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Risk of major nuclear accidents underestimated

Nuclear Monitor Issue: 
Jos Lelieveld

Major reactor accidents of nuclear power plants are rare, yet the consequences are catastrophic. But what is meant by “rare”? The results of a new study indicate that previously the occurrence of INES 7 major accidents and the risks of radioactive contamination have been underestimated. Scientists at the Max Planck Institute for Chemistry in Mainz (Germany) have calculated that such events may occur once every 10 to 20 years (based on the current number of reactors) - some 200 times more often than estimated in the past.

Nuclear accidents associated with the melting of the reactor core are caused by the failure of the cooling systems, and can have major environmental and societal consequences. In total about 20 core melt events have occurred in military and commercial reactors worldwide since the early 1950s (Burns et al., 2012). An accident risk assessment of nuclear power plants by the US Nuclear Regulatory Commission in 1975 estimated the probability of a core melt at 1 in 20 000 per year for a single reactor unit. A follow-up report in 1990 adjusted this number and indicated that the core damage frequency is not a value that can be calculated with certainty, though an appendix presented the following likelihood of a catastrophic accident (NRC: Severe Accident Risks – An Assessment for Five U.S. Nuclear Power Plants, NUREG 1150, 1990):

a. Probability of core melt 1 in 10 000 per year;
b. Probability of containment failure 1 in 100;
c. Probability of unfavourable wind direction 1 in 10;
d. Probability of meteorological inversion 1 in 10;
e. Probability of evacuation failure 1 in 10.

The product of these possibilities is 1 in 1 billion per year for a single reactor (this assumes that factors (a)–(e) are independent, which is not the case, so that the actual risk of a catastrophic accident should be higher than this). Given this, with a total of about 440 active civilian reactors worldwide, and an estimated mean remaining lifetime of 20–25 yr (together ~10 000 reactor years), then the probability of such a major accident occurring in this period would be roughly 1 in 100 000. In light of the uncertainties, the simplicity of this calculation is appealing.

However, based on the evidence over the past decades one may conclude that the combined probabilities (a) and (b) have been underestimated.

To evaluate the global risks, empirical evidence can be used to estimate the factors (a) and (b) from above. In the past decades, four INES level 7 catastrophic nuclear meltdowns have occurred, one in Chernobyl and three in Fukushima. Note again that INES 6 and lower level accidents with partial core melts such as Three Mile Island (USA), Mayak (a plutonium production and reprocessing plant in Siberia) and Sellafield (UK) are not considered. The total number of operational reactor years since the first civilian nuclear power station in Obninsk (1954) until 2011 has been about 14 500 according to the IAEA in 2011. This suggests that the probability of a major reactor accident, i.e., the combined probability of the factors (a) and (b), is much higher than estimated in 1990.

Simply taking the four reactor meltdowns over the 14 500 reactor years would indicate a probability of 1 in 3625 per reactor per year, 275 times larger than the 1990 estimate. However, since 2011 is at a junction in time with impacts of a catastrophic meltdown still unfolding, this direct estimate is high-biased, and it is rounded off to 1 in 5000 per reactor per year for use in the model simulations. This is actually only a factor of two higher than the estimated core melt probability noted above, factor (a), although originally this factor also represented partial core melts, which have occurred more frequently. Based on the past evidence, this principally assumes that if a major accident occurs, the probability of containment before substantial radioactivity release is very small. The researchers thus argue that including the factors (b)–(e) can distort the risk perception. The rounded estimate implies that with 440 civilian reactors worldwide a major accident can be expected to occur about once every few decades, depending on whether counting Fukushima as a triple or a single event.

Furthermore, by using a state-of-the-art global atmospheric model they can directly compute the anticipated dispersion of radionuclides, avoiding the need to guess the factors (c) and (d). In doing so, they find that the vast majority of the radioactivity is transported outside an area of 50 km radius, which can undermine evacuation measures, especially if concentrated deposition occurs at much greater distances from the accident, as was the case for Chernobyl in May 1986. Furthermore, even if an evacuation is successful in terms of saving human lives, large areas around the reactors are made uninhabitable for decades afterwards. Therefore, they argue that such events are catastrophic irrespective of evacuation failure or success, and exclude the factor (e).

In the report, the cumulative, global risk of exposure to radioactivity due to atmospheric dispersion of gases and particles following severe nuclear accidents (the most severe ones, INES 7), are assessed using particulate Cesium-137 and gaseous Iodine-131 as proxies for the fallout..

Using a global model of the atmosphere the scientists compute that on average, in the event of a major reactor accident of any nuclear power plant worldwide, more than 90% of emitted 137Cs would be transported beyond 50 km and about 50% beyond 1000 km distance before being deposited. This corroborates that such accidents have large-scale and transboundary impacts. Although the emission strengths and atmospheric removal processes of 137Cs and 131I are quite different, the radioactive contamination patterns over land and the human exposure due to deposition are computed to be similar. Citizens in the densely populated south-western 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.

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 50 years. According to the IAEA, an area with more than 40 kilobecquerels of radioactivity per square meter is defined as contaminated. But of course, an objective measure for dangerous radioactive contamination is debatable

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. This figure is even higher in southern Asia, due to the dense populations. A major nuclear accident there would affect around 34 million people, while in the eastern USA and in East Asia this would be 14 to 21 million people.

The report 'Global risk of radioactive fallout after major nuclear reactor accidents', by J. Lelieveld, D. Kunkel, and M. G. Lawrence is available at:

Contact: J. Lelieveld (