Probability of nuclear reactor core meltdown higher than expected

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 (NRC) in 1990. The Mainz researchers did not distinguish ages and types of reactors, or other potential contributing factors to accidents such as whether reactors are located in regions of enhanced risks, for example by earthquakes.

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 thirty years and was one of the key elements in the radioactive contamination following the disasters of Chernobyl and Fukushima.

The computer simulations revealed that, on average, only 8 percent of the 137Cs particles are expected to deposit within an area of 50 kilometers around the nuclear accident site. Around 50 percent of the particles would be deposited outside a radius of 1,000 kilometers, and around 25 percent would spread even further than 2,000 kilometers. These results underscore that reactor accidents are likely to cause radioactive contamination well beyond national borders.

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. According to the International Atomic Energy Agency (IAEA), an area with more than forty kilobecquerels of radioactivity per square meter is defined as contaminated.

The team in Mainz found that in Western Europe, where the density of reactors is particularly high, the contamination by more than 40 kilobecquerels per square meter is expected to occur once in about every fifty years. It appears that citizens in the densely populated southwestern part of Germany run the worldwide highest risk of radioactive contamination, associated with the numerous nuclear power plants situated near the borders between France, Belgium, and Germany, and the dominant westerly wind direction.

If a single nuclear meltdown were to occur in Western Europe, around twenty-eight million people on average would be affected by contamination of more than forty kilobecquerels per square meter. This figure is even higher in southern Asia, due to the dense populations. A major nuclear accident there would affect around thirty-four million people, while in the eastern United States and in East Asia this would be fourteen to twenty-one 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 neighbors 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.

— Read more in J. Lelieveld et al., “Global risk of radioactive fallout after major nuclear reactor accidents,” Atmospheric Chemistry and Physics 12 (2012): 4245-58 (doi:10.5194/acp-12-4245-2012)