Fukushima rated at INES Level 7 – what does this mean?

by Barry Brook

Hot in the news is that the Fukushima Nuclear crisis has been upgraded from INES 5 to INES 7. Note that this is not due to some sudden escalation of events  (aftershocks etc.), but rather it is based on an assessment of the cumulative magnitude of the events that have occurred at the site over the past month.

Below I look briefly at what this INES 7 rating means, why it has happened, and to provide a new place to centralise comments on this noteworthy piece of news.

The International Nuclear and Radiological Event Scale (INES) was developed by the International Atomic Energy Agency (IAEA) to rate nuclear accidents. It was formalised in 1990 and then back-dated to events like Chernobyl, Three Mile Island, Windscale and so on. Prior to today, only Chernobyl had been rated at the maximum level of the scale ‘major accident’. A useful 5-page PDF summary description of the INES, by the IAEA, is available here.

A new assessment of Fukushima Daiichi has put this event at INES 7, upgraded from earlier escalating ratings of 3, 4 and then 5. The original intention of the scale was historical/retrospective, and it was not really designed to track real-time crises, so until the accident is fully resolved, any time-specific rating is naturally preliminary.

The criteria used to rate against the INES scale are (from the IAEA documentation):

(i) People and the Environment: considers the radiation doses to people close to the location of the event and the widespread, unplanned release of radioactive material from an installation.

(ii) Radiological Barriers and Control: covers events without any direct impact on people or the environment and only applies inside major facilities. It covers unplanned high radiation levels and spread of significant quantities of radioactive materials confined within the installation.

(iii) Defence-in-Depth: covers events without any direct impact on people or the environment, but for which the range of measures put in place to prevent accidents did not function as intended.

In terms of severity:

Like the scales that describe earthquakes or major storms, each of the INES scale’s seven levels is designed to be ten times more severe that the one before. After below-scale ‘deviations’ with no safety significance, there are three levels of ‘incident’, then four levels of ‘accident’. The selection of a level for a given event is based on three parameters: whether people or the environment have been affected; whether any of the barriers to the release of radiation have been lost; and whether any of the layers of safety systems are lost.

So, on this definitional basis, one might argue that the collective Fukushima Daiichi event (core damage in three units, hydrogen explosions, problems with drying spent fuel ponds, etc.) is ~100 times worse than TMI-2, which was a Level 5.

However, what about when you hit the top of the INES? Does a rating of 7 mean that Fukushima is as bad as Chernobyl? Well, since you can’t get higher than 7 on the scale, it’s impossible to use this numerically to answer such a question on the basis of their categorical INES rating alone. It just tells you that both events are in the ‘major league’. There is simply no event rating 8, or 10, or whatever, or indeed any capacity within the INES system to rank or discriminate events within categories (this is especially telling for 7). For that, you need to look for other diagnostics.

So headlines like ‘Fukushima is now on a par with Chernobyl‘ can be classified as semantically correct and yet also (potentially) downright misleading. Still, it sells newspapers.

There is a really useful summary of the actual ‘news’ of this INES upgrade from World Nuclear News, here. It reports:

Japanese authorities notified the International Atomic Energy Agency of their decision to up the rating: “As a result of re-evaluation, total amount of discharged iodine-131 is estimated at 1.3×10¹⁷ becquerels, and caesium-137 is estimated at 6.1×10¹⁵ becquerels. Hence the Nuclear and Industrial Safety Agency has concluded that the rating of the accident would be equivalent of Level 7.”

More here from the IAEA:

The new provisional rating considers the accidents that occurred at Units 1, 2 and 3 as a single event on INES. Previously, separate INES Level 5 ratings had been applied for Units 1, 2 and 3. The provisional INES Level 3 rating assigned for Unit 4 still applies.

The re-evaluation of the Fukushima Daiichi provisional INES rating resulted from an estimate of the total amount of radioactivity released to the environment from the nuclear plant. NISA estimates that the amount of radioactive material released to the atmosphere is approximately 10 percent of the 1986 Chernobyl accident, which is the only other nuclear accident to have been rated a Level 7 event.

I also discussed the uprating today on radio, and you can listen to the 12-minute interview here for my extended perspective.

So, what are some of the similarities and differences between Fukushima and Chernobyl?

Both have involved breeches of radiological barriers and controls, overwhelming of defence-in-depth measures, and large-scale release of radioactive isotopes into the environment. The causes and sequence of the two events were, however, very different, in terms of reactor designs, the nature of the triggering events, and time-scale for resolution — this is a topic to be explored in more depth in some future post. The obviously big contrast is in the human toll and nature of the radioactive release.

The Chernobyl event killed 28 people directly via the initial explosion or severe radiation sickness, and other ~15 died as directly attributed result of radiation-induced cancer (see the summary provided today by Ben Heard on Opinion Online: Giving Green the red light). Further, Chernobyl led to a significant overexposure of members of the public in the local area and region, especially due to iodine-131 that was dispersed by the reactor fire, and insufficient protection measures by authorities. An increase in thyroid cancers resulted from this.

In Fukushima, by contrast, no workers have been killed by radiation (or explosions), and indeed none have been exposed to doses >250 mSv (with a ~1000 mSv being the dose required for people to exhibit signs of radiation sickness, through to about 50 % of victims dying after being exposed to >5000 mSv [see chart here]). No member of the public has, as yet, been overexposed at Fukushima. Further, much of the radionuclides released into the environment around Fukushima have been a result of water leakages that were flushed into the ocean, rather than attached to carbon and other aerosols from a burning reactor moderator, where they were largely deposited on land, and had the potential to be inhaled (as occurred in Chernobyl).

So is Fukushima another Chernobyl? No. Is it a serious accident? Yes. Two quite different questions — and answers — which should not be carelessly conflated.