Minoru Oogoda of Japan’s Nuclear and Industrial Safety Agency said this morning in Japan, “We have upgraded the severity level to 7 as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean.” The only other nuclear accident to reach level 7 (the highest on the IAEA’s scale) was Chernobyl. But Fukushima isn’t like Chernobyl.
The IAEA has a book 218 pages in length that describes its scale of severity. One criterion is how much total radiation gets released. It defines a level seven rating as “an event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of more than several tens of thousands of TBq [terabequerels] of Iodine-131.”
To figure this out, you add up the amounts of each different radioactive isotope released, multiply by a conversion factor that reflects the characteristics of that particular isotope. Then, you add up all the multiplied numbers.
Japan’s Nuclear and Industrial Safety Agency’s figure is 370,000 TBq, and Japan’s Nuclear Safety Commission measures it at 630,000 TBq. Both are sufficient to rank Fukushima as a level 7 incident, but Chernobyl released 5.3 million TBqs, an order of magnitude greater. There is no such thing as a “high 7” or a “low 7” but clearly there is a quantitative difference here.
Moreover, there are qualitative differences. Paddy Regan, professor of physics at the UK’s University of Surrey said in a BBC interview, “The amount of radiation release is a lot less, and the way it’s released is very different. The Chernobyl fire was putting lots of radioactive material into the atmosphere and taking it over large distances; here [Fukushima], there have been a couple of releases where they’ve vented [gas from] the reactor, and then released some cooling water.”
Another way to look at the differences is in terms of human harm. In the month after the reactor explosion at Chernobyl, 134 workers were hospitalized for acute radiation sickness, and 31 died. At Fukushima, the hospitalizations have been for radiation burns, not acute radiation sickness. And no one has died of radiation exposure (yet).
In the Ukrainian Soviet Socialist Republic and Byelorussian SSR, you had a rural population, while in Japan, the exposed area has a much greater population density. And there’s a huge difference in the way an SSR treated its population compared to Japan.
So, yes, Fukushima and Chernobyl are both ranked as Level 7 incidents. But no, they aren’t identical.
