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More on Level 7

Level 7 About a week ago, the Japanese government rated the event at Fukushima Daiichi at level 7, the highest such level on the International Nuclear and Radiological Event Scale. This put it on par with the accident at the Chernobyl nuclear plant in 1986 (then in the Soviet Union, now Ukraine.)

For some commentators, that meant that Fukushima Daiichi is as serious as Chernobyl. For others, it qualified as a head scratcher, as the two events seem to have many points of departure.

One quick way to find out which view is closer to the truth is to consult the scale itself. It was created and is maintained by the International Atomic Energy Agency. A brochure explaining it can be found here. From the introduction:

The INES Scale is a worldwide tool for communicating to the public in a consistent way the safety significance of nuclear and radiological events.

Just like information on earthquakes or temperature would be difficult to understand without the Richter or Celsius scales, the INES Scale explains the significance of events from a range of activities, including industrial and medical use of radiation sources, operations at nuclear facilities and transport of radioactive material.

Events are classified on the scale at seven levels: Levels 1–3 are called "incidents" and Levels 4–7 "accidents". The scale is designed so that the severity of an event is about ten times greater for each increase in level on the scale. Events without safety significance are called “deviations” and are classified Below Scale / Level 0.

Here’s the INES description of how an accident is rated level 6 and level 7:

ines_Page_4 You can see that Level 7 requires that countermeasures have been taken to fend off “health and environmental effects” on the public, not that these effects need to have occurred before taking action. No one has died from radiation exposure at or around Fukushima while people did die at Chernobyl. But both required countermeasures, thus setting both events at Level 7. (Chernobyl preceded the scale’s creation in 1990 and doubtless influenced its categorization scheme.)


I asked Barbara Hamrick, currently the Radiation Safety Officer at the University of California’s Irvine Medical Center and Secretary-Elect for the Health Physics Society, if she would clarify some elements of INES for us.

What does the INES scale measure in determining an accident or incident?

The INES scale is used to rank nuclear incidents and accidents on a 7-point scale. It was originally developed to facilitate discussions among the member states (countries) of the International Atomic Energy Agency (IAEA) when a nuclear event occurred, in much the same way that the Richter scale is used to convey the severity of an earthquake. 

While examples of each level are provided in guidance, there is still a great deal of subjectivity involved in the final assessment of any nuclear event. It is not uncommon in large events to make “provisional” assessments of the severity level until such time as sampling and other data can be collected and evaluated. I don’t think it would be unusual to see the rating of the event at Fukushima Daiichi to remain provisional for many months.

Many commentators have noted that Fukushima does not seem to rise to the same level as Chernobyl, yet both are rated as level 7. Is it incorrect to look at the scale as a precise measuring tool?

I would not view the INES scale as a “precise” measuring tool. There are only seven levels, ranging from a very minor anomaly, such as exceeding a statutory dose limit, to a major accident involving potential widespread health and environmental effects.

The scale was designed such that a Level 7 event is about 10 times more severe than a Level 6 event. In this case, although the accident at Fukushima Daiichi fits some of the criteria of a Level 7 event, it is 10 times less severe than the Chernobyl accident based on our current understanding of how much radioactive material has been released. In other words, based on what we know right now, it might be more appropriate to consider the Fukushima Daiichi event a Level 7, and promote the Chernobyl event to a Level 8 (if there were such a level).

Conversely, many commentators have called Fukushima clearly as serious as Chernobyl based on INES. Is this a misuse of the scale’s purpose?

I think it is definitely a misuse of the scale. The real consequences of the event will not be known for a long time. In order for the international community to share a common understanding of the magnitude of the event, we use the INES scale to communicate a very rough approximation of the potential consequences.

At this time, it still appears that the Fukushima Daiichi nuclear event is roughly ten times less severe than the Chernobyl event. That may change one way or the other as additional data becomes available. For example, it is possible that due to the meteorology during the event, due to the timing of the evacuations, or due to the food and water restrictions, the actual health effects will be less pervasive than those that we might be tempted to predict based solely on the known releases of radioactive material. It’s simply too early to precisely characterize this event.

INES was created after the Chernobyl accident. Has putting Fukushima and Chernobyl at the same level shown weaknesses in its approach or is it instead proving its value?

I think that’s open for debate. I’m not sure that adding levels would make a lot of difference in how the international nuclear safety community would react or respond, which is really the reason the scale was created; however, if the current scale contributes to public confusion, then a revision may be in order.

Aside from media, who would use the scale and for what purpose?

In my own experience as a regulator, the scale was useful for alerting me to an event and for quickly assessing how much attention I needed to pay to the event. This was particularly true after 2006, when the scale was extended to apply to all manner of nuclear events, and not just those at nuclear power plants and fuel facilities.

INES is overseen by the IAEA, but IAEA does not determine the severity of an accident or incident. Does INES operate on an honor system or some other self-reporting scheme?

I wouldn’t characterize it as an honor system, per se; however, it is true that it is not up to IAEA to establish the level of severity of an event. It is the government of the country in which the event occurs that will generally assess the event and make the call on the severity level.

As we saw with this event, others will frequently weigh in with an opinion, which may influence the rating in the end. Certainly for large events such as the one at Chernobyl, and the current event at Fukushima, even without an immediate pronouncement by the affected country, the global community will eventually detect the event, and may come to their own, possibly different, conclusion.

Keep in mind, the purpose of the scale is to roughly approximate the severity of an event, and it doesn’t really have any other practical effect. At the end of the day, it is in the affected country’s best interest to characterize the event as realistically as possible, as that will impact the international resources made available to them for event management and recovery.


I would agree that Daiichi, while certainly a very significant event, is probably not on a par with Chernobyl. If for no other reason than the differences in the way the then Soviet Union and Japan have reacted to these respective events. For example The Soviet Union was not quick to keep the regional populace from using affected foodstuffs. Specifically as I recall, they allowed the consumption of milk produced regionally which was ultimately linked to many cases of thyroid cancer in the long term. The long term effects of the earthquake/s and tsunami remain to be seen but thus far the Japanese (with a lot of international help) seem to have done a pretty good job at dealing with this particular issue.
Chris M said…
I propose that Level 8 = Blowed up real good! Most of the radioactive material from Daiichi has been released into the ocean, which not only has surface area, but depth. This should dilute the danger significantly. The ocean is over 300 quintillion gallons of water.

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