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Not all nuclear accidents have been rated on the scale

Some of the events on this list predate the scale and do not appear to have been rated. They are listed by Mr. Cochran in chronological order.

Keeping Score on Nuclear Accidents – NYTimes.com, Matthew Wald 12 April 11, Now that Japan has raised its assessment of the Fukushima accident to a 7 on the International Atomic Energy Agency’s scale, equal to the 1986 accident at Chernobyl, it may be time to review past accidents. Thomas B. Cochran, a physicist at the Natural Resources Defense Council, just did that in preparing to testify on Tuesday afternoon before the Senate Environment and Public Works Committee.
Some of the incidents that he lists are technically not meltdowns but rather “core damage accidents.” That term is used when an intact core holds in nearly all of the radioactive materials that are created by a reactor as it splits atoms of uranium and plutonium, leaving behind fragment atoms of materials like cesium, strontium and iodine, which seek to return to stability by giving off radiation. If the core melts, as it did at Fukushima, or explodes, as it did at Chernobyl, that radioactive material is released.

The seven-level scale for the seriousness of the accidents runs from “anomaly,” something that would probably not be mentioned in a newspaper, to “incident,” which might be, to an event with major off-site consequences for health and the environment, like Chernobyl or Fukushima. Some do not involve reactors: Japan, for example, experienced an accident in 1999 at a plant that processes plutonium fuel. The plutonium was stored in a liquid in a vessel that was too large, inadvertently creating a “critical mass,” an amount capable of sustaining a chain reaction. The chain reaction created a shower of radiation and heat, blowing apart the critical mass, but as it cooled, it re-assembled. That rated a level 4.

In fact, the International Atomic Energy Agency pointed out a few hours after Japan announced the 7 rating on Tuesday, the 7 applies to reactors 1, 2 and 3 at Fukushima Daiichi. The accident at Unit 4 is not in the reactor but in the spent fuel pool and is still rated at 5.

Some of the events on this list predate the scale and do not appear to have been rated. They are listed by Mr. Cochran in chronological order.:

1. Sodium Reactor Experiment (SER)
Location: Santa Susana Field Laboratory, California, United States………

Keeping Score on Nuclear Accidents – NYTimes.com

April 13, 2011 - Posted by Christina Macpherson | safety