Catastrophic nuclear accidents such as the core meltdowns at Chernobyl and Fukushima are 200 times more likely to happen than previously believed, say scientists.
Based on the operating hours of all civil nuclear reactors and the number of nuclear meltdowns that have occurred in the past, a team at the Max Planck Institute for Chemistry in Mainz has calculated that such accidents could occur once every 10 to 20 years.
And, they say, when they do, half of the radioactive caesium-137 would spread more than 1,000 kilometers, and a quarter more than 2,000 kilometers.
Western Europe would receive a dosage of more than 40 kilobecquerel of caesium-137 per square meter - the International Atomic Energy Agency official contamination level - about once every 50 years.
In light of the results, the researchers are calling for an in-depth analysis and reassessment of the risks associated with nuclear power plants. Currently, there are 440 nuclear reactors in operation, and 60 more are planned.
To reach their conclusion, the researchers 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, and the number of reactor meltdowns comes to four - one in Chernobyl and three in Fukushima. This translates into one major accident, 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 US Nuclear Regulatory Commission in 1990.
The calculations of how far contamination would spread are a little more complicated, and were based on computer atmosphere models. They revealed that, on average, only eight percent of the 137Cs particles are expected to deposit within an area of 50 kilometers around the nuclear accident site, with around half further than 1,000 kilometres, and a quarter more than 2,000.
The results of the dispersion calculations were combined with the likelihood of a nuclear meltdown and the actual density of reactors worldwide to calculate the current risk of radioactive contamination around the world.
In Western Europe, where the density of reactors is particularly high, contamination by more than 40 kilobecquerels per square meter is expected to occur once in about every 50 years. The densely populated southwestern region of Germany is at greatest risk.
If a single nuclear meltdown were to occur in Western Europe, around 28 million people on average would be affected by contamination of more than 40 kilobecquerels per square meter. In southern Asia, the figure's 34 million people, due to the dense populations. In the eastern US and East Asia, 14 to 21 million people would be contaminated.
"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, director of the Max Planck Institute for Chemistry in Mainz.
"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."