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How They Came Up With That Fukushima Level 7 Rating

David Lewis's picture
  • Member since 2018
  • 353 items added with 37,764 views
  • Apr 13, 2011


It surprises many that Fukushima has been reclassified as a Level 7 Accident under the guidelines of the International Nuclear and Radiological Event  (INES) Scale.  I wondered what was going on. 

 It turned out there is an INES User Manual. Quotes from this manual in what follows appear in italics.  

 Several countries had their own ways of classifying things prior to the creation of the INES scheme, but eventually more than 60 countries agreed on this attempt to create an international standard.  The scale was not designed to rank “incidents“, and “accidents” after the fact as many seem to think. 

 The idea was to have an internationally standard way to “promptly” communicate “the safety significance” of an event where radioactive material is involved as the event happens.  INES wants the public, the media, and the technical community to be, roughly, on the same page at a time like this .  

 “It is important that communications happen promptly; otherwise a confused understanding of the event will occur from the media and public speculation“. 

  INES does not require its member states to report using INES guidelines, or that they use the guidelines in any particular way.  “The purpose of INES is simply to help put into perspective” whatever size of nuclear cataclysm anyone has going.  They merely encourage reporting that follows their guidelines.  

Filling out the standard “Event Reporting Form” (ERF) is like doing your taxes – a typical nuclear apocalypse manager can enter the required information, do some calculations, and the INES number pops out.  There are “worked example” calculations to refer to. 

 Levels 4 to 7 are to be called “Accidents“.  Each step up is supposed to be ten times greater in significance.  If a “5” is a certain sized problem, a “7” is 100 times greater.  Fukushima was just upgraded from 5 to 7.   A “5” can also be described as an “Accident With Wider Consequences“.  A “7” is a “Major Accident“.  In the INES scheme of things, nothing is ever going to exceed what happened at Chernobyl.  . 

Originally, “the key consideration in developing INES rating criteria was to ensure that the significance level of less severe and more localized events were clearly separated” from Chernobyl.  Chernobyl was supposed to define what a “7” is.  Now we have two Level 7 Accidents.   A lot of people point out, correctly, in my mind, that there is a big difference between the two events.  

 The manual is clear this rating scheme is not an exact science:  “It is not appropriate to use INES to compare safety performance between… countries”.   At “Level 2 and above“, INES warns, “the statistically small number of such events” makes it difficult to compare meaningfully.   

A rating system of events you can’t use to compare events with seems useless, or at least it might seem so to the unintiated.  Everyone wants to use this INES scheme to compare Fukushima and Chernobyl.  To do that we need to look at the details of the calculation.   

 Do INES rules indicate Japan should now classify Fukushima as 100 times greater than previously, or did the Japanese do this for some other reason?  The most significant factor in the INES rules which end up having the Japanese classify Fukushima as equal to Chernobyl have to do with the “Activity Released” from the facility.  That’s radioactivity.  

For releases to the atmosphere INES is crystal clear.  You estimate all the different types of radionuclides that have escaped your facility that you think have entered the atmosphere and convert using INES supplied conversion factors to come up with a single number of a standard unit representing the radioactivity release and that is that.  Everything ends up as the total equivalent “terabecquerels (TBq) of  131Iodine“.  (From now one, I’ll use “units” when I mean this standard unit of radioactivity).

 After that things get quite murky.  If you dump radionuclides on the ground or leak them into the ocean, and you are already at Level  5 “detailed guidance cannot be provided here“.  If you kill someone outright with radiation you are immediately at minimum a Level 4, but there isn’t a table for tens, hundreds, tens of thousands, millions, entire planetary populations, and so on.  

And even though a Level 7 Accident was defined by INES as Chernobyl to distinguish between Chernobyl and all other events, the nuts and bolts of the definition are so far exceeded by what happened at Chernobyl that it has turned out that there is room for Fukushima in the category.  

Level 6 is “thousands to tens of thousands” of units of radioactivity released.

Level 7 is “more than several tens of thousands” of units.  

The “Worked Examples” section has a few relevant sample calculations.  Eg:  it shows how the 1956 Kyshtym, Russia event ended up classed as a Level 6 Accident.  

 At Kyshtym, a waste storage tank overheated after its cooling system failed.  The contents exploded with the force of 75 tons of TNT and blew the  8 foot thick concrete lid 100 feet away.   A large quantity of the radioactive isotopes of Strontium and Cesium were released contaminating almost 6,000 square miles of land.  According to the worksheet solution, that release amounted to 20,500 units.  This fit into the “thousands to tens of thousands” of units category, so this accident becomes a Level 6.  

 A lot of other factors that are used to help classify lesser events are ignored once you reach this level of significance – what matters to INES at this level is how much radioactive material escapes your facility.  

 Kyshtym sounds much worse than Fukushima, depending if all that land was rendered uninhabitable and for how long.  If you needed to wait for the radioactive isotopes contaminating the land to decline in power to 1/8, you have to wait about 80 years. INES states the average contamination around Kyshtym was “more than 4 kBq/m2  of 90Sr”.  Like many of you, I have no idea what the significance of that number is.     

But the classification system still makes sense.  Think about a magnitude 9.5 earthquake reported from Antarctica, and an 8.0 centered in Manhattan.  Almost everyone would think the significance of this theoretical Manhattan event would dwarf anything that could have happened in Antarctica, but the experts would still insist on keeping the Richter scale.  

The worksheet example shows how Chernobyl would be calculated.  INES calculates Chernobyl released 5.4 million units..  This is very far beyond the “more than several tens of thousandsof units required to be classed as an INES Level 7 Accident.  

You need to qualify on no other factor of the many that the INES guidelines list, once you have released as much radiative material as was released in these Level 6 and Level 7 accidents.

 Dr. Robert Gale directed the medical effort at Chernobyl.  Because he is one of the leading scientists in the world familiar with what to do when confronted with a disaster of this size, he visited Fukushima to consult with the experts and managers crowding in around there.  He then was asked to brief the Japanese Cabinet.  When he returned from Japan, he described the size of the release at Fukushima as about 10% of Chernobyl in an interview on the Canadian public broadcaster, CBC 1.  This was on April 4.  He was in communication with the highest authorities in Japan, so 10% of Chernobyl  is what they think has escaped from the Fukushima facility.  If Dr. Gale is correct, this, roughly, is a release of about 500,000 units of the radioactive isotope 131Iodine equivalent. (NPR reports today that the release is 370,000 units. I don’t know if they have performed the calculation to convert the cesium to equivalent iodine)

 Clearly, if “more than several tens of thousands” of units of radioactive material released is a Level 7, Fukushima is, without doubt, a Level 7 Accident.  This is true even if a lot of this radioactivity went directly into the ocean, and even though there may well be no detectable harm to humans as opposed to the harm you can calculate using LNT.  There is also the psychological stress caused by the evacuation and widespread radiophobia to consider.  Japan will suffer far less than Russia even if Fukushima actually was as great a release as Chernobyl because the Japanese were quick to protect their population, as opposed to what the Russians did to their people.   But it remains to be seen how the Japanese will react to what became the greatest effect of Chernobyl on the surrounding population:  psychic stress.  Keep in mind also that the INES scale is not just concerned about what happens to people.  It takes account of the “environment”, as it must.  The ocean is part of the “environment”.  

And just as clearly, because ten times the Fukushima amount of radioactive material was released by Chernobyl, it remains a very much bigger eventt. 

 Since the scale is supposed to move up one number as the accident increases in size by a factor of ten, there will be a lot of room for people who want to declare Fukushima is nowhere near the equal of Chernobyl, and because the rules were followed as both events were assigned equal ratings, there is just as much room for people to argue that it is the equal of Chernobyl. 

Because the “key consideration” of the INES classification scheme was “to ensure that the significance level of less severe and more localized events were clearly separated” from Chernobyl, it may be that as the dust settles over Fukushima there will be calls for the maximum significance Chernobyl type level to become an “8”, with Fukushima staying at “7””. 



 1. See the streaming audio:  “Japan Seawater Radiation” a little ways down the page that comes up


(Caution:  information in this post has not been confirmed by the Nuclear Science and Engineering Department of MIT)


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David Lewis's picture
David Lewis on Apr 15, 2011


All countries are strongly encouraged to communicate events internationally (within 24 hours if possible) according to the agreed criteria which are:
—Events rated at Level 2 and above; or
—Events attracting international public interest.
It is recognized that there will be occasions when a longer time scale is 
required to know or estimate the actual consequences of the event. In these 
circumstances, a provisional rating should be given with a final rating provided 
at a later date

Quoting from the User Manual:  

“All countries are strongly encouraged to communicate events internationally (within 24 hours if possible) according to the agreed criteria which are:

—Events rated at Level 2 and above;or

—Events attracting international public interest.

It is recognized that there will be occasions when a longer time scale is required to know or estimate the actual consequences of the event. In these circumstances, a provisional rating should be given with a final rating provided at a later date”

The Japanese could have raised the rating as soon as they realized they’d released “more than several tens of thousands” of terabecquerels” of  131Iodine equivalent to the atmosphere.  INES guidelines would “strongly encourage” them to do so.  But I’m sure they had a lot of things on their “to do” list.  Giving the Fukushima event a “provisional” 5 rating and upping it later to a 7 was the right thing to do.  

INES has nothing to say about how much extra time is acceptable for filing your ERF (Event Reporting Form) to rate your nuclear apocalypse correctly if a tsunami happens to have wiped out in excess of 10,000 of your citizens at the same time.  


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