. They say they’re switching from the fire extinction system to the feed water line, pumping through both in parallel right now. I wonder if this is part of their cooling system plan.

    In unit 1 the assessment has gone from 70% core damage to 55% damage to maybe 100%. The latter number has not been explicitly stated, but it’s the only logical conclusion one can draw from a water level at least 5 m below the upper end of the 4 m tall fuel rod assemblies.

    The big question is going to be about the state of the containment in all three units. TEPCO has stated that they need at least 5 m of water in the dry well to be able to install a new heat removal system. If too much water leaks to be able to achieve that level then they’ll have to come up with a new idea. For now the external work for installing the new cooling system for unit 1 seems to continue. TEPCO say they have no idea how high the current water level is in the unit 1 and they don’t seem to trust units 2 and 3 to still hold pressure.

    I am also curious how this is going to work in unit 2 with its damaged suppression chamber. Won’t the water leak out of the dry well into the S/C and from there wherever the pressure that ruptured the S/C walls escaped? How can that be fixed, especially given the radiation levels in unit 2?

    It’s surprising to me that one hears so little about the spent fuel pools in unit 1, 2 and 3, especially given the severe damage to the top of unit 3 — that building looks in pretty bad shape. How quickly will zirconium rods corrode in sea water exposed to air?

    The highly radioactive debris littered around the reactor site, some of it hundreds of meters away from the exploded units, was contaminated by aerosols from the reactors being vented before the hydrogen explosion. The gas leaked inside the buildings when the pipes leading to the exhaust stacks couldn’t take the pressure of the vented gases. You can actually see the destroyed pipe leading from the side of unit 3 to the exhaust stack on some of the aerial photographs taken by UAVs.

    Some of those bits of debris emit 100s of mSv/h, which is why they’re using remote controlled vehicles to pick up the bits and load them into steel containers (the operators sit inside a cabin on the back of a truck tens of meters away, watching via a video link). This must be also what the upper floors of the units, under the collapsed steel structures, are like.

    I shudder thinking of the people who have to walk into these buildings to try to restore long-term cooling.

  53. CRV9
    May 16, 2011 at 01:34

    Danny-san,I understand all that.
    Data, info weren’t enough or complete. You’d still have to make an educated guess because an accident happened. You’d have to act on your best guess. I’m not saying that anybody else could have done any better than what TEPCO & NISA have done.

    My problem was that one, a half away around the blobe had a better idea. And Areva is not Gundersen. I’m not talking about who’d be right in the end, either. Maybe we need a third party assesser/evaluator or something. Maybe, they are too closed to the situation emotionally, politically, economically. Even I tend to listen to and believe good news/explanations than bad ones. I’d say that NISA should have a third party org. as an official consultant, preferably out of Japan and privately. And I think this should be for any country. If an accident happend in Germany the US or Japan’s org. should be their official consultant to see things in clearer perspective.
    Another ignorant’s rant. This is just a rant.

    • May 16, 2011 at 09:53

      I never thought you were an ignorant; we simply don’t agree on some points.

      1st) NISA and NSC are the “third party” (between producer and consumer), and like any safety authority in the world (for any matter or industry), it’s a governmental agency. IMHO, if anything poses a problem for security is relying in private companies whose interest is one and only one: “increase profit”. Obviously, that’s a very European way of thinking, and US people would tell you if there is anyone you can’t trust that’s “the government”.

      2nd) Whenever there are doubts as far as trust is concerned it’s always nice to have even more third parties validating things. In the Nuclear world, that is ultimately one of the roles of the IAEA, again a governmental agency, though this time international (and therefore likely to be “less biased”).

      3rd) No country should rely on another for any important internal matter, if anything for a very simple reason: Regardless of how naive JPs may be, keep in mind “you have no friends out there”. Every single country and institution works for its own interest, be it personal, corporative, or national (if the “corporation” is a foreign state agency). Only JPs would do what’s best for Japan (it doesn’t mean they can’t do mistakes while trying), you can be pretty sure of it, and ultimately that’s IMO one of the reasons why the whole tragedy media coverage has been so dreadful abroad (none of those medias represent the interests of anything or anyone here).

      4th) By the time a foreigner would have even a slight idea not only on “what’s going on”, but also on “what can be done”, we would all be dead ! (just joking on the consequence). Now without the joke, to take adequate decisions you need to:
      a) get the good info
      b) get enough time to analyze it
      c) know very well what action possibilities you’ve got available (knowing “what should be done” it’s not enough for it to be “possibly done”).

      Again, I don’t know if any other utility would have done better (hopefully we will never find out), but to me it’s really clear that, for causes that escaped their control, TEPCO lacked both the info and time necessary to take “good decisions”. With both time and extra info it’s very easy to say “we could have done better”.

  54. GJBRKS
    May 16, 2011 at 02:41

    Hi ,

    The D/W HVH is NOT at the TOP of the RPV , but UNDERNEATH it , at least according to a recent document from TEPCO :


    • May 16, 2011 at 09:22

      Hey, thanks for noticing that ! (it also makes sense given the fact that HVH is always lower temp than the Bellow Seal); will update the blog right now

  55. May 18, 2011 at 23:45

    New hypothesis concerning the hydrogen explosions.
    If the hydrogen comes from the spent fuel ponds, the SFPs must have dried out very rapidly and one has to hypothesize that the SFPs have to have leaaked massively. But after the explosions the SFPs do not show any indications of leaks. Here is where my hypothesis sets in. I think that reverse flow in the feed line led to the loss of coolant and this reverse flow was stopped by the explosions.
    Details can be found in
    Please let me know wht you think of this hypothesis.

  56. Guest
    May 19, 2011 at 01:06


    FIrst of all thanks for this nice peace of work, meanwhile several people are using it as a base information tool!

    The temperature went down in unit 3 which is a success but its based on the addition of boronic acid which stopps fission reaction. Which is good a bad news. The bad news is fission reaction creates fast neutrons which are not being detected right now and the damaging profile is huge.

    • May 19, 2011 at 09:15

      I’m glad it serves as a tool for others to think.

      As for what you said, where did you get the info on any change of the boron content on injected water?
      AFAIK temp is going down mainly because they more than doubled the water injection flowrate into the RPV.

      Besides, Boron is a neutronic poison that, as you said, stops fission; so I don’t really understand the point you are trying to make.

      • May 19, 2011 at 09:42

        I think he believes that criticality had reoccurred in unit 3 and that this was the cause of the heat. However, AFAIK boron had been added to the cooling water in units 2 and 3 from around the time seawater was still being used for cooling, quite early on. Even if the cooling water being injected boils away the boron should have stayed in the core (along with the sea salt) unless the steel vessel melted through at the bottom.

        I am not a nuclear physicist, but if the cores of all three units melted in the first days of the station blackout, it seems unlikely that criticality would reoccur only in May (when unit 3 temperatures rose again).

        If the uranium oxide melts into a lump at the bottom of the core, one would expect to find bits of neutron-absorbing cadmium rods in there and the neutrons would not be moderated by water as much as they were in the working reactor, hence they would be mostly fast neutrons, which tend to be absorbed by U238 more than split U235 to cause a chain reaction. After all this is not highly enriched, weapons grade uranium, only 5% (when fresh).

      • May 19, 2011 at 10:32

        Well, why the temp. rose and why it went down are two diff questions.
        One can appreciate on the graphs that on #3 around the 3-5th May:
        – RPV and PCV temps rose
        – Water level went down while RPV pressure increased (evaporation increased).

        Now I’m no expert but I don’t think any re-criticality of the fuel is likely after 2 months, yet:
        – we still have to remove decay heat
        – there has to be a reason for the temp ramp up.

        Looking back to the injection flow data (I’ll try to graph that one tomorrow) it seems that TEPCO started lowering it after steady late April behaviour (remember the aim is to keep them cool while minimizing H2O injection). Minimum was reached on 5/1 – 5/2 at 6.5T/h, at which point temp started increasing (and did not stop until TEPCO reached 9T/h).

        IMHO I believe by lowering the water injection too much TEPCO may have reduced steam flow cooling the upper part of the core. As a result they may have melted yet another bit of it, whose influence wasn’t significatively felt until dropping under water, at which point: sudden evaporation/pressure and temp rise were registered. Of course, that’s just my guess with the very limited info we have.

  57. May 19, 2011 at 10:21

    TEPCO’s current (new) plan for the reactor cooling is to pump water to the top of the RPV and have it run down to remove heat.

    The big change from now is that the cooling water will no longer be water from the nearby dam but will be taken from the output of an onsite filtration system built by AREVA that’s supposed to be able to decontaminate 1200 t of water per day from next month (about double the current cooling water usage of the three units combined). It’s claimed to be able to reduce radioactivity by a factor of a thousand, which sounds impressive if it actually works. Without that reduction it would be near lethal to conduct any kind of work around the cooling water pumps or pipes.

    The input of the filtration system is the highly radioactive water in the basements of the turbine halls in units 1, 2 and 3, amongst others. The water leaks into the turbine hall basements from the adjacent reactor building basements. For example, the unit 1 basement was estimated to hold 3000 t of water, with radiation levels of around 1000-2000 mSv/h around there.

    My question is, how does the water get from the reactor core to the reactor basement? I understand the melted core in unit 1 (and probably also 2 and 3) may have pierced the RPV, allowing water to leak out of the bottom of the thick steel vessel. There may also be leaking pipes and pumps connected to the RPV inside the dry well. OK, so that’s how the water gets out into the containment, but how exactly does the water get out of the primary containment?

    Directly underneath the RPV inside the dry well there should be a concrete floor (inside the steel shell). The primary containment is also surrounded by reinforced concrete on all sides, except the lid at the top, the openings for various pipes and the ducts leading down to the suppression chamber (S/C) at the bottom.

    In unit 2 it is assumed that when repeated attempts to vent the containment failed, something in the suppression chamber burst. The steel of the S/C is not surrounded by reinforced concrete as the dry well is, it is exposed to air in the basement. So I can see if for example a welding joint fails, the water would leak from there. I can see the picture for unit 2.

    Apparently, when the #2 S/C burst, that’s when radioactivity at the plant spiked the most. On a late night NHK documentary I watched a couple of days ago, they showed scientists tracking the contamination levels around Fukushima prefecture and they showed weather patterns around the explosions of unit 1, unit 3 and unit 2. According to them it was the bang in unit 2 that led to the massive contamination to the North West of the plant, in Iitate-mura and other rural areas that have seen the highest fallout levels outside the initial 20 km evacuation zone, as the wind was blowing from the SE when that happened and then it rained and snowed over the affected area

    How does the water get out of the containments of units 1 and 3? Did they also fail like unit 2? Is their S/C damaged?

    I saw one mention of pouring concrete into the reactor basement to surround the S/C to stop it from leaking. I suspect that would only work if the basement was dry, but it won’t get dry until the leaking cooling water stops. Bit of a chicken and egg problem, like in that trench near the ocean several weeks ago!

    • May 19, 2011 at 10:47

      Again, thanks for your input.

      1) That’s not a “new” plan but the original one (often Twitt about it, but TEPCO has been preparing for that since Apr17), which had been modified to not only pump water from the Turbine Building (TB) basement but also from within Reactor Buildings (RB).

      2) Wrote it here before, but my believe is water on the turbine basements is coming from condensing all the steam that’s being produced while injecting water. When TEPCO was reporting on the rising levels of trenches under TBs I found out water was raising at identical pace to injected water, and to me that’s too much of a coincidence. Think about it, they’ve been pouring extra ~7T/h of water per reactor since day1, and pressure isn’t rising within RPV/PCV so obviously they are releasing it somewhere… (btw, going through Areva’s ppt agani yest I found an explicit mention to that “steam release”.
      Now radioactivity of fresh water evaporating right after injection is not comparable with that of the water that was originally in the primary circuit fo the plant before the accident, which is the one that had to be leaked once #2 S/C failed.

      3) You can see the radiation peaks linked to the respective explosions in Areva’s presentation; yes n2 was worse.

      4) Repairing any leaking containment will be pretty tricky… far from there yet.

      • May 19, 2011 at 15:57

        The original road map from April 17:

        proposed “Flooding up to top of active fuel”, at least for unit 1 and 3. The water from the basements originally was meant to be processed as much as possible, then shipped to Rokkasho-mura for final cleanup / disposal. That’s what the AREVA water filtration plant was original designed for.

        As for the steam, I would like to know how it gets from inside the containment to inside the basement. Where’s the leak? Or put another way: Where’s the damage to the containment?

        We’ve been told that Fukushima was not going to be anything like Chernobyl because there’s a containment and furthermore, no venting is supposed to have taken place since the days after the accident.

        However, the truth is that the containment has been leaky for at least 20 years, despite how it’s been sold to the public. If anyone remembers the reason for Tepco’s one year suspension in 2002, part of it was a failed (and faked) leak test on precisely this reactor, Fukushima I-1:

        [blockquote]In the routine pressure check conducted at the No. 1 reactor of the Fukushima No. 1 nuclear plant in 1991 and 1992, nitrogen was injected into the container and air pressure was measured over a six-hour period to determine how airtight the unit was.

        The container plays an important role in preventing radioactive materials from being leaked in the event of an accident.

        On the two occasions in question, the pressure readings were unstable, so workers injected air into the container, which is 32 meters high and has a maximum diameter of 18 meters, to make it appear that pressure was being maintained, the sources said.[/blockquote]
        (Japan Times, 2002-10-26)

  58. Guest
    May 19, 2011 at 23:31


    Sorry for being late in replying.

    The Boron addition was mentioned by GRS around the 15th of Mai. I don´t want to say the “whole” reactor changed to fisson reaction again, only parts of it. And yes Boron was added some time ago, but meanwhile several tons of water went trough the unit, so how much is left?

    Now why is it possible? Pu itself can do self fission reaction and the fast neutrons are getting moderated by water. At the beginning of the exothermal reaction the amount of water was increased but there was no effect. After adding the boric acid or the salt the neutrons got absorpted and the temperature went down. This model explains why those things are limited to unit 3 and I´ve no idea on which effect else such a exothermic behavior could be explained.

    As a consequence it is more than obvious to start to measure neutron beam because they are not being detected by any kind of standard dosimeters.

  59. May 20, 2011 at 02:14

    Re-criticality is not possible. Simple answer from neutron kinetics. Boron is not needed but added to increase the safety margin.
    Even if: If there would be recriticality with the geometrical configuration intact it would be immediately stopped due to the negative void.
    No re-criticality in a destructed geometrical configuration. Enrichment is too low to work in such a configuration.
    It must have been hydrogen.

  60. May 20, 2011 at 02:17

    But where did the hydrogen come from?

  61. Guest
    May 20, 2011 at 02:40

    Chain reaction and corresponding increased heat production may progress in parts of the corium if a critical mass can be achieved locally. This condition can be detected by presence of short-life fission products long after the meltdown, in amounts too high to be remaining from the controlled reaction inside the pre-meltdown reactor. As chain reactions generate high amounts of heat and fresh, highly radioactive fission products, this condition is highly undesirable.

    This is being extracted from Wikipedia and the possibility increases with neutron sources from Pu. The strong exothermic prosses in unit 3 must a have a reason and burning hydrogen is from my point of view very unlikely.

  62. May 20, 2011 at 03:20

    How could this be? If enrichment is too low, you cannot get criticality only in the very sophisticated configuration of the core. If this geometry is lost, there is no critcality whatever. By the way, parts of the core cannot turn critical, neutron losses are too high. This can be calculated very accurately, and has been calculated and recalculated over decades.

  63. Bill Dorland
    May 20, 2011 at 03:35

    Criticality does not imply explosion. I agree whole-heartedly that there cannot be a nuclear explosion with the isotope mixture in these power reactors. However, it is trivially possible (in principle) for localized recriticality to occur when the geometry of the fuel is changed. The control rods are placed rather precisely to prevent criticality when they are inserted. Typically, criticality is quenched when one or more rods is not fully inserted (for safety margin).

    In general, however, if the fuel were to become more concentrated as a result of meltdown processes and more distant from the control rods (e.g., by differential slumping) and if non-borated water is circulating around these isolated groups of fuel elements, then there would be recriticality — i.e., a chain reaction.

    The amount of energy that would be released is sufficient in such a scenario that there would very likely be rapid rearrangement of the corium and (evidently) a low probability of a large-scale recriticality event (involving a substantial fraction of the fuel in the reactor). In addition to rearrangement, recriticality in a compromised vessel could boil off the water and shut down the reaction (due to lack of moderation).

    • Bill Dorland
      May 20, 2011 at 03:37

      The isotopic mixes that were measured around April 14 still had improbably high I-131 fractions for there NOT to have been some recriticality events. There is a paper on the ArXiv discussing this point in more detail.

  64. Guest
    May 20, 2011 at 03:51

    I just refer to literature.

    Well, I do not need a sophisticated configuration to get a criticality. A worker in Japan, Tokai – Mura, just added a bit to much of Uranium into a volume and died 3 month later based on neutron beams. This happened 1999. Approx. 60 people got hurt, and several passed later on.

    By the way, nobody knows how this inhomogeneous Corium is being distributed in the reactor and it does not take much to get the observed energy.

    • May 20, 2011 at 08:28

      Guest :
      I just refer to literature.
      Well, I do not need a sophisticated configuration to get a criticality. A worker in Japan, Tokai – Mura, just added a bit to much of Uranium into a volume and died 3 month later based on neutron beams. This happened 1999. Approx. 60 people got hurt, and several passed later on.

      The Tokaimura criticality accident did not involve lowly enriched LWR uranium, it was over 18% U235 and it was as highly soluble uranyl nitrate, not ceramic oxide fuel, which means there was plenty of water for moderation, unlike the corium lump.

  65. Guest
    May 20, 2011 at 04:19


    A discussion about Iodine levels and recriticality is not straight forward, sorry.

    Would you agree in case all Isotopes would be published it would be more than helpful to get an insight whats up in the reactor? I´m refering to short living isotopes they would give evidence for this hyothesis.

    • Bill Dorland
      May 20, 2011 at 06:46

      Guest, here is the paper I was referring to: http://arxiv.org/pdf/1105.0242

      Clearly more information would be better, but I see no obvious holes in the logic of this paper.

  66. Guest
    May 20, 2011 at 16:02

    Yes it was riched U238, I took it as an example that it just could happen without any sophisticated geometry. Regarding the cadmium cotroll rods, cadmium has a low boiling point around 1400°F, it simply distills off, the element which dilutes the corium is Zr which is reacting with water to ZrO2 but this has no impact on neutrons. Means there are not that many substances which could dilute the corium in a first step. When it hits the steel or concrete than it is a different story. As said the recritality is by far not the main reaction but it seems very likely that sometimes it occurs.

  67. May 20, 2011 at 18:22

    Guest :
    Regarding the cadmium cotroll rods, cadmium has a low boiling point around 1400°F, it simply distills off

    Seems like they actually use boron carbide (B4C) with hafnium tips. B4C has a melting point of 2763C (5005F), so probably the steel of its structure is going to melt first, around when the zirconium cladding burns up. With a boiling point of 3500C it’s not going to evaporate from the reactor core any time soon.

  68. guest
    May 20, 2011 at 19:59

    Oh yes, they use B4C with Hf tips, I just read it as well. The B4C is part of a steel rod which tends to melt around 1400°C, so most of it should have molten and will be part of the corium.

    As they entered unit 2 they used a neutron analyser and found nothing so far. From the discussed unit 3 I have no information so far.

  69. Guest
    May 21, 2011 at 00:16

    It´s written in German and was published by GRS might be useful for your charts

    “Am 15.05.2011 um 14:33 Uhr wurde begonnen, Borsäurelösung über die Feuerlöschleitungen in den Reaktor des Blockes 3 einzuspeisen.”

    On May 15th at 2:33pm injection of Boronic Acid was started into the reactor of unit 3…

  70. guest
    May 21, 2011 at 19:05


    Might be of interest to read regarding criticality of unit 3, these are IAEA recommendations.

  71. CRV9
    May 27, 2011 at 11:18

    Hi Daniel, I stopped becasue I didn’t want to turn this into a rant-fest and invite Gundersen crowds. It was my rant anyway and it wouldn’t help the sitiations either.

    I have a crazy idea. You said that caloric output of decays are 0.1 % of its full power. If air were really cold, say like freezing cold, it would be enough to keep fuel rods cool enough?

    They are going to enclose the units from rains and winds. So if they installed huge air-condition units, ones they use for a huge ice rink place, at the same time it would filter out radioactive particles. Would it be good enough so that they would have to use less water, mimimun amounts? In winter time, say Dec to Feb, would be much easier? They could be able to reach to melted fuel rods with robots and somehow contain them?

    I know I’m just wildly thinking. I’m just hoping for the best.

    • May 27, 2011 at 11:30

      CRV9, 0,1% of one of these “small” powerplants is still 1 to 2 MegaWatts of power generated over a small volume… it’s not much when compared with the plants rated power, but it’s an awful lot of heat to remove with a relatively small exchange surface… Cold ext. temp wouldn’t do anything, that’s why they are injecting over 6-7 tones of water per hour to keep them cool…

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  1. April 8, 2011 at 20:16
  2. April 11, 2011 at 10:29
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