Science and technology
It’s the worst; it’s also not so bad
Apr 12th 2011, 18:09 by O.M.
ON APRIL 11th, the Japanese authorities announced that they had reassessed the severity of the ongoing crisis at the Fukushima Dai-ichi nuclear plant. Having previously categorised the incidents at reactors 1, 2 and 3 in the plant as three different accidents classified as level 5 on the International Nuclear Event Scale (INES) they were now going to treat them all together as a level seven accident. According to the International Atomic Energy Agency the fire in a spent-fuel pool at reactor number 4 is still classified independently at level 3 on the INES scale, making it an “incident”, not an “accident”.
The INES scale (the subject of today’s Daily Chart) is designed to communicate the seriousness of a nuclear event in a way that the media and public can understand, as they sort-of-grasp the magnitude scale by which earthquakes are measured. Level 7 is the highest level on the scale, and has previously been awarded only to the Chernobyl disaster of 1986. Thus the new rating seems to make Fukushima in some way as bad as Chernobyl. It isn’t, which means that in this case the use of the INES scale may be as likely to confuse or mislead the public as to clarify matters.
Japan’s Nuclear and Industrial Safety Agency estimates that the emission of radioactive iodine and caesium from the Fukushima plant totals, to date, something equivalent to 370 petabecquerels. (In the confusing world of the measurement of radioactivity, which also features grays, sieverts and curies, among others, one becquerel represents one nuclear decay per second. A petabecquerel is a thousand trillion becquerels.) Japan’s Nuclear Safety Commission has announced a figure a bit less than twice that. The equivalent figure for Chernobyl was about ten times larger still.
Since the INES scale is modelled to some extent on the earthquake scale, its qualitatively assessed levels are, broadly speaking, logarithmic—each is a factor of ten worse, or more severe, than the previous one. On that basis it is not perhaps unreasonable for Fukushima to be classed with an accident ten times worse. But the becquerels don’t tell the whole story. At Chernobyl, evacuation was delayed and a lot of radioactive material got into the food chain and thus into people, where it does the most harm. What is more, though some of it travelled far, Chernobyl’s fallout almost all fell on inhabited land. At Fukushima, some of the emissions have been into water that has gone straight out to sea, and a lot of the airborne emissions have ended up over the sea, too. The sea dilutes things pretty well. That said, food chains can concentrate things, and seafood from the area will certainly merit careful monitoring.
The contaminants that fell on to the land did so mostly but not entirely in nearby places that had already been evacuated. (A line of deposition to the northwest of the plant which extended more than 30km was not fully evacuated straight away.) Food from the most contaminated places is not getting into the food chain. This is important because radioactive iodine, in particular, can be nasty stuff—it is readily absorbed by the body and stored in the thyroid gland, an accumulation that makes it a cause of cancer in children. And the Japanese authorities have been distributing tablets of ordinary, non-radioactive iodine to the local population, on the basis that bodies already well stocked with iodine don’t bother to assimilate any more of the stuff and will excrete, rather than accumulate, the fallout. Thus the consequence of the release in terms of public health looks set to be far, far less than that of Chernobyl, where a clear spike in childhood thyroid cancer was seen.
The story from the plants themselves makes this clear. A month after the Chernobyl disaster, at least 23 reactor operators and firemen had died, mostly of radiation sickness. A month on from the earthquake the death toll at Fukushima remains at three, and those plant workers were killed by the tsunami, not the reactors or their fuel. Three other workers are in hospital after receiving large doses of radiation from standing in contaminated water.
There is no denying that the cumulative releases from Fukushima add up to a “major release” in the way the INES criteria define such things, and in pretty much any other way, too. (Though it remains to be seen how much of that contamination came from the reactor accidents classed as level 7 and how much from the spent fuel pools alongside them.) But as well as talking of a “major release”, INES criteria for level 7 also include “widespread health and environmental effects requiring implementation of planned and extended countermeasures”. At Fukushima, there are undoubtedly widespread environmental effects, and the countermeasures are and will be extended—agricultural restrictions for some areas may remain in place for a long time, and some places may end up depopulated. But because of those countermeasures the likelihood of widespread health effects remains low. (It is also worth remembering that in terms of becquerels things are already a lot better than they were, as iodine-131 has a half-life of only eight days. This means that iodine emitted thirty two days ago has by now lost fifteen-sixteenths of its radioactivity.)
On the basis that simplifying eases communication the INES scale runs together scale and consequence—but good policy can to some extent separate them, and in Japan it seems to have done so. (Good fortune also helps: other wind and rain conditions could have led to a lot more contamination on land.) The point is illustrated by the fact that the radiological threat at Fukushima appears considerably lower than that which followed the only accident currently rated at number 6 on the scale. In the so-called “Kyshtym” accident in the Soviet Union in 1957 a tank of radioactive waste at a military installation exploded. This, according to Richard Wakeford of the University of Manchester, led to 740 petabecquerels of fission products being released. By the time they were evacuated a week or so later 10,000 people had picked up an average effective dose of 120 millisieverts, higher than has yet been seen in any member of the public from around Fukushima. More than 5,000 workers received doses of as much as one sievert within a few hours, Professor Wakeford says, and some 30,000 clean-up workers received doses of more than 250 millisieverts in the following years.
None of this makes Fukushima trivial; it is a grave crisis. Things are still scarily un-locked-down—an earthquake on April 11th interrupted the cooling pumps again, though only briefly. The psychological stress has been intense, and the long-term bill for cleanup will be massive. But in terms of hazard the situation appears to be improving, if in fits and starts. It has been big, yes, perhaps a lot bigger than industry experts originally expected or were willing to admit. But it does not seem, in public health terms, to have turned out too bad.