Researchers show risk of nuclear meltdown drastically undervalued in risk assessment models

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Based on the operating hours of all civil nuclear reactors and the number of nuclear meltdowns that have occurred, scientists at the Max Planck Institute for Chemistry in Mainz have calculated that catastrophic nuclear accidents 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.

Currently, there are 440 nuclear reactors in operation, and 60 more are planned, however in view of their findings, the researchers call for an in-depth analysis and reassessment of the risks associated with nuclear power plants.

High human exposure risks occur around reactors in densely populated regions, notably in West Europe and South Asia, where a major reactor accident can subject around 30 million people to radioactive contamination. The recent decision by Germany to phase out its nuclear reactors will reduce the national risk, though a large risk will still remain from the reactors in neighbouring countries.

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.

Their work found that Western Europe has the worldwide highest risk of radioactive contamination caused by major reactor accidents.

By using a global model of the atmosphere the researchers determined that on average, in the event of a major reactor accident of any nuclear power plant worldwide, more than 90 % of emitted Cesium-137 would be transported beyond 50 km and about 50 % beyond 1000 km distance before being deposited.

Their results show that Western Europe is likely to be contaminated about once in 50 years by more than 40 kilobecquerel of caesium-137 per square meter.

To determine the likelihood of a nuclear meltdown, the researchers applied a simple calculation.

They divided the operating hours of all civilian nnuclear 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.

An accident risk assessment of nuclear power plants (NPPs) 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 (NRC, 1975).

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, 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).

In light of the uncertainties, the simplicity of this calculation is appealing.

The researchers went on to add; “However, based on the evidence over the past decades one may conclude that the combined probabilities (a) and (b) have been underestimated. Furthermore, by using a state-ofthe-art global atmospheric model we can directly compute the anticipated dispersion of radionuclides, avoiding the need to guess the factors (c) and (d).

 In doing so, we 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, we argue that such events are catastrophic irrespective of evacuation failure or success, and exclude the factor (e).”

H/T – CaptD   –   Source: Atmos. Chem. Phys

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