By John Timmer, Ars Technica

Yesterday, the Japanese Nuclear and Industrial Safety Agency revised its preliminary rating of the Fukushima crisis, upgrading it to the highest level on the International Nuclear and Radiological Event Scale. That places it on par with Chernobyl, the only other event to reach the maximum rating of 7 on the INES scale. The two events, however, have some very distinctive properties that raise questions about whether a single numerical rating can really convey the relevant information to the public.

[partner id="arstechnica" align="right"]The INES rating system is maintained by the International Atomic Energy Agency; it's designed to help authorities (either national or industrial) to provide an easy way to inform the public of the severity of an incident. At its low end, it involves things like minor exposures to radioactivity or the theft of short-lived isotopes. Level 7 events, at the other end of the spectrum, involve large releases of radioactive material into the environment and extended cleanup. (Details on the rating system are available in the INES User's Manual.)

Previously, the Japanese authorities were rating each reactor separately, and scored each of them as Level 5, which involved severe damage to the reactor core and a limited release of radioactive material. The new score reflects both a shift to rating everything as a single incident, and the recognition that very large quantities of radioisotopes were released into the ocean. In that sense, the change in score seems completely justified, since large quantities of radiation have been released into the environment, and estimates of the cleanup operation are beginning to run over a decade.

It may, however, be worth questioning whether the rating system is actually doing what it's intended to: providing a convenient way of informing the public. Although Fukushima clearly belongs at Level 7, that rating will inevitably invite comparisons to the only previous event to earn that score, Chernobyl. That event involved materials from the reactor core contaminating the nearby landscape and extensive spread of radioactive materials through the atmosphere, where it affected many people.

Fukushima isn't there yet. So far, most of the material in the core, including the longest-lived isotopes, seems to have stayed there. Far less material entered the atmosphere (only 10 percent of what was released by Chernobyl), and most of that drifted over uninhabited areas of the Pacific. The biggest release occurred directly into the ocean, where it poses less of a threat to humans in the short time before it is diluted into background levels. There have been people exposed to dangerous levels of radioactivity, contamination of nearby land, and threats to the food and water supply. But each of these, so far at least, has been on a smaller scale than in the Ukraine—Fukushima is bad, but it hasn't yet become Chernobyl-level bad.

All of which demonstrates how most rating scales have limited utility when it comes to extreme events. The INES scale brings to mind the World Health Organization's pandemic ratings which accurately captured the rapid global spread of 2009's flu virus, but didn't account for some critical facts: it wasn't especially deadly compared to other strains of the flu, but it tended to hit relatively healthy populations unusually hard. If you're describing an exceptional event, it just might be necessary to actually describe these sorts of details.

In any case, the struggle to get the damaged reactors back under control has continued, as the risk of Chernobyl-style contamination continues. The environmental releases seem to be abating somewhat, although Reuters is reporting the first detection of radioactive strontium, an especially dangerous isotope, in the area.

The need for long-term stabilization of the facility was driven home by a recent large aftershock, which temporarily cut the backup power to the site's cooling systems. Although the removal of radioactive materials from the site may take decades, the risk of further damage to the facility from seismic activities will require a plan to rapidly shore up the damaged reactor containment structures.

Image: SandoCap/Flickr (Note: Original image has been edited for profanity.)

*Source: Ars Technica

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