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When the likelihood of catastrophic failure of a complex system is infinitesimal, the problem of protecting that system from such a failure assumes quite a different complexion from that associated with Â¿everydayÂ¿ (high-probability) failures. Such is the case with nuclear power plants, which, in the most authoritative study to date on the topic of accident risks in commercial nuclear plants,Â¿ are projected to yield risks of only one core melt accident per plant every 17 000 years. The consequences of a core melt accident depend on three factors: the amount of radioactivity released, the way it is dispersed due to prevailing weather conditions, and the number of people exposed. When these three factors in 4800 different combinations were assessed for the U.S. in the aforementioned study, the conclusions were: The probability of 100 or more fatalities is predicted to be about 1 in 1 000 000; the largest calculated value was 2300 fatalities with a probability of about one in a billion.