**A. Appendix**

is tighter than the approximate confidence interval produced earlier and corresponds to the

**Parameter Posterior mean Posterior credible interval Posterior probability**

*γ* −4.67 — *β* × 10<sup>−</sup><sup>5</sup> 5.67 0.990–10.582 95% *β<sup>I</sup>* − *β* 0.048 0.016–0.307 90% *ϕ* 39.37 10.18–90.02 90%

150 Statistics - Growing Data Sets and Growing Demand for Statistics

**Table 2.** Bayes results from the biexponential Poisson model for the Sovacool data.

Whether or not there is a change in the rate of learning over the period, the estimated probabilities of an accident or incident at a reactor in 1963 and 2010 are identical to those obtained

Previous Probabilistic Risk Assessments estimated the probability of a core melt accident to be in the range of one in several 10,000 to one in several 100,000 reactor-years. The real core melt accidents in the past happened with a probability of one in 3,700 years. Much more frequent than anticipated before. Thus, a world with 443 reactors has to expect 2.99 core melt accidents within the next 25 years, a country like the USA with 103 reactors 0.7 core

*The Guardian* data showed that incidents and accidents happen with a probability of approxi-

side of the plant operators. The second investigation based on Sovacool's data shows a

2010. There is also some indication of a stronger learning effect until the beginning of the 1960s, although this is not statistically significant. Between 1963 and 2010, the operating experience increased from 96 to 14,704 reactor-years. So, while operating experience increased by a factor of over 150, the probability of a minor or severe accident at a reactor decreased by

It might be interesting to compare the last results with the empirical core melt probability of

data) or 15 times rarer than other accidents or incidents. Regarding the possible outcomes of a core melt accidents, these differences seem to be unexpectedly low and might indicate that the

This guess finds support in an article by Phillip A. Greenberg. 'Between 1990 and 1992 the US Nuclear Regulatory Commission received more than 6600 "Licensee Event Reports" because US nuclear plants failed to operate as designed and 107 reports because of

datasets used do not contain all incidents and accidents that happened in the past.

. Depending on the dataset, a core melt accident is only 37 times (*The Guardian*

per reactor-year. The data are consistent with no learning effect on the

per reactor-year in 1963 to 0.004 = 4 × 10<sup>−</sup><sup>3</sup>

in

years 1959–1963.

**6. Summary**

melt accidents.

mately 0.001 = 1 × 10<sup>−</sup><sup>3</sup>

merely a factor of 2.5.

1/3700 = 0.27 × 10<sup>−</sup><sup>3</sup>

earlier from the likelihood analysis.

decrease of the accident rate from 0.010 = 10 × 10<sup>−</sup><sup>3</sup>



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**Table A1.** Total nuclear energy produced in TWh [3] (until 31 Dec. 2015) and reactor-years [3] (until 31 Dec. 2011) and number of accidents [7] (until 31 Dec. 2011); countries which started to build reactors which were not operating are excluded.
