A new study has revolutionised our understanding of the likelihood of nuclear accidents such as the core meltdowns in Chernobyl and Fukushima, suggesting that these catastrophic accidents are likely to happen every 10 to 20 years, some 200 times more than had been estimated previously.

The researchers from the Max Planck Institute for Chemistry in Mainz also determined that, in the event of such an accident, half of the radioactive caesium-137 would be deposited over an area of more than 1,000 kilometres away from the nuclear reactor, well beyond national boundaries. Only 8 percent of the caesium-137 would be deposited within an area of 50 kilometres around the accident site, with another 25 percent expected to spread beyond a range of 2,000 kilometres.

Subsequently, their research showed that Western Europe — where the density of reactors is particularly high — is likely to be contaminated approximately once every 50 years by more than 40 kilobecquerel of caesium-137 per square meter (the measurement by which the International Atomic Energy Agency (IAEA) determine an area to be “contaminated”).

The citizens in the southwestern part of Germany run the worldwide highest risk of radioactive contamination, due to their close proximity to the numerous power plants situated near the borders between France, Belgium, and Germany, and when coupled with the dominant westerly wind direction.

If a single nuclear meltdown were to occur in Western Europe, approximately 28 million people would be affected by contamination. Naturally, this figure is higher in Asia where the populations are much more dense, where a major nuclear accident would end up affecting around 34 million people. The figures for the east of America and East Asia would be between 13 to 21 million people.
“Germany’s exit from the nuclear energy program will reduce the national risk of radioactive contamination. However, an even stronger reduction would result if Germany’s neighbours were to switch off their reactors,” says Jos Lelieveld. “Not only do we need an in-depth and public analysis of the actual risks of nuclear accidents. In light of our findings I believe an internationally coordinated phasing out of nuclear energy should also be considered,” adds the atmospheric chemist.

To quote the Max Planck Institute on the researchers’ methodology, “to determine the likelihood of a nuclear meltdown, the researchers applied a simple calculation.”

They divided the operating hours of all civilian nuclear reactors in the world, from the commissioning of the first up to the present, by the number of reactor meltdowns that have actually occurred. The total number of operating hours is 14,500 years, the number of reactor meltdowns comes to four—one in Chernobyl and three in Fukushima. This translates into one major accident, being defined according to the International Nuclear Event Scale (INES), every 3,625 years. Even if this result is conservatively rounded to one major accident every 5,000 reactor years, the risk is 200 times higher than the estimate for catastrophic, non-contained core meltdowns made by the U.S. Nuclear Regulatory Commission in 1990. The Mainz researchers did not distinguish ages and types of reactors, or whether they are located in regions of enhanced risks, for example by earthquakes. After all, nobody had anticipated the reactor catastrophe in Japan.

Subsequently, the researchers determined the geographic distribution of radioactive gases and particles around a possible accident site using a computer model that describes the Earth’s atmosphere. The model calculates meteorological conditions and flows, and also accounts for chemical reactions in the atmosphere. The model can compute the global distribution of trace gases, for example, and can also simulate the spreading of radioactive gases and particles. To approximate the radioactive contamination, the researchers calculated how the particles of radioactive caesium-137 (137Cs) disperse in the atmosphere, where they deposit on the earth’s surface and in what quantities. The 137Cs isotope is a product of the nuclear fission of uranium. It has a half-life of 30 years and was one of the key elements in the radioactive contamination following the disasters of Chernobyl and Fukushima.