Chernobyl, 25 Years Later

26 April 2011—Twenty-five years ago, the day the commentary of the Chernobyl catastrophe reached the West, I was in Washington, D.C., covering one of the annual meetings of the American Physical Society. A leading nuclear physicist was dragooned to speculate in front of a hushed audience about what could have gone so terribly wrong. His tentative explanation had to do with an obscure phenomenon called ”Wigner energy,” which I had never heard of and which I found hard to understand.  Evidently it was an undesirable energy release that can occur in reactors containing a lot of graphite.

That night, sitting at the bar of the funky little hotel I like to stay at when I’m in D.C., I discovered that the man sitting next to me was none other than Walter S. Sullivan, at that time the most eminent science journalist in the United States—considered by some to be the ”Dean” of science journalism. Sullivan, a charming fellow, was able to explain to me what Wigner energy was all about.

What makes this memory interesting today is that Wigner energy turned out to have nothing whatsoever to do with the Chernobyl accident. Everything the eminent American physicist had said was irrelevant, and so was Sullivan’s elegant explanation.

The major lesson of Chernobyl—and the lesson of the 1979 Three Mile Island disaster before it, and very likely that of the still unfolding Fukushima disaster—is to expect the unexpected.

After a detailed analysis of the Chernobyl incident, it turned out that the graphite-moderated reactor in use there had a peculiar design defect: When it was operating at certain power levels, if there was a loss of coolant, the reactivity of the plant would escalate abruptly and sharply. In addition, the inner containment structure of the reactor, instead of being a steel pressure vessel as is the case with most other reactors, was essentially a box with a weakly attached lid. All the fuel and control rods penetrated that lid, so if there was a sudden increase in pressure, the lid would lift and rupture all the rods.

Of course it was hard to fathom how Soviet engineers could have designed a reactor with such singular defects, and even harder to fathom why the Chernobyl operators had deliberately put the reactor into the precise state where it would be most likely to explode. But hardest of all to accept was the possibility that everything could go dreadfully wrong all at once, partly because of some underlying single cause.

”Focusing on individual components” in risk analysis can be profoundly misleading, as the Princeton University researcher M.V. Ramana pointed out in a recent commentary. The seductive but misleading reasoning was that ”a severe accident [couldn’t] happen unless multiple safety systems [failed] simultaneously,” and that ”therefore, a severe accident is exceedingly unlikely.”

The recent Fukushima disaster in Japan once again brought home the real risk of common-cause failures: A single event led to ”the loss of off-site electrical power to the reactor complex, the loss of oil tanks and replacement fuel for diesel generators, the flooding of the electrical switchyard, and perhaps damage to the inlets that brought in cooling water from the ocean,” as Ramana enumerates. ”Fukushima also demonstrated one of the perverse impacts of using multiple systems to ensure greater levels of safety: Redundancy can sometimes make things worse.”

Another outcome of Chernobyl, now to be tragically repeated at Fukushima: Long term, the invisible results of the accident will be even worse than the visible ones. It will take decades to restore the immediate physical environment of the Fukushima plants—an exercise that has not gone well in the region around Chernobyl. Meanwhile, there will be an ongoing death toll over the years from radiation-induced cancers. But these victims will be unidentifiable, not only because it will be impossible to determine if a particular cancer is caused by radiation from Fukushima or by something else, but because the total expected increases in cancer and cancer death rates will be undectable among the extremely large numbers of cancers that will occur in any case.

Right after Chernobyl, the most credible estimates of the long-term worldwide additional leukemias and solid cancers was more than 30 000. At a meeting held at the International Atomic Energy Agency in Vienna 10 years after the accident, the epidemiologist Elisabeth Cardis presented an analysis predicting that about 8500 people would die among the groups most immediately affected by the accident. But because leukemia and cancers are generally so common, it was unlikely that those deaths would be detectable in national health statistics, reported Cardis, who at that time was the head of the radiation group at the World Health Organization’s International Agency for Research on Cancer, in Lyon, France.

More recently, Cardis—now a research professor in epidemiology at the Centre for Research in Environmental Epidemiology, in Barcelona—has presented somewhat more precise estimates of the health impact of Chernobyl on Europeans. ”The risk projections suggest that by now Chernobyl may have caused about 1000 cases of thyroid cancer and 4000 cases of other cancers in Europe, representing about 0.01 percent of all incident cancers since the accident.” In the long run, the same models project a total of about 40 000 additional cancers in Europe and about 20 000 more cancer deaths.

Those estimates are similar to those found in the most recent United Nations report—the 2008 edition of UNSCEAR’s ”Sources and Effects of Ionizing Radiation”— points out Tom Cochran of the Natural Resources Defense Council, taking into account that all such estimates have a factor of 10 error range. The UNSCEAR dose estimates, says Cochran, imply about 40 000 excess cancers about about 20 000 added deaths worldwide. Shortly after Chernobyl, Cochran and Princeton University’s Frank von Hippel produced an estimate of long-term excess cancer deaths of 3500 to 70 000.

Because most cancers are so common, Cardis and her coauthors have reiterated, it was unlikely that any of the Chernobyl cancers would have been detectable. (The only exception is the thyroid cancer incidence, because thyroid disorders are much more rare than leukemia and solid cancers.)

Chernobyl was the worst nuclear accident in strictly physical terms, inasmuch as the reactor exploded and then burned for days, spewing radiation high into the atmosphere. But the effect was to disperse radioactive materials very broadly, from a local area that was thinly populated.

Fukushima shows that in terms of health effects, there may be accidents even worse than Chernobyl. A less extreme reactor accident might disperse radioactivity in more concentrated form locally, in a more populated area. Luckily for the Japanese (one hopes), winds carried most of the radiation released immediately after the accident out to sea.

Ultimately, the challenge of Chernobyl is one that Fukushima also poses: how to adequately account in public policy for risks and costs that can only be estimated but never precisely measured, and for factors that are essentially qualitative.