Was Fukushima Rated Correctly on INES?

By Jeff Merrifield

On April 12, one month after the initiation of the tragedy at the Fukushima Daiichi I nuclear power plant, the Japanese government labeled the event a level seven accident—the most severe rating on the International Nuclear Events Scale (INES). This announcement heightened international concerns about the severity of the event and the potential for circumstances to worsen. This posting is intended to provide some clarification about these events in historical context.

Overview of INES system

In 1989, a group of nuclear safety experts at the International Atomic Energy Agency developed the seven-point INES system to classify the seriousness of nuclear events. Each step on the scale is designed to represent an event 10 times more severe than the previous step. INES has been adopted by the nuclear regulatory authorities of over 60 countries as a communication tool to ensure public safety during a nuclear event.

Using this scale, the Three Mile Island accident was identified as a level five and the Chernobyl accident was identified as a level seven. The seven levels on the INES are described below:

Events at Fukushima

On March 11, a 9.0-magnitude earthquake near the island of Honshu triggered a 46-foot-tall (14 meter) tsunami, which hit the Fukushima Daiichi I plant some 15 minutes later. The initial reports indicate the earthquake caused a loss of offsite power, triggering the startup of the emergency generators at the six-unit site. It appears that backup emergency systems were operating as intended at the time the tsunami occurred.

Unfortunately, the subsequent tsunami disabled the backup diesel generators and flooded the electrical systems at the site, causing the site to enter a station blackout. Emergency batteries powered the systems for eight hours. When the batteries failed, however, it caused the subsequent loss of cooling, triggering a series of explosions, partial meltdowns at the site, and a variety of problems at all six units.

On March 13, the day after the hydrogen explosion at unit 1, the Japan Atomic Energy Agency made an official statement that the event at unit 1 had reached a level four on the INES. The following day, after the hydrogen explosion at unit 3, the French Nuclear Safety Authority (ASN) argued the event should be rated a level five or six on the INES.

On March 18, Japanese authorities announced that the events at units 1, 2, and 3 were being categorized as INES level five accidents and the event at unit 4 would be categorized as a level three incident.

Before April 1, airborne releases of radioactive materials had already begun, individuals within 20 kilometers of the site had been evacuated, and health authorities had prevented the consumption of foods such as milk and spinach that were contaminated with radiation. In addition, on April 2, water from unit 2 was seen flowing into the sea and higher levels of radiation were being identified in seawater near the site.

Impact of the level seven rating

The Japanese government’s decision to rate the Fukushima incident as an INES level seven accident on April 12 raised questions about whether the incident was getting worse and if radiation dangers were equivalent to those of the Chernobyl accident. The chain of events leading up to April 12 indicates a higher INES score should have been issued a week or two sooner. The delayed rating announcement, which heightened concerns about worsening circumstances, was more likely a lagging indicator of events at the site.

As far as the assignment of a level seven rating, which placed Fukushima on the same level as Chernobyl, this may not be in line with reality. While I don’t want to minimize the seriousness of the recent event, there are some clear distinctions from Chernobyl that need to be mentioned.

  • First, evidence suggests the level of radioactivity released from Chernobyl was 10 times greater.
  • Second, because debris from the graphite/uranium fuel fire at Chernobyl entered the jet stream, a significant amount of radioactive material was deposited over a much greater geographical area than the more localized radiation releases at Fukushima.
  • Third, during Chernobyl, individuals living near the site were not evacuated for days after the accident and continued to eat contaminated foods. Following the Fukushima event, Japanese authorities promptly evacuated local populations and took active measures to prevent contaminated food and water from being consumed by local populations.

While there have been significant releases of radioactive material at Fukushima through airborne and contaminated water dispersion, the overall impact on human and environmental exposure pathways appears to be meaningfully less than Chernobyl due to the comparatively lower levels of radioactive release, prompt evacuation orders, and the dilutive effects of the Pacific Ocean. These factors clearly justify the need for a distinction in ratings between the two accidents.

In the weeks and months ahead, we will have an even greater opportunity to analyze this accident and place it into proper historical context. Given the information currently available, it seems that the Japanese government’s INES ranking for Fukushima may be too high and the IAEA may need to consider revisiting the criteria used to classify nuclear events on the INES.



As a member of Shaw Power Group’s executive team, Jeff is focused on enhancing the group’s external relationships, including business development,marketing and communications, government and regulatory affairs, customer relations, and strategic planning. He has been particularly involved in supporting the group’s nuclear-related efforts. Prior to joining Shaw, Merrifield served two terms (1998 to 2007) as a commissioner of the U.S. Nuclear Regulatory Commission. In this role, he was one of five individuals overseeing this independent commission that regulates the safety and security of the 104 operating nuclear power plants in the United States.

7 thoughts on “Was Fukushima Rated Correctly on INES?

  1. Tom Murphy

    @ Mark Norsworthy – Thank you for the informative reply. We already use a logarithmic scale for earthquakes and such a scale would be open-ended and well understood by the public. It would serve to differentiate Fukushima from Chernobyl while not trying to downplay the severity of the former event.

    @ Alan – I think your concept that scale should be based on the “human impact” would be a step backwards. IMO the proper measure is how much radiation was released from the facility to the environment. Many people would disagree with your assertion that the radiation that “went out to sea” should not “count”.

  2. Mark Norsworthy

    This is a somewhat long post, bear with me. Let me begin by stating that I agree the INES scale needs to be revised–the scale should not arbitrarily stop at 7. The IAEA threshold for a 7 is 50,000 TBq or higher of airborne radioactive release; it is the “or higher” part that is the problem. The scale should continue logarithmically to at least a 9 or 10.

    It turns out that the INES User Manual suggested thresholds are based on how much airborne radioactivity is released, measured in TBq of I-131. Other radionuclides have a conversion factor applied to make them equivalent to I-131. The guidelines are that a level 5 is 500 – 5000 TBq, level 6 is 5000 – 50000 TBq, and level 7 is 50000 TBq and up.

    Fukushima released 370,000 – 630,000 TBq according to the April 12 Japanese press release (depending on the agency making the estimates). Either figure would clearly justify a 7 on the INES scale. But why does the scale stop at 7? Chernobyl was ~5.2 million TBq. If the scale continued in powers of 10, then Fukushima would be at most an 8, and Chernobyl a 9. Then it would be explicitly clear that Fukushima, while serious, is still an order of magnitude less severe (in terms of airborne radioactive release) than Chernobyl.

    An added benefit of modifying the INES scale is that it would more accurately reflect the many orders of magnitude difference between TMI and Chernobyl, which I don’t think is captured by the difference of 5 to 7. The minimal releases from TMI did not justify a level 5 classification–the amount of core damage drove that rating.

    Clearly, it would be extremely difficult to create a broadly applicable scale that takes into account the impacts on the public. Rather, the INES rating is based on something relatively objective, and is intended to give an idea of severity in terms of raw amount of radioactivity released.

    The manual explicitly states, “The reason for using quantity released rather than assessed dose is that for these larger releases, the actual dose received will very much depend on the protective action implemented and other environmental conditions. If the protective actions are successful, the doses received will not increase in proportion to the amount released.”

    The flaws in the design of the INES scale were not apparent until Fukushima, because the only level 7 accident was Chernobyl. The IAEA likely thought they were facilitating public communication by capping the scale. There were no events other than Chernobyl greater than 50,000 TBq, so they called it a 7.

    The scale ought to continue logarithmically in order to provide more useful information for the public.

  3. Lucia

    Radiation is still pouring out of Fukushima, so to say that is is only one-tenth that of Chernobyl is not meaningful. How much radiation will ultimately be emitted from the complex? Maybe a lot more than Chernobyl.

  4. Alan

    I think the contention that it should be INES 6 is entirely valid. After all, what IS the rating based on? As I understand it, there are some measures in terms of radioactivity released that we have to go by, and there is also some qualitative definitions for the different levels.

    Problem is, radioactivity released does not directly correlate with the impact to the public. The 3 points mentioned, particularly that the public was better informed, are points I stress over and over again. The majority of the human consequences of Chernobyl could have been prevented through better disaster management – yes, even after the reactor blew its top. Fukushima is also evaluated too high based on radioactive releases due to the fact that most of it went out to sea and didn’t harm people very much.

    Now, there is the argument that extended evacuation measures are needed due to the 2 REM public limit. The use of such measures is a part of the INES 7 definition. You can argue either one, 6 or 7. It would be wrong to tie the definition only to the amount of radiation that was dispersed. That is not a reasonable evaluation tool when communicating to the public.

  5. Brucie B

    So where is a 50MT thermonuclear ‘accident’ explosion on the INES scale?
    INES is a political tool much like the ‘Homeland Security Threat Monitor’.
    A political fallout scale.
    INES is used by gov’t authorities to announce cranked up initial accidents’ rating to public only later minimized by actual scientific radiation inventory studies. Fukushima incident after initial media hype will rate low. Japanese nuclear workers and agencies have done a good job at mitigating effects of a natural catastrophic event.

  6. Tom Murphy

    “As far as the assignment of a level seven rating, which placed Fukushima on the same level as Chernobyl, this may not be in line with reality. ”

    An INES rating of 7 does not mean “as bad as Chernobyl” it means the event was a major accident.

    I believe a more correct way to discuss Fukushima is to say that there were mutilple INES 7 events.

    “Given the information currently available, it seems that the Japanese government’s INES ranking for Fukushima may be too high and the IAEA may need to consider revisiting the criteria used to classify nuclear events on the INES.”

    I completely disagree with the first part of this statement. The information currently available fully supports the INES rating of 7.

    As to the second part, a better scale could be developed. My first concern would be trying to pegged its upper range to what happned at Chernobyl. We could have a worse event in the future. My second concern would be any attempt to develop a scale that causes the events at Fukushima to be percieved as any less than a major accident with long term implications.

    Tom Murphy

  7. Joffan

    If there were no ceiling to the INES scale, what would Chernobyl be ranked at? My guess: 9.

    And I would expect Fukushima to end up as a 6, using most-likely (rather than worst-case) estimates. The frustrating thing about the INES scale is that it is supposed to simplify understandng of how serious an event is, but it seems to have been used in a rather over-technical way on this occasion.

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