[updated 4/13 8:40]

Today the big news all over the world was NISA uprating #FNPP1 accident to INES (International Nuclear Events Scale) level 7. That is the top of such scale, and was so far only “awarded” to Chernobyl.

Even before the news become official, still afterwards, I’ve noticed most people and media don’t understand what that means, and are simply scared by the obvious analogy between Fukushima and Chernobyl.

As such, I’ve just decided to write this post summing up what I said this morning (4/12) in tweeter.


What does INES and today’s uprate mean?


  • INES is not a “nuclear alert level”, it’s a classification of nuclear events gravity based on “what has failed, how it has failed, and how much radioactivity escaped”. [4/13] It’s meant to be a tool to communicate with the public in a simple to understand way (IAEA).
  • INES levels are not evaluated based on “present condition”, but based “on the accident as a whole” on “everything that happened since day1”. In the case of #FNPP1, the worst happened on the early days, and has been progressively brought under (relative) control.
  • As such, NISA has not raised “the alert level”, which would indicate the situation has worsened now, it has “reassessed what happened short after 3/11”. As a matter of fact:
    • #FNPP1 reactors remain stable and plant situation keeps slowly improving day by day.
    • Nation wide (and #FNPP1) radioactivity monitoring posts keep showing the same downwards trend as every day since the H2 explosions (save the 1st rain).
  • NISA has not been lying, nor “made a wrong assessment”, they’ve simply re-evaluated the gravity of the accident based on new evidence which was only available after 3/18. They were not specific on that today, but going through the accident chronology seems to point towards all the radioactive water and ocean leaks found around the plant; before only postH2 explosions airborne measurements were available. Obviously, further evidence and more complete analysis may brought up further revisions of what was published today.
  • INES level evaluation is not a simple task, and as such all ratings up to now remain “tentative” or “temporary”: they represent the best assessments that can be done with current information.
  • If you really want to put your hands into how to assess INES level, you are welcome to read this 218 p document (INES User’s manual). That is though a difficult task that will be carried by experts committees once the accident is closed and all the information is available.

Why did NISA raised accident to INES 7 ?


  • There are many different criteria to determine INES level, but NISA tentative rating is based only on the radioactivity levels that escaped the plant, which I tried to represent on the following chart (sources: NISA PR p2 & INES manual p17).

Please do note that:

  • INES manual doesn’t establish any “fixed limits” to go from one level to another, though suggested values are half way through the intervals I noted on the graph (500 ;  5,000 ; 50,000).
  • I have purposely divided level7 in two intervals, showing that if one follows INES logic (10x increase jumps one level) Chernobyl should be in a hypothetical “level8”. Of course, INES doesn’t make such difference and everything above 50,000 to 100,000 TBq fits within level7.
  • [4/13] At this point NISA seems unable to tell what part of those 370k TBq came from which reactor, and the level7 rating applies for the accident as a whole. Some say it shouldn’t be just “one level7 but 3 or 4”, yet I don’t get the logic: if you divide those 370 TBq by 3 or 4 you may end up having 3 or 4 level 6 accidents…
  • [4/13] There are experts like Prof Oka from Tokyo’s Waseda University who believe NISA/JNS preliminary calculations are “too conservative” while pointing to level6 (about 1/100 from Chernobyl) being more likely. As I said, still far from final INES decission.

Not all level 7 accidents are the same


As stated previously, Fukushima tentative uprating is solely based on the released radioactivity levels, but that is far from being the only important thing for considering the gravity of the situation. Indeed, despite being at the same INES level, there are fundamental differences between Fukushima and Chernobyl accidents. Here are what I believe to be the 3 main ones:

  1. Released radioactivity is 10x smaller (based on NISA/JNS estimations), enough to be classified one hypothetical level bellow.
  2. Radioactive material spread is completely different: In Chernobyl the whole graphite core exploded and was thrown into the atmosphere/surroundings in ashes/dust; In Fukushima reactors containment remains mostly sound, and leaked radioactivity has been mostly from liquid effluents that haven’t left the premises of the plant. Of course some of it leaked to the ocean and some was released to the atmosphere during H2 explosions, but the majority is still confined within the immediate proximity of the plant.
  3. Because of 2, Fukushima plant remains “operable”, meaning that work to contain the radioactivity and limit the impact on the environment remains both possible and effective; Chernobyl couldn’t be approached after the accident.

Obviously, as a result of those differences, there have still not be a single victim of radioactivity because of #FNPP1 accident (it doesn’t mean “there won’t”, simply that “there have not been”). Some may argue we’ll only see those in terms of long-term cancers for most exposed workers, but it’s a weak argument in societies where tobacco and air pollution results in millions of those cancers and nobody seems to be concerned about.



And that’s about it. On a final note, please understand this is far from being my speciality, and I did not intend to be too technical, just pointing out simple things I believe anyone could understand if willing to.

  1. April 12, 2011 at 23:20

    Gracias por la publicación, está muy buena.

  2. ChrisMac
    April 13, 2011 at 12:30

    Lets not down play this. Sure it wasn’t a Chernobyl, but this isn’t a picnic at the park. Nuclear power is not safe and NEVER will be.

    • April 13, 2011 at 13:48

      Since when an INES7 accident that has required evacuation of people and business around 30km, put in risk the lives of hundreds of workers, and stopped sales of food products, is “a picnic in the park”?

      My family lives within 100km of #FNPP1, which is why I started to track radioact. data to begin with. I don’t know what is your concept of walking in a park, but I can say that not many people in NorthWestern Kantou area enjoys it much nowadays (though obviously the main reason for that isn’t #FNPP1)

      Otherwise, I agree with you, Nuclear power is not (absolutely) safe and will be extremely difficult (if even possible) to ever make it 100% safe. It is still pretty much at the bottom of risks I take in my life; also at the bottom of risks that modern societies assume to ensure their well-being and maintain their way of life.

      PS: This is just a warning, I won’t let this blog become a place for opinionated debate. There are many other places for that. Any comments on the facts or data I post are welcomed, everything else won’t be published.

  3. April 14, 2011 at 15:33

    Super stuff here! Thanks for this blog. Please consider a domain-name for your blog and link it within wordpress to the domain-name. I am happy to pay for that.

  4. Bill Dorland
    April 15, 2011 at 19:33

    Very helpful, thank you.

    I wonder if the final INES ratings will be lower for Unit 1-3, which have plausibly maintained certain levels of containment, so that leaks to the environment have been mainly in solution (water). This is a much lower threat to life than airborne. I have not found any good analysis of the equivalence factors required to calculate the airborne I-131 equivalent releases for the water releases from Unit 2, for example, but surely the effects are much less serious even if the amount of TBq is large. Unit four, on the other hand, is rated as INES-3 according to the single criterion of defense-in-depth. I have not seen an estimate of the radiation released from the Unit 4 SFP. If a large fraction of the measured releases in air came from Unit 4, then the final INES ratings might be something like 6-6-6-7 or 5-5-5-7. Comments?

    • April 16, 2011 at 20:43

      I don’t know why there is so much fear on unit 4 sfp on the internet forums… release from there reached the maximum after the correspondent h2 explosion and recorded radioactivity peaks were rather small when compared with previous explosions (notably those registered at units 2 and 3). You can find that info in many places, but again it’s available on AREVA’s presentation (p27).

      Otherwise, the pool was confirmed to be filled up again rather quick, and right now radioactivity on the water surface was recently estimated at merely 85 uSv/h (TEPCO data; though they are going to repeat the measure to be sure). With the fuel covered by water there is not much escaping from it … ; by far unit2 is the one having most problems and leaking most of what’s being recorded both on air/soil/sea.

      • Bill Dorland
        April 16, 2011 at 23:04

        It is hard to understand exactly where his arrows point to, but in the Areva presentation, I read the labels to mean the highest peak was associated with Unit 4; two peaks on either side of that are pointed to as Unit 2.

        My interest in 4 is mainly because it is outside containment. The other reactors are not in cold shutdown yet, but they have a lot of containment structures still intact. Also, the greatest lessons that can be learned seem to be associated with Unit 4, since there is usually no good reason to have the entire reactor core in the spent fuel pond for months at a time. That was a risk taken that could be reduced at other sites for no capital cost, as long as it is understood that it was a risk with consequences.

        Thank you again for your site.

  1. April 12, 2011 at 23:47
  2. April 13, 2011 at 09:27

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